I
I
ILLUMINATING
ENGINEERING PRACTICE
LECTURES
ON ILLUMINATING ENGINEERING
DELIVERED AT THE
UNIVERSITY OF PENNSYLVANIA
PHILADELPHIA, SEPTEMBER 20 TO 28, 1916
UNDER THE JOINT AUSPICES OF
THE UNIVERSITY AND THE
ILLUMINATING ENGINEERING
SOCIETY
McGRAW-HILL BOOK COMPANY, INC.
239 WEST 39TH STREET. NEW YORK
LONDON: HILL PUBLISHING CO., Lnx
6 & 8 BOUVERIE ST., E. C.
1917
VI PREFACE
volume, were delivered at the University of Pennsylvania between the
dates September 20 and September 28 inclusive, by men peculiarly
qualified by training and experience to present the most advanced
treatment of illumination problems.
It is worthy of record here that there were 180 subscriptions to
the entire course and that in addition 59 tickets to individual
lectures were sold. Supplementing the lectures an exhibit was
arranged which exemplified modern methods of illumination and
illustrated modern lighting appliances. An inspection tour was also
organized in connection with the lectures, including visits to places
of interest to lighting men, in Pittsburgh, Washington, Philadelphia,
Atlantic City, New York, Boston, Schenectady, Buffalo, Cleveland
and Chicago.
EDWARD P. HYDE.
THE INCEPTION OF THE 1916 ILLUMINATING ENGINEERING COURSE
In considering special activities when undertaking the Presidency
of the Illuminating Engineering Society in the summer of 1915, I
conceived the idea of a course of lectures on illuminating engineering
which would be supplementary to the course held at The Johns
Hopkins University in 1910, and which would emphasize the practical
rather than the theoretical aspect of the subject. Later it developed
that members of the faculty of the University of Pennsylvania had
discussed a like project. Happily these two ideas, of independent
origin, were brought together before the Council of the Illuminating
Engineering Society, and the lecture course was duly consummated.
The result has been very gratifying to the Illuminating Engineering
Society. The value of the course was demonstrated at the time of
its presentation. This book is expected to extend that value
materially.
CHARLES P. STEINMETZ.
OPENING EXERCISES
The lecture course followed immediately upon the adjournment
of the 1916 Annual Convention of the Illuminating Engineering
Society, which was held in Philadelphia. On the evening preceding
the first lecture, and following the closing session of the Convention,
a meeting was held in the auditorium of the Museum of the Uni-
versity of Pennsylvania, to which meeting the public was invited.
The following interesting program was carried out:
Address CHARLES P. STEINMETZ,
President Illuminating Engineering Society.
Address EDGAR F. SMITH,
Provost University of Pennsylvania.
Address EDWARD P. HYDE,
Chairman 1910 and 1916 I.E.S. Committees on
Lectures.
Popular Lecture Subject, "Controlled Light"
WM. A. DURGIN,
Director Illuminating Engineering Society.
A large and enthusiastic audience greeted the distinguished
speakers. Representations from the faculty and undergraduate
body of the University, from the membership of the Illuminating
Engineering Society, and from the local lighting organizations,
combined to make the occasion auspicious.
Expression of Appreciation Tendered by the Illuminating Engineering
Society to the University of Pennsylvania
The very able and cordial cooperation of the faculty and staff
of the University of Pennsylvania which contributed largely to the
success of the Illuminating Engineering Lecture Course prompted
the Council of the Illuminating Engineering Society to forward
to Provost Smith of the University an engrossed "appreciation"
couched in the following terms:
The Council of the Illuminating Engineering Society expresses
vii
Vlll OPENING EXERCISES
its appreciation of the courteous cooperation of the Provost and
Faculty of the University of Pennsylvania in the joint organization
and conduct of the Illuminating Engineering Lecture Course,
September 2ist to 28th, 1916.
(Signed) G. H. STICKNEY, (Signed) WM J. SERRILL,
General Secretary. President.
December 14, 1916.
CONTENTS
PAGE
PREFACE v
COMMITTEE ON LECTURES x
? Illumination Units and Calculations i
By A. S. MCALLISTER.
The Principles of Interior Illumination, Parts I and II 37
Committee: J. R. CRAVATH, WARD HARRISON, R. ff. PIERCE.
The Principles of Exterior Illumination 77
By Louis BELL.
Modern Photometry go.
By CLAYTON H. SHARP.
Recent Developments in Electric Lighting Appliances 131
By G. H. STICKXEY.
Recent Developments in Gas Lighting Appliances 165
By ROBERT ff. PIERCE.
Modern Lighting Accessories 183
By W. F. LITTLE.
Light Projection: Its Applications 213
By E. J. EDWARDS and H. H. MAGDSICK.
The Architectural and Decorative Aspects of Lighting 253
By GUY LOWELL.
Color in Lighting 267
By M. LUCKIESH.
Church Lighting Requirements 1 297
By E. G. PERROT.
, The Lighting of Schools, Libraries and Auditoriums 307
By F. A. VAUGHN.
The Lighting of Factories, Mills and Workshops 337
By C. E. CLEWELL.
The Lighting of Offices, Stores and Shop Windows 363
By NORMAN MACBETH.
The Lighting of the Home 395
By H. W. JORDAN.
The Lighting- of Streets (Part I) 415
By PRESTON S. MILLAR.
Street Lighting (Part II) 461
By C. F. LACOMBE.
Railway Car Lighting 493
By GEORGE H. HULSE.
The Lighting of Yards, Docks and Other Outside Works 513
By J. L. MINICK.
Sign Lighting 535
By L. G. SHEPARD.
ix
2 ILLUMINATING ENGINEERING PRACTICE
curves and can be converted into " light curves" only after making
proper modifications in accordance with certain well-defined solid
geometrical relations. It seems appropriate to give emphasis to
this statement by defining the solid geometrical relations referred
to, which are equally as simple as plane geometrical or trigonomet-
rical relations.
SOLID GEOMETRICAL RELATIONS
Of the several space geometrical relations with which an illuminat-
ing engineer should be familiar, by far the most important, and
happily the simplest, is that existing between the external area or
zonal area of a sphere and its diameter or zonal width. This rela-
tion is one of direct proportion. That is to say, the external area
Fig. I. Spherical geometrical relations.
of a zone of any chosen sphere varies directly with the width of the
zone, and the total external area is that of a zone having a width
equal to the diameter of the sphere.
In almost all cases of application to illumination problems, one
is interested in the relative values rather than the actual values of
the various zonal areas and the above mentioned proportion is all
that he needs to take into consideration. However, one can de-
termine the actual as well as the relative values with extreme sim-
plicity by means of certain plane geometrical or trigonometrical
relations applied to the sphere.
In Fig. i, which represents a sphere cut along a vertical plane
through the center O, the zone of infinitesimal vertical width* ED,
along the diameter, has an external area represented by the sloping
MCALLISTER: ILLUMINATION UNITS 3
width at C multiplied by the circumference of the zonal circle passing
horizontally through C. Now the circumference of the horizontal
circle through C bears to that of the horizontal circle through B
(that is, the " great circle" of the sphere), the relation of cos < to i.
Likewise the sloping width of the zone at C bears to the vertical
width ED the inverse ratio, i to cos <.
Since these two ratios, one the inverse of the other, are to- be
multiplied together in determining the zonal area, it is obvious
that the external area of the zone having a width ED along the
diameter is equal to the product of this width by the circumference
of the "great circle." Similarly the total external area of the sphere
is found by multiplying the sphere diameter (= total width of all
zones) by the circumference of the great circle; or is equal to
dXird = 7rd z = 4irr 2 where d is the diameter and r the radius of
the sphere.
Familiarity with the above fundamental spherical (space) geomet-
rical relations is absolutely essential to a proper understanding of
the significance of the curves showing the space distribution of the
candle-power of light sources; to the derivation or interpretation
of diagrams showing^ the light from sources whose candle-power
curves are known, and to the solution of problems relating to plane
surface or extended surface sources.
It is noteworthy in this connection that the modern tendency is
away from point sources, and point-source candle-power methods
of calculation, towards extended source and lumen-output calculat-
ing methods, so that the importance of becoming familiar with space
geometrical relations is ever on the increase.
UNIT SOLID ANGLE THE STERADIAN
Although the illuminating engineer is seldom called upon to make
use of solid angular dimensions expressed in terms of any unit of
solid angular measurement, because almost all of the calculations
in which he is interested can be based on ratios rather than, actual
values of solid angles, yet it may at times be found convenient to
refer to some solid angular measurement in terms of a unit of
measurement. Two distinct units have been employed for this
purpose, one represented by the whole sphere and the other by
a value i -5- 4?r as large. For the former no special name has
been standardized, while to the latter the name "steradian" is
applied.
4 ILLUMINATING ENGINEERING PRACTICE
v.
From its definition it will be seen that any zone on a sphere having
a diametrical width such that W = d -r- 471-, where d is the diame-
ter of the sphere, will subtend a solid angle of one steradian, and
that 4?r = 12.57 -f steradians equal one sphere in solid angular
measurement.
Since the external surface of a sphere of unit radius is equal to
47r units of area, it follows that a steradian is an angle having such
a value as to subtend unit area on the surface of a sphere of unit
radius, or an area equal numerically to the radius squared on a
sphere of any dimension whatsoever expressed in any unit of length
or area.
It is sometimes stated that the solid angle subtended by a chosen
area when viewed from a chosen position can be calculated in
steradians by dividing the numerical value of the area by the square
of the distance between the point selected and the area. This
statement is correct only when applied to an area every infinitesimal
element of which occupies the same distance from the point of
observation; that is, when the area lies on the circumference of a
sphere having its center at the point chosen.
RELATION BETWEEN LIGHT AND CANDLE-POWER DISTRIBUTION
In order to present most clearly the exact significance of the
candle-power curve, explain most readily the diagram for showing
the distribution and summation of the light flux (lumens) from the
source, and to give proper emphasis to the necessary distinction
between candle-power distribution and light distribution use will
be made of the curve of candle-power of a source giving light in
only one hemisphere.
In order definitely to fix ideas it will be assumed that the maxi-
mum candle-power of the source is 100 and that the candle-power
decreases uniformly according to a cosine function of the angle of
vision to zero at 90 degrees from the position of maximum candle-
power^ The curve showing the distribution of candle-power of
such a source (which could be for example, an infinitesimal plane
radiating in accordance with the " cosine law" of space distribution
of candle-power) is represented in Fig. 2.
Assume now that the source is placed at the center of a hollow
sphere of unit radius the interior surface of which is illuminated by
the source, as indicated in Fig. 2. The illumination on each ele-
mentary area of the surrounding sphere will at each point be numeric-
MCALLISTER: ILLUMINATION UNITS
5
ally equal to the candle-power of the source when observed from
that point expressed in foot-candles if the radius of the sphere is
one foot; in meter-candles if the radius is one meter, etc. Hence
to determine the lumens incident upon any chosen section of the
surrounding sphere it is necessary merely to multiply the area of
that section by the mean candle-power of the source effective over
that section.
It is convenient not only for present purposes but also for purposes
of subsequent comparisons, to express the area of sections of the
surrounding sphere in terms of the zones cut off by various angles
below (and above) the horizontal.
Surface
Source of
Light
Figs. 2 and 3. Space distribution of candle-power and light flux from infinitesimal
surface source.
It should here be observed that, for sake of convenience in deriva-
tion and explanation, the angles indicated herein are measured
(in both the plus and the minus direction) from the horizontal plane,
whereas in actual curves of candle-power distribution the angles
of elevation are "counted positively from the nadir as zero to the
zenith as 180 degrees." That is to say, whereas in the curves herein
shown the vertical angles are measured through zero from minus
90 degrees to plus 90 degrees, it is the more usual plan to make all
measurements in the positive direction from zero plotted at the
bottom of the curve to 180 degrees at the top.
The zonal areas measured from the horizontal plane are as
follows :
ILLUMINATING ENGINEERING PRACTICE
Zonal angle from
horizontal
Zonal width
sine of angle
Zonal area
2v zonal width
Max. C. P. of zone
0-15
0.259
1.6 3
25-9
0-30
0.500
3-14
50.0
o-45
0.7070
4-44
70.7
0-60
0.866
5-44
86.6
o-75
0.969
6.06
96.6
0-90
i .000
6.28
IOO.O
Candle-Power
Lumens
Zone
Area
Max.
Min.
Mean
Area X CP
0-30
30-60
60-90
3-14
2.30
0.84
50.0
86.6
IOO.O
00.0
50.0
86.6
25.0
68.3
93-3
78.5
I57-I
78.4
Total
6 28
Total.. . .
314.0
3 4
The vertical widths of the separate zones are represented by the
vertical line at the extreme right in Fig. 3. Along this line have
been erected certain perpendiculars for representing the candle-
power values over each part of the zone width. The product of the
candle-power at each point by the zone area at that point which
bears the constant relation of 2ir -r- i to the vertical width of each
zone, gives the lumens over that zone to a certain scale. Obviously
the area of the triangular figure at the right in Fig. 3 represents (to a
scale involving the candle-power scale, the distance scale and the
constant 2ir) the total lumens radiated by the source. From this
figure, known as the Rousseau diagram, the lumens effective over
any chosen zone can be computed at once from the intercepted area
on the diagram. This is not an approximate, but an absolutely
exact method of calculation. Any errors involved in using the
method can be attributed to inaccuracies in measuring or plotting
the candle-power or in determining the areas from the diagram;
that is, to inexactness in carrying out the method rather than to the
method itself.
By using the Rousseau diagram merely as an aid in visualizing
the problem and resorting to plane or spherical geometrical or
trigonometrical calculations for actual determinations, one can often
eliminate all inaccuracies other than those inherent in the photometric
testing of the lighting source.
MCALLISTER: ILLUMINATION UNITS 7
If the candle-power had been uniform throughout the lower hemi-
sphere at a value equal to the actual maximum of 100 the total
number of lumens would have been 628, or just twice the actual
value. Similarly, if the uniform candle-power of 100 has been
active throughout both the upper and the lower hemisphere, the
lumens output from the source would have totalled 1256, or four
times the actual value determined above by slide-rule computation.
The mathematically exact result would be,
Area X c.p. = 4^ X 100 = 1256.64 +.
The exact ratio between the total lumens produced by the light-
ing source having the candle-power distribution indicated in Fig.
2, and the lumens that would be produced by a source giving
uniform candle-power in all directions equal to the maximum in
Fig. 2, is i -T- 4. Obviously this ratio, which is called the "spherical
reduction factor," in any practical case depends upon the shape of
the candle-power distribution curve, becoming indefinitely small in
the case of a concentrated beam and reaching a maximum of i.o
in the case of a source of uniform candle-power such as a spherical
surface source.
It may be well at this point to call attention to the fact that the
"mean spherical candle-power" of a surface source of any shape
whatsoever is equal to one-fourth of the product of the effective
radiating area by the maximum candle-power of an (infinitesimal)
unit area of the source, provided only that each infinitesimal area
radiates in space according to the cosine law of space-distribution
and all infinitesimal areas have the same maximum value of candle-
power. The total effective candle-power in any chosen direction
observed at any chosen position from such a source is equal to the
product of the candle-power per unit area by the "projected area"
of the source as viewed from the direction (and exact position)
chosen. These facts will be discussed in greater detail later in
connection with the subjects of "brightness" "output" and
"appearance."
On account of the fact that such curves as those shown in Fig. 2
are often loosely referred to as "light-distribution" curves, rather
than "candle-power-distribution" curves, certain misconceptions
have been produced in the minds of persons not familiar with the
exact physical significance of the geometrical representation of the
photometric relations.
In order to lay proper emphasis on the distinction that must be
8
ILLUMINATING ENGINEERING PRACTICE
made between " light distribution" and " candle-power distribution,"
a comparison will be made with the actual distribution of light in
each vertical zone (as accurately shown by the Rousseau diagram
of Fig. 3) and the distribution of light which would exist if the curve
of Fig. 2 were in reality a "light distribution" rather than a "candle-
power distribution" curve. This curve is reproduced in Fig. 4,
where it is treated as representing "light distribution," and on the
basis of this interpretation the Rousseau diagram of Fig. 5, has been
constructed by the methods already explained. A comparison of
the incorrect diagram of Fig. 5, with the correct diagram of Fig. 3
will serve to show the inaccuracy in treating a "candle-power
distribution" curve as a "light distribution" curve.
Figs. 4 and 5. Space distribution of light from an assumed source and corresponding flux
summation diagram.
CANDLE POWER DISTRIBUTION FROM CYLINDRICAL AND
SPHERICAL SURFACE SOURCES
In Fig. 6, the smaller double circles show the space distribution
of candle power around an infinitesimal cylindrical surface source
having a vertical axis. In Fig. 7, the elliptical area is the Rousseau
diagram showing the light flux produced over various zones of the
sphere surrounding the light source, as explained above.
In Fig. 6, the large central circle shows the candle-power distribu-
tion around a spherical surface source; the corresponding Rousseau
diagram is represented by the rectangular area in Fig. 7. The
separate curves of Fig. 6 have been so drawn that the rectangular
area of Fig. 7 is equal to the elliptical area of the same figure. That
MCALLISTER: ILLUMINATION UNITS 9
is, the light output from the cylindrical surface has been made equal
to the light output from the spherical surface source.
It will be recalled, from well-known trigonometrical and geomet-
rical relations, that the area of an ellipse is equal to Tr/4 times the
product of the major and minor axes, whereas that of a rectangle
is equal to the product of the major and minor sides. It follows
therefore that the minor side of the rectangle in Fig. 7 is equal to
-90
Figs. 6 and 7. Space distribution of candle-power from infinitesimal cylindrical and spher-
ical sources and corresponding flux summation diagrams.
7T/4 times the minor axis of the ellipse, and hence the maximum
(uniform) candle-power of the spherical surface source is equal to
ir/4 times the maximum (horizontal) candle-power of the cylindrical
surface source in Fig. 6. That is to say, the "spherical reduction
factor" of a cylindrical surface source is equal to 7r/4 = 0.7854.
This is the value usually assigned to a so-called "line-source,"
which has no existence in reality, its nearest approach in practice
10 ILLUMINATING ENGINEERING PRACTICE
being the cylindrical surface of a lamp filament having an inappreciable
diameter.
SPACE REPRESENTATION OF CANDLE-POWER DISTRIBUTION
By means of models representing solids of revolution of the
candle-power curves about the axis of reference one can obtain a
better idea of the real significance of the space distribution of the
candle-power than can be obtained from the flat candle-power curve
which must in any event be interpreted as showing merely a cross-
sectional view of such a space-model. In interpreting a candle-
power distribution model care must be exercised in giving signifi-
cance to the quantities represented. Special emphasis must be
placed on the fact that neither the volumetric content of the model
nor the superficial area has any immediate relation to the flux of
light from the source giving the candle-power indicated by the
model. A striking illustration of this fact is afforded by a com-
parison of the centrally located candle-power circle in Fig. 6 with
the completely displaced candle-power circle in Fig. 2.
As already shown by means of the Rousseau diagrams of Fig. 7
and Fig. 3, the flux produced by the source giving the circular candle-
power curve of Fig. 6 is exactly equal to that produced by the
source giving the circular candle-power curve of Fig. 2, and hence
the solid of revolution of Fig. 6 represents exactly the same amount
of flux as does the solid of revolution of Fig. 2.
The diameter of the circle in Fig. 2 is exactly twice as great
as that in Fig. 6; the superficial area of the solid of revolution of
Fig. 2 is four times that of Fig. 6, and its volumetric content is
eight times as large.
A certain percentage of the volumetric content or superficial area
of any chosen solid of revolution represents the same percentage of
the total flux of light from the source only in the limiting cases of
uniform candle-power in all directions as shown by the centrally
located circle of Fig. 6 or of a section of the sphere cut vertically
throughout the whole depth.
From the two illustrations chosen above, it will be observed that
even when the scale of candle-power is defined, the total flux repre-
sented by a given solid of revolution is known only when the exact
location of the light source within the sphere is known. With the
source at the center, the sphere represents the maximum of light
flux; when the source is at the surface (as in Fig. 2) the light flux
MCALLISTER: ILLUMINATION UNITS n
has only one-half of the maximum value, all other quantities,
dimensions and scales remaining the same.
SPHERICAL SURFACE: THE SO-CALLED "POINT "-SOURCE
For many purposes it has been found convenient to refer to a
source of light as though it were a " point" (that is, without dimen-
sions) and by certain mathematical transformations certain equa-
tions applicable exclusively to surface sources have been treated
as though they related to true point-sources. When dealing with
illumination effects at a distance, no measurable errors are involved
in such assumptions and transformations, but when one attempts
to define the "brightness" or appearance of the source to the eye
on the basis of an assumed point-source, the assumptions are found
to be at conflict with the most significant physical fact, which is that
the brightness is a function of the area, whereas 'points (even an in-
finite number of them) are devoid of dimensions or area.
By treating the so-called "point-source," not as a true point but
as an infinitesimal surface having all of the physical characteristics
of a surface source the mathematical difficulties can be overcome,
but by far the simplest and most satisfactory method is to treat the
source initially, finally and all the time, as a surface source having
true surface source characteristics.
Consider, therefore, a spherical surface source of unit radius (i
cm.) emitting 100 candle-power uniformly in all directions. The
total output from the source will be 4?r X 100 = 1257 lumens.
The superficial area of the source is 4irr 2 = 12.57 sq. cm., and
hence the output is equal to 100 lumens per square centimeter.
At any appreciable distance from the source the "projected area"
of the source viewed from this distance is equal to irr 2 = 3.14 sq.
cm. and hence the "apparent candle-power per unit of projected
area" is 100 -5- 3.14 = 31.9 a value which in the past has been
called "brightness," but no name has been adopted for designating
the unit. For the unit quantity "apparent output" from the
source expressed in "apparent lumens per sq. cm." the term "lam-
bert" has been adopted. This term is applicable equally to the
"brightness" (or appearance to the eye) of any surface whether
radiating, transmitting, or reflecting, and whether or not it acts as
a perfectly diffusing surface, but the unit is defined by, and receives
its magnitude from, the appearance to the eye of "a perfectly
diffusing surface radiating or reflecting one lumen per sq. cm."
12 ILLUMINATING ENGINEERING PRACTICE
As is well known, according to the so-called " inverse square
law" the illumination (or luminous flux density) on a plane at any
chosen distance from a " point-source " varies inversely with the
distance from the source. If it were possible to obtain a true
point-source, it would be possible to produce infinite illumination by
bringing the plane within an infinitesimal distance from the source.
With a spherical surface source the "inverse square" law holds true
provided only that the distance from the source is measured from
the center thereof. In this case the minimum distance from the
source is equal to the radius of the sphere. With a spherical sur-
face source i cm. in radius producing 100 c.p. uniformly in all direc-
tions the maximum illumination (at minimum distance) is equal to
100 -f- r 2 = 100 lumens per sq. cm. This means that the maximum
possible illumination in lumens per sq. cm. is equal to the "bright-
ness" of the source expressed in "lamberts." This relation holds
true for surface sources of all kinds and shape, being absolutely
fundamental. Any assumption that would lead to results contrary
thereto can be said not to be in accord with the physical fact.
In order always to have before one a correct mental picture of
the true physical conditions of lighting sources, it is best always to
assume that the so-called "point- source" is in reality a spherical
surface source (having finite dimensions), and to base all calculations
on the surface source rather than point-source conception. That
is to say, it is not necessary to employ the "point-source conception"
in order to take advantage of the "inverse square law" and similar
relations developed and employed on the basis of the assumed
"point-source," because the same relations are applicable even
more accurately and completely to the spherical surface source.
Moreover, there are certain relations between the output density
of the surface sources and the illumination (flux density) produced
on surfaces illuminated thereby, which can be utilized immediately
when all calculations are based on the surface source conception
but which must be ignored in effect when the point-source concep-
tion is used. This fact is becoming of increasing importance as the
indirect or semi-indirect system of lighting is being substituted for
the direct.
FLUX-SUMMATION ON MEAN SPHERICAL CANDLE-POWER
DIAGRAM
Reference has already been made to the Rousseau diagram for
representing by means of an area the total flux produced by a light
MCALLISTER: ILLUMINATION UNITS 13
source of which the candle-power distribution curve is known. As
a matter of actual practice in illumination calculations use may
be said always to be made for the purpose indicated of either the
Rousseau diagram or some one of several modifications thereof that
have been developed for eliminating the necessity of a planimeter
for determining the area or its equivalent.
Figs. 8 and 9 have been drawn to show one of the methods em-
ployed for representing the equivalent of an area by means of a
straight line. The irregular curve XbeY of Fig. 8 is a candle-power
Figs. 8 and 9. Linear and area representations of zonal flux.
distribution curve of which Fig. 9 is the corresponding Rousseau,
or flux-summation, diagram. Consider the small area ABCTPS
of Fig. 9. If such a section be so selected that its mean width
is equal to PB then the small area ABCTPS is equal to the
product of AC (the height) by PB (the width). The problem is to
select some one line which, by geometrical construction, is pro-
portional to the product of AC and PB. In Fig. 8 such a line is
shown by A'C', which by construction, bears to AC (of Fig. 9) the
direct ratio of Ob to OP (of Fig.8). That is to say, it is propor-
tional directly to the area ABCTPS, the proportionality constant
14 ILLUMINATING ENGINEERING PRACTICE
being dependent upon the linear candle-power scale and the
diameter of V' the circle of reference, or rather the enclosing sphere.
The summation of all the various part-areas of Fig. 9 as indicated
by A'C, E'F', etc., of Fig. 8, would produce a single linear dimen-
sion directly proportional to the total area of Fig. 9; that is, directly
proportional to the total flux from the source of which the irregular
curve of Fig. 8 shows the space distribution of the candle-power.
One can easily define the proportionality constant by applying the
method here outlined to the determination of the total flux from the
candle-power curve of a " spherical surface" source producing equal
candle-power in all directions. It will be seen at once that the
total of the vertical lengths (corresponding to A'C and E'F', etc.)
would then equal twice the length chosen to represent the uniform
candle-power of the " spherical surface" source; now the total flux
is equal to 4?r 7 whereas the summatio length is 2 7 and hence the
proportionality constant is 2ir.
That is to say, independent in every respect of the irregularities
of the candle-power curve, the linear summation method outlined
above gives at once a value equal (if sufficiently small sections are
selected for summation) to twice the mean spherical candle-power
of the source, measured on the candle-power scale, and this value
multiplied by 2ir equals (with the same degree of accuracy) the total
flux from the source expressed in lumens.
It will be noted that, contrary to the relations involved in the
Rousseau diagram, the diameter of the circle (or sphere) of reference
cancels out from the proportionality constant in the linear summa-
tion of Fig. 8, whereas it appears as a direct factor in the area
summation of Fig. 9.
LINEAR SUMMATION BY GRAPHICAL CONSTRUCTION
In Fig. 1 1 is reproduced the candle-power curve of an infinitesimal
cylindrical surface source of which the Rousseau flux diagram
(elliptical) is shown in Fig. 12, identical except as to dimensions with
the elliptical diagram in Fig. 7. In Fig. 10 is shown a graphical
method for adding together the vertical linear equivalents of the
separate 30 degree areas in the Rousseau diagram of Fig. 12, the
equivalents in each case being determined by the geometrical method
already outlined in connection with Fig. 8. It will be noted that
the 3o-degree, 6o-degree and 90-degree angle lines have been so
transposed, while retaining their equivalent lengths, that the corre-
MCALLISTER: ILLUMINATION UNITS
spending vertical distances are directly added one to the other to
produce at once the total length of QQ', which (according to the
proportionality constant derived above) is equivalent to twice the
mean spherical candle-power represented by the candle-power
distribution curve of Fig. n or the Rousseau diagram of Fig. 12.
The linear summation diagram briefly outlined in connection with
Fig. 10 was developed by Dr. A. E. Kennelly, past president of the
Illuminating Engineering Society, and is known as the Kennelly
Diagram.
Q
+ 90'
+ 60'
-I- SO'
-30
Figs. 10, ii, and 12. Kennelly linear summation diagram; candle-power curve of cylindrical
surface source; Rousseau area summation diagram.
ABSORPTION-OF-LIGHT METHOD OF CALCULATION
One of the most convenient and an absolutely reliable method of
calculation in illumination problems is that based on the law of
conservation. According to this law the total flux (lumens) of
light absorbed by the illuminated surfaces within any chosen en-
closure of any size, shape or character is exactly equal to the total
amount of flux (lumens) produced by the sources of the lumination.
This law is fundamental and calculations based upon it give ab-
solutely accurate results when the assumptions as to absorption,
etc., are correct. That is to say, by adding together the value of
the lumens separately absorbed by the various surfaces illuminated
one obtains at once an exact measure of the lumens produced by the
sources of light.
In order to determine the absorbed flux, it is necessary to know
only the value of the incident flux and the absorption coefficient;
i6
ILLUMINATING ENGINEERING PRACTICE
the product of these two represents accurately the lumens absorbed.
Any error found in applying this method is to be attributed to the
inability to determine either the value of the incident flux, or the
absorption coefficient, or both, but not to the method itself.
For example, assume a room 25 ft. wide, 80 ft. long, 10 ft. high
having a white ceiling with an absorption coefficient of 0.20; light
walls with an absorption of 0.50; and a dark floor with an absorption
of 0.90, to be so lighted that the incident illumination on the ceiling
is i foot-candle, that on the walls 2-foot candles and on the floor
3 foot-candles. The following summation shows the amount of
lumens absorbed:
Area
Incident
Absorption
Sq. ft.
Ft. C.
Flux
Coef.
Flux
Ceiling
2OOO
2IOO
2000
I
2
3
2000
4200
6000
o. 20
0.50
0.90
400
2IOO
5400
Walls
Floor
Total lumens absorbed = 7900.
Total mean spherical candle-power equals 7900 -7-4^ = 630.
This method is not approximate ; it is absolutely exact. However,
it should not be assumed that results in practice can be obtained
with such ready facility as here indicated, because the absorption
coefficients of ceiling, wall and floor materials are not known to a
high degree of accuracy; various surfaces in addition to those here
considered intercept and absorb much of the light, and the light is
not uniformly distributed over the various surfaces. In regard to
the last mentioned limitation it is worthy of note that the mere lack
of uniformity in the distribution of light flux does not affect the
accuracy of the absorption method provided only that the true
mean effective values of the incident illumination and of the
absorption coefficient are assumed in each case.
The actual distribution of the incident flux can be approximated
by means of some of the point-by-point methods of calculations,
while the absorption coefficient must be based on the results of
tests relating to the materials composing the absorbing surfaces.
Values for such coefficients will be given in connection with other
lectures dealing with the practical application of the methods of
calculation herein described.
MCALLISTER: ILLUMINATION UNITS
IXTER-REFLECTIONS BETWEEN WALLS, CEILING AND FLOOR
Some idea concerning the bearing of reflection upon illumination
can be gained readily from a brief study of the values derived from
the above absorption problem.
The total incident flux on the ceiling, walls and floor equal 2000 +
4200 -f- 6000 = 12,200 lumens, whereas the lighting units are re-
quired to produce only 7900 lumens. The " mean effective absorp-
tion coefficient" of the room as a whole is, therefore, 7900 -f- 12,200
= 0.65. Of the total of 12,200 lumens incident upon the surfaces
only 7900 come directly from the lamps, 12,200 7900 = 4300
lumens being attributable to inter-reflection between the surfaces.
Since only 2000 lumens are directed toward the ceiling (where
400 are absorbed and 1600 are reflected), whereas 6000 are directed
toward the floor, it is apparent at once that the room selected is
lighted by lamps which produced considerably more light in the
lower than in the upper hemisphere; that is to say use is not made
of the indirect system of lighting.
For sake of comparison, consider now the same room with the same
absorption coefficients with the same total amount of incident flux
upon the floor and walls but with such an amount directed toward
the ceiling that the reflection therefrom equals the amount absorbed
by the floor. In other words assume that, in effect, use is made of
the " totally indirect" system so far as the ceiling and floor are
concerned.
The light flux reflected from the ceiling (with its 0.20 absorption
= 0.80 reflection) must equal the 5400 lumens absorbed by the floor.
Hence, 5400 + 0.80 = 6750 equals the flux incident upon the
ceiling. The tabulation will then be as follows:
Area
Incident
Absorption
Sq. ft.
Flux
Ft. C.
Coef.
Flux
Ceiling
2OOO
2IOO
2000
6750
4200
6000
3-37
2.OO
3.00
o. 20
0.50
0.90
1350
2100
5400
Walls
Floor
Total lumens absorbed = 8850.
Total mean spherical candle-power 8850 -5- 4*- = 705.
The total incident flux is equal to 6750 + 4200 + 6000 = 16,950
lumens, as compared with the former 12,200 lumens. Thus with
1 8 ILLUMINATING ENGINEERING PRACTICE
an increase of 1 1 .9 per cent, in the candle-power of the lighting units,
there is an increase of 16,950 12,200 = 4750 or 39 per cent, in
the total incident flux in the room, with an increase of 1350
400 = 950 or 237 per cent, in the ceiling illumination.
In referring above to the change in the system of lighting equip-
ment use was made of the term "totally indirect," in order to con-
centrate ideas on the immediate problem at hand rather than to
describe the system actually required to produce the results indi-
cated. With only 6750 lumens incident upon the ceiling which
absorbs 1350 lumens, and a total of 4200 lumens incident upon the
walls which absorb 2100 lumens, it is evident that the lighting units
must supply considerable flux directly to the walls, and hence a
" totally indirect" system of lighting would not produce the results
required.
As already stated, in actual practice conditions are not so readily
denned as assumed above, and the absorption method cannot be
applied practically with the degree of simplicity that might be in-
ferred from the above examples, but it can be looked upon as a most
reliable check upon the more complicated methods of calculation
and as an invaluable aid in solving problems connected with the
illuminating of reflecting surfaces, investigating quantitatively the
effect of inter-reflection between surfaces, and ascertaining the
limits in the distribution of light flux between illuminated surfaces.
UTILIZATION FACTOR
In actual practical problems in illumination design it has been
found quite convenient to make use of the direct relations between
the so-called "total lumens utilized" and the lumens produced by
the lighting sources, because the former can be considered to be the
known quantity and the latter the unknown quantity in one phase
of the practical illumination problem. The "lumens utilized" are
assumed to be equal to the mean illumination (in, say, foot-candles)
over the reference plane (say 30 in. above the floor) multiplied by
the area of the floor (in square feet). The ratio between this
quantity of lumens to the lumens produced by the source is called
the "utilization factor," or "coefficient of utilization."
Referring to the two examples given above it will be seen that
(if the illumination on the reference plane be assumed to be equal
to that at the floor level) the utilization factor in the so-called " direct
lighting" problem would be 6000 -f- 7900 = 0.76, whereas in the
"indirect" problem it would be 6000 -f- 8850 = 0.68.
MCALLISTER: ILLUMINATION UNITS 19
A study of the above problems in the light of the above definition
will show that the "utilization factor" depends on not only the
system of lighting and the absorption by the ceiling and walls but
also on the absorption by the floor. The fact of the matter is that
with highly reflecting floor, walls and ceiling the "utilization factor"
would have a value greater than unity. This condition would seldom
be reached in practice but would be closely approached in the case
of a dining-room decorated in light colors, with a wide expanse of
table linen and light floor covering. The value of the utilization
factor depends upon the character of the lighting units, relative
dimensions of the room, color and material of the ceiling, walls and
floor. Utilization factors, as determined by actual tests under
service conditions, will be discussed fully in other lectures, and need
not be dwelt upon herein.
ILLUMINATION BY DAYLIGHT
Mention has already been made of the simple solution of problems
that would otherwise prove quite complex by means of certain
solid angular relations. This statement applies with particular
force to problems relating to the illumination from either artificial
or natural sky-light through either ceiling or side-wall windows.
In view of the fact that as a lighting source the sky is located at
an indefinite, if not infinite, distance from the objects illuminated,
it is obvious at once that resort cannot be had to the method of
calculation based upon the so-called "inverse-square law." For
purposes of calculation the sky can best be considered as an extended
surface source of undefined shape at an indefinite distance from and
completely surrounding the observer, being visible (except for local
obstructions) throughout the upper hemisphere above the horizontal
plane occupied by the observer. The first and most important step
is to establish the relation between the illumination produced at
any chosen point by such a source and the solid angle subtended by
the source when viewed from that point; or rather first to show
that the solid angular relations are in strict agreement with the
''inverse-square law" and that by basing the calculations exclusively
on the former the latter may be eliminated.
Referring to Fig. 13, consider the perfectly general case of a small
section (dA) of a surface lighting source of any shape or inclination
(a) situated at any distance (R) from any chosen point (P). Let
c be the normal emitting density (here used as "apparent candle-
2O
ILLUMINATING ENGINEERING PRACTICE
power per unit area") of this source. The illumination produced
at point P, from the inverse square and cosine laws, is,
= c
(dA) cos a
Consider now the illumination that would be produced at the
same point P, by a surface source (da) at the circumference of the
imaginary enclosing sphere subtending the same solid angle as
Surface of any shape
in any position
Fig. 13. Photometric relations based on equality of solid angles.
(dA) and having an equal normal emitting density c. The illumina-
tion at the central point, P, would be
From simple geometrical relations, the correctness of which will
be appreciated at once from a glance at Fig. 13, it is seen that the
areas (da) and (dA) bear to each other such a ratio that
(da) = ~(dA) cos a
(3)
Combining equations (3) and (2) and comparing the result with
equation (i), there is obtained
c(dA) cos a
(4)
MCALLISTER: ILLUMINATION UNITS 21
Equation (4) shows that when dealing with surface lighting sources
(such as the sky, artificial windows, or indirect lighting systems)
the illumination at any chosen point is fully defined when the emit-
ting density of the source and the solid angle subtended by th/e
source as viewed from the point chosen are known. Upon this
relation can be based some extremely simple graphical solutions of
problems relating to illumination by daylight or by surface lighting
sources in general.
From the relations derived above it will be seen that in calculating
the illumination produced by a surface source it is unnecessary to
know either the candle-power of the source or the distance of the
source from the point of observation, provided only that the solid
angle subtended by the source and the emitting density (expressed
preferably in lumens per unit area) are known. It is obvious there-
fore that, so far as calculations are concerned, any surface source of
indefinite shape, size or location (such as the exposed sky surface)
can be treated as equivalent to a definitely located source of definite
shape and size provided only that such values are assigned to the
dimensions and position of the substituted surface source that the
solid angles are the same as before and the assumed emitting density
of the substituted source is identical with that of the original.
Hence in day-lighting problems it may be assumed that a plane sur-
face source of sky- value emitting density having the exact dimensions
of the exposed area of either a ceiling or a side- wall window can safely
be substituted for the sky.
From all points within a room receiving an unobstructed view of
the sky through a window, the window itself can be treated as the
surface lighting source having an emitting density in lumens per
unit area exactly equal to that of the sky. At point where the sky
is partly hid from view through the window, the solid angle is corre-
spondingly reduced for the full sky density, and a lower density
must be assigned to the remaining portion of the original solid angle
in accordance with the relative reflection coefficients of the ob-
structing areas on the side exposed to view through the window.
CIRCULAR SKY-WINDOW SOURCES
The above described method of substituting a surface source of
known dimensions and location for some other source of more com-
plex dimensions and uncertain location is invaluable in determining
the illumination produced by the light received through ceiling
22
ILLUMINATING ENGINEERING PRACTICE
windows from either natural or artificial sources. For this purpose,
it is most convenient to substitute for any square, rectangular or
irregularly shaped window source a circular or elliptical source of
equal emitting density, equivalent in area and in practical solid
angular relations.
In Fig. 14, let ACB represent an edgewise view of a flat circular
source, assumed to be in the ceiling of a room, having any chosen
value of uniform emitting density and any desired radius. If
through the edges A and B there be passed an imaginary sphere
of any chosen size whatsoever such as ADB with its center at
5R-
10 11 12 13 14 16
Pig. 14. Equilux spheres with illuminating values in per cent, for spheres passing through
points i, 2, 3, 4, etc., length units below source.
some point on a vertical line passing through the center c, the inner
surface of this imaginary sphere below the lighting source will
receive an illumination which will be uniform in intensity normal
to the surface of the sphere throughout the whole interior of the
imaginary sphere.
The statement just made is not based on the equality of the solid
angles subtended by the source when viewed from various points
along the interior of the imaginary sphere; in fact, the solid angle is
not constant but varies directly with the cosine of the plane angular
deviation of the point of observation from the position directly
below the center of the source. However, the above statement
MCALLISTER: ILLUMINATION UNITS 23
applies not to the illumination density on a plane normal to the line
of observation of the source from the point chosen but to the
density normal to the imaginary sphere at this point. The ratio
between these two densities varies inversely with the cosine of the
angular deviation just mentioned, so that the final product is con-
stant, and hence the density normal to the imaginary sphere is
constant.
Evidently the exact value of the density (in lumens per unit
area) of the normal illumination against the inner surface of the
sphere will bear to the emitting density (in lumens per unit area) of
the circular surface source the inverse ratio of the interior area of the
exposed zone of the sphere to the area of the lighting source, since
the lumens produced must equal those utilized. From solid geo-
metrical relations it will be seen that this ratio equals the square of
the radius AC to the diagonal AD. When the radius of the circular
source is taken as the unit of length for the measurement of all dis-
tances, and the unit of illumination density (lumens per unit area) is
taken as the emitting density of the source, then the percentage value
of the illumination density on the interior of the sphere is equal to
100 divided by the square of the diagonal AD.
For convenience any sphere passing through the edges A and B, as
just indicated, can be referred to as an "equilux" sphere (the "lux"
being one of the several units of illumination). Equilux spheres of
the proper sizes being employed, one is enabled "to explore the
whole region" illuminated by the source, and to ascertain immedi-
ately for any desired point within the space explored the exact value
of that component of the light flux which is normal to the particular
equilux sphere passing through that point.
In Fig. 14 are indicated numerous equilux spheres and the points
of intersection of these spheres with horizontal planes (floors) at
light distances of 3.5 units and 7 units of length (radii) below the
source, and with vertical planes (walls) 3 and 5 length units distant
from the center of the source.
Points of intersection of the two assumed horizontal planes with
the equilux spheres evidently lie on circles having as the common
center the point on the floor immediately below the center of the
circular ceiling lighting source.
It is an interesting fact that at any point on the floor the compo-
nent of the flux normal to the floor is equal to the component normal
to the equilux sphere at that point, so that the values of equilux
density are simultaneously the values of light flux density normal
24 ILLUMINATING ENGINEERING PRACTICE
to the horizontal plane (floor). Expressed in other words, the illu-
mination along the floor at any point is known at once when one has
determined the value of light flux density on the equilux sphere
passing through that point. Hence, the whole problem of floor
illumination density and distribution determination is completely
solved when the equilux spheres and intersecting lines have been
constructed. One could not well wish for a simpler solution.
In Fig. 15 are shown results obtained directly from Fig. 14. It
will be noted that with a ceiling height equal to 3.5 times the radius
of the lighting source the light flux density at a point on the floor
immediately below the center of the source reaches a value of about
012345678
Distance from Point below Center of Source
Fig. 15. Graphs of illuminations on floors with two different ceiling heights.
7.55 per cent, of the emitting density of the source, while the density
along the floor decreases rapidly with increase in the distance from
the point of maximum density. With a ceiling twice as high as
formerly the maximum light flux density is reduced to 2 per cent.,
but the rate of decrease with increase of distance from the point
below the center of the source is much less; in fact, at distance greater
than 5 units of length (radii) the light from the high source is greater
than that from the low source. This fact will be appreciated when
it is recalled that the "solid angle" subtended by the source when
viewed from the floor at a great distance from the center is larger
with the high ceiling than with the low ceiling.
Even in the case of ceiling sources it is at times desirable to calcu-
MCALLISTER: ILLUMINATION UNITS 25
late the illumination on the side-walls, and it is well to have avail-
able some method for this purpose. When it is remembered that
any method developed for use with ceiling window sources can be
supplied at once to side window sources, it will be appreciated that
the method of calculating the wall illumination with ceiling window
sources becomes that of calculating the floor illumination with side
window sources, and the desirability of having a simple method
will be apparent. Such a method, for convenience described in con-
nection with ceiling sources, is as follows:
At any point on any vertical plane as far below the ceiling source
as this plane is distant from the center of the source in the normal
(nearest) direction, the illumination normal to the vertical plane at
that point is equal to that normal to the horizontal plane at this point.
At any other point the normal illumination on the vertical plane bears
to the normal illumination on the horizontal plane at this point, the
ratio of the distance of the vertical to the distance of the horizontal
plane from the center of the source, each distance being measured
in a direction normal (shortest) to the plane considered. When
solving problems relating to plane circular lighting sources by means
of the equilux spheres one can easily determine the illumination
normal to any horizontal plane and can then calculate the illumi-
nation on any vertical plane by direct proportion.
By obvious modifications the above described methods for deter-
mining the floor and wall illumination produced by circular ceiling
sources, can be applied to similar problems relating to ceiling and
side-wall sources of any shape or size, and to problems of all kinds
relating to daylight illumination.
BRIGHTNESS UNIT THE LAMBERT
Although it is not unusual to refer to an isolated lighting unit of
the "point" type (that is, of the type treated as equivalent to a
" point source " as distinguished from a " surface source "), as possess-
ing a certain candle-power, yet it is recognized that the lighting
characteristics of the source are not fully defined until the whole
space distribution of the candle-power is so specified that the
"mean spherical candle-power" or the output in lumens can be
determined. Illumination calculations have been greatly simplified
by the introduction of the "lumen" as a unit in which to express
not only the output from the source but also the absorption by the
surfaces illuminated, the total lumens produced by the source being
in every case equal to the total lumens absorbed by the surfaces.
26 ILLUMINATING ENGINEERING PRACTICE
Moreover, certain seemingly complicated problems relating to sur-
face sources, inter-reflecting walls, ceilings, etc., permit of the simp-
lest possible solution when use is made of the lumen rather than the
candle-power conception in expressing the output, the output den-
sity and the "appearance" of the source to the eye when viewed from
various directions. The ratios involved in the substitution are
fundamental and do not depend upon the character of the source,
being the same for " non-mat" as for "mat" surfaces.
With a perfectly diffusing "mat" surface source, multiplying the
constant value of "lumens per square foot" of the source by the
total area of the source in square feet gives the exact value of the
output in lumens independent in every respect of the shape or size
of the source.
When use is made of the "apparent candle-power per square
inch" in this connection multiplying this value by the whole area
of the source in square inches does not give the "mean spherical
candle-power of the source; it gives a value differing therefrom
in the ratio of i to 4 under all conditions. In the single case of a
perfectly diffusing spherical source of which the projected area
equals one-fourth the total area the total mean spherical candle-
power is equal to the product of the "apparent candle-power per
square inch" by the projected area in square inches.
It is evident, therefore, that so far as concerns the output of
"mat" surfaces, the expression lumens per unit area has a definite
significance and cannot be misinterpreted, while the term "ap-
parent candle-power per unit area" may or may not be correctly
interpreted.
It is frequently assumed, tacitly, that a surface source is made
up of an infinite number of "point sources." If such were the case,
plain surface sources would emit in all directions rather than in one
hemisphere, and the "cosine law" of emission would be invalid.
The fact is that a surface source is made up of an infinite number of
infinitesimal plane surface elements, each of which radiates in a
single hemisphere and does not act like a point source. When the
"cosine law" is applicable the "mean spherical candle-power" of
each element of the source is equal to one-fourth of the maximum
apparent candle-power of the element, or is equivalent to one-
fourth of the total area of the element multiplied by the "apparent
candle-power" per unit projected area viewed from any direction
within the radiating hemisphere. Hence the universal i to 4 ratio
noted above for "mat" surfaces.
MCALLISTER: ILLUMINATION UNITS 27
In view of the fact that as a rnatter of actual practice almost all
surface sources are either of the "mat" type or are treated as though
they obeyed the "cosine law of emission," it would seem that the
very great simplification in calculation brought about by substitut-
ing the lumen conception for the candle-power conception would
fully justify the substitution even if the results obtained were some-
what inaccurate in the case of "non-mat" surfaces. The fact is,
however, that the ratio involved in the substitution is absolutely
fundamental and does not depend upon the character of the emitting
surface.
As has already been shown the ratio of the "output in lumens per
square foot" to the "apparent candle-power per square foot" of
all "mat" surface sources is the constant IT -r- i. It is equally
true that the "apparent foot-candles" of a source of any character
whatsoever viewed from any chosen direction bears to the "ap-
parent candle-power per foot" of the same source viewed from the
same direction the identical ratio IT -4- i the "TT ratio" not being
dependent in any respect upon the "cosine law of emission." The
fact of the matter is that the TT ratio is based on solid geomet-
rical relations, and is independent of the space distribution of the
candle-power in any direction except that toward the point under
consideration.
That is to say, there is a definite numerical ratio between the
apparent foot-candle density of a source and its apparent candle-
power per square foot, which ratio is the same under all conceivable
conditions of space distribution of the candle-power.
It can, therefore, be stated that any illumination photometer of
the "pyrometer" type calibrated to. read in "apparent emitted
foot-candles" or "lamberts" will when pointed toward a bright
surface give an exact measure of the "apparent candle-power per
square inch" of the source provided only that the "apparent
foot-candle" value is divided by 144^ or TT as the case may be a
constant in no way dependent upon the "cosine law."
Hence in order to determine the "apparent candle-power per
square inch" of a surface source in a chosen direction, it is unneces-
sary to measure the apparent candle-power in this direction of a
limited isolated section of known projected area; the identical
result can be obtained much more conveniently by observing the
"apparent foot-candle density" apparent lumens per square foot) or
"lamberts (lumens per sq. cm.) of the source when viewed from the
chosen direction and dividing this value by the constant 144^ or TT.
28 ILLUMINATING ENGINEERING PRACTICE
It is to be noted that, independent in every respect of the name
given to the quantity dealt with, the measurable value of the
"apparent foot-candle" density of a surface source differs from the
measurable" apparent candle-power per square inch" of the same
source viewed in the same direction in a definite numerical ratio,
without regard to the character of the surface. The ''apparent
candle-power per square inch" is known as " brightness" and the
"apparent foot-candle density" observed from the same direction
is also "brightness."
The unit of "brightness," the "lambert" is equal to neither the
"apparent foot-candle" nor the "apparent candle-power per square
inch." It is identical with the "apparent lumen per square centi-
meter," being 929 times as bright as the "apparent lumen per square
foot" or its equivalent the "apparent foot-candle." Hence a per-
fectly diffusing surface emitting or reflecting one lumen per square
foot (one apparent foot-candle) will have a brightness of 1.076
millilamberts.
The mean effective value of the output density of a surface source
(and all practical sources are surfaces) can best be found by dividing
the total output in lumens by the total area of the source expressed
in some convenient unit. This numerical value is absolutely
identical with the mean effective value of the appearance of the source
in apparent lumens per square foot ("apparent foot-candles") or
apparent lumens per sq. cm. ("lamberts"), the latter being the
standardized unit for expressing the "appearance" or "brightness"
of a surface source.
Only in the case of a perfectly "mat" source is either the "out-
put density" or the "appearance" uniform in all directions, but no
error is involved in the solution of problems dealing with non-uni-
form sources when mean effective values are substituted for the
variable space values, provided only that the solution is recognized
as being expressed in mean effective values.
For example, one can determine quickly by the use of mean effect-
ive values the average illumination produced over, say, the whole
floor area of a room, but when he wishes to know the space variations
in the illumination throughout a room he must resort to some more
laborious point-by-point method. In most problems of today,
with lighting units giving widely distributed flux the prime essential
feature is no longer the proper space distribution of the illumina-
tion, but rather the production of adequate average illumination
without excessive brightness in the field of view.
MCALLISTER: ILLUMINATION UNITS 29
PRESENT DAY CALCULATING METHODS
The adoption of the method of expressing the "brightness" or
"appearance" of a lighting source in terms of physical reality based
on the "surface-source" conception, rather than using the mathe-
matically derived expression of brightness in terms of tacitly as-
sumed point-sources with surface-source characteristics, represents
a step in the progress of illumination calculations from the methods
of the mathematical-physicist to those of the engineer similar
in results accomplished to the adoption of the now universally em-
ployed magnetic flux and flux-density conceptions for the earlier
isolated magnetic pole conception in the evolution of electrical
calculations from those of the physicist to those of the engineer.
Similarly the practical abandonment of the laborious point-by-
point methods of illumination determination in favor of the much
more rapid output-utilization methods based on the law of conserva-
tion, converts the calculations of the lighting expert from those of
the physicist to those of the engineer. Just as the mathematical
physicist will continue to deal with fictitious isolated magnetic poles
and by careful transformation of his equations will derive results
in exact accord with physical facts, so will he continue t'o employ
point-source conception and the point-by-point methods of calcula-
tions and his results will be true to nature, but the practical illuminat-
ing engineer will train his mind to think in terms of surface sources
rather than point-sources, and will base the few equations needed
by him in his everyday work on the law of conservation either
directly or indirectly recognized. The results obtained by him with
the minimum of exertion will be absolutely identical with the results
derived much more laboriously by the physicist employing the time-
honored methods with which he is familiar.
ILLUMINATION UNITS AND NOMENCLATURE
In order to render most serviceable for reference the book in which
these lectures are reprinted there is here presented the latest (1916)
list of units, definitions and abbreviations of the Committee on
Nomenclature and Standards of the Illuminating Engineering
Society.
DEFINITIONS
1. Luminous Flux is radiant power evaluated according to its visibility;
i.e., its capacity to produce the sensation of light.
30 ILLUMINATING ENGINEERING PRACTICE
2. The visibility, K x of radiation, of a particular wave-length, is the
ratio of the luminous flux to the radiant power producing it.
3. The mean value of the visibility, K m , over any range of wave-lengths,
or for the whole visible spectrum of any source, is the ratio of the total
luminous flux (in lumens) to the total radiant power (in ergs per second,
but more commonly in watts).
4. The luminous intensity, I, of a point source of light is the solid angular
density of the luminous flux emitted by the source in the direction con-
sidered; or it is the flux per unit solid angle from that source.
Defining equation:
or, if the intensity is uniform,
[-%
O)
where co is the solid angle.
5. Strictly speaking no point source exists, but any source of dimensions
which are negligibly small by comparison with the distance at which it is
observed may be treated as a point source.
6. Illumination, on a surface, is the luminous flux-density on that sur-
face, or the flux per unit of intercepting area.
Defining equation:
or, when uniform,
where 5 is the area of the intercepting surface.
7. Candle the unit of luminous intensity maintained by the national
laboratories of France, Great Britain, and the United States. 1
8. Candlepower luminous intensity expressed in candles.
9. Lumen the unit of luminous flux, equal to the flux emitted in a
unit solid angle (steradian) by a point source of one candle-power. 2
10. Lux a unit of illumination equal to one lumen per square meter.
The cgs. unit of illumination is one lumen per square centimeter. For this
unit Blondel has proposed the name "Phot." One millilumen per square
centimeter (milliphot) is a practical derivative of the cgs. system. One
foot-candle is one lumen per square foot and is equal to 1.0764 milliphots.
The milliphot is recommended for scientific records.
11. Exposure the product of an illumination by the time. Blondel
has proposed the name "phot-second" for the unit of exposure in the cgs.
system. The microphot second (o.oooooi phot-second) is a convenient
unit for photographic plate exposure.
1 This unit, which is used also by many other countries, is frequently referred to as the
international candle.
1 A uniform source of one candle emits 4 if lumens.
MCALLISTER: ILLUMINATION UNITS 31
12. Specific luminous radiation, E 1 the luminous flux-density emitted
by a surface, or the flux emitted per unit of emissive area. It is expressed
in lumens per square centimeter.
Denning equation:
For surfaces obeying Lambert's cosine law of emission,
E' = T&O.
13. Brightness, b,ot an element of a luminous surface from a given posi-
tion, may be expressed in terms of the luminous intensity per unit area
of the surface projected on a plane perpendicular to the line of sight, and
including only a surface of dimensions negligibly small in comparison with
the distance at which it is observed. It is measured in candles per square
centimeter of the projected area.
Defining equation:
A dl
~ dS cos 0'
(where 6 is the angle between the normal to the surface and the line of
sight).
14. Normal brightness, b Q , of an element of a surface (sometimes called
specific luminous intensity) is the brightness taken in a direction normal
to the surface. 1
Defining equation:
;, dl
bo = dS'
or, when uniform,
b = S'
16. Brightness may also be expressed in terms of the specific luminous
radiation of an ideal surface of perfect diffusing qualities, i.e., one obeying
Lambert's cosine law.
16. Lambert the cgs. unit of brightness, the brightness of a perfectly
diffusing surface radiating or reflecting one lumen per square centimeter.
This is equivalent to the brightness of a perfectly diffusing surface having
a coefficient of reflection equal to unity and an illumination of one phot.
For most purposes, the millilambert (o.ooi lambert) is the preferable
practical unit.
A perfectly diffusing surface emitting one lumen per square foot will
have a brightness of 1.076 millilamberts.
Brightness expressed in candles per square centimeter may be reduced
to lamberts by multiplying by IT = 3.14.
Brightness expressed in candles per square inch may be reduced to foot-
candle brightness by multipyling by the factor 144^ = 452.
1 In practice, the brightness & of a luminous surface or element thereof is observed and
not the normal brightness 60. For surfaces for which the cosine law of emission holds,
the quantities b and &o are equal.
32 ILLUMINATING ENGINEERING PRACTICE
Brightness-expressed in candles per square inch may be reduced to lam-
berts by multiplying by Tr/6.45 = 0.4868.
In practice, no surface obeys exactly Lambert's cosine law of emission;
hence the brightness of a surface in lamberts is, in general, not numerically
equal to its specific luminous radiation in lumens per square centimeter.
Defining equations:
/ - dF
L ~dS
or, when uniform,
17. Coefficient of reflection the ratio of the total luminous flux
reflected by a surface to the total luminous flux incident upon it. It is a
simple numeric. The reflection from a surface may be regular, diffuse
or mixed. In perfect regular reflection, all of the flux is reflected from the
surface at an angle of reflection equal to the angle of incidence. In perfect
diffuse reflection the flux is reflected from the surface in all directions in
accordance with Lambert's cosine law. In most practical case^ there is a
superposition of regular and diffuse reflection.
18. Coefficient of regular reflection is the ratio of the luminous flux
reflected regularly to the total incident flux.
19. Coefficient of diffuse reflection is the ratio of the luminous flux
reflected diffusely to the total incident flux.
Defining equation:
Let m be the coefficient of reflection (regular or diffuse).
Then, for any given portion of the surface,
E'
20. Lamp a generic term for an artificial source of light.
21. Primary luminous standard a recognized standard luminous
source reproducible from specifications.
22. Representative luminous standard a standard of luminous inten-
sity adopted as the authoritative custodian of the accepted value of the
unit.
23. Reference standard a standard calibrated in terms of the unit
from either a primary or representative standard and used for the cali-
bration of working standards.
24. Working standard any standardized luminous source for daily use
in photometry.
25. Comparison lamp a lamp of constant but not necessarily known
candlepower against which a working standard and test lamp are succes-
sively compared in a photometer.
26. Test lamp, in a photometer a lamp to be tested.
MCALLISTER: ILLUMINATION UNITS 33
27. Performance curve a curve representing the behavior of a lamp
in any particular (candlepower, consumption, etc.) at different periods dur-
ing its life.
28. Characteristic curve a curve expressing a relation between two
variable properties of a luminous source, as candlepower and volts, candle-
power and rate of fuel consumption, etc.
29. Horizontal distribution curve a polar curve representing the
luminous intensity of a lamp, or lighting unit, in a plane perpendicular to
the axis of the unit, and with the unit at the origin.
30. Vertical distribution curve a polar curve representing the lumin-
ous intensity of a lamp, or lighting unit, in a plane passing through the
axis of the unit and with the unit at the origin. Unless otherwise specified,
a vertical distribution curve is assumed to be an average vertical distri-
bution curve, such as may in many cases be obtained by rotating the unit
about its axis, and measuring the average intensities at the different eleva-
tions. It is recommended that in vertical distribution curves, angles of
elevation shall be counted positively from the nadir as zero, to the zenith
as 1 80. In the case of incandescent lamps, it is assumed that the vertical
distribution curve is taken with the tip downward.
31. Mean horizontal candlepower of a lamp the average candlepower
in the horizontal plane passing through the luminous center of the lamp.
It is here assumed that the lamp (or other light source) is mounted in
the usual manner, or, as in the case of an incandescent lamp, with its axis
of symmetry vertical.
32. Mean spherical candlepower of a lamp the average candle-power
of a lamp in all directions in space. It is equal to the total luminous flux
of the lamp in lumens divided by 4?r.
33. Mean hemispherical candlepower of a lamp (upper or lower) the
average candlepower of a lamp in the hemisphere considered. It is equal
to the total luminous flux emitted by the lamp in that hemisphere divided
by 27r.
34. Mean zonal candlepower of a lamp the average candlepower of a
lamp over the given zone. It is equal to the total luminous flux emitted
by the lamp in that zone divided by the solid angle of the zone.
35. Spherical reduction factor of a lamp the ratio of the mean spherical
to the mean horizontal candlepower of the lamp. 1
36. Photometric tests in which the results are stated in candlepower
should be made at such a distance from the source of light that the latter
may be regarded as practically a point. Where tests are made in the
measurement of lamps with reflectors, or other accessories at distances
such that the inverse-square law does not apply, the results should always
be given as "apparent candlepower" at the distance employed, which
distance should always be specifically stated.
1 In the case of a uniform point-source, this factor would be unity, and for a straight
cylindrical filament obeying the cosine law it would be ir/4.
3
34 ILLUMINATING ENGINEERING PRACTICE
The output of all illuminants should be expressed in lumens.
37. Illuminants should be rated upon a lumen basis instead of a candle-
power basis.
38. The specific output of electric lamps should be stated in terms of
lumens per watt and the specific output of illuminants depending upon
combustion should be stated in lumens per British thermal unit per hour.
The use of the term "efficiency" in this connection should be discouraged.
When auxiliary devices are necessarily employed in circuit with a lamp,
the input should be taken to include both that in the lamp and that in the
auxiliary devices. For example, the watts lost in the ballast resistance
of an arc lamp are properly chargeable to the lamp.
39. The specific consumption of an electric lamp is its watt consump-
tion per lumen. "Watts per candle" is a term used commercially in con-
nection with electric incandescent lamps, and denotes watts per mean
horizontal candle.
40. Life tests Electric incandescent lamps of a given type may be
assumed to operate under comparable conditions only when their lumens
per watt consumed are the same. Life test results, in order to be com-
pared must 'be either conducted under, or reduced to, comparable condi-
tions of operation.
41. In comparing different luminous sources, not only should their
candlepower be compared, but also their relative form, brightness, distri-
bution of illumination and character of light.
42. Lamp Accessories. A reflector is an appliance the chief use of
which is to redirect the luminous flux of a lamp in a desired direction or
directions.
43. A shade is an appliance the chief use of which is to diminish or to
interrupt the flux of a lamp in certain directions where such flux is not
desirable. The function of a shade is commonly combined with that of a
reflector.
44. A globe is an enclosing appliance of clear or diffusing material the
chief use of which is either to protect the lamp or to diffuse its light.
45. Photometric Units and Abbreviations.
Abbreviation
Photometric Name of Symbols and defin- for name
quantity unit ing equations of unit
1. Luminous flux Lumen F.^ i
d d^f
2. Luminous intensity Candle I = ^, T -^ cp.
, lunation ^ E - f - J cos ph. f,
[ Phot-second
4. Exposure i Micro phot- Ei phs. /xphs.
second
MCALLISTER: ILLUMINATION UNITS 35
Photometric Name ok Symbols and defin- for name
quantity unit ing equations of unit
5. Brightness
Apparent candle
per sq.cm. , _ dl
Apparent -candle dS cos 6
per sq. in. _ rfF
Lambert dS
'XT i L i_ f Candles per sq.cm. , dl
6. Normal brightness { _ 4 . b = - _
[ Candles per sq. in. dS
7. Specific luminous j Lumens per sq.cm. _ ,
radiation [Lumens per sq. in. '
pr
8. Coefficient of reflection m =
E
9. Mean spherical candlepower scp.
10. Mean lower hemispherical candlepower Icp.
11. Mean upper hemispherical candlepower ucp.
12. Mean zonal candlepower zcp.
13. Mean horizontal candlepower mhch.
14. 1 lumen is emitted by 0.07958 spherical candlepower.
15. i spherical candlepower emits 12.57 lumens. '
1 6. i lux = i lumen incident per square meter = o.oooi phot = o.i milliphot.
17. i phot = i lumen incident per square centimeter = 10,000 lux = 1,000
milliphots = 1,000,000 microphots.
1 8. i milliphot = o.ooi phot = 0.929 foot-candle.
19. i foot-candle = i lumen incident per square foot = 1.076 milliphots =
10.76 lux.
20. i lambert = i lumen emitted per square centimeter of a perfectly diffusing
surface.
21. i millilambert = o.ooi lambert.
22. i lumen, emitted, per square foot 1 = 1.076 millilamberts.
23. i millilambert = 0.929 lumen, emitted, per square foot. 1
24. i lambert = 0.3183 candle per square centimeter = 2.054 candles per
square inch.
25. i candle per square centimeter = 3.1416 lamberts.
26. i candle per square inch = 0.4868 lambert = 486.8 millilamberts.
46. Symbols. In view of the fact that the symbols heretofore pro-
posed by this committee conflict in some cases with symbols adopted
for electric units by the International Electrotechnical Commission, it
is proposed that where the possibility of any confusion exists in the
use of electrical and photometrical symbols, an alternative system of
symbols for photometrical quantities should be employed. These
should be derived exclusively from the Greek alphabet, for instance:
Luminous intensity ....................................... T
Luminous flux ........................................... V
Illumination ....................... '. .....................
1 Perfect diffusion assumed.
PRINCIPLES OF INTERIOR ILLUMINATION
BY A COMMITTEE
J. R. CRAVATH, CHAIRMAN
WARD HARRISON
ROBERT ff. PIERCE
PART I. ELEMENTS OF DESIGN
As the subject of illumination units and calculations is treated in
a separate lecture only those parts of this subject of immediate
practical application to design will be taken up here, and no attempt
will be made to explain the derivation of units, or the terms or
diagrams here mentioned in connection with calculations.
CALCULATIONS
Measurement and Expression of Light Output from Sources. One
of the first things necessary in illumination calculations for interiors
is a knowledge of the light output or luminous performance of various
sources of light available for lighting the interior in question. In
connection with the light output of a source it is important that we
should know: (a) how the light is distributed from the source, that
is, the candle-power distribution or intensity in various directions;
(b) the flux of light in lumens or mean spherical candle-power; and
(c) the brightness per unit area of the source of light.
Candle-power Distribution. The polar coordinate curve, Fig. i,
is the common means of expressing the intensity of candle-power of
light in various directions from a source. Such a curve (in which
the candle-power is shown by the distance of the curve from the
reference point or light source) gives at a glance a good idea of the
characteristics of light distribution from the source, provided the
distribution of light is symmetrical around a vertical axis. If it is
not symmetrical, of course, several curves plotted from candle-power
readings in different planes are necessary.
The practising engineer should be an industrious student and
collector of curves of this kind.
37
30 ILLUMINATING ENGINEERING PRACTICE
Light Flux. The total output or flux of light in lumens (which
is 12.57 times the mean spherical candle-power) is sometimes
graphically expressed by a Rousseau diagram but more frequently
by numerals showing the lumens emitted in different zones together
with the total lumens.
The mathematical derivation of light flux from the polar co-
ordinate curve is out of the scope of this lecture except that one
short-cut method of great practical convenience for quickly determin-
ing the light flux in any zone or zones from a common polar co-
Fig, i. Polar coordinate candle-power curve.
ordinate curve should be mentioned. The method is based on the
principle that on a polar coordinate curve the light flux in various
zones is proportional to the length of a perpendicular line drawn
from the candle-power curve at the middle of the zone to the ver-
tical axis. If we take the sum of the perpendicular distances for
lo-deg. zones (such as AB plus CD plus EF etc., in Fig. i)
from the curve to the vertical as measured from the center of
each lo-deg. zone (measuring these distances by the same scale as
the candle-power scale of the curve) and add 10 per cent, to this
sum, the result will be the total lumens in the zones under considera-
PRINCIPLES OF INTERIOR ILLUMINATION
39
tion. The quick way to get this sum is by the use of a strip of paper
and a sharp pencil. Starting at a marked zero point measure the
perpendicular distance from the curve to the vertical ic-deg. zone
at the 5-deg. point (AB Fig. i) marking it on the strip. Then
with the last mark as a starting place measure the distance for the
second zone, CD from the vertical to the curve at the 1 5-deg.
point, and so on adding each perpendicular distance for every
10 degrees to the one before, over the whole 180 degrees. Then by
using the candle-power scale of the curve to measure the total length
of the slip of paper so measured off and adding 10 per cent., the
numerical value of the lumens emitted in any zone or for the entire
sphere o to 180 degrees is quickly ascertained. Obviously the same
method applies to any one or more of the lo-deg. zones into which
the sphere is divided by this method, so that the lumens can be
thus determined for any one or more lo-deg. zones.
The brightness over the area of the source of light (or of the source
of light with its enclosing equipment such as a globe or reflector)
is of much importance in connection with the hygiene of the eye in
designing interior illumination. Such brightness has been ex-
pressed in many units, such as candles per square centimeter, candles
per square inch, candles per square foot, etc., but practice is rapidly
settling to the new unit approved by our Society, namely, the "lam-
bert" and its loooth part, the millilambert. The latter is about
equal to the brightness of white blotting paper when illuminated
with 1.25 foot-candles. Table i shows the relation of various
brightness units.
TABLE I. CONVERSION TABLE FOR VARIOUS BRIGHTNESS VALUES
Values in units in this column X
conversion factor value in
units at top of column
1.
.**
V
h
1$
at u
O
Candles per
sq. meter
Candles per
sq. foot
Lamberts (apJ
parent lum.
per sq. cm.)
Ft.-candles(apJ
parent lumend
per sq. foot)
Millilamberts
Candles per sq. cm.
I
6.451
10,000
929 . 03
3 . 14
2918
3141.6
Candles per sq. inch
155
I
1550
144
.4867*
45*
486.7
Candles per sq. meter
Candles per sq. foot
Lamberts (apparent lumens per
sq. cm.)
.0001
.00108
318
. 00064-
51
.0069
2.O54
10.70
3180
.0929
I
295 .8
.000314
.00330-
12
I
.2918
3 14
929.03
31416
3 3912
1000
Foot-candles (apparent lumens
per sq. foot)
000343
.00214
3.40
..318
.00108
1 .076
Millilamberts
. 0003 I 8
.002054
3. 180
2958
.001
.929
I
ILLUMINATING ENGINEERING PRACTICE
Luminous Output of Bare Light Sources. Although in good practice
in the lighting of interiors, the lamps are seldom used bare without
reflectors, shades or globes of any kind, it is nevertheless of funda-
mental importance to the engineer to know the luminous output of
the various sources of light without auxiliary equipment. Then he
can proceed with his calculations by allowing the proper percentage
of loss for whatever equipment is used around the lamps.
The luminous output of different kinds of lamps per unit of input
has been rapidly changing during the past few years owing to im-
provements in the art and will probably continue to change so that
any data given here must be taken with the idea that they must be
revised from various reliable sources at frequent intervals.
Table II shows the lumens and the lumens per watt for a number
TABLE II. LUMENS OUTPUT OF AMERICAN TUNGSTEN INCANDESCENT LAMPS
JULY i, 1916
Watts
Watts per
spherical c.p.
Lumens per
watt
Total lumens
IO
15
20
105-125 VOLT MAZDA B LAMPS
1.67
1.47
1.41
7-50
8-55
8.90
75
128
I 7 8
25
40
50
i-35 "
1.32
i-3i
9-30
9-50
9.60
234
380
480
60
1.28
9.80
590
IOO
75
IOO
I . 22
10.3
1,030
105-125 VOLT MAZDA C LAMPS
1.09
i .00
n-5
12.6
86 5
1,260
20O
0.90
14.0
2,8oo
300
4OO
500
0.82
0.82
0.78
15-3
15-3
16.1
4,600
6,150
8,050
750
I,OOO
0.74
o. 70
17.0
18.0
I2,8oo
18,000
NOTE. 220 Volt lamps are about 10 per cent, less efficient.
PRINCIPLES OF INTERIOR ILLUMINATION 41
of the commonest sizes and types of tungsten filament incandescent
lamps, new, as made and used in the United States, August, 1916,
when operated at a voltage giving an average rated life of 1000 hours.
From this it is seen that the lumens per watt range from 7.5 for
the lo-watt size to 18 for the loco-watt size.
Gas mantle burners, new, and properly adjusted range in specific
output from 200 to 325 lumens per cubic foot of gas per hour in
sizes giving 400 to 3000 lumens. These figures vary with the compo-
sition of the gas and many other factors.
The amount of light obtained from the old-fashioned open flame
burner gas jet depends upon the richness of the gas in certain hydro-
carbons which produce a yellow flame in the open jet. This quality
is commonly known as the candle-power of the gas and was at one
time the common standard by which gas was rated. With the gas
mantle, however, the candle-power according to the old standards
has nothing to do with the light output of the burner which in this
case depends on the composition of the gas.
The efficiencies of lamps burning acetylene, Blau gas, alcohol,
kerosene and gasoline vary considerably, depending upon the design
of the burner, the purity of the illuminant and the conditions of
supply. The following figures have been actually obtained under
favorable conditions, but do not necessarily represent the maximum
obtainable. On the other hand, the average results in the case of
kerosene and gasoline are probably much below the stated values.
Lumen, hours,
per cu. ft.
Acetylene (open flame)
Acetylene (mantle)
500
QOO
Blau gas (mantle)
4OO
Kerosene (round wick open flame) . .
Per gallon
0,OOO
Kerosene (mantle)
24,000
Kerosene (mantle-pressure tvpe)
8o,OOO
Gasoline (mantle-low pressure).
8o,OOO
Alcohol (mantle)
l6,OOO
Kerosene lamps in particular suffer a considerable decrease in
efficiency during burning.
The older carbon filament incandescent lamp gave a specific
output of from 2.5 to 4 lumens per watt.
44 ILLUMINATING ENGINEERING PRACTICE
Prismatic reflectors offer a control of light which approaches that
of the mirror. Considerable light passes through the reflector at the
tops and bottoms of the prisms.
For indirect lighting and semi-indirect with dense reflectors it can
be shown theoretically that the best reflector for the purpose would
distribute light evenly over the whole ceiling area served from one
fixture.
That is, in a small room, with one central fixture, the whole ceiling
would be evenly illuminated; or in a large room with a fixture in the
center of each bay each reflector would evenly illuminate that bay.
By confining a considerable portion of the light flux to the center
of the ceiling with a fixture hung in the middle of the room, more of
the light flux will reach the working plane after one reflection from
the ceiling than if the distribution over the ceiling were more uniform.
The more even the distribution the greater the amount of light lost
by absorption at the walls. However, from the standpoint of the
desk worker there is some advantage in having the ceiling evenly
illuminated as there is some tendency to specular reflection from the
brightest portions of the ceiling causing a slight veiling glare. This
glare is not so pronounced if the ceiling is evenly illuminated. An
indirect reflector giving uniform ceiling distribution must be of the
deep bowl type, but this type has a very sharp " cut-off " or transition
from high to low illumination at the edge of the reflector. This
causes a shadow on the ceiling which is objectionable and calls for
some modification of uniform ceiling distribution. Two principal
ways of overcoming this have been worked out in practice which
work well with non-concentrated light sources. One is to use a
shape similar to the deep bowl distributing type for the lower part
of the reflector and a flaring bell-shaped one for the upper part. The
other plan is to use a large reflector of the shallow bowl-shape. The
former plan is used mainly with mirrored reflectors where it is desir-
able on account of first cost, to keep down the size while the other
plan is used with white enamel reflectors and for semi-direct lighting
with large glass bowls. While it may be immaterial for the engineer
who plans the lighting installation how the result of eliminating
dark shadows from the ceiling is accomplished it must nevertheless
always be kept in mind that good design calls for the elimination of
these shadows to a large extent by tapering off the brightness from
the center to the edges of the illuminated area covered by each
reflector.
For semi-indirect lighting a plain bowl somewhat shallower than
PRINCIPLES OF INTERIOR ILLUMINATION 45
a hemisphere is likely to give the best results in efficiency. Orna-
mental designs in which the maximum diameter of the bowl is greater
than the diameter at the top cause considerable loss of light because
of the light which is intercepted by the part of the bowl projecting
inward. Therefore when such designs are used this extra loss should
be recognized in the calculations and a decision reached whether the
ornamental effect attained is sufficient to justify the loss.
While the placing of lamps and shaping of semi-indirect bowls is
not as important as in the case of indirect reflectors of the opaque
type, it is not by any means a matter of indifference. The lamps
should be placed in a position not to cause undue shadows on ceilings
or walls or too uneven illumination on the bowls as viewed from
below.
Angle reflectors may be obtained giving a number of different
types of distribution for special purposes such as show window light-
ing, bulletin board lighting and other cases where more light is
wanted on one side of the plane through the lamp axis than on the
other. They cannot be classified into general types as there is such
a variety. Makers data should be thoroughly studied as to the forms
available.
Shifting the position of a lamp in a reflector by the use of different
forms of shade holders may materially change the light distribution.
In the selection of reflectors for any purpose it is always well to
remember the fundamental principle that control of the light flux
is the end to be desired if the flux is not to be wasted by escaping to
places where it is not needed or positively undesirable. The
larger the percentage of the total flux of light from the lamp which
the reflector intercepts and reflects in desired directions the higher
the efficiency; unless, however, the natural undirected flux from the
lamp approximates the distribution desired. With reflectors which
must confine the light flux of the lamp within rather restricted areas
as in show windows and for localized lighting of work benches and
the like it is important to use reflectors large enough to intercept
a considerable portion of the light flux. There is apt to be a tend-
ency to cut down reflector sizes to save first cost but such reduction
usually means a permanent impairment of efficiency. This is also
true in the lighting of a large high room of the armory or coliseum
type where the lamps must be placed high and all light eminating from
reflectors at angles only a little below the horizontal is likely to
undergo serious loss by striking dark roof and walls.
Sky Brightness Characteristics useful for design of natural illumina-
44 ILLUMINATING ENGINEERING PRACTICE
Prismatic reflectors offer a control of light which approaches that
of the mirror. Considerable light passes through the reflector at the
tops and bottoms of the prisms.
For indirect lighting and semi-indirect with dense reflectors it can
be shown theoretically that the best reflector for the purpose would
distribute light evenly over the whole ceiling area served from one
fixture.
That is, in a small room, with one central fixture, the whole ceiling
would be evenly illuminated; or in a large room with a fixture in the
center of each bay each reflector would evenly illuminate that bay.
By confining a considerable portion of the light flux to the center
of the ceiling with a fixture hung in the middle of the room, more of
the light flux will reach the working plane after one reflection from
the ceiling than if the distribution over the ceiling were more uniform.
The more even the distribution the greater the amount of light lost
by absorption at the walls. However, from the standpoint of the
desk worker there is some advantage in having the ceiling evenly
illuminated as there is some tendency to specular reflection from the
brightest portions of the ceiling causing a slight veiling glare. This
glare is not so pronounced if the ceiling is evenly illuminated. An
indirect reflector giving uniform ceiling distribution must be of the
deep bowl type, but this type has a very sharp " cut-off " or transition
from high to low illumination at the edge of the reflector. This
causes a shadow on the ceiling which is objectionable and calls for
some modification of uniform ceiling distribution. Two principal
ways of overcoming this have been worked out in practice which
work well with non-concentrated light sources. One is to use a
shape similar to the deep bowl distributing type for the lower part
of the reflector and a flaring bell-shaped one for the upper part. The
other plan is to use a large reflector of the shallow bowl-shape. The
former plan is used mainly with mirrored reflectors where it is desir-
able on account of first cost, to keep down the size while the other
plan is used with white enamel reflectors and for semi-direct lighting
with large glass bowls. While it may be immaterial for the engineer
who plans the lighting installation how the result of eliminating
dark shadows from the ceiling is accomplished it must nevertheless
always be kept in mind that good design calls for the elimination of
these shadows to a large extent by tapering off the brightness from
the center to the edges of the illuminated area covered by each
reflector.
For semi-indirect lighting a plain bowl somewhat shallower than
PRINCIPLES OF INTERIOR ILLUMINATION 45
a hemisphere is likely to give the best results in efficiency. Orna-
mental designs in which the maximum diameter of the bowl is greater
than the diameter at the top cause considerable loss of light because
of the light which is intercepted by the part of the bowl projecting
inward. Therefore when such designs are used this extra loss should
be recognized in the calculations and a decision reached whether the
ornamental effect attained is sufficient to justify the loss.
While the placing of lamps and shaping of semi-indirect bowls is
not as important as in the case of indirect reflectors of the opaque
type, it is not by any means a matter of indifference. The lamps
should be placed in a position not to cause undue shadows on ceilings
or walls or too uneven illumination on the bowls as viewed from
below.
Angle reflectors may be obtained giving a number of different
types of distribution for special purposes such as show window light-
ing, bulletin board lighting and other cases where more light is
wanted on one side of the plane through the lamp axis than on the
other. They cannot be classified into general types as there is such
a variety. Makers data should be thoroughly studied as to the forms
available.
Shifting the position of a lamp in a reflector by the use of different
forms of shade holders may materially change the light distribution.
In the selection of reflectors for any purpose it is always well to
remember the fundamental principle that control of the light flux
is the end to be desired if the flux is not to be wasted by escaping to
places where it is not needed or positively undesirable. The
larger the percentage of the total flux of light from the lamp which
the reflector intercepts and reflects in desired directions the higher
the efficiency; unless, however, the natural undirected flux from the
lamp approximates the distribution desired. With reflectors which
must confine the light flux of the lamp within rather restricted areas
as in show windows and for localized lighting of work benches and
the like it is important to use reflectors large enough to intercept
a considerable portion of the light flux. There is apt to be a tend-
ency to cut down reflector sizes to save first cost but such reduction
usually means a permanent impairment of efficiency. This is also
true in the lighting of a large high room of the armory or coliseum
type where the lamps must be placed high and all light eminating from
reflectors at angles only a little below the horizontal is likely to
undergo serious loss by striking dark roof and walls.
Sky Brightness Characteristics useful for design of natural illumina-
46 ILLUMINATING ENGINEERING PRACTICE
tion are given in Table III. It will be seen that there is an enormous
variation in the brightness of the sky during what are ordinarily
TABLE III. SKY BRIGHTNESS
Sky, with light clouds
Sky, clouds predominating, generally cumulus . .
Sky, blue predominating, clouds cirrus
Sky, cloudless, either clear blue or hazy
Sky, cloudy, storm near or present
Walls, typical rooms, ordinary range diffused
daylight through window
Brightness in
millilamberts
2,OOO
1,900
1,500
I,OOO
700 to 70
50 to
considered daylight hours. Calculations of daylight illumination
of interiors should therefore be made on the basis of maximum and
minimum values.
The sky is the principal source of daylight illumination of in-
teriors, as the illumination obtained directly from the sun may be
considered as purely incidental and frequently avoided by the use
of shades.
In connection with daylight we have first to consider the amount
of sky exposure through side or ceiling windows; the amount of
illumination (excluding reflection from walls and other buildings
on any point) varying directly according to the area of the exposure
as projected from the point in question. Part of the window area
may be obstructed by buildings and in certain cases the reflection
of light from these buildings (or in other words their brightness)
must also be taken into account as well as that of the sky.
Illumination from Direct Sunlight in the open has been found to
reach approximately 9000 foot-candles, in Virginia, during the
summer months as measured on a horizontal plane. Extensive
measurements made there by Prof. Herbert H. Kimball, of the U. S.
Weather Bureau, show that the total illumination from sun and sky
during the middle of the day consists of about 20 per cent, skylight
and 80 per cent, direct sunlight. Sunlight shining into interiors
therefore may have about 80 per cent, of its outdoor value.
With clear glass windows the only sky brightness which is useful
for illumination of the room is that directly visible from the interior
of the room. If the window is obstructed by buildings the sky
brightness is not available. Where a window is exposed to sky
PRINCIPLES OF INTERIOR ILLUMINATION 47
area either above or at one side, and the illumination from such area
does not reach back into the 'room far enough, prisms and diffusing
glasses of various kinds are applicable. The action of the prism
glass window is to bend the light rays so that they strike back further
into the room than if a clear glass window were used. Rough and
ribbed glasses accomplish the same end with less precision and
effectiveness. They diffuse the light rays passing through, and a
certain portion of such rays are directed back into the room. For
some locations louvers or shutters consisting of partially or wholly
opaque strips which can be tilted at any angle make it possible to
regulate the relative amount of sun and skylight or cut out direct
sunlight without too serious a reduction in the skylight. The com-
mon method of controlling sunlight is by translucent shades but this
method for some interiors (such as art galleries) does not offer very
accurate control.
The Brightness or Intrinsic Brilliancy of Various Artificial Light
Sources and also the brightness of some sources equipped with dif-
fusing glassware for the protection of the eyes is shown in Table IV.
TABLE IV. BRIGHTNESS or ARTIFICIAL LIGHT SOURCES
Brightness in
milHlamberts
Crater, carbon arc 40,800,000
Flaming arc, clear globe . . . . . . . 2,435,000
Magnetite arc, clear globe 1,945,000
Gas-filled tungsten electric light filament 1,400,000
Incandescent electric tungsten, 1.25 watts per
candle 516,000
Quartz tube, mercury vapor arc , 486,700-292,000
Incandescent electric carbon filament, 3.1 watts
per candle 236,000
Acetylene flame (i foot burner) 25,800
Welsbach mantle 15,080
Cooper Hewitt glass tube mercury vapor lamp. . 6,800
Kerosene flame 1,946-4,380
25 watt frosted tungsten lamp, side 2,920
Candle flame 1,460-1,945
Gas flame (fish tail) 1,314
10" opal ball, over 100 watt tungsten lamp 306
Ceilings over indirect lighting fixtures (usual
range, brightest part as viewed by occupants
of room) 73~4
Glass bowls used for semi-direct lighting 1,000-35
4 8
ILLUMINATING ENGINEERING PRACTICE
Depreciation due to dirt on glass and reflecting surfaces and to
inherent characteristics of the lamp's used must be recognized in
design.
Both the total lumens and the lumens per watt of tungsten fila-
ment electric lamps drop with use, partly by the blackening inside
the bulb and partly by disintegration and increase in resistance of
the filament. Such lamps operated at the specific outputs shown in
Table II, fall off in lumens output about 15 per cent, in 1000 hours
service. With electric arc lamps and gas mantle burners so much
12 16 20 24
Elapsed Time in Weeks
Fig. 2. Depreciation caused by dirt.
32
36
40
depends upon the adjustment and other variable factors that no
depreciation figure inherent in the lamp can be given, but unless
maintenance is especially good more must be allowed than for the
internal depreciation of the tungsten filament electric lamp.
The accumulation of dirt on the surrounding glassware and on the
globe or reflector is an important cause of loss of light and should
also always be reckoned with in preliminary calculations. It is
necessary to assume some probable maximum depreciation figure
from this cause and in making such an assumption of course the sur-
rounding conditions and the probable frequency of cleaning must be
considered. In Table V is given a compilation of results of various
tests made in different places by different observers on the effect
of the accumulation of dirt, and Fig. 2 shows the depreciation over
an extended period for a given set of reflectors.
The effect of accumulation of dirt on side and ceiling windows
is probably about the same as on lamps.
Utilization of the Generated Light Flux. There are various methods
of calculating the resultant illumination at the desired point with a
PRINCIPLES OF INTERIOR ILLUMINATION
49
TABLE V. Loss OF LIGHT BY ACCUMULATION OF DIRT
Authority and
reference
Conditions and
surroundings
Lamps, globes and
reflectors
3-ga
|6S
ill
"1 G
(x >
8
|
* 0.
Durgin & Jackson,
Down town Chicago
Semi-direct, dense
3wk.
76
35 o
Trans. I. E.S., 1915.
office building dust-
bowls and tungsten
p. 707.
iest rooms. 1 lamps.
Aldrich & Malia,
Office building in Chi-
Prismatic reflectors,
12 wk.
25
8.5
Trans. I.E. S.. 1914.
cago Stock yards.
satin finish and tungs-
p. 112.
ten lamps. Direct.
Do.
Do.
Mirror reflectors and
9 wk.
25
II .0
tungsten lamps. In-
direct.
Do.
Do.
Opal bowl reflectors
2wk.
5
IO.O
and tungsten lamps.
Semi-indirect.
C. E. Clewell, Fac-
Suburban factory
Prismatic, satin finish
14 wk.
42
13 5
tory Lighting, p. 46.
office.
reflectors and tungs-
ten lamp. Direct.
Do.
Do.
Prismatic clear reflec-
17 wk.
17
45
tors and tungsten
lamps.
Do.
Suburban factory.
Do.
9 wk.
28
14.0
Do.
Do.
Do.
ii wk.
29
II. 5
Do.
Do.
Do.
13 wk.
40
13-4
Edwards & Harrison,
Office corridor Subur-
Enclosing prismatic.
8 wk.
II
6.2
Trans. I. E. S., 1914.
ban district, Cleve-
p. 176.
land.
NOTE. Since depreciation is more rapid at first, as shown by the curves the decline per
month here given would not apply to longer periods.
NOTE. For extensive additional tests see' paper by A. L. Eustice, Trans. I. E. S., 1909,
p. 849-
given generated light flux. Before making such calculations it is
of course important to reach an intelligent decision as to the points
where the desired illumination is needed and whether it is best to
consider the illumination measured in a horizontal plane, or vertical
plane, or a plane in some other angle, suited to the particular re-
quirements in question. Common practice in calculating and
measuring the illumination in most interiors is to ascertain the
illumination in a horizontal plane from 2.5 to 3.5 ft. above the floor,
or about the height of desks, counters and benches. For the
4
50 ILLUMINATING ENGINEERING PRACTICE
majority of interiors this consideration of the horizontal plane serves
the purpose sufficiently except for special localized lighting around
machinery. If the illumination in the horizontal plane, commonly
known as the "working plane" is to be taken as the criterion, it is
possible to measure the average illumination in this plane over an
entire room by measuring the illumination with a portable photom-
eter at the center of a number of equal-sized rectangles into which
the room may be divided. Dividing this average light flux by the
light flux generated by the lamp gives what is known as the per-
centage efficiency of utilization, or utilization factor. Of course any
other plane might be used for figuring efficiency of utilization pro-
vided the position of the plane were the position where the light was
wanted. For example in an Art Gallery the efficiency of utilization
might well be figured from the light flux incident upon wall spaces
devoted to pictures and in a show window it would be figured from
the flux through a curved surface corresponding to the line of trim
of the window.
The point-by-point method of calculation (that is, if dealing in
English units, dividing the candle-power by the square of the dis-
tance in feet to the point in question and multiplying this by the
cosine of the angle of the incident ray to the surface in question to
get the foot-candles incident illumination) is now chiefly used only
for calculating the illumination at a few points from a single or small
number of light sources. It is too time-consuming and laborious
a method for the calculation of the illumination of large interiors with
many light sources. It has the further limitation that it takes no
account of reflection from ceiling, walls and floors and considers only
the illumination direct from the lamp and its accessories.
The point-by-point method may be of considerable use in forecast-
ing the differences in daylight illumination and at different points of
interiors where the sky exposure and reflection coefficient of the
buildings visible from any point in question are definitely known.
The foot-candles illumination at various points as one proceeds back
into a room from a window with unobstructed sky exposure may for
the rough purpose of practical calculations be taken as inversely
proportional to the square of the distance from the window to the
given point. In applying this rule the fact should be kept in mind
that frequently the window is far from an unobstructed sky exposure
and that the sky exposure changes as seen from various points further
back into the room. The effective exposure is the projected area of
the sky seen by one looking at the window from the given point.
PRINCIPLES OF INTERIOR ILLUMINATION 51
A practical short-cut in the use of the point-by-point method in
calculating horizontal illumination which obviates the necessity of a
table of cosines and makes possible calculations with only the aid of
a polar candle-power curve of the light sources, is the following,
which is a graphic method of applying the cosine factor. In the usual
rule for getting horizontal illumination the illumination is equal to
the candle-power at the given angle divided by the square of the dis-
tance multiplied by the cosine of the angle between the ray in
question and the vertical. Now if we draw a perpendicular from the
photometric curve at the angle in question to the vertical and take as
the candle-power the candle-power scale reading at the point where
this perpendicular intersects the vertical, we apply the cosine
factor at the outset and by simply dividing this candle-power at the
intersection with the horizontal, by the square of the distance the
illumination is determined.
In calculations of illumination by the zone flux method all of the
lumens emitted in a certain zone, say from o to 60 degrees or from
o to 70 degrees, are figured as falling upon the working plane in
the general lighting of an interior. This method, of course, takes no
account of the uniformity of illumination and where approximate
uniformity is desired must be used only with lamps and reflectors
giving a type of distribution which will be sufficiently uniform. The
zone flux method is chiefly applicable to illumination calculations
with opaque reflectors where ah 1 of the flux is emitted in downward
directions and little reliance is placed upon walls and ceilings to bring
up the general illumination. Some industrial plants and foundries
present such conditions. In the application of this method care
must be taken not to select such a large zone as a basis that too much
of the light strikes walls or other obstructions. At the same time in
large interiors it is not necessary to confine the zone to simply those
which would cover the floor near by. In show-window lighting if the
reflector selected is such as to confine its flux to the plane it is desired
to illuminate the method may sometimes be used for approximation.
Empirical methods of calculation based on actual experience and
tests of existing installations form by far the most important basis for
most calculations. With the other methods certain assumptions are
necessary which may or may not be correct. With the empirical
method based on experience, the only sources of error are those due to
erroneously assuming conditions in the case to be calculated to be
similar to those in the tested cases. Tables VI and VII and Figs. 3
to 9 inclusive give utilization factors or ratio of generated lumens to
ILLUMINATING ENGINEERING PRACTICE
TABLE VI. UTILIZATION FACTORS
Ceiling, reflection coefficient
Light 70 per cent.
Medium 50 per
cent.
Walls, reflection coefficient
Light
50
per cent.
Medium
35
per cent.
Dark
20
per cent.
Medium
35
per cent.
Dark
20
per cent.
Lighting Equipment:
Direct, Prismatic
6"?
6l
^o
<8
<6
W J
40
37
oV
36
o"
36
o u
35
Direct, Light Opal
cj7
C-2
CQ
48
46
o /
33
oo
28
o w
27
<^W
26
*T W
24
Direct, Dense Opal
61
58
57
56
53
40
35
34
34
32
Direct, Steel Bowl, Enamel or Alu-
minum.
57
55
54
54
53
39
36
35
35
34
Direct, Steel Dome, Enamel
70
67
65
67
65
46
42
39
42
39
Totally indirect, Mirrored
4.O
^8
36
27
26
T.V-;
24
o"
21
o"
20
z /
IS
H
Semi-indirect, Light Opal
47
45
43
39
35
30
25
24
22
20
Semi-indirect, Dense Opal. . . .
42
4-1
4b
31
20
T"O
27
T- A
25
T- W
22
O A
18
O^
17
Totally enclosing
4 6
42
40
38
35
Light Opal
25
19
18
18
15
The values in this table have reference to square rooms equipped with a sufficient number
of lighting units and so placed as to produce reasonably uniform illumination. In each case
the upper figure applies to an extended area, namely, one in which the horizontal dimension
is at least five times the distance from floor to ceiling. The lower figure applies to a con-
fined area, one in which the floor dimension is but five-fourths of the ceiling height. The
utilization factor for a rectangular room is approximately the average of the factors for
two square rooms of the large and small floor dimension respectively.
lumens incident upon the working plane for a number of typical con-
ditions. A study of these tables shows the marked influence of size
of room and ceiling and wall colors on efficiency. The figures on
utilization factors Figs. 3 to 9 will hold for all rooms of the same
relative proportions, as to shape, without regard to sizes.
PRINCIPLES OF INTERIOR ILLUMINATION
53
HYGIENE
The hygienic aspect of illumination is chiefly that of the effect
on the eyes. It is also known that sunlight and other kinds of
light having ultra-violet rays have a germicidal effect useful in kill-
ing disease organisims. There is also a psychological effect of light.
TABLE VII. UTILIZATION FACTORS OBTAINED BY LANSINGH & ROLPH
Page 586. Transactions I. E. S., 1908. Room 11.5 by 10.1 ft. high. All
lamps at ceiling. Reflectors (where used) were of clear prismatic type.
Ceiling
Walls
Floor
Per cent,
utilization
i Bare lamp
Dark
Dark
Dark
16 4
i Lamp in reflector
Dark
Dark
Dark
Ti 6
i Bare lamp
Light
Dark
Dark
20 4
i Lamp in reflector
i Bare lamp
Light
Light
Dark
Light
Dark
Dark
42.0
a.8 6
i Lamp in reflector
i Bare lamp
Light
Light
Light
Light
Dark
Light
55-0
60 o
i Lamp in reflector
3 Bare lamps
Light
Dark
Light
Dark
Light
Dark
79-o
14. O
3 Lamps in reflector
3 Bare lamps
Dark
Light
Dark
Dark
Dark
Dark
26.0
26 o
3 Lamps in reflector
3 Bare lamps
Light
Light
Dark
Light
Dark
Park
34-o
4.6 o
3 Lamps in reflector
Light
Light
Dark
^o.o
3 Bare lamps . .
Light
Light
Light
e6 O
3 Lamps in reflector
Light
Light
Light
66 o
The germicidal effect of sunlight has led to legislation requiring
sunlight in living and sleeping rooms in some cities. It is evident,
however, that an intelligent application of this to design requires
considerable definite knowledge as to the amount of sunlight in
a room which will cause appreciable germicidal effect and on this
scientific evidence is still lacking.
As to the psychological effects there is a still greater need of defi-
nite knowledge. Points which may be considered psychological
by some are taken up later under the head of aesthetic effects.
The eye is concerned chiefly with two things (a) sufficient bright-
ness of visualized objects, resulting from sufficient illumination and
(b) with the distribution of brightness within the entire field of vision.
Ordinary requirements for efficient vision are:
i. Sufficient quantity of steady diffusely reflected light from the object
viewed.
54
ILLUMINATING ENGINEERING PRACTICE
2. Minimum flux of light emitted in the direction of the eye by specular
or spread reflection from the objects viewed.
Per cent Utilization
oS88SS2
.
T
j&
1
Walls I
i
1
"36
J"
1
Roor
Rati
Walls Med
ieflection Coefficients
Ceiling Varied
Walls Black 4.3*
Medium 42.5?
" White 81.0*
Flooj Wood I4.0SJ
Q A
j Spacing of Units
= 1.59
ite - 81*
1
rt
K-13'6-->j
Slack- 4.3*'
Height above Plane
urn -42.5* Walls Wt
.
D
+*
~
I
ire 1
t
I)i
rect
X^
>* V
/"
B
a.
9
TStt
*&
?
L^i
ifc^
^
> V>
']>?
^
%2
<?
^
^f^
XI
l.O.
w.
^
<4 i,-
.v.
^
L
ir.
20 40 60 80 100 20 40 60 80 100 20 40 60 80 100
Reflection Coefficleat ol Ceiling
Fig. 3. Utilization factors.
70
T
i
T
'CO
7
.i.
Walls 1
i
Re
c
11
F
Room
Ratio
Walls Medium
flection Coefficients
eiling Varied
^alls Black 4.3*
> Medium 42.5*
.. White 81.0*
loor Wood 14.0*
B
Spacing of Units
= 1.04
81*
n a
Height above Plane
-42.5* Walls White -
?lack-4.3*
40
I-
20
10
rcct
--*-*
^
j^_
a
1)
irec
a-
T*i
cct
-
<*>
^
^
"*^7 P "
^.
9
*/
^i
H
-
^
V
^
*
;
.0.
>r.
ftffns
.0.
r.
^L
D.a t . i
.0.
r.
20 40 60 80 100 20 40 00 80 100 20 40
Reflection Coefficient ol Ceiling
Fig. 4. Utilization factors.
3. Absence of violent brightness contrasts within the field of vision.
4. Freedom from sharp shadows.
PRINCIPLES OF INTERIOR ILLUMINATION
55
Glare Defined. The 1915 Committee on Glare of the Illuminating
Engineering Society in its report on Interior Illumination, page
36, 1. E. S. Transactions, 1916, tentatively offered the following defi-
i I ------
3
B
H
ti
D
?
h6'9*^
B H
tr
D
Reflection Coefficients
Ceiling Varied
Walls Black 4.3*
Medium 42.5*
,, White 81.0*
Floor Wood 14.0*
Boom O
Ratio
Walls Black -4.3*
Height above Plane
Walls Mediom- 42.5* Walls White -81*
20 40 80 100 20 40 60 80 100 20 40 60 80 100
Reflection. Coefficient ol Ceiling
Fig- 5- Utilization factors.
-1
------ - -
_L2'2" Beflection Coefficients
f Ceiling Varied
Walls Black 4.3*
T~
I
*'&-" " B H
White 81.0*
Floor Wood 14.0*
T
i
Ratio s i> acin K of Tnits
< 27 >
Height above Plane
Walls Black-4.3* Walls Medium -42.5* Walls White-81*
100 20 40 60 80 100 20 40 60 SO 100
Reflection Coefficient of Ceiling
Fig. 6. Utilization factors.
nitions which express more definitely than heretofore attempted
what constitutes glare. Three alternative definitions were offered
as follows:
ILLUMINATING ENGINEERING PRACTICE
A A
?
36'
f
Reflection Coefficients
Ceiling Varied
Walls Black 4.3*
M Medium 42.5*
' White 81.0*
Floor Wood 14.0*
20 40 60 SO 100 20 40 60 80 100 20 40 60 80 100
Reflection Coefficient of Ceiling
Fig. 7. Utilization factors.
Roorn-A-lsYx ISY* u' 1 Unit
Eoom-D- 13 6 x 27 x 6 18 Units
Reflection Coefficients
Walls Varied
Ceiling Black -4.3*
" Dark Gray -33*
Light Gray-64* Reflection Coefficients
" White -81*
Floor Wood
Walls Varied
Ceiling Black -4.3%
" Dark Gray-33jt
Light Gray-64t
White -81%
Floor Wood -14%
20 40 60 80 100 20 40 60 80 100
Reflection Coefficient of Walla
Fig. 8. Utilization factors.
20 40 60 80 100 20 40 60 80 100
Reflection Coefficient of Walls
Fig. 9. Utilization factors.
PRINCIPLES OF INTERIOR ILLUMINATION 57
Glare. i. Brightness within the field of view of such excessive
character as to cause discomfort, annoyance, or interference with
vision.
2. Excess brightness of or flux of light from the whole or any por-
tion of the field of view, resulting in reduced vision, fatigue or dis-
comfort of the eye.
3. Light shining into the eye in such a way, or of sufficient quantity,
as to cause discomfort, annoyance or interference with vision.
Contrast glare is a kind of glare commonly experienced in de-
fective lighting of interiors. That is, the contrast between the
brightness of the sources of light and other objects in the visual
field is so great as to cause discomfort, annoyance or interference
with vision. As far as we know there is no measurable interference
with vision when the glaring bright source of light is removed 25
to 30 degrees away from the center of vision. It may, however, cause
discomfort, annoyance and eye fatigue if it is anywhere within the
visual field. Therefore while a design which removed the lamp more
than 25 degrees from the ordinary range of the center of vision might
be satisfactory as far as measurable interference or reduced ability
to see is concerned, it might not be satisfactory to work or live under
continuously because of the fatigue and annoyance resulting.
A review of all of the available data and observations of cases
where eye fatigue and annoyance have been complained of together
with numerous eye fatigue tests by the Ferree method indicates that
to avoid glare effects visible light sources should not be more than 200
times as bright as their background and preferably not over 100
times, in ordinary artificial lighting of interiors where the average
illumination of the working plane is from 3 to 6 foot candles. As
most of the tests on this point have been made at about this mag-
nitude of brightness it is not entirely certain what ratio should be
adopted for other magnitudes, but from tests made by Nutting
(I. E. S. Transactions, 1916 Convention) on the lower limits of
annoying glare (which limits of brightness are of course much
higher than for fatiguing glare) as well as from certain well known
common experience there is reason to believe that for higher illu-
minations than 6 foot candles this limit of contrast should be less
than loo to i while for lower limits it may be more than 100 to i.
Brightness for bowls and globes for locations where they are con-
tinuously within the field of vision, with from 3 to 6 foot candles on the
working plane should be kept approximately below 300 millilamberts
in rooms with light-colored (50 per cent, reflection coefficient) walls to
58 ILLUMINATING ENGINEERING PRACTICE
safely conform to the 100 to i contrast limit. The brightness should
be diminished as the reflection coefficient of the walls is decreased.
Outdoors where brightness magnitudes are much higher it is worth
while noting that contrasts do not often exceed twenty to one; while
at night, outdoors, much greater contrasts are well known to be
tolerable.
Brightness glare is glare due to an excessive general brightness of
the field of view. It is seldom experienced in interior illumination
except possibly from the reflection of sunlight from a sheet of white
paper.
Temporary glare resulting from flicker is a condition caused by the
lack of brightness accommodation of the retina of the eye to such
sudden changes in brightness.
Specular reflection or -veiling glare from glossy paper, polished
metal work and the like are very common conditions with all sys-
tems of lighting and are likely to be especially pronounced with arti-
ficial illumination, from relatively small sources. The polished
surface reflects a glaring image of the source of light. The actual
brightness of the glare on the paper as far as it can be measured is
not likely to be over 1.5 times that of the background but this seems
to be enough to make trouble in this location though it would hardly
be noticed elsewhere. Frequently the ink or pencil marks on paper
have more specular reflection than the paper and in the glare posi-
tions these marks may be equally as bright as the paper, and hence
invisible or nearly so.
Shadows may cause interference or trouble with work if the illumi-
nation in the shadow is insufficient or if the contrast between the
parts in shadow and those out of the shadow makes the shadowed
places appear dark by contrast. Shadows caused by bright light
sources with direct lighting have sharp edges and may cause annoy-
ance while an equal shadow with a large source of light or indirect
lighting where the transition from the middle of the shadow to the
edge is gradual may not be perceptible e'xcept to the expert.
Shadows are to be most carefully considered in large office and
factory spaces lighted by general lighting, where the location of the
work with reference to the light cannot be adjusted or charged and
the illumination must be sufficiently good at any point in any posi-
tion to permit of efficient work.
The ratio of illumination in the shadow to illumination just out-
side of the shadow with large sources or indirect light may be as
high as one to two without causing annoyance provided the illu-
PRINCIPLES OF INTERIOR ILLUMINATION 59
mination in the shadow is sufficient for the purpose in hand. Be-
cause of the nature of these shadows with indirect lighting the or-
dinary person is apt to think there are no shadows and to attempt the
closest work in the shadows of his head and body, not realizing that
the illumination is better away from this shadow. With the sharper
shadows common to direct lighting systems this would not be the
case. However, owing to the sharpness of these latter shadows the
same shadow ratio might sometimes cause some annoyance.
In a large room with a number of lighting units the actual magni-
tude of the shadow, that is the ratio of illumination in the shadow
to that out of it, is likely to be about the same with an indirect sys-
tem as with a direct, provided the spacing of the outlets is the same
in both cases. The direct lighting shadows have sharp edges, how-
ever, which makes them easily apparent where the others are not.
Quantity of Illumination. It is customary to discuss problems
concerning the quantity of illumination required for different pur-
poses in terms of the illumination incident upon the work. This in-
cident illumination, however, is the cause which produces the desired
effect, namely, brightness of the object viewed, and it is this effect
that is the real end desired. Table VIII calculated by Dr. P. G. Nutt-
ing from work by Konig and himself shows the sensibility of the eye
TABLE VIII. EYE SENSIBILITY AT DIFFERENT MAGNITUDES OF SURROUNDING
BRIGHTNESS
P. G. Nutting
Average brightness
magnitudes, milli-
lamberts
Perceptible percent-
age difference in
brightness
Exterior, daylight
Interiors, daylight
IOOO.O
IO.O
0.0176
0.030
Interiors, night
.....' o.i
O. 123
at different typical brightness magnitudes. As a matter of fact of
course the brightness magnitudes of interiors both at night and day
vary considerably from the average brightness value given. From
this table it will be seen that increasing the illumination one hundred
fold from a rather poor lighted interior at night to an interior by
daylight makes the eye able to perceive a percentage difference in
brightness about one-third of that it is able to perceive in the former
case. This gain is apparently rather small but if the eye is working
near the limit it may be important.
60 ILLUMINATING ENGINEERING PRACTICE
The eye sees by virtue of differences of brightness and color.
The question of a sufficient quantity of illumination for a given kind
of work is not altogether that of delivering a certain number of foot-
candles on a certain plane where the work is being done. The ques-
tion is fundamentally one of producing a sufficient contrast of bright-
ness for the eye to perceive readily objects with a given brightness of
surroundings. In the case of reading printed or written letters on
paper we have a considerable contrast between the paper and ink
or pencil which makes them easy to distinguish with any kind of
illumination which does not produce specular reflection or glare from
the paper or ink, provided the illumination is of sufficient quantity.
In the case of sewing on either dark or light goods there is very little
contrast between the thread and the goods so that the problem of
producing sufficient shadows and specular reflection to enable the
thread and the texture to be seen easily is important. For this
purpose localized lighting coming mainly from one direction is
necessary.
Many tables have been published of the intensity of illumination
required for various purposes but all should be used with allowance
for the fact that color and direction must be considered as must also
the general brightness of the surroundings. The latter is~ especially
true when there is a large window exposure but the particular
spot to be illuminated does not get the benefit of the window
illumination.
The indications of scientific research so far are that the eye works
best when the object upon which vision is centered is of about the
same general magnitude of brightness as the surroundings. This is
what one might expect from the conditions under which the eye has
been evolved.
Table IX shows the approximate foot-candles illumination consid-
ered about right by a number of authorities for various classes of
interior lighting.
The question of proper quantity of illumination for reading has
been investigated much more thoroughly than that for other pur-
poses. Tests show considerable difference between individuals
although the same individuals show consistent repetition of the
quantities considered sufficient. If the direction and diffusion of light
is such as to cause veiling glare from the paper or ink more illumina-
tion is required although it cannot be said that with veiling glare
present it is ever possible to produce as satisfactory and comfort-
able illumination, no matter what the intensity, as can be obtained
with veiling glare practically absent.
PRINCIPLES OF INTERIOR ILLUMINATION 6 1
TABLE IX. ILLUMINATION FOR VARIOUS PURPOSES
Foot-candles
Reading: U. S. Government Postal Car minimum require-
ments.
Clerical and office work
Drafting
Drafting, tracing on blue prints or faint pencil drawings.
Factory work, coarse .'.
Factory work, fine
Corridors
Stores, ordinary practice
Stores, first floors, large cities
Audience rooms
Show windows
2 . 8 Note a.
3-7
5-10
10-20 Note 6.
1.25-2.5 Note c
3.5 -10 Note c.
0.25-1
3-7
5-10 Note d.
i-3
5-40 Note d.
NOTES. (a) Some individuals are satisfied with half this while others, especially the
aged and those not properly fitted with glasses and those whose eyes are sub-normal for
any reason may be satisfied only with values considerably higher than this; perhaps 5 to
10 foot-candles. When such individuals are to be satisfied this fact must be remembered
in the design.
(6) Illumination from below is preferable, using a translucent table.
{c) Depends also on color.
(d) Depends on surrounding competition.
As a result of extensive tests of postal clerks and others on the
light required for reading under postal car lighting conditions the
United States Government now specifies a minimum illumination of
2.8 foot-candles at points where reading of letter addresses is to be
done by postal clerks.
There is no conclusive evidence at the present that there is any
marked hygienic advantage in color of one artificial illuminant over
another. This statement refers to purely physiological results
rather than to aesthetic effects. An exception to this which should
be noted, however, is that there is good evidence that the chromatic
abberation of the eye causes a certain lack of clearness with most
natural and artificial illuminants so that for seeing fine details a light
which is nearly monochromatic like the mercury-vapor light is
preferable.
ESTHETIC EFFECTS
It is not the function of this portion of the lecture to give a dis-
sertation on art but rather to call attention to methods by which
certain desirable effects can be produced and undesirable ones
avoided.
62 ILLUMINATING ENGINEERING PRACTICE
The function of illumination is to provide light and shade, as it is
artistically called, on various objects. From the standpoint of
appearance much depends on how the light and shade are regulated
or in more scientific language upon the direction and diffusion of the
light. By diffused light is here meant light coming from many di-
rections or from large surfaces like the sky or illuminated ceilings.
Much of the pleasing or displeasing effect of a design of interior illumi-
nation depends upon the proper use or misuse of shadows, and tastes
differ decidedly as to what light and shade effects are most pleasing.
Heavy shadows are produced by light coming mainly from one
direction with very little general diffused light. While for some par-
ticular purposes extreme contrasts are considered desirable by some
persons, others think them to be unpleasant. It is possible to
eliminate shadows so completely by having light coming from many
directions that there remains only the difference in the coefficient of
reflection of different parts of the illuminated object to enable the eye
to distinguish it. If the object is a piece of white statuary or
moulding of uniform color and reflecting power, perfectly uniform or
diffuse illumination will obliterate all details.
Direct lighting systems with small sources produce sharp shadows
like those produced by sunlight. With indirect lighting systems and
semi-direct systems with very dense glassware the shadows are very
similar to those obtained from skylight. They differ from window
daylight in direction when the ceiling is the main reflecting surface,
but if the wall is the main surface window direction and diffusion is
imitated. Sky- and window-light shadows are gradual transitions
from light to dark.
Exposed light sources have been used for many years for decora-
tive effect and will doubtless continue to be used. It is for the illumi-
nating engineer to recognize this fact and to guide the use into the
proper hygienic channels. With the tiny sources of light available
up to the introduction of electricity and gas mantle burners the bad
effects of glare with decorative lighting of this kind were not much
felt. With the brighter and more powerful light sources now
common, adequate shading precautions must be taken. The bare light
source of to-day is not only hygienically bad but it is so crude as to
be unartistic. There are so many opportunities to produce pleasing
effects with diffused light by the use of colored glass, cloth, or paper
shades, leaving the main light for useful purposes to be obtained in
other ways that there is no longer much excuse for the type of fixture
which in spite of its great expense offers nothing better for light
PRINCIPLES OF INTERIOR ILLUMINATION 63
diffusion than a lot of loosely hung prisms interspersed with bare
lamps.
In the use of lamps for decorative effects the same rule as to low
brightness values should be adhered to as is laid down under the head
of hygiene. The lamp shade or globe which must be faced continu-
ally should be not more than 200 times as bright as its background,
and no light source of this kind should be bright enough to be annoy-
ing or noticeably glaring.
Although white daylight cannot be said to have an unpleasant
effect on countenances it is notable that among lamps, those which
give light yellowish in color rather than those offering considerable
green and blue are the most pleasant. Red and yellow light bring
out the agreeable color of the face while the absence of those colors
and prominence of blue and green give the countenance a ghastly hue.
There is some difference of opinion as to how far red and yellow and
amber colors should be sought in light, especially in residence light-
ing. Some even go so far as to color the tungsten lamp purposely to
get nearer the yellow color of the old carbon lamp. However, this
result can also be obtained by the use of ceiling and wall colors and
proper glassware. If indirect or semi-indirect lighting is used, the
color of the ceiling has much to do with the resultant illumination in
the room. The ceiling can be so tinted as to make the room illumi-
nation as yellow as desired. The small amount of illumination which
should be allowed to come directly through the bowl of a semi-
indirect fixture should not have much effect in the general total.
A very yellow light like that of the old carbon incandescent and
open gas flame or more modern illuminants with yellow globes brings
out certain yellowish hues in decorations and paintings so as to give
a richer effect than would be obtained with white light. At the same
time it must be remembered that these are deficient in green and blue
and the green and blue in paintings and decorations suffer accord-
ingly. Either the decorations should be suited to the color of light
or the color of the light to the decorations. As to which course
should be pursued depends entirely on the particular conditions of
the case.
At the present time almost any color desired in artificial lighting
can be obtained with a sufficient expenditure of money. Where it is
desirable to bring out all of the colors as in daylight several methods
are open. The Moore carbon dioxide tube lamp and the intensified
carbon arc lamp uncorrected give practically white light. The gas
filled tungsten lamp and the gas mantle burner with a special mantle
64 ILLUMINATING ENGINEERING PRACTICE
can be used with a glass having the proper selective absorption to
filter out the excess of certain colors and give a white light. The
same process can be used with other yellow illuminants. Since this
process involves throwing away the excess yellow over and above
that needed to maintain a proper balance for white light it is, of
course, somewhat wasteful.
In considering the question whether art may clash with hygiene
and utility, it may be appropriate to ask whether anything can be
considered artistic which is unhygienic and ill-suited to the use for
which it is intended. Nevertheless it may.be proper to mention
some points where so-called art and comfort and the health of the
user may clash. When an architect designs an interior so that noth-
ing but exposed glaring lamps on brackets will satisfy his idea of the
artistic one is tempted to ask where the art conies in as far as the
user of the room is concerned. When an audience room or council
chamber is finished in dark colors with elaborate chandeliers of a
design which permit of nothing but a great quantity of glare one is
again tempted to make the same inquiry. Cases can be cited with-
out number where the ideas of some person as to what is artistic are
given precedence over health and comfort. There are no reasons
why these three elements cannot be combined.
PART II. THE PROCESS OF DESIGN
The process of illumination design usually consists of the following
steps:
1. Selection of the general scheme of lighting, and the type of
lighting units.
2. Calculations of the quantity of light flux required.
3. Final selection of the location and size of the lighting units.
In making each of these steps we must fall back upon the basic
information given in Part I.
The selection of the general type of light source must, of course,
depend on the kind of lamps available. This depends on local
conditions and need not be discussed here. Then the hygienic and
artistic requirements and limitations should be considered.
Both the electric incandescent lamp and the gas mantle burner
are adapted to the illumination of almost any kind of interior from
the roughest to the most refined. For the illumination of offices and
industrial plants there also comes up for consideration the mercury-
vapor lamp. For some of, the roughest industrial plants such as
foundries and steel mills the flame arc lamp can also be considered,
PRINCIPLES OF INTERIOR ILLUMINATION 65
although in offices and stores the fumes emitted are not allowable.
The color of the light from the mercury-vapor lamp, of course, is an
objection from the artistic standpoint although hygienically no case
has been found against it. For work on fine black and white detail
the better visual acuity it gives tends to offset the psychological
effect of its color.
In choosing between the tungsten electric and gas mantle burner
lamps the following points must be considered for each case:
'(a) The cost per 1000 lumen hours for electricity versus gas at the
current prices. In making such comparison allowance should be
made for the probable depreciation of the lamp below the labora-
tory performance figures given in Part I. In the case of elec-
tricity there is a blackening and increase in resistance in the lamp
internally and the accumulation of dirt externally to cause depre-
ciation, and in the case of gas in practice the burner adjustment is
seldom as good as that obtained in the laboratory and there is the
possibility of worn and defective mantles. These depreciation figures
can easily lower the electric lamp output by from 20 to 50 per cent,
below laboratory figures and the gas lamp output to by from 30
to 60 per cent, below the laboratory figures. Of course, the engineer
should take into account the maintenance conditions that are likely
to exist in the completed installation. The better the mainte-
nance the lower the necessary percentage allowance for depreciation.
(b) The relative convenience of control under the two methods.
If the gas installation is to be arranged for a control practically
equivalent to that of electric, the comparative total cost of the two
systems should be considered.
(c) Additional blackening of ceilings and walls with gas as com-
pared to electricity should be weighed against the cost of elec-
tricity along with the cost of gas.
(d) The probable relative steadiness of the two illuminants under
the particular local conditions under consideration. The voltage
of the electric system may be very unsteady and the pressure of the
gas very steady or the reverse.
(e) The cost of glassware and lamp renewals for electric lamps
and the cost of glassware and mantle renewals or maintenance
service for gas lamps should be figured.
No illuminant should be chosen which does not permit the use of
the proper globe, shade or reflector equipment to conform to the
hygienic requirements spoken of later.
Glare Elimination. The necessity of the elimination of glare de-
66 ILLUMINATING ENGINEERING PRACTICE
pends largely on the .purpose to which the room is to be put. In
a living room or a general office or an audience room where persons
sit for long periods in one position it is of first importance to avoid
glare in the eyes of the occupant. On the other hand if the eye is
not to be exposed to the glare for long periods, some temporary
glare is permissible in many cases to keep down the cost of con-
struction and operation.
Glare may be kept from the eyes of the occupants of a room by
limiting the brightness contrast ratios to which the eye is subjected.
In the case of artificial light this is done by inserting opaque re-
flectors or a diffusing medium between the lamp and the possible
positions from which it can be seen.
Practically all sources of artificial light now in common use are
too bright for continuous exposure to the eye with the background
illuminated no better than is common practice to-day.
In eliminating glare by the insertion of diffusing glass or other
material between the light source and the eye three general methods
have been used. An opaque reflector or one of dense trarislucent
glass, cloth or paper can be placed over the lamp far enough to pro-
tect the eyes of occupants of the room and yet allow direct light from
the lamp and reflector to fall on objects under and near the lamp.
Another method is to reverse this process, putting opaque or dense
translucent reflectors under the lamp to reflect the light to a light
colored ceiling or wall and so obtain a diffused light from the ceiling
or wall. As the light is spread out on the ceiling its brightness is
comparatively low and the brightness contrast ratios are cut down to
bring them within the limit of tolerance of the eye. A third method
is to put around the lamp an enclosing globe that will diffuse the
light going in all directions. While this is a very common method it
is an incomplete solution of the problem of the most modern illumi-
nants because a diffusing globe which will cut the brightness down
to a proper figure is either so large as to be prohibitively expensive
or so dense as to cause a prohibitive loss of light.
The second method, that of using indirect lighting, or semi-direct
lighting with bowls of very low brightness, is the only reasonably
economical and practical method which conforms fully to the hy-
gienic requirements in most cases where low brightness of the units
is required. Even if the ceiling is dark in color it may be more feas-
ible to light the room indirectly from a dark-colored ceiling than to
put in enough outlets to supply general illumination from the en-
closing globes.
PRINCIPLES OF INTERIOR ILLUMINATION 67
A method which partially eliminates glare, adopted in many cases
in which indirect lighting would be considered too expensive on
account of the poor reflecting qualities of the ceiling, involves the use
over the lamps of reflectors deep enough to hide most of the source of
light, the lamp being placed as high as possible to get it out of the
ordinary range of vision. This method is extensively employed both
with translucent reflectors of various types of opal and with opaque
reflectors of white enamel steel, aluminum-finished metal and mir-
rored glass. This method is necessarily an incomplete solution of the
problem of eliminating glare because it is possible to see the lamp
filaments, mantles or frosted tips of the lamp and the interior sur-
faces of the reflectors, any of which is bright enough to cause contrast
glare. It is however much more efficient and less glaring than the
use of bare lamps or flat reflectors.
In the lighting of industrial plants where the ceilings are consider-
ably broken up and not very white, and in large rooms of the coli-
seum or armory type with high roof and open roof trusses, the
operating expense of indirect lighting would be usually considered
prohibitive, and the method of using bowl reflectors of various
depths with lamps placed high is the most common in the best
practice to-day.
Opinion differs somewhat as to whether the bowl reflectors used
in this way should be opaque or translucent like opal. Opaque re-
flectors have been extensively used partly because of the greater
strength of the opaque metal reflector and partly because it was felt
that light striking such dark colored ceilings would be so largely
wasted that a reflector directing all of the light flux below the hori-
zontal might better be used. The latter is a mistaken view. A
dense opal reflector directs as much light flux below the horizontal as
a good white enameled reflector, so that the light passing through
the opal reflector to light the ceiling and upper walls represents clear
gain. Illuminating the ceiling and upper walls reduces the contrast
glare, makes the room more cheerful, and adds to the diffused light.
In an armory or a coliseum type of building there is another
method of partially reducing the contrast glare effect which combines
some of the elements of the methods previously mentioned. This
is to use reflectors of an extra deep bowl type confining most of the
light flux within about 40 degrees of the vertical. This of course
reduced the number of' light sources which are within the field of
vision at one time and those sources which can be seen are near the
edge of the visual field. With such deep reflectors a mirrored sur-
68 ILLUMINATING ENGINEERING PRACTICE
face is more necessary to the exact control of light and high efficiency
than where the reflectors are shallower. The reflector should not
be too concentrating or the illumination on vertical surfaces will be
poor.
Along with this plan of using deep reflectors in buildings of this
type it is frequently considered desirable to provide for some illumi-
nation of the roof and upper walls to reduce the contrast glare
effect between the illuminated interior at the lower part of the re-
flector and the roof background. This can be done by providing
indirect lighting for the roof from separate lamps and reflectors but
is most easily accomplished by simply allowing enough light to es-
cape out of the top of the deep reflector to illuminate the roof.
The avoidance of glare with natural lighting from side and ceiling
windows is partly a matter of the proper selection of window glass,
louvres and shades but it is also very much dependent upon the
general arrangement and color scheme of the room.
Diffusing glass of various kinds such as ribbed, prism, frosted,
corrugated and roughed glass have been used to some extent to in-
crease the illumination in a room (as already explained in Part I)
and may do this very effectively if they are kept clean. In the
application of such glass care should be taken not to place diffusing
glass below the eye level. In an ordinary type of window where
the sill is much below the eye level the lower sash should not be
provided with diffusing glass. The effect of diffusing glass is to
receive light from the sky and transmit it by diffusion into the room.
The result is a great increase in brightness of the lower window, to
such an extent that the brightness is much greater than that to
which the eye is accustomed in such a location. While the eye is
accustomed to the brightness of the sky and clouds above a hori-
zontal plane it is not accustomed to such a high order of brightness
below the horizontal plane. Although it is occasionally subjected to
it when outdoors with sunlight on snow or on white macadam roads
or desert sand all of these conditions cause eye discomfort.
It is quite possible for the architect to render glare unavoidable
either by night or by day and so defeat all later attempts at good
lighting. Conditions are more easily controlled as regards artificial
illumination, however, than as regards natural illumination. In
the case of artificial illumination, interiors with a very dark finish
with corners where there is a small amount of illumination introduce
large contrasts which are uncomfortable, if lighted by ordinary
methods with exposed lamp or lamps with enclosed globes. Such
PRINCIPLES OF INTERIOR ILLUMINATION 69
interiors can be lighted by the expenditure of sufficient luminous
energy upon dark ceilings and walls to bring up the general illumina-
tion to a satisfactory point. This method, however, is not in ac-
cordance with the general scheme of design of such interiors. The
only method of treatment of such interiors which is satisfactory and
is in accordance with the general architectural scheme is the use of
localized light from thoroughly shaded sources and this usually
means that there must be a large number of sources.
In the case of daylight illumination from windows, one of the prin-
cipal things to be avoided is an architectural arrangement which
makes it necessary for persons to be seated facing windows with a
bright sky visible through the window in contrast to a dark space
around the window. Facing the window may not be objectionable
when seated very near to the window so that the sky occupies a
considerable portion of the field of vision but as one recedes into the
room the sky occupies a smaller portion of the visual field and in
painful contrast with it are the walls of the room which are very
much less bright.
In office work the direction of diffusion of light has much to do
with the amount of glare from papers on desk tops. Daylight com-
ing from windows at one side of the desk gives the best working
conditions, partly because of the large diffusing surface (the sky)
from which the light comes and partly because of the fact that it
comes from one side so that all of the veiling glare on the paper is
in a direction where it is not often observed by the worker.
The most effective method of eh'minating veiling glare in office
work with either daylight or artificial lighting is the use of nothing
but matte or soft finish paper. Of course this is not feasible in most
cases at the present time. Under present conditions such glare can
be eliminated only by so placing the source of light that the angle
from the source to the paper can never equal the angle from the paper
to the eye. Under these conditions the only veiling glare present
is that due to a reflection from the paper of the moderate illumina-
tion from the walls and ceilings. Such a position is usually only
feasible with a drop cord or wall bracket lamp placed at one side and
slightly back of the worker. With any kind of local desk lamp near
the work it is difficult to avoid glare from the paper altogether as
there are so many positions from which the light can be received.
Furthermore with either a desk lamp or a wall bracket lamp properly
placed for one worker, direct glare from the lamp, or glare by re-
flection from the paper, is almost sure to be experienced by other
70 ILLUMINATING ENGINEERING PRACTICE
workers in the room. With indirect lighting for general office work
a slight amount of veiling glare consisting of reflection of the ceiling
from the paper is received in many working positions but this is not
so serious as the glare with the other arrangements described.
Complaint is sometimes made that daylight and artificial light do
not mix well in color or direction and that there is a period at dusk
when there is likely to be trouble with an artificial lighting arrange-
ment that is satisfactory after dark. This trouble is usually due
simply to insufficient artificial light for the best work, but is some-
times further aggravated by the presence of sky areas visible to the
worker but shaded from the work. In the latter case the eye is
adapted to the sky brightness rather than the desk top brightness.
As already seen most of the available modern light sources are
very bright. In order to conform to the hygienic requirements, if
the reflectors or shades used are not opaque, they must at least be
dense. Semi-direct lighting usually requires a bowl which is rather
thick, not only to withstand the mechanical strain but to give a
sufficient thickness of glass to cut down the brightness. Various
glass mixtures have been compounded for such bowls. Some of the
blown glass bowls for this purpose consist of two or three layers,
forming what is technically known as a cased glass. Specific limita-
tions for bowl brightness have already been noted.
From the efficiency standpoint the prime requisite for a semi-
direct lighting bowl is a pure white highly polished interior surface
which will give a high percentage of reflection from its surface and
a sufficiently dense glass medium so that the light that is not re-
flected shall be considerably reduced in brightness.
In the manufacture of heavy diffusing glasses of this kind there is
much opportunity for development of pleasing artistic effect by the
use of tints and coloring. To most people yellowish tints are more
pleasing than those of blue or green.
The eyebrows of the average person shade the eyes from rays
falling as near perpendicular as 25 degrees from the vertical or less,
but for rays emanating from light sources above this angle artificial
shading must be provided if the lamp is overhead. If the edge of
the lamp shade is below or near the level of the eye any kind of shade
which will intercept all rays above the horizontal will protect the eye.
With daylight illumination it is common for window curtains and
draperies to cut off about 50 per cent, of the total light and for the
roller shade to be left where it will cut off from 30 per cent, to 40
per cent, of the remainder. Large effective window areas in pro-
PRINCIPLES OF INTERIOR ILLUMINATION 71
portion to the size of the room are conducive to the most hygienic
daylight conditions. Dark curtains or draperies around the edges
of a window tend to increase the contrast glare effect. The bright-
ness of the sky seen through the central part of the window is not
changed by such draperies and the total illumination in the room
is materially changed so that the contrast between the sky and the
interior surface of the room is increased. Practices of this kind
should be discouraged for hygienic reasons. For similar reasons large
window spaces are desirable. Legislation for schoolroom construc-
tion frequently names a window area of from % to % of the floor
area.
In selecting a window shade it is well to consider the purpose for
which the shade is most likely to be used. A dark dense shade is
frequently objectionable for shutting out sunlight in an office build-
ing, factory or schoolroom because it shuts out altogether too much
light. If a very dark shade is used to shut out sunlight, a small
area of brightly illuminated space is left near the window while the
rest of the room is in strong contrast to this bright space and the
effect is to introduce contrast glare and make the illumination of
the room seem insufficient. Moreover, such a preponderance of
brightness below the eye level is unnatural, as before explained, and
will of itself cause discomfort if sufficiently pronounced. If on the
other hand, use is made of light colored window shades which allow
considerable diffused light to pass through, the illumination sent
back into the room is not so seriously interfered with when they are
pulled down and the contrasts of brightness within the room are
not so great and the whole effect is more comfortable and hygienic.
Having selected a general type of lighting source to be employed
and the lamp equipment to be installed the next step is the selection
of the exact size and location of the lighting units. Two general
characters of problems are presented in practice. One of these is
where the general illumination is desired within minimum and maxi-
mum limits and the other is where the principal consideration is a
local illumination of a certain intensity without much regard to the
quantity of illumination elsewhere in the room.
In problems of the latter class where the illumination at some
particular point is the main thing desired, the point-by point method
of calculation has its advantages. If it be assumed that a certain
number of foot-candles illumination is required at a certain point
this illumination multiplied by the square of the distance in feet will
give the candle-power which must be emitted from the unit in that
72 ILLUMINATING ENGINEERING PRACTICE
direction. The general type of unit and its shading equipment hav-
ing been already selected it then becomes a matter of determining
what size of lamp will most nearly give the candle-power required
at that particular angle. This is done from photometric curves
of the lamp equipment. If curves are not available for all sizes of
lamps that can usually be calculated with sufficient approximation
from curves made with one size of lamp.
Most of the problems however are those requiring a certain aver-
age general illumination. The selection of such an average however
always implies that the minimum in the working area shall not
fall too far below the average. In modern practice it is comparatively
easy to keep this minimum within 25 per cent, of the average with
proper design. Having assumed the average illumination required and
assuming also that the spacing to be selected will be such as to give
a reasonable degree of uniformity the next step is the calculation of
the total light flux required to be generated by the lamp. Using
the empirical method. This is obtained by the simple formula
Where i equals foot-candles average illumination upon the
working plane.
a equals area of the working plane in square feet.
e equals the efficiency or utilization factor or percentage of lumens
generated which become effective upon the working plane with the
lamp equipment and room conditions under consideration.
L equals the total lumens to be generated by the lamp.
In the foregoing formula, ia, of course, equals the total lumens ef-
fective upon the working plane.
In applying the foregoing formula of course the important thing
is to select the proper value for e, the efficiency or utilization factor.
This can best be done by consulting the various tables and curves
of utilization factors, Tables VI and VII and Figs. 3 to 9 or any other
good authority and selecting conditions which most nearly corre-
spond with those in the room under calculation. In applying
these factors they should be reduced by the amount corresponding
to the depreciation due to dirt and age of lamp. Such depreciation
figures for various conditions have already been noted.
If the value of e is not obtainable from experience and use is
to be made of opaque direct reflectors, e can be determined for most
large interiors from the distribution curve of the lamp and reflector
PRINCIPLES OF INTERIOR ILLUMINATION 73
by dividing the total lumens emitted by the lamp by the lumens
emitted in the zone from o to 70 degrees. For smaller rooms a
smaller zone should be used.
Having determined the total lumens required to be generated by
the lamp by the foregoing formula there remains the determination
and decision as to how this total flux is to be divided, or in other
words the sizes of the lamps and their locations.
In most cases there are certain natural divisions of the rooms by
ceiling panels or other architectural features so that it is necessary
in the interest of good appearance to make the lighting outlets
symmetrical with reference to these panels. The ideal condition
to be sought after is to divide the ceiling into a number of squares
with an outlet at the center of each square. Frequently it is not
possible to do this, but it is well to maintain the divisions as nearly
squares as possible. In other words if an oblong division is necessary
long and narrow rectangles should be avoided.
Height. To secure proper uniformity either with indirect light
or with direct lighting reflectors giving the most extensive type of
distribution the height of the sources of light should not be less than
half their distance apart, taking the height of the sources of light as the
height of the ceiling in the case of indirect lighting and as that of the
lamp in the case of direct light. Spacing at shorter intervals than the
maximum permissible is desirable both in the case of direct and in-
direct lighting in order to secure greater uniformity, freedom from
annoying shadows, and a reduction in the amount of specular
reflection or veiling glare from papers and polished metals. Shorter
spacing is imperative if concentrating direct reflectors are used.
When the spacing has been determined in a way which will
fit in symmetrically with the architecture and at the same time an-
swer the uniformity requirements, the number of outlets is ascer-
tained and this number, divided into the total lumens to be generated
by the lamp, gives the lumens per lamp. From the proper up-to-
date manufacturer 's information the lamp size most nearly answer-
ing the requirements must be selected.
Indirect fixtures should be hung a sufficient distance from the
celling to avoid a very spotted lighting effect. The nearer to the
ceiling they hang the greater the concentration of light under the
fixture.
EXAMPLES OF THE PROCESS OF DESIGN
The following typical examples on the process of design are given
to illustrate the principles that have been laid down.
74 ILLUMINATING ENGINEERING PRACTICE
Example i. A large room area 100 by 100 feet with 14.5 foot ceil-
ing used for general office purposes and clerical work, having light
colored walls and ceilings. The entire area is covered by desks and
filing cases. Since practically the entire room has to be illuminated
sufficiently for working purposes, localized lighting is not to be
considered except possibly for a few billing machines having lamps
on portions of the machine that might be in shadow. In order to
avoid glare the system must be indirect or nearly so, so that the semi-
indirect with very dense bowls will be selected, as the office is of a
prominent concern where the decorative effect of the illuminated
bowls is desirable. As it is necessary to seek first the highest effi-
ciency of the employees (as saving in the consumption of energy for
lighting would be a very small percentage of the amount spent for
pay-roll) the lighting intensity should be such as to be beyond
criticism or question as to sufficiency. An average illumination of
6 foot-candles will, therefore, be selected with the understanding that
the minimum is not to fall below 4.5.
From the utilization factor table we see that a large interior of this
kind has a utilization factor of about 48 per cent, before allow-
ing for depreciation and dirt. We will allow 15 per cent, deprecia-
tion by dirt on electric lamps and reflectors, and assume that the
system of cleaning and maintenance will be such that this will be
a maximum figure. We will also allow 10 per cent, depreciation
for falling off in luminous output of the lamp. This gives a total
figure of 25 per cent, to be allowed for dirt and depreciation in
service, so that our 48 per cent, utilization factor is reduced to 36
per cent.
The room having a floor area of 10,000 square feet, multiplying
this by 6 foot-candles average illumination gives 60,000 lumens re-
quired on the working plane. 60,000 lumens divided by 36 per
cent, gives 166,600 lumens to be generated at the lamps. Taking
up the spacing of the lamps we find the room divided into bays
20 X 20 feet and as those bays are not too large to give good uni-
formity with an outlet in the middle of each bay with this ceiling
height we will put an outlet in the middle of each bay. With this
division 25 outlets will be required. The total 166,600 lumens at
the lamps divided among 25 outlets equals 6660 lumens per lamp.
The nearest sizes to this in electric lamps are the 400- watt 6150
lumen lamp and the 5oo-watt 8050 lumen lamp. In gas lamps an
inherent depreciation figure of 20 per cent, more than the electric
had probably better be assumed. An output of 325 lumens per
PRINCIPLES OF INTERIOR ILLUMINATION 75
cubic foot per hour less 20 per cent, equals 260 lumens. Twelve
inverted mantles taking 2.5 cu. ft. of gas each per hour would then
give 7800 lumens.
The size of lamps having been determined, the bowl can be selected
for the semi-direct lighting fixture of a glass having a density prefer-
ably such that the bowl brightness will not be over 300 millilamberts,
as that will not be over 100 times as bright as of the 3 millilamberts
on the wall illuminated to about 6 foot-candles. The brightness of
a wall in millilamberts equals the incident foot-candles times 1.07
times the coefficient of reflection of the wall.
Example 2. A small office room 10 feet wide and 10.5 ft. high by
20 feet deep with light ceilings and walls, typical of thousand of
rooms in large office buildings. The character of the occupancy
cannot be predicted but the usual arrangement is desks near the
window facing each side-wall. These desks maybe either flat or roll
top. Another possible arrangement is to place the desks so that the
back of the worker is to the window. There would also probably be
a typewriter desk farther back in the room, usually along one of
the walls. The building is to be provided with electricity only for
lighting. The two plans for artificial lighting for such an office which
must naturally receive consideration are the following:
A, General lighting, supplemented by local desk lighting. B,
General lighting for all purposes without localized lamps. The
economy of modern lamps has done away with much of the necessity
of using localized lighting for the sake of economy as formerly.
For most office work localized lighting is not as satisfactory as general
lighting, because of the veiling glare from papers, etc. However, if
general lighting is depended upon alone use must be made of a system
which will not cause annoyance from shadows.
If this is in a typical modern office building it is desirable to have
as few outlets as possible on partitions as the occupancy and loca-
tion of partitions may change. If general lighting is to be ac-
complished from ceiling fixtures centrally located a system indirect
or nearly so will provide for most contingencies in variation of desk
location, etc., and if the desks are located facing each wall the
shadows of heads will cause the least annoyance. On account of the
importance of reducing the shadows to their lowest terms an in-
direct system will be selected, rather than semi-direct. The office
can conveniently be assumed as divided into squares each 10 by 10
feet and an outlet located in the center of each square. This arrange-
ment provides for ample illumination of the rear of the room
76 ILLUMINATING ENGINEERING PRACTICE
farthest from the windows. On account of the possibility of shadows
and veiling glare being more annoying with only two sources of
light and with the possibilities of workers being seated with their
backs to the illuminated ceiling so as to cause maximum shadows,
more light should be provided at the lamp per square foot of floor
area than in the case of the general office in Example i. How-
ever, if there were only one desk in the room and that located
directly under a lighting unit the reverse would be true and less light
would have to be provided, because the maximum light would be
received directly under the outlet.
In this case, therefore, we will allow for an average illumination of
7 foot-candles which may fall to 4 or 5 foot candles along the walls
in shadows. Seven foot-candles times 200 square feet equals 1400
total lumens to be generated and delivered upon the working plane.
The efficiency of utilization in such a room will probably be around
29 per cent., which, when reduced by 25 per cent, for dirt and lamp
depreciation as in Example i, would mean a factor of 22.5 per cent.
The 1400 lumens needed divided by the 22.7 per cent, equals 6600
lumens to be generated at two outlets or 3300 lumens per outlet.
The nearest single lamp to this in output is the 28oo-lumen, 200-
watt lamp. Since we have been rather liberal in our allowances as
to the foot-candles required at the start the use of this lamp would be
permissible.
If a room of this type were a little wider it would be best to have
two rows of fixtures in spite of the spacing rule given because of the
desirability of minimizing the shadows at the desks near the walls.
Example 3. If the general office of Example i were an industrial
plant having the same dimensions but with a darker, more broken
up ceiling the method of treatment would be the same except that
deep bowl opal reflectors or opaque shallow or deep bowl reflectors
might be used. The utilization factor would be changed and per-
haps more allowance should be made for dirt.
Working Out Cost Comparisons. In making comparison of oper-
ating and maintainance cost for different illuminants or systems of
lighting the following items should enter for any given period.
(a) Cost of energy or fuel (electricity, gas, oil, etc.).
(b) Renewals of lamps, lamp parts or burners (mantles, lamps,
trimmings, etc.).
(c) Cost of cleaning lamps and accessories.
(d) Cost of cleaning or redecorating walls and ceilings.
(e) Interest and depreciation on cost of system in building.
PRINCIPLES OF EXTERIOR ILLUMINATION
DR. LOUIS BELL
Exterior illumination is, speaking broadly, the generalized case
of application of artificial light. In interior lighting, that applied,
for example, within the limitations of a room, the light flux is con-
fined within the bounding surfaces where it is subject to reflection
and absorption, the amount of which has to be taken into rigorous
account in reckoning the final result in lumens available for service.
There are no such restrictions necessary in exterior lighting since its
problems have to be dealt with chiefly in terms of the radient un-
limited by artificial boundaries. In general the case is that of a
luminous source required to produce a certain flux density on a
single arbitrary plane which may be horizontal, as in the case of
street lighting, or vertical, as when one illuminates the facade of
a building. In rare instances one deals with both a horizontal and
one or more vertical planes simultaneously, as when light is directed
into a street or public square of limited extent, but there is always
one general direction and more commonly several in which no limiting
surfaces are interposed and the solid angles pertaining to which
must be regarded as representing regions of complete absorption.
The use of reflectors with exterior illuminants is merely an effort
to limit this absorption angle by the partial interposition of a reflect-
ing surface effective roughly in proportion to the solid angle which
it subtends from the source. On this point of view it is immediately
evident why in employing such reflectors their equivalent solid angle
is a matter of great importance, so that it frequently happens that
the last few inches of radius on a reflector determine whether it is
to be good or bad in redirecting the light. Further, whether one or
more bounding surfaces must be taken into account in planning for
exterior illumination, the effect on the conditions of illumination is
altogether different from that found in interior lighting. Here the
surfaces are frequently fairly light so that they present low coefficients
of absorption. One surface, the ceiling, is almost always light and
one, the floor, is generally dark, but no darker, however, than the
ground which serves as the working plane in exterior lighting. In
this latter case, one works under serious disadvantages in the appli-
77
78 ILLUMINATING ENGINEERING PRACTICE
cation of the light, since the surface to be illuminated is usually
rather dark with high absorption. The remainder of the surfaces
toward which light flux is directed are practically also of high absorp-
tion, save in exceptional cases, so that one cannot depend in exterior
lighting upon that measure of assistance often equivalent to an in-
crease of from 50 to 100 per cent, in the effective flux.
One is generally dealing out of doors with directed light flux from
the radient somewhat modified by the shades or reflectors that may
be applied thereto, and as a rule only one or a few such radients have
to be considered. Hence the numerical computations in the case
of exterior lighting are fairly simple, and the working out of exterior
problems is rendered fairly easy by the fact that the intensity of
illumination demanded is generally less than with interior lighting
and the conditions with respect to uniformity are also considerably
less severe. Within doors the illumination demanded is determined
by the things which have to be done by its aid and some of these are
tasks which require close vision on unfavorable details, so that com-
mon intensities of illumination run all the way from 10 to 50 lux
(i to 5 foot-candles), and in rare instances much higher. In exterior
lighting, save for deliberately scenic purposes, 10 lux is rarely ex-
ceeded and the usual standard intensities run from about 0.5 to
perhaps 5 lux (0.05 to 0.5 foot-candle). Broadly, in exterior lighting
the conditions of distribution are less favorable than in interior
lighting, but the requirements of intensity and uniformity are much
less severe.
The amount of illumination required in exterior work depends on
its use, but this is never such as to call for illumination good enough
to facilitate the observation of fine detail. At most one may have
to read a program or an address card. Ordinarily it is sufficient to
distinguish people and vehicles easily, to note obstructions on the
roadway or sidewalk, to recognize persons and things at a moderate
distance, and perform other simple tasks requiring no close discrimi-
nation. One recognizes objects on road or sidewalk chiefly by their
shadows. If their color tone be nearly that of the road surface they
are almost invisible, except when so illuminated as to show a shadow.
One also sees at night the contrast of light and dark masses, like
the silhouette of a cart against an illuminated roadway, or of a
white-clad person against a hedge or fence. The eye, therefore, is
not called upon to do any fine work and hence does not require a
degree of illumination sufficient greatly to develop its full discrimina-
tory powers.
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 79
Only in such exterior work as has to do with the deliberate illumi-
nation of particular objects, as in some spectacular lighting, is it
necessary to push the intensity near to the point common in interior
lighting. This is fortunate since with immense spaces to light and
unfavorable conditions as regards reflecting surfaces exterior lighting
is only economically possible in virtue of the modest necessities of
the case. Luckily the human eye works about equally well for the
purpose of seeing over a very wide range of illumination. From the
full sun shine of noon to twilight, the illumination may vary in the
ratio of 1000 : i and yet the eye can do most of its work comfortably
at either extreme. It is not the absolute amount of light which
counts, but the relative amount as between two things to be dis-
criminated. Speaking in general terms one can distinguish as
varying in shade two adjacent surfaces, the illumination of which
varies by a little less than i per cent., whether the actual intensity of
the lighting be of the order of magnitude of 10 or 1000 lux. A con-
trast of 10 per cent, is conspicuous even when the illumination falls
much below 10 lux. The power of the eye to discriminate both
shades and small details even in black and white falls off rapidly
under ordinary visual conditions, so that at a few tenths lux (or
hundredths of a foot-candle) even a contrast of 25 or 30 per cent,
between surface and surface may not be easily visible unless the
surfaces are on a very large scale, and one fails to read even very
coarse type. In such lighting obstacles are difficult to see, persons
difficult to recognize and, while one can still see to move about, the
conditions are bad if any traffic is to be considered. Such is the
situation even in pretty good moonlight which may run to say from
o.i to 0.25 lux (o.oi to 0.025 foot-candle).
One of the many valuable properties of the eye is that it possesses,
however, an extraordinary power of adaptation, that is, of getting
used to great variation in the intensity of the lighting and still
being able to see fairly well. It may be light-adapted, as when the
pupil shrinks to its minimum diameter, and the eye adjusts itself
to a very bright light, or it may be dark-adapted, when the pupil
opens very wide and the retina itself becomes adjusted to condi-
tions of very low illumination. This latter process is largely a
physiological one which requires some little time to accomplish,
but it is tremendously efficient. After ten or fifteen minutes in
complete darkness, for instance, the eye is many hundred times
more sensitive to faint illumination than in its light-adapted condi-
tion, and as a matter of fact with this long dark adaptation the same
80 ILLUMINATING ENGINEERING PRACTICE
keenness of discrimination which ordinarily exists at 10 to 50 lux
may be found even at a hundredth of this amount. This is par-
ticularly true as regards vision of surfaces differing slightly in illu-
mination, less true for the observation of detail like printing.
It is for this reason that one can see much better by moonlight, to
which one gradually gets adapted in the absence of other illumina-
tion than is possible with artificial lighting where one continually
comes under the more intense illumination near lamps. The power
of adaptation of the eye, therefore, rises to considerable practical
importance in external lighting. If dark adaptation is not spoiled
by glaring sources of light one can see astonishingly well at low
illumination. Hence under lighting conditions where one has to
work with a meager amount of light, a source which would be
entirely unobjectionable in the case of the brilliant illumination
found, for instance, in a public square, becomes unpleasantly glaring
and unfits the eye for good vision. This is what happens when one
drives an automobile under a low hung and brilliant street lamp.
The vision must again adjust itself to the less brilliantly illuminated
regions, only to get another rebuff from the next lamp.
This would look as though uniformity in lighting roads and other
large areas might be very important. Its value is lessened by the
fact already referred to, that is, that we see objects, generally, in a
moderate illumination, chiefly through their shadows. A perfectly
uniform low illumination, could it be attained conveniently, would
be good from the standpoint of adaptation and bad from lack of
contrast due to shadows. The contrast directly under a strong light
source may be actually much less than in a faint light directed cross-
wise so that the visible contrast is not between the object itself and
its surroundings, but between its shadow or shadowed parts and
the surroundings. These facts were very beautifully brought out
in experiments tried a couple of years ago for the National Electric
Light Association.
For most purposes of exterior lighting the best results are obtained
by lamps rather well shaded, so as to reduce the intrinsic brilliancy
of fairly good power, and so located as to produce only a moderate
amount of uniformity in the resulting illumination. In situations
where the intensity for one reason or another must be considerably
increased, the value of directed light as against flat uniformity is
very considerable. The front of a building, for example, can be
flattened distressingly by too uniform lighting or brought out with
brilliant effect by a little judicious cross-illumination, a condition
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 81
precisely analogous to that found in the interior lighting, for instance,
of a church, in which the high altar requires oblique illumination to
bring out its relief. The same practical application of contrast
appears in that class of exterior illumination which has to do with
decorative and spectacular illumination of places or things.
Now and then most remarkable effects can be produced by close
attention to regulating the quantity, quality and direction of the
light applied. This is on a large scale exactly what is done in the
setting of theatrical scenes where effects of spaciousness or of dis-
tance are produced upon the very limited area of a stage. 1 Brilliant
and uniform illumination tends to give an effect of nearness and
lack of relief. Faint and carefully directed lighting on the contrary
may be made to produce an effect of vague distance. When many
light sources are in view a decreasing spacing gives a spurious effect
of distance, uniform or increasing spacing from the foreground back,
the reverse effect. Lamps of decreasing brilliancy along the line
of view likewise produce an impression of far perspective, while
increasing brilliancy gives an illusion of nearness. The cases where
these principles need to be applied are not common enough to justify
going into the matter to any great extent, but most astonishing
results can be reached in the way of forced perspective wherever
scenic effect is desirable.
The problems encountered in exterior lighting are of a very diverse
character involving many different sets of conditions, each of which
must be met in a systematic and definite way. One can divide the
total roughly so that each group possesses somewhat similar char-
acteristics, for instance, perhaps the simplest case of exterior light-
ing is that of a public square which presents a somewhat close
analog to certain types of interior lighting. Here, as a rule, from
the nature of the surroundings and the density of the traffic, the
illumination has to be considerably higher than usual, rising even
to 10 or 20 lux (one or several foot-candles) and averaging there-
fore almost as high as certain interiors. A square roughly approxi-
mates a large and not very high interior having a very dark ceiling
and side walls of rough texture and very varied reflecting power.
For all practical purposes the sky above is almost completely absorb-
ing, while the entering streets take the place of a few great windows,
from which practically no light is reflected, but which may receive
a little from the outside, that is, from down the street. If such a
square is illuminated from sources provided with over head reflectors
having angles wide enough to intercept rays which pass above the
82 ILLUMINATING ENGINEERING PRACTICE
house tops, the full downward flux from all the lamps may be
considered as concentrated on the walls and floor. Absence of
ceiling reflection somewhat diminishes the amount of aid given to
the general illumination by secondary reflections. If then, we know
the efficiency of the reflector system which keeps the light from going
skyward and therefore the total downward flux of the lamps, the
illumination on the working plane can be reckoned practically as in
a case of interior lighting. In the latter case suitable reflecting
systems will turn quite half the total light flux from the sources upon
the working plane, from which, knowing the area, the average
illumination can be found at once by a process which will be out-
lined later. The uniformity of the lighting will be determined
by the number and place of individual radiants and the light curve
derived from each. A pure flux method leads to the general average
illumination, a point-by-point method to the maximum and
minimum.
Second on the list comes street lighting, so important that it
will be dealt with in this course by special lectures. Here the
interior analog would be a very long hall with a black ceiling and
it is usually necessary to determine the illumination by consider-
ing the effect of individual radiants, since they are seldom close
enough together to require the addition of the luminous effects from
more than a very few lamps. As a rule the average lighting of a
street, except in the case of one carrying very heavy traffic, does not
require the intensity desirable in public squares. Indeed the neces-
sary illumination in certain classes of streets may fall to a point
where the lamps are little more than markers of the way. Only
in densely built regions can any gain be counted upon from reflec-
tion from sides of buildings which, however, it is sometimes desirable
to light rather brightly for the general effect. As the lamps are
usually placed considerably lower than the buildings the solid or
spherical angle subtended by the reflector, if there is one, may be
considerably less than in the case of large open spaces.
Next in order comes the lighting of building exteriors which in
the case of public squares and of streets is incidental rather than
primary. The lighting of the facade of a building for utilitarian
or decorative purposes or the lighting of a public monument is a
case of direct illumination, in which the light from one or more
reflecting systems is concentrated on a definite area, be it large or
small, to produce specific results over that surface. In the same
category falls the lighting of spaces like railroad yards, docks and
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 83
work of construction. Such lighting may have to rise to a bril-
liancy as great as or greater than, that desirable in public squares,
or may fall to the average of rather mediocre street lighting accord-
ing to the purpose intended, but in all cases it is directed for special
rather than general results. It requires ordinarily lighting units
equipped with reflectors of comparatively large spherical angle, so
as to direct a large percentage of the luminous flux, and if the
properties of the reflectors are approximately known the results can
be calculated, as will be presently shown, very easily, by a simple
flux method.
Finally, one has to meet the special conditions imposed by parks
and other very large open spaces. These are peculiar in that no
help can be received from any lateral surfaces and in that conserva-
tion of resources demands generally so small an amount of illumina-
tion that the preservation of suitable dark adaptation in the eye
becomes of paramount importance. Only now and then is high
intensity desirable and that only locally, where the lighting may
assume something of a decorative aspect.
To take up in more detail the illumination necessary in these dif-
ferent cases the highest limit is touched by the lighting of public
squares. In such areas, which are generally centers of streets carry-
ing dense traffic, the average illumination on the reference plane,
three or four feet above the ground, should be as shown by ex-
perience, one or several lux (a few tenths of a foot-candle). In some
cases it has been pushed even to 10 lux over a considerable area.
The exact density required is evidently determined by the nature of
the situation, but any average less than one lux (o.i foot-candle)
must be regarded as undesirably low. In practice the average
should generally run to at least double this amount in order to pre-
serve a suitable minimum while using only a moderate number of
lighting units. Certainly anything less than 0.5 lux must be re-
garded as unsatisfactory as a minimum figure and it is not easy to
secure in a large area this minimum without having an average
exceeding i lux, and a considerable area of maxima of at least double
this amount. One needs to see well in a public square where many
people congregate and many vehicles pass, and the amount of
illumination must therefore be pushed to a high limit with some
special effort at uniformity in order to prevent the appearance of dark
areas in the general effect. Likewise in such situations the buildings
deserve more than the usual illumination since they are commonly
of importance and of decorative value when properly lighted.
84 ILLUMINATING ENGINEERING PRACTICE
As in every case of exterior illumination the actual amount of
light to be furnished in a public square depends on the nature of
the situation. The figure just given ought to be regarded as an
irreducible minimum for areas in which there is any considerable
amount of traffic even of pedestrians. Where vehicles are frequent
and the space generally more crowded it is necessary to increase
the illumination considerably, rising as high perhaps as 5 lux or
more under extreme conditions. Large open areas through which a
continuous stream of street 'cars, automobiles, and pedestrians are
pouring, particularly in the evening hours, can hardly be too strongly
illuminated for safety and convenience.
The method selected to provide the illumination depends intrin-
sically on the particular area to be dealt with. As a general rule
the lamps, whatever their size, should be carried relatively high in
order to secure a fairly even light distribution without going to an
abnormal number of supporting structures. It is a good rule to keep
a public square as free of lamp posts and other obstructions as
possible, which indicates the wisdom of avoiding a multiplicity of
small lamps and of utilizing a few tall standards which may be given
a high decorative importance and lead to a simpler and more effective
installation. In a few instances where the open area is large and
the traffic is chiefly around its margin, lamps carried on the curbs
as in ordinary street lighting prove to be the best sources of distribu-
tion. In a case of this kind the light should be where the traffic is,
and consequently the lighting of the center of the area can be reduced
in intensity while that on its bounding streets should be correspond-
ingly increased. For simplicity of installation and efficiency of
light production large units are desirable in this as in every case of
a requirement for brilliant illumination. Arc lamps of 1000 or
2000 candle-power or incandescent lamps of nearly or quite equiva-
lent candle-power lend themselves readily to this particular use.
They should always, of course, be enclosed in diffusing globes, and
it should be remembered that the gas-filled incandescent lamp is
almost as glaring as an unshielded arc. If in such a square there are
any important monuments, as sometimes happens, the illumination
should be directed high enough to include them. There is no need
here to deal with the details of calculating the illumination, since
this subject has been admirably handled in a previous lecture.
Broadly the problem can be attacked along two related lines.
First, the area and the desired illumination gives at once the
effective lumens required to obtain the average result. It will
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 85
generally be found that the figure thus given is a minimum since
the ordinary criterion of proper illumination considers not alone
the average but also the minimum so that the required light flux
must be distributed so as fully to meet the minimum requirement,
when incidentally it will carry a somewhat high maximum. The
simplest way of solving the problem is to determine from the gen-
eral light flux required the type of unit which will be desirable to
use, that is, one not so large as to involve great difficulty in proper
placement or so small as to require undue multiplication of supports.
From the polar candle-power curve of such a unit the equilucial
lines corresponding to the minimum permissible illumination can
be plotted for various heights of placement and then these areas
arranged so as to overlap enough to insure keeping comfortably
above the minimum at all points. With ordinary light-sources and
reflecting equipment one will rarely select a height of placement less
than 30 ft. in open squares, although this figure may be somewhat
reduced in cases where the margin of the area is the chief region
of traffic. As a rule the more powerful the unit the higher it may
be advantageously placed.
As to whether single or multiple units should be used on a single
standard, the decision is chiefly a matter of taste. With the large
incandescent lamps in particular the efficiency varies very little
with output so that one may freely use standards carrying two or
more lamps at comparatively slight loss of efficiency. Clusters are
generally not to be recommended since the several globes with their
supports are in each other's way; moreover, the low lying clusters,
which have been frequently used in the past, are seldom either effi-
cient or artistic. In so-called ornamental lighting both arc and
incandescent lamps are generally mounted much too low for efficient
light distribution, a few distinguished exceptions like the recent
exposition lighting at San Francisco to the contrary. The rule of
artistry in the lighting of public places is to keep the lamp carriers
in scale with the general architectural environment and to place
lamps of sufficient candle-power to give the necessary result in
illumination approximately as here indicated. There is a particular
need of studying such problems in lighting since they cannot well
be solved by the ordinary apparatus of street lighting, placed, as it
is, usually along the curbs near to the fagades of buildings, and de-
signed to be at its best in illuminating a rather narrow street. The
public square is a place by itself as regards the requirements for
illumination.
86 ILLUMINATING ENGINEERING PRACTICE
In street lighting proper the chief area of illumination is that
of the street surface itself where the vehicular traffic is located.
Secondarily, sidewalks and crosswalks must be adequately lighted,
and finally, where the building line is near the street, the fronts
of buildings themselves cannot be left out of consideration; first
because they need to be lighted for the general effect; and second,
because they may, if light in color, add something to the general
effectiveness of the street illumination. The commonest mistake
made in street lighting is to follow a uniform method and type of
illuminant irrespective of the individual needs of the street. A very
common method of lighting in the earlier days consisted in placing
at each street intersection, irrespective of the length of the block
or the character of the street, an arc lamp, usually of insufficient
candle-power. This gave a fine uniformity of lighting units, but
extremely bad illumination except in parts of the city where the
intersections were very close together. The almost inevitable
result later has been the thrusting of incandescent or gas lamps into
the intermediate spaces, finally producing a mixture of kinds and
sizes of lamps both bad in appearance and unsatisfactory for the
purpose intended.
If a street is to carry dense traffic for a considerable period each
night, that street requires thoroughly good illumination, as good
even as the better class of public squares. If the traffic is not heavy
and pedestrians are occasional, vehicles are chiefly to be considered,
the same degree of lighting is totally unnecessary as well as waste-
ful. An active business street for this reason, even if not of the
first class, demands brighter illumination than an ordinary residence
street, and this in turn better illumination than a suburban road.
Speaking generally the minimum requirement for street lighting is
that demanded for proper policing, the maximum, that required
for active business accompanied by dense traffic after darkness falls.
The attempt to illuminate all streets in approximately the same way
and to about the same amount means that if the important streets
are really well lighted a great deal of waste will occur in the unim-
portant ones, or if the illumination be standardized for the latter
the former will suffer greatly. One of the most difficult tasks in
arranging the proper illumination of a city is to bring the public to
the appreciation of the fact that light is for general civic service and
not for the uniform distribution of lighting expense through every
mile of street or every ward and precinct.
In his own practice the speaker customarily divides streets into
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 87
first, second and third class, with respect not to the popular idea of
their merits or the cost of the buildings upon them, but strictly on
the basis of the needs disclosed by their use during the hours of
darkness. For the purpose of lighting, a first-class street may be
a boulevard leading through the center of the city and containing
many of the important business houses, or a street running along
the water front, congested with vehicles and streams of humans,
or even a side street through which from one necessity or another
a great volume of traffic passes.
A second-class street may be a side or subsidiary street of business
houses, a residence street of fine mansions, a back street of swarming
tenements or a long road running out of the city but constituting
a main avenue of automobile traffic. The third-class streets will
form the residuum after the first two classes are well marked out,
the ordinary rank and file of city and suburban streets not much
frequented, and never at all congested.
Most first-class streets are thoroughly obvious except for those
which form short cuts or for some particular reason are crowded after
nightfall. These, however, are easily discovered by a very brief
investigation of traffic. The second-class streets require more skill
in selection. Some of them suggest themselves at once, but a con-
ference with the chief of police will usually open up the situation in
a very interesting manner, and it is just at this point that the
greatest difficulty in satisfying the public occurs. It is not polite
to tell the alderman of the Nth ward that a couple of shabby streets
in his district needed to be extremely well lighted on account of the
semi-criminal character of his constituents, while the quiet residence
street on which he lives may be relegated to the third-class. Person-
ally I have tried to make a practice of extending good second-class
lighting to all regions of churches and schools and other districts
where for one reason or another the streets might be much used at
night. Some singular anomalies may be found in making classifica-
tions. For example, an active business street down town, which
at first thought would be put in the first class, may turn out to be
very little used after dusk and so be relegated to the second. The
proper classification is a matter of tact and local study.
It is sometimes wise to add a fourth group of streets and roads to
those already mentioned, in which the street lamps are hardly more
than markers of the way. Almost every town has running out of
it long roads not heavily populated, but which still are main lines
of traffic to neighboring districts. To light these even as a third-
88 ILLUMINATING ENGINEERING PRACTICE
class street should be lighted is uneconomical, but a very modest
equipment indeed may work great improvement in the traffic
conditions. It is extraordinary how much even small lamps widely
spaced will do to assist traffic on a dark night. To use the lamps as
markers rather than for illumination then becomes practically a
rather important measure of public convenience, although from the
standpoint of light flux the provision may seem almost a practical
joke. Some engineers attempt even a somewhat further subdivi-
sion, having in mind a gradation between the second and third class,
but it is not generally necessary, since there is trouble enough in
establishing a sound basis of classification in even three groups.
As respects the actual amount of illumination required for street
work the figures given depend on the agreement as to the way in
which this illumination shall be measured. Abroad it has been the
custom to reckon the illumination as the total received upon and
resolved upon a horizontal reference plane usually taken as a meter
above the ground. This means that the light received from each
source must be resolved according to the cosine law on the plane
and the total received from all the sources added. In American
practice it has been the custom to reckon the illumination as that
received from one direction only upon a plane normal to the ray.
On account of the obliquity of the illumination the former method
generally gives a lower numerical value for the illumination, a fact
which must be borne in mind in interpreting foreign specifications.
For the special case in which only two lamps are considered,
spaced at four times their height, the numerical results will coincide
by the two methods and such relations of spacing to height is not
uncommon especially abroad.
For street lighting proper, the writer prefers the usual American
method on the ground that the most trying tests of street lighting
in practice, such as reading an address, or recognizing a person, do
not depend upon supposing either page or person to be extended flat
upon the ground, but do depend on the light that fairly strikes them
from the lamp. Either method of reckoning is perfectly safe pro-
vided it is consistently used.
Based on the usual American reckoning the illumination in first
class streets should run nearly or quite as high as in public squares,
that is, should amount to a minimum of at least 0.5 lux (0.05 foot-
candle) and preferably double this amount, with an average two
or three times as great. The chief streets of well-lighted foreign
cities before the war averaged fully up to this standard. Indeed I
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 89
have often taken as a rough test of the proper illumination the ability
to read a Baedecker when walking or riding along the street, that
excellent volume being utilized merely as one generally at hand.
First-class streets differ among themselves to a considerable extent,
but if the minimum is kept up to i lux the maximum will usually run
high enough to give an average of from 2 to 4 lux with a maximum
anywhere from 5 to 20.
In street lighting the difference between the minimum and maxi-
mum illumination is generally conspicuously great. Any ratio less
than i : 10 requires very special efforts to secure uniformity and
streets which are practically very well lighted indeed may show
ratios of i : 25 or even i : 50. In such instances the darkest spots
will usually be very small in area and due to special circumstances
and the average will be high. Streets here designated as second-
class ordinarily require about half the intensity ascribed to first-
class streets, that is, an average of 0.5 to i.o lux. Third-class
streets, again, may have advantageously about half the intensity
of the typical second-class streets, with the proviso that anything as
low as 0.25 lux as an average would unquestionably bring a mini-
mum so low as to be almost negligible midway between lamps.
Finally where street lamps are used merely to mark the way the
illumination will be so small except near the lamps as to be hardly
worth considering, the function of the lamps being to define rather
than to illuminate the road. A committee appointed a few years
ago in London to make recommendations as to street illumination
drew up the following recommendations:
Classification of streets
Minimum horizontal illu-
mination in foot-candles
Class A
O.OI
0.025
O.O4
O.o6
0.10
Class B. ....
Class C
Class D. .
Class E.
which correspond pretty closely to that here suggested.
The class E streets of this table correspond to the first-class streets
just described, classes C and D include the second-class streets, and
classes A and B the third-class. Bearing in mind the difference in
the conventional measurement the results are of practically the same
order of magnitude.
90 ILLUMINATING ENGINEERING PRACTICE
Variations in intensity such as here required depend on two things,
the height and the spacing of the illuminants and, other things being
equal, the diversity ratio between maximum and minimum depends
on the relation of the height to the spacing. Powerful light sources
need to be placed high in order to avoid both too great difference
between maxima and minima and too great obliquity of the more
distant rays. Small lamps may be placed correspondingly lower
and also require closer spacing to meet the minimum requirements.
With big units approximating 1000 candle-power the height, of
placement should be not less than 25 ft. to obtain the most useful
distribution of light, while the smaller units either electric or gas
are usually most effective when placed from 15 to 20 ft. high with
the spacings ordinarily employed. Occasionally, in using particularly
well-screened large units closely placed in order to obtain a very
powerful illumination, the figures here given may be somewhat
reduced, the point being to adjust the spacing with reference to the
direction of maximum intensity, so that this may fall nearly at the
midway point between lamps.
Extreme uniformity of illumination is in general not worth the
effort in street lighting, the main point being that for streets carry-
ing any material amount of traffic the minimum illumination must
be high enough to give reasonably good results. This matter will
be taken up in the lecture dealing with the technique of street light-
ing, so that I need not further mention it here except to say that there
is definite evidence of too great uniformity tending to prevent the
quick vision of obstacles, and also tending to lessen the attentiveness,
for instance, of the driver of a motor car. This point was admirably
brought out in the psychological work done under the direction of
Prof. Munsterberg for the N.E.L.A. street lighting tests.
As to the conditions of placement of street lamps the main practi-
cal factor is the nature of the street. A narrow street, particularly
if well built up, may be admirably lighted in the usual manner by
placing the lamp posts upon the curb. A street much shaded by
trees loses too much from shadows with this positioning and use
must be made of long brackets, mast arms, or cross suspensions, in
this country usually the mast arms. Very broad streets sometimes
can be advantageously lighted by means of a row of posts down the
center perhaps on isles of safety, in extreme cases in conjunction
with curb lighting as well.
A word here with reference to glare from street lamps, which
sometimes becomes very unpleasant, especially with high efficiency
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 91
illuminants. Lamps mounted high are in general much less trouble-
some than those placed low, and even when powerful light sources
so placed fall well within the field of vision at the midway point
between them, the gross intensity of the light reaching the eye is so
considerably reduced as not to be serious. One does not par-
ticularly mind the glare from even a very powerful arc at 200 or 300
ft. distance, while it may be most offensive when nearer. In fact
it is not always easy to tell at long range whether a high power lamp
has or has not a diffusing globe, but in either case the light does not
produce serious glare. Even the smaller lamps now used for street
lighting, especially the almost universal high-efficiency incandescent
lamps, which are often placed low, may produce very offensive
glare and seriously hinder the utilization of the illumination derived
from them. Certainly in the larger types of these lamps the use of
frosted bulbs or thin diffusing globes is highly desirable and proves
of practical benefit.
Passing now from street lighting, of which the details will be fully
set forth in a separate lecture, we come to a rather special case of
illumination, namely, the lighting of public monuments and the
facades of buildings. I will not here enter at length into the technique
of this matter since it will form part of the subject matter of another
lecture of this course, but will merely point out some of the general
requirements and the means for meeting them. Where the facade
of a building is to be illuminated the method employed has to depend
on the character of the building and its distance from available situa-
tions for lamps. Sometimes suitable ornamental lamp posts with
powerful illuminants may be placed on the curb of a wide sidewalk,
fairly high, in such position as to give an admirable effect in lighting
the front of a building. In cases where this is not feasible and yet
for one reason or another good illumination of the facade is desired
the modern projector using high-intensity incandescent lamps meets
the requirements with admirable effect. The difficulty here is the
proper placement for the lamps, which can seldom be found on
the building itself and more generally has to be sought on opposite
or near-by roofs. The same conditions hold for the task occasionally
required, of lighting public monuments. Many of these had better
be left under the concealing wing of night, but occasionally a fine
example appears which fully deserves all the attention that can be
bestowed upon it.
This again is a case for flood lighting which can rarely be carried
out from the base of the monument or the immediate vicinity. It
Q2 ILLUMINATING ENGINEERING PRACTICE
usually required a suitable placement of the lamps at a distance
several times the height of the monument. As reflectors for in-
candescent lamps can now be obtained which give a fairly concen-
trated beam, a suitable point of attack can always be found, even
if it has to be a couple of hundred feet away. It is, in fact, rather
easier to get a projector with a fairly narrow beam than it is to
obtain one with a beam of moderately great angle, well distributed
and concentrated, but progress is now being made to assure good
results in almost any condition that can be found.
I will not here go at length into the topic of flood lighting, but
will content myself with pointing out that with such reasonably
exact knowledge of the reflecting system as should be at hand in any
well designed commercial lamp it is a perfectly simple matter to
calculate the wattage required to produce any given amount of
illumination which circumstances demand.
In doing this I shall merely amplify the out-
line of the theory which I gave in the
Baltimore Lectures of six years ago, apply-
ing the added data which are now available.
Consider a source, x, placed in the focus
of an approximately parabolic reflector.
All the light within the spherical Z <f> passes
out in a scattered secondary beam. The
primary beam delivered by the mirror takes
Fig. i. Beam candle-power. r J J
the light from an Z 471- (< + 8) and is
diminished by the absorption and scattering at the mirror surface.
Let the beam fall normally upon a surface producing a circle of
illumination of radius r. Then
Trr r 2
Or, if the circle is projected into an ellipse,
E = j-t as average.
Here rj is the specific efficiency of the source in Us reflecting system
and w is watts used, 17 = apt.
Where a is the specific output of the source in spherical candles
per watt,
p is the coefficient of reflection of the surface,
K is percentage of total sphere effective = 471- (0 + 0).
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 93
Now obviously the larger the parabola and the shorter the focal
length the smaller is < but care must be taken that in case of small
focal length 8 does not unduly increase, due to lamp socket or sup-
port. As a practical matter K ranges from 0.5 or less in shallow
parabolas to 0.8 or even 0.9 in deep ones of focus say of only one-
tenth the diameter of the opening.
p ranges from perhaps 0.6 with cheap metal reflectors to 0.8 or
0.85 in high grade silvered glass mirrors.
<7 ranges from i to nearly 1.5 in various lamps. A lamp is at its
best when its main axis of filament is in the axis of the mirror. Its
distribution is then a tore (Fig. 2) a doughnut with a very small
hole, and the main body of light is well reflected.
Reflectors with depolished surface or fronted with a depolished
screen scatter the light from each element of surface and increase
the scattered secondary beam at the expense of the primary closely
directed beam. They thus may throw light over an angle of 90 or
so and cannot give high concentration, although showing very low
intrinsic brilliancy and having a distinctly useful place in illumina-
tion, for instance of a tennis court.
In practical flood lighting the value of rj is likely / \( j
to run from 0.5 to 0.75, more nearly the former figure ^^^-^
when dealing with reflectors and lamps in their average Fi &- 2. Light
condition. In lamps of the (arc) carefully designed
search light type 77 may rise to or somewhat above unity on account
of the large proportion of light delivered from the advantageously
placed crater to the mirror and the small proportion of light ob-
structed by the source.
Assuming rj = 0.5 for an ordinary flood light one reaches the follow-
ing very simple formulae for the relation between illumination,
energy and circle of illumination.
2W
r* = ^ (3)
Dividing lumens by area gives illumination in lux, if area is in
square meters or in foot-candles if in square feet. Hence the follow-
ing examples.
Required; illumination on a circle of 10 in. radius from a 1000 watt
lamp and reflector.
94 ILLUMINATING ENGINEERING PRACTICE
2 X 1000
h, = - - = 20 lux
100
Required, watts to give 50 lux on a circle of 5 meters radius
Required, circle which 4000 watts will light to 20 lux.
8000
r 2 = - - = 400
20
r = 20 n.
It will be observed that the distance does not enter these reckonings,
for the simple reason that so long as all the flux of the primary beam
falls on the required surface distance does not count save as it may
involve atmospheric absorption which is of small moment at flood-
lighting distances. Only when the spread of the beam gets it off the
object does distance become important in reckoning the illumination.
At very short range the secondary beam proceeding directly from
the source may not be negligible, and this follows the ordinary inverse
square law.
Sufficient has been said already to outline the general method of
reckoning exterior illumination. The fundamental principle is to
reckon average illumination from the flux theory according to
methods which have been already laid down in a previous lecture,
and then to make sure of a sufficient amount of uniformity and a
sufficiently large minimum by computing the illumination directly
from the candle-power curve of the illuminants concerned at any
point. In open squares several sources, in small squares all the
sources have to be considered. In street work one need very rarely
add the illumination from more than two sources on one side of the
point of reckoning, the symmetrical sources on the other side being
obvious in their effect. The computations involve no special diffi-
culties- and are fully taken care of by the general theory once the
candle-power distribution curve of the source is known. With
reasonable care in the placement of lamps a very good estimate of
exterior lighting including street lighting can be made from light
flux alone, the ordinary practice of placing and spacing the lamps
being sufficient to secure the necessary light distribution.
It should be mentioned in this connection that the N.E.L.A.
Committee, which investigated the details of street lighting, found
that for practical purposes the useful illumination was pretty nearly
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 95
proportional to the light flux as might have been anticipated. In
most instances it is the lower hemispherical flux which is concerned.
In narrow streets well built up where the limiting walls have a per-
ceptible effect on the distribution of the light one might include the
total flux which for lamps with reflectors is roughly proportional to
the lower hemispherical flux in any case. Street illuminants there-
fore can rather fairly be rated in terms of the total lumens which
they give, subject to the requirement that reasonable intelligence
must be used in locating the sources.
In exterior illumination even more than in interior it is the adapta-
tion of means to ends which makes the difference between good and
bad results. One cannot safely travel on a hard and fast theory in
such matters. He cannot, for example, say, I will take the abed
lamp of (n) candle-power as my standard and I will adapt all things
to it. If he does so the result is quite certain to be mediocre in
quality. It is practically necessary in meeting the great range in
intensities required in exterior lighting to depend upon not one kind
or size of unit but at least several sizes and perhaps several kinds.
At the present moment for light sources materially below some-
where about 1000 candle-power the large high efficiency incandescent
lamps have the call. In larger outputs than this the big luminous
and flame arc lamps still hold their own well. A few smaller arc
lamp units are used for strictly ornamental lighting, but the carbon
arc lamps of every kind and even the smaller flame and luminous
arc lamps are rapidly passing to the scrap heap. How far the ten-
dency just indicated can go on, and whether the arc lamp is to have a
permanent place in exterior lighting is somewhat open to doubt.
My own opinion is that, particularly on account of the conspicuous
difference in color, the best of the flaming and luminous arc lamps
have at least a considerable period of usefulness still before them, but
in the smaller sizes the hand-writing is certainly upon the wall.
Ordinary public lighting is generally found as a matter of practice
to include the use of at least three sizes of units, two of which will
generally be incandescent electric lamps or the equivalent gas
mantles.
For the lighting of public squares and first-class streets the big
units, whether arc or incandescent, .are altogether desirable. For
second-class streets one may either retain the same size and type of
unit expanding the spacing a bit, or may pass to a smaller unit. The
latter is the more common practice, although some transitional
streets may be very well treated in the former fashion. The smaller
96 ILLUMINATING ENGINEERING PRACTICE
units may pass into some of the lighting of third-class streets, the
distances of spacing being stretched a bit in response to the smaller
necessities. It is quite usual, however, to employ a still smaller
unit for much of the third-class lighting as well as for all the cases
requiring lamps merely as markers. More than three sizes of lamp
are very rarely indicated, and extremely good work can be done with
two, although the gain in simplicity so attainable does not amount
to much.
In shades and glassware one commonly finds that each type of
unit has its own requirement. All powerful radiants like arc lamps
and very large incandescent lamps should be provided with diffus-
ing globes. These can now be obtained giving good diffusion with-
out much loss of light. Lighting units of more moderate output,
say from 100 to 300 candle-power, in many cases require screening
to obtain the best results, particularly toward the upper limit of
size just mentioned. An incandescent lamp of a couple of hundred
candle-power, unshielded, is rather an offense to the eyes, and
diffusing glassware or frosted bulbs very much improve the actual
lighting effect, although they sometimes create an entirely false
impression of insufficient light. Most people still judge a street
lamp by its intrinsic brilliancy rather than its actual power, and this
psychological fact must be kept in mind. Lamps of smaller output
than 100 candle-power seldom need screening, for while they may be
unpleasantly bright when viewed from very nearby, in the position,
actually occupied by them they may be comparatively inoffensive.
COLOR IN LIGHTING
In the lighting of buildings and monuments and flood lighting
problems generally, and to a less extent in some types of street
lighting, the matter of color may rise to considerable importance.
Save in rare instances color in illumination can only be obtained at
a considerable and sometimes almost prohibitive cost of energy.
One can get very efficiently a bright yellow from the flame arcs, a
color perfectly good for utilitarian purposes, but not lending itself
to any decorative effects. It is possible to produce flaming elec-
trodes giving striking colors at some loss of efficiency, but yet at
an efficiency probably exceeding anything that can be obtained by
screens or colored globes. At the Boston Electrical Show of 1912
red and green flame arcs, owing their color only to the impregnation
of the electrodes, were used with rather beautiful effect, but such
BELL: PRINCIPLES OF EXTERIOR ILLUMINATION 97
electrodes cannot be obtained commercially, and the illuminating
engineer has to fallback practically upon screens for obtaining colored
effects.
Color in lighting may be utilized to intensify the hue of objects
already colored or to impart color to things not already possessing it.
Light as nearly white as possible brings out the natural color values
in a fairly uniform way. A single color gains in brilliancy from flood-
ing with the same color, while illumination with the wrong color
may utterly spoil the effect. These things are, of course, perfectly
familiar in interior lighting. The decorative value of color has been
comparatively little appreciated or utilized in exterior illumination.
The most striking instance of its employment on a large scale was at
the Panama-Pacific International Exposition of last year, at San
Francisco, in which for both day and night effects color played a
predominant part. In regular flood lighting work a monument or
even a sign may be so tinted as to gain from the application of a
particular color in its illumination. But instances where this can
be advantageously applied are rather rare.
Perhaps of more general importance is the possibility of producing
highly decorative results in the illumination of facades of buildings
by giving them color values which relieve the monotony of the effect
otherwise attainable. Comparatively little has been done in this
line, although the writer tried it out experimentally on the facade
of the Massachusetts State House and of the building of the Edison
Illuminating Company last year far enough to learn something
of its possibilities. The chief difficulty in such work, which can be
carried out with very beautiful effects, is to obtain the necessary
illumination without too great cost in energy. Screens of the
colored film used in theatrical work can readily be arranged in con-
junction with lamps for flood lighting. In the case of the experi-
ments just referred to the screens were fitted into frames in racks
just in front of the lamps. In theatrical working the areas to be
covered are small and the available intensities are so great that a
considerable range of color can be successfully employed. This
range is much limited in the larger problems of exterior lighting unless
at great cost of energy. Light yellow screens fail to produce any
striking effect. Even amber tints, although losing considerable
light, do not seem to produce a good hue on the surface illuminated.
Light reds work better and light rose pinks also are very successful.
Greens and blues are not very striking unless deep in color and
consequently wasting much light.
g8 ILLUMINATING ENGINEERING PRACTICE
In general terms the loss of light in colored screens of hue deep
enough to produce any material effect is from 50 to 80 per cent, so
that one has to allow from 3 to 4 or 5 times the intensities which
would ordinarily be utilized for flood lighting. It is not necessary
to fit all the reflectors with screens in doing such work. A ground
illumination can be produced in the oridinary way and then tints
laid on by banks of special reflectors directed either so as to overlay
the whole or any part of it with warm color.
Considerable experimenting is needed to produce screens which
will give the maximum of tinting effect with minimum loss of light
and which will retain their color without fading. Of course, the films
used for theatrical purposes will not withstand moisture so that when
used out of doors they must either be screened in with glass or with-
drawn in rainy weather. The colored applications are interesting,
and probably will be made an important adjunct in flood lighting,
but the whole matter is still in the experimental stage.
MODERN PHOTOMETRY
BY CLAYTON H. SHARP
The present lecture is to be looked upon as in a measure a con-
tinuation of the lectures on the measurement of light given in the
1910 I. E. S. course at Johns-Hopkins University. It is intended
to supplement those lectures not only by introducing an account of
the developments in photometry since 1910, but also by treating
of certain matters which were either insufficiently treated or were
omitted entirely from the 1910 lectures. It should be understood,
however, that it is the intention of the lecturer not to attempt a
complete review of photometric advance during recent years, but
rather to confine himself to the practical features which properly
belong in this essentially practical course.
The practice of to-day in the measurement of light involves innova-
tions and improvements which the change of conditions since 1910
has brought forward. Since 1910 the introduction of the gas-filled
tungsten filament incandescent lamp with its whiter light has made
the photometric difficulties due to color differences a more important
factor in the art and has been a direct incentive toward the prose-
cution of the investigation of the problem of heterochromatic
photometry and of the introduction of means to solve it, while the
increasing demand for accuracy in photometric measurements, and
particularly the growth of the idea of the measurement of luminous
output of all lamps in terms of their total luminous flux rather than
in terms of their candle-power, has given a great incentive to the use
of the integrating sphere. During the six-year interval new and
improved types of apparatus have been constructed and put into
use.
PHYSICAL PHOTOMETER
The physical photometer, an apparatus which will measure the
light from any illuminant and give the result in terms identical
with those which would be obtained by the use of a photometer by
a person of normal color vision, has been realized. This physical
photometer has been constructed and practically used by Ives 1
who uses a sensitive thermopile as a means for measuring the radiant
energy. He has two methods for selecting the radiation from the
99
100 ILLUMINATING ENGINEERING PRACTICE
lamp in accordance with the luminosity curve of the average
human eye. The first of these methods involves passing the light
through a spectroscope equipped with a shield or screen which is
cut out in the form of the luminosity curve. The spectrum, which
is thereby reduced to a luminosity curve spectrum, is reunited, and
the total energy passing through the screen, which is then propor-
tional to the light of the lamp, is thrown on the thermopile. The
second method, which for experimental purposes is undoubtedly
simpler, involves passing the light through a glass cell having a
thickness of one centimeter containing the following solution:
Cupric chloride 60 . o grams
Potassium ammonium sulphate 14. 5 grams
Potassium chromate 1.9 grams
Nitric acid, gravity 1.05 . 18.0 c.c.
Water added to make one liter.
Between the solution and the lamp is interposed another water cell
to prevent overheating of the solution. The transmission of this
solution is according to Ives identical with the luminosity curve of
the average eye.
FLICKER PHOTOMETER
Ives 1 has recommended a system of heterochromatic photometry
involving the use of a standardized form of flicker photometer and the
investigation of the color vision of the observers using it. The
flicker photometer as recommended by him has a field two degrees
in diameter with a surrounding field of large dimensions illuminated
to approximately the same degree. As the standard illumination
for the flicker field he recommends 25 meter-candles.
A simple attachment to be used on an ordinary Lummer-Brodhun
photometer to convert it to a flicker photometer corresponding to
these specifications has been described by Kingsbury 2 and is ex-
pected shortly to be commercially available. Ives has shown both
theoretically and experimentally that the settings of observers using
a flicker photometer are affected by peculiarities of their color vision.
He has, therefore, proposed a criterion for normality of color vision of
observers using the flicker photometer. This consists in measuring
the light of a 4-wpc. carbon lamp through a one centimeter layer
of each of two different solutions. The, first consists of 72 grams
of potassium bichromate in water to make one liter. The trans-
1 Ives, I. E. S. Transactions, 1915, page 315. A bibliography of the subject is there given.
2 Kingsbury, Journal of Franklin Institute, August, ipiS-
SHARP: MODERN PHOTOMETRY
101
mitted light with this solution is yellowish. The other solution
consists of 53 grams of cupric sulphate in water to make one liter.
This gives a bluish color. The solutions are to be used at 2OC.
Ives has shown that a person with perfectly normal color vision will
find with a flicker photometer the same value for a 4-wpc. lamp with
either solution. His proposal then is to make color measurements
using the flicker photometer and a group of observers so selected that
on the average their value for the transmission of the yellow solu-
tion is the same as that of the blue solution, such a group having
according to his measurements normal color vision. This proposal
has been thoroughly investigated by Crittenden and Richtmyer 3
who by studying the peculiarities of a large number of observers
using a Lummer-Brodhun photometer have shown that identical
photometric results are obtained by a selected small number of
observers having on the average normal color vision as determined
by Ives' criterion and using a flicker photometer.
CROVA'S METHOD
Ives 4 has shown that an incandescent gas mantle can be compared
without error with a 4-wpc. carbon lamp using an ordinary photo-
meter and interposing between the eye and the photometer a 25
mm. layer of the first of the following solutions. To effect a com-
parison between a 4-wpc. carbon lamp and other incandescent electric
illuminants the second of the following solutions is used:
For mantle
burners
For incandescent
electric lamps
Cupric chloride
Potassium bichromate
90 grams
?o grams
86 grams
60 grams
Nitric acid (1.05 gravity)
Water added to make one liter.
40 c.c.
40 c.c.
When using the first solution with a mantle burner against a
4.85-w.p.scp. carbon standard, the standard has a value which is
one divided by 1.065 times its true value. No correction is neces-
sary in using the second solution. The use of this solution has the
great advantage of eliminating not only the color difference between
the lights as seen in the photometer field, but also the effects of pecu-
1 Crittenden and Richtmyer, I. E. S. Transactions, vol. n, page 331, 1916.
4 Ives, Physical Review, page 716, 1915. Ives and Kingsbury, I. E. S. Transactions,
vol. 10, page 716, 1915.
IO2 ILLUMINATING ENGINEERING PRACTICE
liarities of color vision on the part of the observer. It suffers from
the disadvantage, which under many conditions is a very serious
one, of cutting down the brightness of the photometer field to about
one-tenth the value which it otherwise would have. This necessi-
tates either a rearrangement of the photometric apparatus so that
the photometric field shall be much brighter than otherwise is
necessary, a procedure which is attended with certain practical
difficulties, or requiring the observer to work with a faint field and
consequently to keep his eyes shielded from extraneous light so that
their photometric sensibility may be sufficiently great.
LIGHT FILTERS
The difficulties of heterochromatic photometry may be effec-
tually overcome by interposing between the photometer and one
of the light sources a colored screen which will cause the illumination
on both sides of the photometer disc to have the same color. The
use of this expedient presupposes, however, that the amount of
light absorbed by such a filter when used with the light in question
is known. The determination of the transmission factors of light
filters involves all the difficulties of heterochromatic photometry, but
relegates them to the domain of the standardizing laboratory, where
they can be overcome by the experimental means at hand. The
use of light filters, since it reduces the practice of the compari-
son of lights of different color to the same degree of simplicity
as the comparison of lights of the same color, and by means at
once convenient and free from liability to error, is becoming very
extended and may be rightly described as the most commonly
accepted method in practical photometry. These light filters may
be of translucent solids or may be in the form of solutions. Ives
and Kingsbury 5 ' 6 have investigated yellow and blue solutions for
use in this way and have given equations whereby their transmission
may be computed. Such solutions may, with suitable precautions
be used as reference standards. Mees 7 has produced a line of care-
fully constructed light filters using colored gelatins, these filters
covering the entire range of the ordinary lights to be measured.
It is found practicable to get colored glasses serving as light filters
for nearly all purposes. For instance a blue glass may be obtained
which when interposed between a 4-wpc. standard and a pho-
* Ives and Kingsbury, I. E. S. Transactions, Vol. 9, page 795, 1914.
Ives and Kingsbury, I. E. S. Transactions, Vol. 10, page 253, 1915.
7 Mees, I. E. S. Transactions, Vol. 9, page 990, 1914.
SHARP: MODERN PHOTOMETRY 103
tometer will give a color match with a i-wpc. tungsten lamp, or a
pinkish glass may be found which when interposed between a i-
wpc. tungsten lamp will give a color match with a 4-wpc. carbon
standard. Glasses also may be obtained to give a color match of
gas-filled tungsten lamps with vacuum tungsten lamps, etc.
As has been said the calibration of these glasses rests with a
standardizing laboratory and involves all the difficulties of hetero-
chromatic photometry. Through an extensive set of measure-
ments of certain light filters made by a number of laboratories under
the lead of the Bureau of Standards, 8 certain light filters in the pos-
session of the Bureau of Standards have come to have an unusually
accurate calibration. It is possible for other laboratories to have
standards calibrated by comparison directly with those at the Bureau
of Standards or indirectly through other laboratories deriving their
standards from the Bureau. Through this procedure light filters
of carefully known value may readily be obtained by any photo-
metrist, and by the use of these filters the difficulties of hetero-
chromatic photometry can in nearly all cases be overcome and the
same degree of concordance attained in the photometry of different
colored lights which is expected in the photometry of lights of the
same color.
EXTRAPOLATION OF LAMP VALUES
Middlekauff and Skogland 9 have shown that a curve or equation
giving the relation between the voltage and current or candle-
power of tungsten lamps can be established which holds within close
limits for tungsten filament lamps of all ordinary sizes and styles
of construction; so that knowing the candle-power of any normal
tungsten lamp by calibration at a voltage at which its color matches
the color of the standard, its candle-power at some other voltage at
which it gives a color corresponding to the lamp under test may be
accurately computed. This method, which has the endorsement of
the U. S. Bureau of Standards, should be of great practical utility.
STANDARD LAMPS
Since 1910 the drawn wire tungsten lamp has supplanted the
pressed filament lamp, and lamps of drawn wire are now used for
purposes of photometric standards. In the smaller sizes of lamps
Middlekauff and Skogland, I. E. S. Transactions, Vol. 9, page 734; also Bulletin of
Bureau of Standards, Vol. 3, p. 287.
Middlekauff and Skogland, I. E. S. Transactions, Vol. 11, page 164; also Bulletin of
Bureau of Standards, Vol. n, p. 483.
104 ILLUMINATING ENGINEERING PRACTICE
small variations in candle-power are likely to be discovered due to the
variations in contact between the filament and the wire supports.
It is therefore necessary that, for the smaller sizes at least, lamps
should be of special construction, avoiding this variation in contact
with its consequent variable loss of heat to the anchor wires. Either
the anchor wires are pinched tightly over the filament or the filament
is drawn so tightly over the wires that no variability can ensue. The
constancy of the candle-power of the drawn wire lamps, together with
their mechanical strength, etc., is such as to fit them eminently
well for service as standards. They may be standardized not
only at a voltage approximating their operating voltage, but also
at lower voltages; for instance at such a voltage that they give a
color match with a 4-wpc. carbon standard. It is a question whether
all things considered, the tungsten standards are not more reliable
than the old carbon standards, but the time has not yet come
when this question can be finally answered. It is to be noted, how-
ever, that inasmuch as the incandescent lamps most used to-day are
of the tungsten class, the use of tungsten standards enables photo-
metric measurements to be made without the difficulties of hetero-
chromatic photometry. A photometric laboratory may carry side
by side a series of 4-wpc. carbon standards and a series of ap-
proximately i.2-wpc. tungsten standards, each set of standards to
be used with its corresponding class of lamps. The introduction
of the gas-filled lamp, however, has given rise to a situation where
heterochromatic photometry is difficult to avoid. The filaments of
these lamps are made up in the form of fine spirals. The candle-
power of a spiral wound filament can vary not only because of
alteration in its physical state or in the conditions surrounding it
in the bulb (convection currents, etc.) but also on account of any
change in the spacing of the spires of the helices in which the filament
is formed. If on account of sagging at the high temperature at which
the filaments are operated, the little spirals open up somewhat at
any point, the candle-power will be found to be reduced at this
point, since there the convection currents carry off more heat.
Moreover, the question of the conduction of the heat from the filament
by the anchor wires is one which may intervene to cause variable
candle-power. Hence it is that it is a more difficult thing to get from
gas-filled lamps the entire constancy of candle-power at given voltage
or current which is demanded of a real standard. Lamps of this
type are sometimes calibrated as " check lamps," intended to be used
as standards in the industrial photometry of gas-filled lamps, but
SHARP: MODERN PHOTOMETRY , 105
not dignified with the name of standards. It is to be hoped that
methods of construction will be found whereby entire constancy may
be insured in the candle-power of gas-filled lamps specially designed
for use as standards. Until this is done the real standards against
which gas-filled lamps have to be compared are vacuum tungsten
lamps and this comparison involves color differences which, however,
can be removed by the use of suitable light filters.
lo-C.P. HARCOURT LAMP
This important primary standard has been subjected to thorough
investigation at the Bureau of Standards and there has been found
a well-defined difference between the pentane lamps of English and
American manufacture. Moreover, the newer American lamps are
differentiated from the older ones by certain operating requirements.
For instance, the time required for the lamp to reach its full candle-
power is less than 15 minutes in the case of the English lamp, whereas
20 minutes must be allowed with the newer American lamps and
30 minutes with the older ones. The Bureau authorities have found
that the control of the density of the pentane is of considerable
importance and that to empty the saturater once a month, as
should be done according to the instructions of the London Gas
Referees, is quite insufficient when the lamp is used as much as three
times a day, since the density of the residual pentane would be
considerably greater and its candle-power greater. At the Bureau of
Standards the density of the pentane used is always kept below
0.635. The saturater should be from one- third to two- thirds full
of pentane at starting, and the height of liquid as seen against the
window of the saturater should never be less than ^ inch. It is
recommended that in the photometer room a hood or chimney should
be arranged in the ceiling above the lamp in such a way as to carry
the products of combustion directly out of the room. The correc-
tion for water vapor is made in accordance with the following:
/ = 7 8 [/ + (8 - /)o.oos6 7 ].
Where 7g represents the candle-power of the lamp with normal water
vapor content, namely, 8 liters of water per cubic meter of dry air,
and / represents the actual humidity. In order to determine the
value / a hygrometer of the wet and dry bulb type is used. The
most precise instrument is the Assmann psychrometer which con-
sists of two finely divided mercury thermometers, mounted side
by side on a stand and with a tube surrounding the bulb of each.
106 , ILLUMINATING ENGINEERING PRACTICE
At the top is a small spring-driven suction pump which draws a
rapid current of air over both bulbs. One bulb is surrounded by a
cloth which is wet with water, while the other is dry. From the
difference between the readings of the two, by reference to hygro-
metric tables, such as for instance the tables issued by the United
States Weather Bureau, the pressure of the water vapor is determined.
A simpler apparatus than the Assmann psychrometer is the sling
psychrometer, used extensively by the Weather Bureau. This is
a relatively inexpensive apparatus consisting of two thermometers
mounted on a handle so that they can be swung rapidly in a circle.
One of them being wet and the other dry, a difference is obtained
which corresponds to the humidity of the atmosphere. Knowing e,
the partial pressure of the water vapor, and the barometric height, b,
in millimeters, the water vapor in liters per cubic meter of dry air
is found from the equation
>
I =T ~ X 1000
b e
The effect of variation in atmospheric pressure on the candle-power
of flame standards has been investigated by Butterfield, Haldane
and Trotter in London and also by Ott 10 in Zurich. Ott confirmed
the old formula of Liebenthal, as follows:
/ = 1.049 ~~ -55^ H~ 0.06011(6 760)
Butterfield, Haldane and Trotter found a relation which is depicted
in curves of Fig. i.
A late investigation by the U. S. Bureau of Standards 11 yielded
the curves of Fig. 2.
The variation of standard flames with barometric pressure is of
vital importance when dealing with these standards in places located
at considerable altitudes above sea level, and affects us particularly
in this country where there are a number of important cities at
relatively high altitudes.
PORTABLE ELECTRIC STANDARD
A portable electric standard lamp outfit which has been used to a
limited extent in gas. photometry is illustrated in Fig. 3. The lamp
used has a single loop tungsten filament and is of such a rating that
when burned so as to give two candle-power, it has a color which
10 Ott, Journal of Gas Lighting, Nov. 16, 1915.
11 Transactions I. E. S., Vol. X, page 843, 1915.
SHARP: MODERN PHOTOMETRY
107
,110
iioo
J
40
100
Fig. i.
50 60 70 80 90
Barometric Pressure Cm. of Mercury
Variations of flame candle-power with atmospheric pressure. (Butterfield, Haldane
and Trotter.)
40
110
50 60 70 80 90 100
Barometric Pressure: - Cm of Mercury
Fig. 2. Variations of flame candle-power with atmospheric pressure, (i) Hefner lamp; (2)
Pentane lamp; (3) No. 7 Bray slit union gas burner. (Bureau of Standards.)
io8
ILLUMINATING ENGINEERING PRACTICE
Fig. 7. Diagram of bridge of portable
standard.
matches that of gas burned in an open burner. It then consumes
approximately one ampere with four volts potential difference be-
tween its terminals. The current for the lamp is furnished by a
portable six-volt storage battery such as is used frequently for gas-
engine ignition purposes and of 40 ampere-hour rating. The bat-
tery, therefore, when fully charged is capable of supplying current to
the lamp for a considerable time. On the controller box are mounted
suitable rheostats and also a Wheatstone bridge and galvanometer
for setting the lamp to its standard
candle-power. As is well known,
the tungsten filament has a large
positive temperature-resistance co-
efficient. If the lamp, therefore, is
made one arm of a Wheatstone
bridge, the other arms being con-
stituted of zero temperature coeffi-
cient wire, as shown in Fig. 7, the
resistances may be so adjusted that
the bridge is in balance when the
lamp is operating at its normal
candle-power. As soon as the current through the lamp varies, its
resistance also varies and the bridge falls out of balance. The
balance of the bridge is indicated by a pivoted galvanometer which
is shown in the figure, or it may be determined by means of a tele-
phone and an interrupter which gives a slight click in the telephone
if the bridge is out of balance. This method of adjusting the stand-
ard lamp is a very sensitive one, particularly when a galvanometer
is used; far more sensitive than a direct reading ammeter or volt-
meter, even of the laboratory standard type. In order to permit
adjustment, there is a portion of the bridge in the form of a slide
wire, the galvanometer circuit making contact with this slide wire
at any position desired. The position may be recorded from a
divided scale with which the slide wire is equipped.
The apparatus was given the form here described in order that an
unskilled man unfamiliar with electrical apparatus should be able
to operate the standard. He has merely to close the switch and
adjust the rheostat until the galvanometer comes to zero. It
should be noted, however, that in a general case a test of candle-
power of gas will agree with a test made against a lo-c.p. pentane
lamp only when the conditions of atmospheric humidity are standard
for the lamp, that is, eight liters of water per cubic meter of dry air.
Fig. 3. Portable electric standard.
Fig. 4. One-meter sphere for industrial photometry of incandescent lamps.
(Facing page 107.)
Fig. s. Industrial sphere photometer, showing arrangement of sphere doors and photom-
eter scale.
FIG. 6. Sharp- Millar calibrator in position on photometer.
SHARP: MODERN PHOTOMETRY 109
If the hygrometric condition is different from this a different result
will be obtained. It is, therefore, necessary in using the electric
standard to observe with, for example, a sling psychrometer the
hygrometric condition, and to apply a correction to the invariable
electric standard to make it agree with what the pentane standard
would show under similar conditions. The operation involved in
this correction can be reduced to great simplicity. 12
Total Flux Standards. Standards of flux or of mean spherical
candle-power have to be derived from ordinary standards of candle-
power. In the primary standardization of lamps in lumens, there-
fore, it is necessary to adopt some method whereby the total lumi-
nous flux can be computed from a series of candle-power measure-
ments in a sufficient number of directions. In the practice of the
Electrical Testing Laboratories in making lumen standards, the
lamp is first standardized carefully as an ordinary standard for
mean horizontal candle-power. Then its candle-power distribution
curve is determined by measurements at various angles in the verti-
cal plane and the lamp's spherical reduction factor, or the relation
between its spherical candle-power and its horizontal candle-power
is computed. The known horizontal candle-power multiplied by the
spherical reduction factor gives then the spherical candle-power.
The latter multiplied by 4?r gives the total lumens. Having estab-
lished standards by this procedure, copies sufficiently accurate for
industrial purposes can be made by the more direct method of
the integrating sphere.
BAR PHOTOMETER
The bar photometer, the classic apparatus of the photometrist,
is being used mgre and more according -to methods which were but
little recognized a few years ago. The standard method in the use
of the bar photometer was for many years to fix the light sources at
the ends of the bar and to move the photometer between them until
the point of balance was obtained. It is becoming 'now more com-
mon practice to allow the photometer head to remain stationary and
at a fixed distance from the light source to be photometered; that is,
the test lamp, while the photometric balance is effected by moving
the comparison lamp. This method of using the bar is an almost
necessary one in the photometry of large sources of light, particu-
larly of lamps with reflectors having concentrating properties, in the
photometry of projectors, etc., where it is important to measure the
12 Sharp and Schaaf. American Gas Light Journal, Vol. VIII, p. 325, 1913.
IIO ILLUMINATING ENGINEERING PRACTICE
apparent candle-power of a source at a fixed distance. It has been
found feasible and desirable from the point of view of convenience,
to diminish the length of the bar and this has been made by the use
of small low voltage tungsten filament lamps for comparison lamps.
A low voltage tungsten lamp has a filament so small that it can be
considered as a point source of light when very much closer to the
photometer disc than is possible with the ordinary lamp. On this
account the comparison lamp can be brought up much closer to the
disc and the whole bar very greatly shortened without any practical
decrease in accuracy of the apparatus. In doing this the bar photo-
meter approaches the construction which is well known in the case
of portable photometers intended primarily for the measurement of
illumination; in fact it is found that in a great deal of practical work
a portable photometer may be substituted for much more elaborate
and cumbersome photometer bars. In precision work it is desirable
that the brightness of the disc shall have a known and constant
value. In order to attain this condition the comparison lamp is
fastened to the carriage on which the photometer head is mounted
and the distance of the two from the test lamp is varied in order to
get the photometric balance. In this case again the use of the small
tungsten lamp as a comparison lamp enables a simplification to be
made in that the lamp can be mounted on a short arm which is
rigidly attached to the photometer carriage, thereby avoiding the
necessity of an additional carriage.
INTEGRATING SPHERE
The use of the integrating sphere is extending very rapidly. The
Committee on Nomenclature and Standards of the Illuminating
Engineering Society has made recommendations as follows:
Illuminants should be rated upon a lumen basis instead of a candle-
power basis.
The specific output of electric lamps should be stated in terms of lumens
per watt and the specific output of illuminants depending upon combustion
should be stated in lumens per British thermal unit per hour.
When auxiliary devices are necessarily employed in circuit with a
lamp, the input should be taken to include both that in the lamp and
that in the auxiliary devices. For example, the watts lost in the ballast
resistance of an arc lamp are properly chargeable to the lamp.
The specific consumption of an electric lamp is its watt consumption per
lumen. "Watts per candle" is a term used commercially in connection
with electric incandescent lamps, and denotes watts per mean horizontal
candle.
SHARP: MODERN PHOTOMETRY in
These recommendations have been adopted by the American
Institute of Electrical Engineers and by the National Electric Light
Association. The measurement of the horizontal candle-power of
gas-filled lamps has, for reasons which are discussed later, been found
unsatisfactory. All of these facts tend to bring the integrating
sphere into a position of greater importance in practical photometry.
Little has been added to the theory of the integrating sphere or
to the principles of its practice since Ulbricht's treatment of the
same, but there has been a considerable development of the sphere
in the way of making it a more practical apparatus for routine
photometric work. Inasmuch as the theory of the sphere was
merely hinted at in the 1910 lectures, it may be well here to say
more about it.
It has been shown that a diffusing glass window on the surface
of the sphere is illuminated by each element of surface of the sphere
to a degree dependent only on the brightness of that element and
independent of its position. This presupposes that the direct light
of the lamp in the sphere is not allowed to shine on the window.
No other form of enclosure, such as a box, conforms to this theoretical
law and hence all other forms are imperfect integrators as com-
pared with the sphere.
To prevent the direct light of the lamp from falling on to the
window a white diffusing screen is interposed. The presence of the
screen is a disturbing factor in the sphere for two reasons. First,
the light from the lamp falling directly on the screen must be re-
flected from the latter before it can reach the sphere and hence this
part of the flux is diminished by the absorption of the screen before
it comes to the sphere surface from which it is reflected to the
window. Second, a portion of the sphere surface is hidden from
the window by the screen, and the light falling on this portion must
be reflected before it reaches a part of the sphere which is reflecting
directly on the window. Hence this portion of the total flux suffers
a diminution due to the absorption of the sphere coating. As a
partial compensation for these two losses we have the light from the
sphere reflected by the side of the screen turned toward the window.
It is not difl&cult to calculate the flux falling directly on the screen
and the flux on the hidden part of the sphere. The position of the
lamp and of the screen should be such as to make the sum of these
two elements of flux a minimum. As a practical matter the amount
of this re-reflected flux depends on the distribution of flux from the
lamp and hence the position of the screen most favorable for one
112
ILLUMINATING ENGINEERING PRACTICE
lamp would not be best for another. In the case of incandescent
lamps in general the best position of the window is at the top of
the sphere with the lamp vertical on the vertical diameter, for in
this position the lamp base which casts a shadow anyhow, casts it
on the screen and so the flux on the screen is less than it would be in
any other position. This arrangement is inconvenient and should
be resorted to only when the highest precision is desired. In any
case the screen error can be made a very small one by using a sphere
of adequate size. Increase in the size of the sphere reduces the error
in two ways; first, by reducing
the relative area of the screen
as compared with that of the
sphere, and second by per-
mitting the screen to be placed
further from the lamp whereby
the flux on it is decreased. If
the lamp is not too near to
the wall of the sphere and to
the screen and the sphere is of
sufficient size, no danger of an
excessive screen error is to be
apprehended. In any case
the use of the substitution
method when lamps of more
or less similar candle-power distribution characteristics are being
photometered, ensures the partial or complete elimination of the
error.
When bulky lamps, such as arc lamps, or lamps with shades or
reflectors, are to be photometered, the sphere must be standardized
or its constant determined with the test lamp in place in the sphere.
This requires that the test lamp and the standard lamp have separate
locations in the sphere. The standard lamp should be left in place
in the sphere while the test lamp is being measured. There must
be a screen between the standard lamp and the window and another
between the test lamp and the window. There should also be a
screen protecting the test lamp and its parts from the direct light
of the standard lamp. A scheme of the arrangement is shown in
Fig. 8. The reason for the latter screen is as follows: The test lamp
emits a certain flux of light. The parts of the lamp, which are foreign
bodies or obstacles in the sphere, interrupt and absorb a certain
fraction of this flux, which, therefore, never escapes from the confines
Fig. 8. Integrating sphere with large lamp
and screen.
SHARP: MODERN PHOTOMETRY 113
of the lamp as useful light, and should not be measured. The
lamp parts, however, interrupt a portion of the reflected flux in the
sphere which otherwise would increase the brightness of the window
and thus needs to be accounted for. If the direct light of the
standard lamp is screened off, the parts or appurtenances of the
test lamp will absorb approximately the same fraction of the re-
flected light of the standard lamp as that of the test lamp, and no
great error is incurred.
To determine the effect of the added screen between the standard
lamp and the test lamp, a photometer setting is made with the sphere
containing only the standard lamp with, and again without, the
screen; in other words the total flux of the lamp with the screen is
measured against itself without the screen as standard.
The improvements in the integrating sphere have been in the direc-
tion of providing for easy and quick introduction and removal of
lamps and in the adaptation of the sphere to the photometering
of gas lamps. Speaking of the latter point first, it has been found
that with a sphere of ample size, from 2.0, to 2.5 meters in diame-
ter, provided with suitable ventilating openings at the top and
bottom, gas lamps, even of very large size, can be photometered
without difficulty. The ventilating openings need to be made so
as to rob the sphere of as little of its white interior surface as possible,
and at the same time to prevent the escape of light from the interior
and the ingress of light from the exterior. This is quite simply
done by covering the opening with a circular disc set down a short
distance from the surface, so as to leave a sufficient passage for the
air, while cutting off the light.
To facilitate the handling of incandescent lamps, a number of
plans have been employed. One quick handling device for the sphere
intended for the photometering incandescent lamps was treated
of in the 1910 lectures. A device has been used at the U. S. Bureau
of Standards and at the Physical Laboratory of the National Lamp
Works of the General Electric Company, whereby the act of opening
the door of the sphere swings an arm carrying a lamp socket out to
the opening so that lamps are readily changed. When the door is
closed this arm swings back again and places the lamp at the center
of the sphere. A complete sphere photometer has been designed and
constructed at the Electrical Testing Laboratories which seems to
meet the requirements of routine photometry of incandescent lamps
of all sizes. Inasmuch as no other device of this kind seems to have
been described in the literature, a fairly complete description may be
8
114 ILLUMINATING ENGINEERING PRACTICE
given here (Figs. 4 and 5) . The sphere is of one meter diameter and
has been variously constructed of sheet metal, of cast aluminum and
cast iron. The cast aluminum is the most desirable material, but
the price of it at the present time is almost prohibitive. The cast
sphere is mounted on three legs of two-inch iron pipe with floor
flanges, and all of the auxiliary parts are screwed or bolted to the
sphere itself which, therefore, forms the carcass or frame of the in-
strument. Referring to Fig. 5 an opening about 40 by 58 centi-
meters is cut out of the sphere and two doors, either of which will
fit this opening, are mounted on a vertical shaft. To each of these
doors is fastened a bracket carrying a lamp socket. Thus when the
opening of the sphere is filled by one of the doors, the lamp socket
attached to it is in the sphere carrying the lamp to be photometered,
while the other lamp socket is 'outside the sphere ready to have its
lamp inserted. When the photometering of the lamp in the sphere
is completed, the vertical shaft is rotated a half turn, whereby the
already photometered lamp is withdrawn from the sphere and the
one to be photometered is put inside. By means of a special
switch attached to the vertical shaft the lamp inside the sphere is
automatically connected to the source of current. Thus while the
lamp in the sphere is being photometered, the lamp which has been
photometered can be removed and a fresh one substituted for it.
Very heavy filament lamps require the current to be flowing through
them for a little time until they reach their ultimate temperature,
and consequently their ultimate candle-power. For instance, gas-
filled lamps of 20 amperes rating should be photometered after they
have been heating for at least one minute. With the sphere here
described an auxiliary preheating circuit can be attached so that
the lamp which is outside the sphere is being heated during part of
the time when the lamp inside the sphere is being photometered.
The photometric arrangements are made part of the sphere itself.
The photometer bar which is supported by a cast-iron bracket per-
mits the travel of the comparison lamp over a distance of 46.3
centimeters. The comparison lamp is a low voltage tungsten lamp
so selected that it will have the requisite candle-power when it is
operating at an efficiency which will cause it to match in color the
regular vacuum tungsten filament lamps. There are four scales to
the photometer, giving the three following ranges: 30 to 240
lumens, 200 to 1600 lumens and 1200 to 9600 lumens. These scales
are directly above each other and are made by contact printing on a
photographic plate. The scales are translucent and are read by the
SHARP: MODERN PHOTOMETRY 115
photometer operator who changes the lamp and who sees the shadow
of a stretched wire on the scale thrown by the test lamp itself. The
photometer operator who makes the settings is ignorant of the scale
readings and is thereby protected from any possible bias. Only
one scale is visible at a time, the rest being covered by a movable
shutter. In order to enable the sphere to be used without change
of calibration of the standard lamp, the following arrangement is
employed.
Over the diffusing window of the sphere, which has a diameter of
8 centimeters, is placed a hemisphere of the same diameter. > This
hemisphere contains in turn a small diffusing window which consti-
, tutes one side of the photometer disc. In a narrow slot between the
hemisphere and the diffusing window of the large sphere is placed
a slide with four openings. The largest of the openings has the
same diameter as the hemisphere. The next opening has such a
diameter that when it is introduced, the brightness of the window
of the small hemisphere is cut down to such a degree that the pho-
tometer is direct reading on the second of its scales rather than on its
first. Inasmuch as the first scale reads from 30 lumens to 240 lumens
and the second scale from 200 lumens to 1600 lumens, the amount
of brightness reduction on interposing the second opening is in the
ratio of 20 to 3. The first scale enables vacuum tungsten lamps of
the smaller sizes (7^ to 25 watt) to be photometered. For larger
lamps it is necessary to go to the second scale which covers the range
approximately of 25 watts to 200 watts. Hence by the use of these
two ranges all of the ordinary sizes of vacuum tungsten lamps are
covered. The other two apertures are intended for the photometry
of gas-filled lamps where the whiter color of the lamp introduces an
additional photometric difficulty. To reduce this color to the color
of the comparison lamp, filters of pinkish glass are placed in aper-
tures 3 and 4. These apertures again are so dimensioned that the
photometer is direct reading without readjustment of the comparison
lamp. The scale used with aperture 3 is identical with the scale
used with aperture 2. The scale used with aperture 4 has a range of
1 200 lumens to 9600 lumens and covers therefore gas-filled lamps
up to 500 watts. For still larger lamps an additional diaphragm is
placed in aperture 4 and the range thereby extended to take in 1000-
watt gas-filled lamps. Hence with one and the same setting of the
comparison lamps any ordinary incandescent lamp may be read
directly. The slide containing the apertures 1,2,3 an d 4 is mechanic-
ally connected to the shutter over the scales, so that when the
n6
ILLUMINATING ENGINEERING PRACTICE
slide is moved, the shutter is also moved to expose the proper
scale.
Photometric measurements are made by the aid of a Lummer-
Brodhun prism. The comparison lamp is held at its standard value
by means of a Wheatstone bridge arrangement. Such is described
above under the Portable Electric Standard. With this apparatus
it is found that incandescent lamps can be photometered more
rapidly than on a photometer bar with a rotator.
APPLICATION TO THE PHOTOMETERING OF STREET LAMPS
The sphere has also been used in practice in the determination of
the candle-power of street lamps. The photometering of lamps in
the street by ordinary methods is admittedly unsatisfactory. How-
ever, by the use of the arrangement which is referred to in the first
lecture on Street Lighting, where a i meter sphere is mounted on
a truck and brought directly underneath the lamp which is then
lowered into the sphere, this class of measurement is made practically
as accurate as an indoor measurement.
INSTRUMENTS FOR MEASUREMENT OF ILLUMINATION
In addition to the instruments described in the 1910 lectures
certain new ones have entered the field and will here be described.
Fig. 9. Sectional view of Macbeth illuminator.
Macbeth Illuminometer . This is a small, light weight instrument
differing only in details of construction but not in principle from other
well-known instruments. As shown in Fig. 9 the photometric
device is a Lummer-Brodhun cube which is looked at through a lens.
A lamp is carried on a rod projecting from the end of the tube and
can be moved back and forth by means of a rack and pinion. The
SHARP: MODERN PHOTOMETRY
117
scale is drawn on the exposed portion of this rod. The light from
the lamp falls on a small translucent screen which is seen on one
side of the field. The other side of the field is a reflecting test-
plate located at the point where the illumination is to be measured.
The calibration of the scale is in accordance with the inverse square
law, the theoretical law of the instrument. A small housing about
the lamp provides for the exclusion of stray light from the sides of
the tube. The lamp is held at standard condition by means of an
ammeter which is contained in a separate box which also carries the
necessary rheostats. With
this instrument is provided a
so-called reference standard
which is shown in cross-section
in Fig. 10. This reference
standard is arranged to be
placed on the reflecting test-
plate, and the tube surrounding
the sighting aperture is inserted
at D so that this test-plate may
be viewed under the light of
the small standardized lamp
contained in the reference
standard. When a given cur-
rent is passed through the
standard lamp, known illumina-
tion is produced on the test-
plate, and against this known
illumination the photometer
can be standardized. It is to
be noted, however, that any
error of the ammeter is involved
in the use of the reference standard and hence the necessity for
maintaining the ammeter in correct calibration. The range of
the Macbeth illuminometer is normally from i to 25 foot-candles.
It is increased by the insertion of neutral glass screens on either one
side or the other of the Lummer-Brodhun cube. The total range
of the instrument with the two screens ordinarily provided with it
is said to be from about 0.02 to 1200 foot-candles.
Sharp-Millar Photometer Small Model. In this smaller model of
the photometer described in the 1910 lectures (see Fig. n), the size
has been reduced to approximately 12}^ inches in length and 2*/
Fig. 10. Section view of Macbeth reference
standard.
n8
ILLUMINATING ENGINEERING PRACTICE
by 2^ inches in cross-section. The box is of metal rather than of
wood and the scale which is used has also been reduced to one-half.
Like the previous instrument, this is adapted to the measurement of
illumination, candle-power and surface brightness. Instead of
using an ammeter or a voltmeter for holding the comparison lamp
at its proper candle-power, a Wheatstone bridge arrangement such
as is described above under the portable electric standard is used.
Instead of a galvanometer with -the bridge, a telephone receiver is
used as a detecting instrument. If the bridge is out of balance,
making and breaking the circuit through the telephone gives a
series of audible clicks. When the current is reduced to zero, as is
Fig. ii. Sharp- Millar photometer, small model.
indicated by a state of silence in the telephone, the proper current
is flowing through the comparison lamp. The telephone method,
while not so sensitive as the galvanometer method, yet is sufficiently
sensitive so that a careful observer can set the current to its correct
value within closer limits than is possible with a small portable
ammeter or voltmeter. The use of the bridge and telephone en-
hances the portability of the instrument very greatly inasmuch as
no other auxiliary apparatus is required than two dry cells or a
small storage cell for the purpose of furnishing, the current. The
instrument is provided with the usual absorbing screens for extend-
ing its range and can easily be held in the hand when used. Either
an attached transmitting test-plate or a detached reflecting test-
plate may be used.
Calibrator. For use with the above instrument and also with the
ordinary model of the Sharp-Millar photometer, a calibrator has
been devised whereby the accuracy of the calibration of the instru-
ment may be checked at one point. This consists of a short tube
(Fig. 6) to be set on the test-plate of the photometer. This tube
carries near its upper end a seasoned incandescent lamp which is put
in a bridge connection similar to that described above. The bridge
is non-adjustable. In connection with the tube is also a rheostat so
that the current through the lamp may be varied until the bridge is
SHARP: MODERN PHOTOMETRY
119
in balance. When the bridge is balanced the lamp throws a known
illumination on the test-plate and the photometer may be adjusted
to give the corresponding reading on its scale. Inasmuch as the
scale is known to be correct throughout its length, a check at
one point insures its accuracy at all points.
Compensated Test-plates. All illumination photometers measure
illumination on the assumption that the light reflected or trans-
Fig. 12. Principle of compensated test-plate.
Fig. 13. Compensated test-plate on a photometer.
mitted from the test-plate follows Lambert's cosine law. As a
matter of fact no substance yet found will diffuse light in exact
accordance with this law. The light received at high angles pro-
duces a brightness of the test-plate which is too small as compared
with the similar flux of light incident at small angles. The failure of
the test-plate to conform to this law is therefore reflected in an
error of the photometer which differs according to the conditions
120
ILLUMINATING ENGINEERING PRACTICE
under which the photometer is used. In an attempt to obviate this
error, a so-called compensated test-plate as illustrated in Fig. 1 2 has
been produced. 13 In this figure the light incident upon the upper
surface of the test-plate P is reinforced by light admitted through
a translucent glass ring A , so that at all angles the brightness of the
under surface of P due to the combined action of the light trans-
Fig. 14. Principle of compensated reflecting test-plate.
mitted from above and the light incident upon it from beneath,
corresponds to the theoretical amount. It will be noted that the
amount of compensation increases with the angle of incidence and
hence can be made practically complete for all angles up to the very
-40
10'
20
40 50 60 70 v
Angle of Incidence
90 100 l
Fig. 15. Errors of various test-plates viewed normally. A, Depolished transmitting
plate; B, polished transmitting plate; C, depolished glass reflecting plate; D, compensated
transmitting plate; E, compensated transmitting plate.
high ones. The intrusion of light to the ring A when the rays are
parallel to P is prevented by the screen S. The test-plate is attached
to a photometer as shown in Fig. 13. The same principle is shown
applied to a reflecting test-plate in Fig. 14. In this case the reflect-
ing test-plate consists of a sheet of depolished white glass. This is
set a small distance from another diffusing white surface C. Com-
pensating light falling upon the plate C is reflected on the lower sur-
13 Sharp and Little, I. E. S. Trans., Vol. 10, p. 727, 191$-
SHARP: MODERN PHOTOMETRY
121
face of P, transmitted to the upper surface of P, and reinforces the
light reflected from P to exactly the right amount to insure compen-
sation for the deficiencies of P at high angles. The behavior of
various test plates is summed up in the curves of Fig. 15. It is
evident that where high precision is required in illumination pho-
tometry, and where test-plate errors cannot be computed and
allowed for, the use of some form of corrective test-plate is of
vital importance.
Static Illumination Tester.^ This instrument provides a means for
the ready measurement of illumination to a relatively low degree of
r
Fig. 1 6. Principle of simplified illumination tester.
precision by an instrument of extreme simplicity in construction
and use. The principle which it involves may be described as fol-
lows: The field of uniformly graded brightness produced by the
comparison lamp, instead of being formed in the air where it is
invisible, is formed on a sheet of diffusing material, which thereby
is given a continuously graded brightness. The light which is to
be measured falls on a diffusing sheet in juxtaposition to this field,
so that the point can be readily seen where the brightness of the
one field equals that of the other. The graded field being cali-
brated, the brightness of the unknown field is determined by finding
with the eye the point where the brightness of the unknown field
equals the brightness of the known graded field.
The illumination tester embodying this principle is shown in
Fig. 1 6. This figure shows a rectangular box B approximately
2.5X2.5 cm. in cross-section by 20 cm. long, containing at one end
a small tungsten filament lamp L behind an opal glass screen. The
top of the box over the rest of its length is made up of a sheet of
i Sharp, Electrical World, September 16, 1916.
122 ILLUMINATING ENGINEERING PRACTICE
clear glass to which is pasted an arrangement of papers P which
constitutes what may be described as a continuous photometer
disc of the Leeson type, extending from one end of the glass to the
other. The interior of the box is painted white, except for the far
distant end which is black. The photometric element P consists of
a sheet of fairly heavy paper with a slit cut out of it having saw
tooth edges. Over the entire arrangement is then pasted a sheet of
thinner translucent paper having a mat surface. When the lamp is
lighted, the end of the slit nearest the opal glass is seen to be very
bright, and this brightness fades away gradually toward the other
end of the slit. When an exterior illumination falls on this photo-
metric device, the outer portions are illuminated almost wholly
by this exterior illumination, while the slit is illuminated chiefly by
the light from the inside of the box. At the point where the bright-
ness of the exterior portion is the same as the brightness of the slit,
the saw teeth fade away and are hard to distinguish. This point
which can be recognized without difficulty provided the papers are
properly selected for the purpose, indicates the photometric value on
the scale.
The completed apparatus in its experimental form is shown in
Fig. 17. In this figure the photometric box is seen mounted on a
larger box which contains a single dry cell serving as the source of
current. The box also carries a small precision voltmeter and a
rheostat. The photometric box is so arranged that it can be re-
moved from the rest of the apparatus, the flexible cord conductor
with which it is connected being stowed away in the larger box when
the two are used as a unit. Photometric readings are taken in a
direction at right angles to the axis of the box. The exact angle to
the vertical at which these readings are made seems to be of rela-
tively little importance in the general case.
By slight structural modifications the instrument can be adapted
to the measurement of the brightness of surfaces as well as the
illumination incident upon them.
Mr. R. ff. Pierce 15 has also produced an instrument of this same
eneral class.
ILLUMINATION MEASUREMENTS
Practice in illumination measurements is so varied according to
conditions governing any particular test, that to go into anything
approaching a complete discussion would be beyond the scope of
American Gas Light Journal, page 67, August 14, 1916.
SHARP: MODERN PHOTOMETRY 123
this lecture. Certain . general precautions, however, need to be
taken in practically every case. Among the most important ones
are the following:
To be sure that the comparison lamp in the photometer is giving
its correct candle-power. This involves a comparatively recent
standardization of the same taken in connection with its electrical
measuring instrument, and an assurance that the electrical measur-
ing instrument is sufficiently reliable and accurate for its purpose.
To use the photometer in such a way that there is no undue loss
of light on the test-plate due to the presence of the operator or other
person.
To select the test stations properly according to the design of the
test.
To see that the test-plate is level and in the proper position.
To see that the scale readings are properly recorded and any con-
dition such as the introduction of a neutral glass screen is noted.
The subject of precautions to be taken in illumination measure-
ments has been quite fully treated by Little 16 in an Illuminating
Engineering Society paper.
MEASUREMENT OF BRIGHTNESS
For many purposes it is desirable to know the brightness values
of objects or of walls and ceiling in a room or of a shade or reflector
of a lamp. The standard forms of portable photometers designed
to measure illumination enable these measurements to be made
simply by removing the test-plate, it there be one, and sighting the
photometer directly on the object in question. Photometric
balance is then secured between the object and the diffusing plate
in the photometer. The reading of the scale needs to be multiplied
by a constant to give the brightness value either in candle-power
per square inch or in rnillilamberts. 17 The determination of this
constant is a matter for the standardizing laboratory and is not
particularly easy, inasmuch as it involves illuminating to a known
degree a surface of known area which is then photometered as a
source of light. It should be noted that the brightness constant of
a photometer is a function only of the test-plate which is used with it,
and that with changes in the calibration of a photometer this con-
stant is unaffected, provided the test-plate is unchanged. The
relation between the brightness constant expressed in apparent
'Little, Transactions I. E. S., Vol. 10. page 766, 1915.
17 The millilambert is a unit of brightness, and is equal to the brightness of a perfectly
reflecting and perfectly diffusing surface on which one millilumen per square cenfimeter falls.
124 ILLUMINATING ENGINEERING PRACTICE
lumens emitted per square foot, and the illumination which is
the lumens incident per square foot, is evidently the transmitting
or reflecting power of the test-plate according as the test-plate is of
the transmitting or reflecting type. In practice in the standard-
izing laboratory it is convenient to have carefully preserved a stand-
ard test-plate of known brightness constant which can serve as a
reference plate for the calibration of other plates.
DAYLIGHT MEASUREMENTS
For measuring daylight the use of a light filter to secure an ap-
proximate color match is indispensable. Inasmuch as the quality
of daylight varies greatly, dependent upon the character of the sky,
no one filter will enable a match to be made, but a single filter may
in practice be used because the outstanding differences are not so
excessive as to prevent fairly good measurements being made. On
account of the very high values of illumination usually given by
daylight, it is more convenient to put the filter on the daylight side
rather than on the side of the comparison lamp. In the practice
of the Electrical Testing Laboratories a sheet of suitably colored
gelatin is sometimes utilized such as is employed in spot-lighting
in theatrical work. Daylight foot-candle values alone are fre-
quently, perhaps usually, of subsidiary importance because
illumination values vary so much from time to time with the
outdoor or sky conditions. Rather the photometrist must give
a value at the place which is studied, coupled up in some was
with a value representing outdoor conditions. The condition
most commonly chosen is the brightness of some portion or of all
of the visible sky, or the illumination produced by some portion of
all of the visible sky. For this purpose various types of apparatus
have been produced by which the brightness of the sky can be com-
pared with the brightness of a test-plate in a room, for instance.
As illustrative of this class of problems, the methods employed by the
Electrical Testing Laboratories in studying the obstruction of day-
light to buildings caused by alterations in a structure in the street
will be instructive. In this work one photometer with a vertical
test-plate was placed close to the window-line of the building. Along-
side of it was another photometer having a fan-shaped arrangement
placed over the test-plate whereby the test-plate received only the
light from the unobstructed portion of the sky. (See Fig. 18.)
Readings were made simultaneously with these photometers and
thereby a relation was obtained between the illumination produced
Fig. 17. Simplified illumination tester.
Fig. 1 8. Simultaneous measurements with two photometers in determining daylight
conditions.
(Facing page 124.)
SHARP: MODERN PHOTOMETRY
125
by the.sky independently of all structures, and the light entering the
building. After alterations were completed, measurements of the
same kind were repeated and the change in the ratio was taken to
indicate the degree of light obstruction caused by the alterations.
It was found that even with these precautions it was necessary to
work only with an overcast sky of practically uniform brightness.
Otherwise the relations did not hold. As an aid to carrying out
measurements of this kind the instrument shown in Fig. 21 was
Fig. 21. Instrument for daylight comparisons.
devised and constructed. In this instrument a direct comparison
is obtained between the light falling on the vertical test-plate on one
side and the test-plate turned toward the sky with a sky limiting
device on the other.
PHOTOMETRY OF GAS-FILLED LAMPS
It was found at the Electrical Testing Laboratories that gas-filled
lamps when rotated for the purpose of determining their mean
horizontal candle-power, changed both in current consumption and
in candle-power, and that this change varied with the speed of rota-
tion. With high speed of rotation, centrifugal force causes the
cooler portions of the gas in the interior of the bulb to be thrown off
to the periphery of the bulb, leaving the filament surrounded by
hotter gases than if it were stationary. Hence the temperature of
the filament with the same watts input increases, and with it the
candle-power and efficiency of the lamp. 17 This effect was discovered
about the same time also by Middlekauff and Skogland 18 who found
further that with very low speeds of rotation the candle-power of
these lamps decreased, while it increased with higher speeds, so that
for every lamp a speed can be found at which the candle-power and
11 Sharp Photometry of gas-filled incandescent lamps, Trans. I. E. S., Vol. 9, page 1021,
1914-
11 Photometry of the gas-filled lamp, Bulletin of Bureau of Standards, Vol. 12, page 589.
126 ILLUMINATING ENGINEERING PRACTICE
watts are the same as when stationary. The determination of the
horizontal candle-power of gas-filled lamps is at the present time a
matter of little importance, but it can best be accomplished by
rotating the lamp quite slowly at or near this critical speed and plac-
ing behind it two mirrors 120 degrees apart so that the photometer
disc is illuminated not only by the lamp itself but by its two re-
flected images resulting from beams equally directed about the
periphery of the lamp. The two mirrors are employed to obviate
the violent flicker on the photometer disc which would otherwise
occur.
PHOTOMETRY OF LAMPS WITH SHADES AND REFLECTORS
The measurement of distribution of light about illumination ac-
cessories is carried on by methods which are well known and which
were described in the Johns-Hopkins lectures. A new apparatus
for the purpose designed by Little is shown in Fig. 19. In this
apparatus there are only two mirrors and the record of the photom-
eter settings is made on a paper fastened to a flat board in front of
which the photometer carriage moves. The position of the board is
changed for each change in the angle of the movable mirror. This
apparatus is equally well adapted to the photometry of gas mantle
burners and of incandescent electric lamps.
The determination of the diffusing power and of the absorption
losses in lighting glassware is receiving more of the attention which
it deserves. No standard method for the measurement of diffusion
has yet been decided on, but a very good idea of the diffusing powers
of glassware may be obtained by taking two measurements of the
brightness of the glassware with its normal lamp inside of it; one
with the photometer looking directly at the lamp and the other
looking at a position on the globe. about 45 degrees distant from
the first position. It is desirable that methods of measuring diffu-
sion should be further investigated and finally standardized.
The determination of absorption of globes or reflectors may be
made through a comparison of the total flux of the light from a
lamp without the globe and with it. The total flux may be found
from distribution values worked up by means of the Rousseau
or the Kennelly diagram, or by direct computation. More con-
venient, however, is the use of the integrating sphere. A good
arrangement of the standard lamp and of the accessory in the
sphere for determining the loss of light in the accessory is shown in
Fig. 22. It will be noted that the standard lamp is placed with its
SHARP: MODERN PHOTOMETRY
127
socket turned toward the globe and that the base of the globe is
turned toward the standard lamp so that the sphere losses are mini-
mized. The procedure then is as follows: With the globe removed
from the sphere, the sphere is standardized or the photometer is
adjusted to give a reading corresponding to the total flux of light
from the standard lamp. Then the standard lamp is extinguished
and the globe lamp is lighted. Reading the photometer then shows
this total flux of light. It then is put out and the globe is placed
over it, the standard lamp is
again lighted, and a reading is
taken. This reading should be
equal to the first one, except-
ing for the reflected flux in the
sphere which is intercepted by
the globe. Finally the stand-
ard lamp is extinguished and
the globe lamp is lighted.
This reading gives by compar-
ison with the previous one the
total flux of light issuing from
the globe. This total flux of
light compared with the total
flux of light of the lamp without
the globe, reading No. 2, shows the absorption by the globe. It is
very necessary to notice all the precautions which must be taken
in this class of work as experimenters have been led into error by
neglect of some of them.
22. Arrangment for measuring globe
absorption in integrating sphere.
REFLECTION AND TRANSMISSION MEASUREMENTS
Measurement of the transmission of light through a transparent
medium such as a sheet of glass is most simply made by means of a
bar photometer or portable photometer, measuring the candle-power
of a lamp first without and then with the glass interposed in the beam.
Similarly the reflecting power of a mirror can most readily be
measured. When it comes to the measurement of diffusing media,
either transmitting or reflecting, the measurement is more difficult.
Inasmuch as diffusing media not only diminish the light but also
change it from unidirectional into multidirectional light, some in-
tegrating device is in this case required. In this class of measure-
ments the integrating sphere may very suitably be used.
128
ILLUMINATING ENGINEERING PRACTICE
In Fig. 20 is shown a view of a 1 2-inch sphere set up to measure
the transmission of a diffusing glass. There is an opening of definite
diameter in the top of the sphere, limited by a circular metal dia-
phragm, and the light from the lamp outside the sphere shines
through this into the interior. Photometric measurement gives
the value of this luminous flux. Then the diffusing glass is placed
directly beneath the limiting diaphragm and another measurement
is made which gives the amount of flux traversing the glass. In the
case of diffuse reflectors the procedure is somewhat different. Fig.
23 shows the arrangement. The diffuse reflector which, as will be
Fig. 23. Measurement of coefficient of Fig. 24. Nutting's apparatus for measuring
diffuse reflection, using an integrating coefficients of reflection,
sphere.
seen, is placed at the center of the sphere with its reflecting side
turned at an angle of 45 degrees to the light and away from the
photometer. Thus no screen is needed in the sphere. The amount
of flux admitted by the diaphragm being known, and the amount
reflected from the diffusing surface being measured, the reflection
coefficient at this angle of incidence can be computed. One method
by which the amount of light admitted to the sphere can be checked
up under similar conditions to those of the measurement of the
reflected flux, is to place a mirror of known coefficient of reflection
in the position occupied by the diffuse reflector. The amount of
flux then measured divided by the known coefficient of reflection
of the mirror, gives the amount of flux incident upon the diffuse
reflector.
SHARP: MODERN PHOTOMETRY 129
A singularly ingenious and elegant piece of apparatus for the
measurement of coefficients of diffuse reflection has been devised
by Nutting. 20 Thisinstrument is shown in plan in Fig. 24. The
ring in the figure is covered on the upper surface by a dense milk
glass. On the under surface it is covered by the diffuse reflector
which is to be tested. A special photometric device is supplied
whereby the brightness of the under surface of the milk glass may be
compared with the brightness of the diffuse reflector. If a diffuse
reflector had 100 per cent, reflecting power, its brightness would be
the same as that of the milk glass. Any deficiency is due to its
absorption. The photometer, which is a Martens-Konig polariza-
tion apparatus, gives a comparison, between the two directly, and
hence shows the reflecting power of the unknown surface. Direct
and reverse readings must be taken in order to eliminate polarization
errors.
MEASUREMENTS OF PROJECTION APPARATUS
The elementary theory of a projector having a convex lens or
a parabolic mirror and a nearly point source shows that when the
source is placed at the principal focus of the mirror, the light rays
leave the surface of the mirror with an angle of divergence which is
equal to the angle subtended at that particular point of the mirror
by the source of light. Therefore the illumination obtained from an*
apparatus of this kind diminishes with the distance, and if the
distance is great enough, the exponent of the distance with which it
diminishes is two. In other words, the illumination from the entire
apparatus follows the inverse square law. With a small projecting
apparatus accurately focused for "parallel" beam, it is not neces-
sary to take any very great distance away in order to have the in-
verse square law apply. A goodly distance is, however, in all cases
advisable, and in some cases imperative. For example, in the case
of head-lamps which are focused so as to throw an imperfect image
of the source of light a distance of 200 or 300 feet ahead, it is evident
that the inverse square law could not be assumed without taking a
distance considerably greater than 200 or 300 feet. Sometimes the
focusing distance is shorter than this. In any case, in the photo-
metric investigation of an apparatus which is to be used approxi-
mately at a certain distance, it is advisable to focus it for that dis-
tance and to make the measurement at that distance. These
measurements can then be expressed as apparent candle-power of the
20 Nutting, Trans. I. E. S., Vol. 8, page 412, 1912.
9
130 ILLUMINATING ENGINEERING PRACTICE
apparatus at that distance, and in so doing the inverse square law
is not assumed. It is evident that measurements of this character
must perforce be made at night and that the portable photome-
ter is practically the only apparatus that can be used for the pur-
pose. It is advantageous to set the projector on a stand which can
be rotated about a vertical axis and which has a divided scale whereby
the angle at which the measurement is taken may be read off. By
taking a series of measurements covering a few degrees on either side
of the axis of the beam, data may be gathered whereby a candle-
power distribution curve may be plotted. In view of the narrow-
ness of such a beam a plot in polar coordinates is as a general thing
of little use. It is good practice" to plot these measurements in rec-
tangular coordinates, putting angles in the axis of X and apparent
candle-power in the axis of F. If this distribution curve is carried
far enough, it may be integrated according to the Rousseau method
and the total flux of light emitted by the apparatus thereby de-
termined. This, compared with the total flux of the lamp alone,
gives the loss of light in the apparatus. A plot so made enables
the exact position of the maximum candle-power, which should be the
beam candle-power, to be determined. In the case of lamps for
flood lighting the efficiency of the apparatus is of great importance
and hence a minimum loss of light in it should be striven for, more
perhaps than is the case with projectors. The value of the lost
light may in this case most readily be determined by the use of the
integrating sphere.
RECENT DEVELOPMENTS IN ELECTRIC LAMPS
BY G. H. STICKNEY
INTRODUCTION
Lectures by Drs. Steinmetz, 1 Hyde 2 and Whitney, 3 in the 1910
Course, treated of the physical and chemical principles of light
production, and described the electric illuminants from the scientific
standpoint.
On this foundation it is the purpose of this lecture to trace the
more important of the recent developments and describe briefly the
principal lamps now in common use.
From the great mass of available data, an attempt is made to
present such information as will be of most practical value in select-
ing and applying electric lamps.
Since arc lamps are usually furnished as complete units they are
so treated. Incandescent lamps, however, are equipped with a
great variety of reflectors and other accessories, which are furnished
separately. It has, therefore, been found most expedient to pro-
vide a separate lecture on such accessories 4 and give but slight refer-
ence to them in this lecture.
LAMP DEVELOPMENT
The basis of all artificial lighting is the means for converting
electrical or other energy into light. Advances in the lighting art
have followed in the wake of the improved, practical light source,
and it is here that the greatest possibility for future advance lies.
The most efficient illuminants are still very far below the ideals of
efficiency, while many of them offer much opportunity for improve-
ments, as regards reliability, convenience and maintenance.
Few, if any of the recent improvements in light producers have
come by chance. They have rather been the result of arduous and
expensive research by trained physicists, chemists and engineers in
well organized laboratories. Even when an improved principle of
light production has been discovered, practical devices have had to
be designed, machinery for manufacturing economically and in
quantity developed, sizes and other characteristics determined
upon, in order that the improvement could be utilized to advantage.
132 ILLUMINATING ENGINEERING PRACTICE
While all these items cannot be perfected in advance of the prac-
tical application of the appliance, it is remarkable how few changes
are necessary. It is a tribute to Thomas A. Edison that so many of
his standards still hold.
PROGRESS SINCE 1910
In general, the progress since 1910 may be summed up in (a)
improved efficiency, (b) reliability and safety, (c) economy of main-
tenance, (d) adaptability, (e) simplicity and convenience.
Accompanying these improvements there has been a corre-
sponding increase in intrinsic brilliancy of light sources, which, while
advantageous for certain applications, has in general been undesir-
able. Fortunately, however, diffusing devices can be readily ap-
plied, giving an over-all result much in favor of the improved
illuminants.
TENDENCY AS TO TYPES
Among the incandescent units the tungsten filament or "Mazda"
lamp has assumed predominence. The tantalum and Nernst
lamps have practically disappeared from manufacture, while the use
of metallized-carbon filament or "Gem" and carbon lamps has
decreased very rapidly in the last four years.
The actual percentages reported by the National Electric Light
Association 5 show that approximately 80 per cent, of all incandes-
cent lamps sold during 1915 in this country were of the tungsten
filament type. Incandescent lamps, as a whole, have increased in
importance, encroaching on fields of lighting formerly assigned to
other illuminants.
While many enclosed carbon arc lamps are still in use, especially
in street lighting, their manufacture has dwindled to a very small
number, giving way to more efficient illuminants. The flaming
arc has been changed from an open to an enclosed lamp, and has
been applied to street, industrial and photographic lighting whereas
formerly its principal application was spectacular lighting.
The "luminous," "magnetite" or "metallic flame" arc lamp has
become one of the leading street illuminants, especially since the
ornamental types became available, while the multiple lamp used
in industrial lighting is not now exploited.
CLASSIFICATION
Steinmetz 1 classified electric illuminants as (a) solid conductor,
(b) gaseous conductor, (c) arc conductor, and (d) vacuum arc. For
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 133
the present lecture, a similar classification, with the more common
names, is used, namely, (a) incandescent lamp (Mazda, etc.), (b)
Moore lamp and X-Ray tube, (c) arc lamp (luminous and flame),
(d) mercury vapor lamp (Cooper Hewitt).
INCANDESCENT LAMPS
Of the incandescent lamps, the only types meriting our considera-
tion are the tungsten-filament lamps, designated by the principal
American manufacturers as " Mazda."
The principal distinct developments since 1910 are (a) drawn
tungsten filaments, (b) coiled filaments, (c) concentrated filaments,
(d) chemical "getters," (e) gas-filled construction.
In addition to these, however, there have been innumerable
minor improvements, which have resulted in large aggregate gains
in efficiency and have tended toward uniformity, increased strength 8
and reduced cost. 9
For example, it is very largely due to the minor improvements
that the 60- watt lamp of to-day costs one-third less than in 1910
and gives 20 per cent, more light.
Drawn Wire Filaments. Drawn wire filaments superseded the
former pressed filaments in about 1911. The ductile form of tung-
sten 6 was finally produced in the Research Laboratory at Schenec-
tady, 7 after extended experiments and many discouraging failures.
It revolutionized lamp manufacture, simplifying the processes
very considerably and reducing the cost. Further, it became pos-
sible to draw filaments exactly to size, which in turn, increased the
practical efficiency by eliminating weak points, and also made it
possible for the first time in lamp manufacture to produce all lamps
of a lot for the predetermined voltage. The economic influence of
this last factor is being felt to-day in the demand for standardization
of circuit voltages.
With the development of the drawn wire processes, the lamps be-
came much more rugged, so that to-day they are widely used in steam
and trolley cars, automobiles, on moving machinery and in other
relatively rough service.
The drawn wire could also be made more slender, so that the 10-
watt and even the 7.5-watt, no-volt lamps became practicable.
Coiled Filaments. Another result of the use of ductile tungsten
was the possibility of winding the wire around a mandrel, thereby
producing the helically coiled filament (See Fig, i).
The first application of this was in the so-called " focus" type
134
ILLUMINATING ENGINEERING PRACTICE
lamp, in which the filament was concentrated into a small space,
more or less approximating the point source. The automobile and
locomotive headlight lamps and a much more effective stereopticon
lamp became practicable. 10
The advantage of the concentrated light source in connection with
lenses and reflectors is illustrated in Table I, which shows the maxi-
mum beam candle-power obtained with a i6-in. parabolic reflector
(G. E. Floodlighting projector Form L-i) with lamps of approxi-
mately 100 watts, but with widely varying filament dimensions. In
these tests the lamps were focused so as to give maximum beam
candle-power and operated at 100 mean spherical candle-power.
TABLE I. BEAM CANDLE-POWERS
Light source
Mazda lamp used
dimensions
Beam
m.m.
candle-
Volt
Watt
Bulb
Type
Dia.
Length
6
108
G- 3 o
C headlight
2.0
6-5
462,000
32
100
0-30
C headlight
5-0
5-o
223,000
no
IOO
G-2 5
C stereopticon
6-5
6-5
142,000
no
100
0-30
B stereopticon
8.0
8.0
32,600
no
IOO
PS-25
C regular
25
o.S
12,700
no
IOO
G-35
B regular
30
68
3,800
The most important effect of the coiled filament, however, is in
connection with the gas-filled construction.
Chemical "Getters" This refers to the introduction of various
chemicals, sometimes called "getters," within the lamp. Some of
these chemicals act while the lamp is being exhausted, while others
continue to act throughout the life of the lamp. Some of the impor-
tant effects of these chemicals are:
1. Regeneration, that is, redepositing evaporated material on the filament.
2. Combination with material depositing upon the bulb to form more trans-
parent compounds.
These combined actions permit increased efficiency, reduce bulb
blackening, 11 and help maintain the candle-power of the lamp
throughout its rated burning life.
GAS-FILLED LAMPS
With the elimination of several weak points, it had been possible
to raise filament temperatures of vacuum lamps, and hence efficien-
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 135
cies, to a point corresponding to about one watt per candle-power,
beyond which filament evaporation seemed to preclude much further
advance.
The announcement in 1913, of lamps consuming approximately
one-half watt per candle-power was, therefore, rather astounding to
the lighting world. This came as the result of a remarkable re-
search 12 in the same laboratory that produced ductile tungsten.
The new principle which involved the gas-filled construction, de-
pended upon the fact that when operated under a moderate gas pres-
sure, the tungsten filament could be maintained at a higher tempera-
ture without excessive evaporation.
The introduction of gas within the bulb, however, incurred a new
loss; namely, convection, that is, heat carried off by gas currents.
Such losses are relatively less on filaments of large diameter, so that
high current lamps are more efficient than low. By using the heli-
cally coiled filament, as already referred to, and thereby simulating
large diameter, it was possible to apply the principle to practical
lamps (see Fig. i). Later, by selection of gas of low heat conduc-
tion, it became practicable to extend it to lower currents; for ex-
ample, zoo-watt and 75-watt i lo-volt multiple lamps. The gas-filled
construction is most advantageous with high current series lamps
and high wattage multiple lamps. On the lower wattage multiple
lamps, it as yet gives lower efficiency than the vacuum type of con-
struction, and hence is not employed. In the larger sizes, however,
the gas-filled lamps, which are designated by the leading American
manufacturers as "Mazda C," are extensively used, their high
candle-power and efficiency being responsible for extending the
application into fields not formerly occupied by incandescent lamps.
Owing to the higher filament temperature the light is perceptibly
whiter 13 and more actinic, than that of the vacuum type " Mazda B "
lamps. Lamps having ratings of up to and including 1000 watts
(18,000 lumens) are in regular production. Larger lamps have been
made and could readily be provided if there was sufficient commercial
demand.
Since all the series lamps in common use are of relatively high cur-
rent, the gas filled lamps are especially advantageous, and have
superseded the vacuum lamps all along the line.
A still further gain is secured for the higher power series lamps by
providing 15- and 20-amp. lamps, to be operated irom the usual
alternating current series circuits; namely, 6.6 and 7.5 amp., by
means of individual auto-transformers or series transformers.
136 ILLUMINATING ENGINEERING PRACTICE
The concentrated arrangement of filament permits of a more
effective control of the candle-power distribution with refracting
globes and small diameter reflectors.
Candle-power Distribution. Formerly all clear incandescent lamps
had practically the same distribution of candle-power, so that the
mean horizontal candle-power bore a practically fixed relation to
the mean spherical candle-power, and to the total light output.
With the recent development, several forms of filaments, having vari-
ous candle-power distributions (see Fig. 2) are used in the different
lamps. Therefore, the mean horizontal candle-power is no longer a
representative measure of light output.
Position of Operation. As in the past, the smaller lamps can be
operated in any position. It has been found advantageous, however,
to construct some of the larger lamps (for example, multiple lamps
of 200 or more watts) without bottom anchors on the filaments.
Such lamps may not operate satisfactorily in other than an approxi-
mately pendant position.
It is seldom desirable to operate these larger lamps in horizontal,
tip-up, or inclined positions, but where such is the case, special
lamps can be obtained, if the position of operation is specified.
Some of the high-power focus type lamps, on the other hand,
should not be operated within 45 of the pendent position. Such
lamps are usually operated tip up or horizontally. In order to
economize space in housings, these lamps are made short so that
if used pendant it is not practicable to protect the stems from
heated gases rising from the filament.
Accessories for focus type lamps should therefore be planned for
proper lamp position according to information given by the lamp
manufacturers.
Circuits. It is highly desirable to operate incandescent lamps at
rated voltage or current. While low voltage does no harm, beyond
lowering the light output and efficiency, and also changing the color
of the light, continued low voltage is often a source of complaint
from light users. Over-voltage shortens the life of the lamps and
if excessive may destroy the filament.
While lamps have sufficient leeway to permit operation at a reason-
able over-voltage and so operated are usually more economical, the
practice of running lamps at labeled voltage is generally preferred
and is recommended by the manufacturers.
Incandescent lamps operate interchangeably and equally well
on alternating-current and direct-current circuits. The only ex-
Fig. i. Helically coiled filament of tungsten wire. (Magnified to show turns.) Illus-
tration also shows concentrated arrangement of filament for a focus type lamp. Note the
cooling effect of supporting anchors on the heated filament.
Candlepower Distribution in Vertical Plane, Multiple
Mazda Lamps, with Different Forms of Filaments
Clear Bulbs, no Reflectors, 1000 Lumens.
S.R.F. = Spherical Reduction Factor = ii.Horiz.C.P.
Fig. 2. Curves of candle-power distribution in vertical plane, multiple Mazda lamps, with
different forms of filament.
(Facing page 136.)
<
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 137
captions to this are the non-vacuum series lamps which can be
operated to best advantage on alternating-current circuits.
Although the lamps give satisfactory life on series direct current,
on the failure of the lamp, there is sometimes maintained a lower
voltage arc, which may burn the socket contacts before the protec-
tive film acts.
On account of the low heat capacity of slender filaments, no volts
of 25 watts or less (220 volt lamps of 60 watts or less) show percep-
tible flicker on 25-cycle circuits, 15 which may be objectionable.
Lamps made for higher amperage avoid this effect. In general no-
volt lamps are a little more efficient and of lower cost than 220-
105-125 Volts
(Lamps 1 4 Scale)
Fig. 4. Regular (Mazda B) vacuum lamps for no-volt circuits.
volt lamps. While the 2 20- volt lamps are made for the same oper-
ating life, no-volt circuits should usually be preferred.
Styles and Types. Where possible lamps as listed by the manu-
facturer should be used. Special lamps should be avoided. Higher
costs, slower deliveries and poorer quality -may be expected on special
lamps. The present lists include lamps to cover practically all
needs.
Data on some of the principal types of Mazda lamps are given in
Table II. These data are subject to some change as improvements
become available.
The variety of incandescent lamps is so great that it is imprac-
ticable to give full lists. It is worth while to call attention to some
of the more special types which come into common use, but are not
so well known as the regular types.
138
ILLUMINATING ENGINEERING PRACTICE
TABLE II. ENGINEERING DATA ON MAZDA LAMPS, JULY, 1916
.3
1
Input Watts
per spherical
c.p.
1
sis
f
i
9
a
3
3
'o
h
Reduction facJ
tor
Bulb
ill
III
Base
Standard
package
quantity
Position of
burning
g
**j
bo y
C C
s-=
&
IK
E-
.j
S'^
105-125 VOLT "B" STRAIGHT SIDE BULBS (Fig. 4)
IO
.67
7-SO
75
0.78
S-I7
2H
4%
Med. screw
IOO
Any
IS
47
8.55
128
0.78
S-i 7
2J,
Med. screw
IOO
Any
20
25
41
35
8.90
9-30
178
234
0.78
0.78
S-I7
S-IQ
8
4 5 /i
Med. screw
Med. screw
IOO
IOO
Any
Any
40
32
9-50
380
0.78
S-I9
2%
In
Med. screw
IOO
Any
SO
31
9.60
480
0.78
8-19
2%
sK
Med. screw
IOO
Any
60
.28
9.80
590
0.78
S-2I
2%
Med. screw
IOO
Any
IOO
.22
10.3
1030
0.78
S-30
77/6
Med. sc. sk.
24
Any
105-125 VOLT "C" PEAR-SHAPE BULBS (Fig. 3)
75
1 .09
ii. 5
865
PS-22
2%
6H
Med. screw
50
Any
49
IOO
1. 00
12.6
1260
PS-2S
3H
7H
Med. screw
24
Any
5M6
200
0.90
14.0
2800
PS-30
3 3 A
m
Med. sc. sk.
24
Tip down
6
300
0.82
15.3
4600
PS-35
4%
9H
Mog. screw
24
Tip down
7
400
0.82
15.3
6150
PS-40
5
10
Mog. screw
12
Tip down
7
500
0.78
16.!
8050
PS-40
5
10
Mog. screw
12
Tip down
7
750
IOOO
0.74
0.70
17.0
18.0
12800
18000
....
PS-52
PS-52
6H
6^
I3H
13K
Mog. screw
Mog. screw
8
8
Tip down
Tip down
>H
>H
220-250 VOLT "B STRAIGHT SIDE BULBS
25
.65
7.60
191
0.79
S-I9
2^
5
Med. screw
IOO
Any
40
.42
8.85
354
0.79
S-I9
2%
$N
Med. screw
IOO
Any
60
-39
9 05
540
0.79
S-2I
2%
w
Med. screw
IOO
Any
IOO
.2?
9-90
990
0.79
S-30
3*
7%
Med. sc. sk.
24
Any
150
.27
9-90
1480
0.79
S-35
4%
m
Med. sc. sk.
24
Any
250
.20
10. S
2620
0.79
8-40
5
10
Med. sc. sk.
12
Any
220-250 VOLT "C" PEAR-SHAPE BULBS
200
I .00
12.6
2520
PS-30
3%
8%
Med. sc. sk.
24
Tip down
6
300
o .92
13.7
4100
PS-35
4%
9 ; H
Mog. screw
24
Tip down
7
400
0.90
14.0
5600
PS-40
5
10
Mog. screw
12
Tip down
7
500
0.85
14.8
7400
PS-40
5
10
Mog. screw
12
Tip down
7
750
0.82
15.3
11500
PS-52
6^
139*
Mog. screw
8
Tip down
rii
IOOO
0.78
16.1
16100
PS-52
6}^
13%
Mog. screw
8
Tip down
SH
105-125 VOLT "B" ROUND BULBS (Fig. 5)
15
.53
8.20
123
0.80
G-i8^
aMe
3%
Med. screw
IOO
Any
15
43
8.80
132
0.80
G-25
3tf
4 S A
Med. screw
50
Any
25
.41
8.90
222
0.80
G-i8^
2 Me
3H
Med. screw
IOO
Any
25
31
9.60
240
0.80
G-25
3Vi
4%
Med. screw
50
Any
40
30
9.65
386
0.80
G-25
3tf
4%
Med. screw
50
Any
60
.20
10. s
630
0.80
G-30
3%
SW
Med. screw
24
Any
IOO
14
II.
I IOO
0.80
G-35
m
7N
Med. sc. sk.
24
Any
220-250 VOLT "B" ROUND BULBS
25
40
1.50
1.41
8.40
8.90
2IO
356
O.80
0.80
G-25
G-25
3H
3tt
4H
4X
Med. screw
Med. screw
50
50
Any
Any
105-125 VOLT "B" TUBULAR BULBS (Fig. 5)
25
1. 35
9-30
232
0.78
T-io
iH
&t
Med. screw
IOO
Any
25
1.44
8.75
218
T-8
i
12
Med. screw
50
Any
40
1.39
9-OS
362
T-8
i
12
Med. screw
50
Any
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 139
TABLE II. ENGINEERING DATA ON MAZDA LAMPS, JULY, 1916. (Continued)
SIGN, STEREOPTICON AND FLOODLIGHTING LAMPS
Watts per
spherical c.p.
Lumens per
watt
Total lumens
Reduction fac-
tor
Bulb
Max. over-
all length
(inches)
Base
Standard
package
quantity
Position of
burning
Light center
length
(inches)
|
>,
H
ll
VOLT "B" SIGN STRAIGHT SIDE BULBS
5
1. 52
1.46
8.25
8.60
20.6
43-0
0.79
0.79
8-14
8-14
$;
4
4
Med. screw 1 100
Med. screw]) 100
Any
Any
50-65 VOLT "B" SIGN STRAIGHT SIDE BULBS
5
1-73
7.25 |36.2
o.?8| 8-14
*H
4
Med. screw
IOO
Any
105-125 VOLT "B" SIGN STRAIGHT SIDE BULBS
10
1.92
1.73
6.55
7-25
49.0
72.5
0.78
0.78
S-I 4
8-14
m
4
4
Med. screw
Med. screw
IOO
IOO
Any
Any
105-125 VOLT "C" STEREOPTICON ROUND BULBS
IOO
250
500
1. 00
0.80
0.67
12.6
15-7
18.8
1260
3950
9400
. . . .
G-2S
G-30
0-40
3%
5
7H
Med. screw
Med. screw
Mog. screw
50
24
12
*
*
4M
105-125 VOLT FLOOD LIGHTING "C"
200
400
0.95
0.85
13.2
14.8
2640
5920
....
G-3O
G-40
3%
5
Stt
Med. screw
Mog. screw
24
12
3H
Can be operated in any position except within 45 degrees of vertical, base up.
MAZDA STREET LIGHTING LAMPS
Nominal rated
c.p.
Total lumens
Average volts
Average watts
Input watts
per spherical
c.p.
Output lumens
per watt
Bulb
Max. over-
all length
(inches)
Base
Standard
package
quantity
Position of
burning
Light center
length
(inches) 1
1
B!
si
5.5-AMP. "C" STREET SERIES STRAIGHT SIDE AND PEAR-SHAPE BULBS
(Fig. 7)
60
80
600
800
8.5
10.8
46.8
59-5
0.98
0.93
12.8
13.5
38
$;
7V4
Mog screw
Mog. screw
50
50
Any
Any
SH
sH
IOO
1000
13.0
71-5
0.90
14.0
S 24^
3 Vifl
7^4
Mog. screw
50
Any
s**
250
2500
29.7
163.0
0.82
IS. 3
PS-35
4H
934
Mog. screw
24
Tip down
7
400
4000
47-4
260.0
0.82
15-3
PS-40
5 !IO
Mog. screw
12
Tip down
7
6.6-AMP. "C" STREET SERIES STRAIGHT SIDE AND PEAR-SHAPE BULBS
(Fig. 7)
60
80
IOO
250
400
000
600
800
1000
2500
4000
6000
7.1
9-1
10.9
23-5
37.1
55-7
46.8
60.0
72.0
155.0
244.0
368.0
0.99
0-94
0.90
0.78
0.77
0.77
12.7
13-4
14-0
16.1
16.3
16.3
S-24h
S-2 4 h
S-2 4 h
PS-35
PS-40
PS-40
3H
3Me
3Hfl
4H
5
5
7H
7W
7H
9%
10
IO
Mog. screw
Mog. screw
Mog. screw
Mog. screw
Mog. screw
Mog. screw
50
50
50
24
12
12
Any
Any
Any
Tip down
Tip down
Tip down
5H
5H
sH
7
7
7
7.5-AMP. "C" STREET SERIES STRAIGHT SIDE AND PEAR-SHAPE BULBS
(Fig. 7)
60
80
IOO
250
400
000
600
800
1000
2500
4000
6000
6-4
8.0
9-6
19.6
30.5
45-8
48.0
60.0
72.0
147-0
228.0
344-0
I .00
0.94
0.90
0.74
0.72
0.72
12.6
13-4
14.0
17.0
17-5
17.5
S-24H
PS?3S
PS-40
PS-40
i
4V
5
5
10
IO
Mog. screw
Mog. screw
Mog. screw
Mog. screw
Mog. screw
Mog. screw
50
50
50
24
12
12
Any
Any
Any
Tip down
Tip down
Tip down
lit
7
7
7
15-AMP. "C" STREET SERIES PEAR-SHAPE BULBS (Fig. 7)
400 1 4000 j 14.4! 216 | o.68| 18.5] PS-40 | 5 |i2H | Mog. screw! 12 I Tip down |
20-AMP. "C" STREET SERIES PEAR-SHAPE BULBS (Fig. 7)
600
1000
6000! 15 .5
10000 25.9
310
520
0.65
0.65
19.3
19 3
PS-40
PS-40
5
5
I2#
12^
Mog. screw
Mog. screw
12
12
Tip down
Tip down
9V*
9H
140 ILLUMINATING ENGINEERING PRACTICE
TABLE II. ENGINEERING DATA ON MAZDA LAMPS, JULY, 1916. (Continued)
MAZDA TRAIN LIGHTING LAMPS
Input (Output
al lumens
d
o
Is
Bulb
%
s-s
sf 3 -
Base
Standard
package
quantity
Position of
burning
fc
y
1!I
>J~
I's
w'C
-S o .
S **
Type
Diam.
(Inches)
_**
e
o> rt
(***
25-34 VOLT AND 50-65 VOLT -"B" TRAIN LIGHTING ROUND BULBS
IO
44
8.75
8?
0.81
G-i8^
2M
3*
Med. screw
100
Any
is
.38
9.10
137
0.81
G-i8^
2Me
3*
Med. screw
IOO
Any
20
.36
9-25
185
0.81
G-i8^
2Me
3%
Med. screw
100
Any
25
36
9-25
232
0.81
G-i8^
2Mfl
3%
Med. screw
IOO
Any
40
.22
10.3
412
0.82
G-30
M
6V4
Med. sc. sk.
24
Any
*75
.16
10.8
810
0.82
G-30
3%
6H
Med. sc. sk.
24
Any
25-34 VOLT AND 50-65 VOLT TRAIN LIGHTING STRAIGHT SIDE BULBS
1
10
50
8.40
84!0.78
S-I 7
2H
4*6
Med. screw
IOO
Any
15
44
8. 75
131 0.78
S-I7
2H
4H
Med. screw
IOO
Any
20
41
8. 9
I78J0.78
S-I7
2^
\%
Med. screw
IOO
Any
25
41
8.90
222 0.78
8-19
2H
sH
Med. screw
IOO
Any
40
.28
9.80
392 0.78
S-I9
2%
5M
Med. screw
IOO
Any
AND 6 VOLT "C" LOCOMOTIVE HEADLIGHT ROUND BULBS
36
0.85
14.8
*530
G-i&M
2Me
3%
Med. screw
IOO
Any
23f 6
72
0.80
iS-7
*H30
G-25
3H
fo
Med. screw
So
Any
234
108
0.75
16.8
*i8io
G-30
3%
5 7 /i
Mog. screw
24
Any
zVi
30-34 VOLT "C" LOCOMOTIVE HEADLIGHT ROUND BULBS
IOO
I .00
12.6
1260
G-25
3W
4%
Med. screw
SO
Any
2%
ISO
0.90
14.0
2100
G-25
JM
4$*
Med. screw
50
Any
2%
250
0.80
15.7
3920
G-30
3K
SX
Med. screw
24
t
3W
* 6 volt lamp only; 5^ volt lamp, 6^ per cent, less.' 1
t Can be operated in any position except within 45 degrees of vertical, base up.
t 30-34 and 60-65 volts.
MAZDA STREET RAILWAY LAMPS
Input
Output
Bulb
d
*-je
g
"rt
Watts pei
spherica
c.p.
I
JM
ll
Reductio
factor
Type
Diam.
(Inches
SSj
^g
~
S*~
Base
Standard
package
quantity
Position o
burning
III
105, 110, 115, 120, 125 AND 130 VOLT "B" STREET RAILWAY STRAIGHT
SIDE BULBS
t23
t36
t56'
t94
1.42
1.40
1.31
1.24
8.85
9.00
9.60
10. I
*2l8
*354
*S70
*IOOO
0.78
0.78
0.78
0.78
S-I9
S-I9
S-2I
8-24^
2%
2%
2ft
sH
5/4
Med. screw
Med. screw
Med. screw
Med. sc. sk.
IOO
IOO
IOO
50
Any
Any
Any
Any
* 115 volt lamps only, other lamps in proportion to their volts.
t Nominal watts.
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
141
(Lamps U Scale)
G-25 T-10
15, 25 and 40 25 Watts
Watts 105-125 Volts
105-125 Volts
T-8
25 and 40 Watts
105-125 Volts
Fig. 5. Round bulb and tubular (Mazda B) vacuum lamps for no- volt circuits.
(Lamps l /4 Scale)
LJr
G-25
100 Watt
105-125 Volts
Stereopticon
G-30
250 Watt Stereopticon &
200 Watt Flood Lighting
105-125 Volts
G-40
500 Watts Stereopticon &
400 Watt Flood Lighting
105-125 Volts
Fig. 6. Floodlighting and Stereopticon (Mazda C) gas-filled lamps for no-volt circuits*
142
ILLUMINATING ENGINEERING PRACTICE
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 143
Focus Type Lamps. These lamps are especially designed for
use with lenses and parabolic reflectors. They are used with
stereopticons, small moving-picture machines, signals, and for
spotlighting, floodlighting, headlighting, etc. 10 The essential fea-
ture of the lamps is the concentration of the filament to approxi-
mate the "point source."
Miniature Lamps. This term is applied to a wide variety of
small lamps used for many special purposes. Such lamps are usu-
(All Lamps V 2 Scale)
B-9H D-10 T-8 G-W 2 S-12 l / 2
Candelabra Candelabra Candelabra Candelabra Decorative
Style B Style D Style E Style G Style F
Fig. 8. Candelabra (Mazda B) vacuum lamps for multiple circuits.
ally for low voltage and provided with small bases, such as the can-
delabra or bayonet types. Among these lamps are those for auto-
mobile and electric vehicle service. The no-volt candelabra lamps
shown in Fig. 8 are becoming popular for decorative purposes, as,
for example, electric candles. Frosted lamps are generally preferred.
The Christmas tree lamps, which were designed originally to
eliminate the fire risk in Christmas tree lighting, are now being em-
ployed extensively for producing special decorative effects, where the
lamps are used as ornaments rather than to produce any consider-
able illumination. Many special forms of bulbs, such as fruits,
flowers, etc., are made. These lamps, which usually operate eight
in series on 100 volts, are now made with tungsten rather than
carbon filaments.
Important among the battery types of miniature lamps are those
for small " flashlights "; while among the recent developments are
the miner's lamps, specified by the U. S. Bureau of Mines.
Colored Lamps. For color matching, photography, theatrical and
144
ILLUMINATING ENGINEERING PRACTICE
decorative purposes, various colored lamps are obtainable. The
color is introduced either by means of a dip or by the use of colored
glass bulbs. The former is less expensive, but the latter is more
permanent. Some of the lamps are special and not usually obtain-
able on short notice. Bowl-frosted or all-frosted lamps are more
commonly used in the small sizes. All-frosted lamps are not recom-
mended in the high wattage lamps.
Complete Equipment. The incandescent lamp is not generally to
be regarded as a complete lighting unit. For most purposes it is
desirable to provide suitable reflectors, shades or globes, for direct-
ing and diffusing the light in accordance with particular require-
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 145
ments. The most effective illumination is secured when the proper
accessory is selected.
Previous to the advent of the high-power lamps, little attention
was necessary, from the lamp standpoint, in the design of the fixture,
beyond assuring general suitability. Now, however, owing to the
large amount of light and heat emitted in a small space, certain pre-
cautions are necessary to insure proper performance of lamps. 14
This problem is similar to that encountered with other high-power
illuminants. While the majority of fixtures take care of these re-
quirements, there are some fixtures in which suitable provision has
not been made.
High-candle-power filaments are too brilliant to be viewed with
comfort, and fixtures should have provision for shielding the eyes
and diffusing the light, olepending upon the application of the equip-
ment.
Ventilation must be provided to carry off the heat and avoid ex-
cessive temperature at the top of the lamp. Suitable sockets and
leading-in wires should be provided. For high wattage lamps, used
out of doors, it is highly important that the fixtures be weatherproof
so as to exclude moisture; otherwise during rain and snow storms,
water will enter. A drop of water falling on the heated glass, near
the top of the lamp, is liable to produce a crack, which will result in
failure of the lamp.
Fig. 9 shows the candle-power performance of the 5oo-watt gas-
filled lamp with a few of the most commonly used equipments. For
larger or smaller lamps the candle-power can be approximated by
proportioning the values to the respective total lumens of the lamps.
MOORE COLOR-MATCHING LAMP
The principle of producing light by electrical discharge, through
a gas of very low pressure, enclosed in a glass tube, was applied by
D. McFarlan Moore. Both long and short tube lamps were devel-
oped. The color of the light from such a lamp depends upon the
gas used. For example, nitrogen produces a pinkish light, carbon
dioxide a white light, neon 17 a reddish light.
The short carbon dioxide tube is the only type in active commer-
cial manufacture in this country at the present time. While this
lamp is not widely used it is notable because of the superiority of
its light where very accurate color matching is required, as, for ex-
ample, in dying silk, wool, etc. An entirely new form, 16 eliminating
146 ILLUMINATING ENGINEERING PRACTICE
the gas valves and other complicating features, has been developed.
This lamp, which is shown in Fig. 10, consumes about 250 watts and
operates on alternating-current circuits. While the overall efficiency
is relatively low, the light is distributed according to the require-
Fig. 10. Moore color matching lamp.
ments of the accurate color matchers, and intensities up to about
200 foot-candles can be secured over a small area.
X-RAY TUBES
Illuminants of this class do not generally interest illuminating
engineers directly, though they play an important part in surgery
and various physical and chemical researches, in which the peculiar
quality of these radiations reveal what cannot otherwise be observed.
The recent development by Dr. Coolidge, which has been char-
acterized as the most important advance since the original discovery,
has been summed up as follows: 18
"Briefly, the device consists of a tube exhausted of all gases to the ex-
treme possible limit, in which is supported the cathode, so arranged that
it may be heated electrically; an electrically conducting cylinder or ring
connected to the heated cathode, and so located with reference to it as to
focus the cathode rays on the target; and the anti-cathode, or target. The
advantages of the tube are complete and immediate control, of the inten-
sity and the penetrating power of the Rontgen rays, continuous operation
without change in the intensity or character of the rays; absence of fluores-
cence of the glass; and the realization of homogeneous primary Rontgen
rays of any desired penetrating power."
ARC LAMPS
The common forms of arc lamp include the open and enclosed
carbon electrode lamp, the open and enclosed flaming carbon elec-
trode lamp and the luminous, magnetite or metallic arc lamps.
The large variety of arc lamps now in active use is indicated by
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 147
Fig. ii, which shows the types of electrodes regularly furnished by
the National Carbon Company. The engineering data of the prin-
cipal forms of arc lamps ior general lighting service are given in
Table III.
Open and Enclosed Carbon Arc Lamps. For general lighting pur-
poses these lamps are generally considered to be superseded, although
there are a considerable number still in use, especially for street
lighting.
The open arc is the standard illuminant for high power projection
lighting, 10 as, for example, with large stereopticons, moving picture
machines, and for search lighting and spotlighting. On direct cur-
rent the brilliant homogeneous crater of the positive is the most
effective approximation of the "point source." A heavily impreg-
nated flame carbon electrode is used in the most powerful search-
lighting equipments.
Recent developments have done much to increase the effective-
ness of the open arc, especially for searchlight work, by surrounding
the crater with a cooling atmosphere. 19
Introduction of chemicals has steadied the arc and the use of
small diameter copper or duplex coated negative electrodes has
served to reduce electrode shadows.
Flaming Arc Lamps. The flaming arc lamp has the lowest specific
consumption of all the common illuminants. It differs from the
ordinary carbon arc in that the addition of certain metallic salts
changes the process of light production, the light emanating from
the arc steam rather than from the craters. The composition of the
electrodes determines the color of the light and to a considerable
extent the efficiency. Both yellow and white light electrodes are
in common use. Red, blue and green electrodes are used for special
medical purposes.
Both the open and enclosed (white) flame arcs are used extensively
in photo-engraving and other photographic purposes, including
moving picture studios, as well as for fading tests of dyes and paints.
Some commercial forms for photo-engraving and similar purposes
are illustrated in Fig. 12.
The inclined electrode type of lamp, formerly used for spectacular
lighting, has in general given way to the enclosed lamp, while the
field has extended to street and industrial lighting. White electrodes,
are usually employed on street and photographic lighting, and yellow
electrodes for industrial lighting. 25
While enclosed flame arc lamps had been produced in 1910, they
148
ILLUMINATING ENGINEERING PRACTICE
did not come into common use in this country until 191 1. 20 The
early lamps gave an unsteady light, and the solid residue from the
electrodes formed an absorbing coating on the enclosing globe.
Many improvements have been made in the past few years. Im-
proved condensing chambers have minimized the accumulation on
the globes. 21 Probably the greatest recent improvement has been
A = Clear Inner, Alba Outer Globe > White Flame
= Clear Inner, Clear Outer Globe ) Carbons
- Clear Inner, Alba Outer Globe ) Yellow Flame
= Clear Inner, Clear Outer Globe ) Carbons
Fig. 13. Direct current multiple 6.5 ampere no volt, enclosed flame arc lamps.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
with regard to the composition of electrodes, 23 tending to steady
the arc and increase the efficiency. The effectiveness of the photo-
graphic arcs has been especially increased.
An ornamental type of enclosed flame lamp for street lighting
has been developed and is exploited by a leading manufacturer. 24
The principal types of enclosed flame arc lamps now on the market
for general illumination, are listed below. Their photometric
curves are shown in the Figs. 13 to 17 as indicated:
Luminous, Magnetite, or Metallic Flame Arc Lamp. While no
radical changes have been made in these lamps since 1910, the effi-
ciency, steadiness and light control have been much improved. 27 An
ornamental form has been developed which is receiving quite ex-
II
5 jj
*o g
Is
f
Fig. 12. Typical open and enclosed flame arc lamps floor types, for photo-engraving and
other photographic purposes.
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
149
A- Clear Inner, Alba Outer Globe \ White Flame
- Clear Inner, Clear Outer Globe f Carbons
C- Clear Inner, Alba Outer Globe ) Yellow Flame
Clear Inner, Clear Outer Globe ) Carbons
14. Alternating current multiple 7.5 ampere, no volt enclosed flame arc lamp.
(Internal auto-transformer gives 10.5 amperes at arc).
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
A - Clear Inner. Alba Outer Globe I
= Clear Inner. Clear Outer Globe )
C- Clear Inner, Alba Outer Globe \
= Clear Inner, Clear Outer Globe f
White Flame
Carbons
Yellow Flame
Carbons
Pig. 15. Alternating current series 6.6 (or 7.5) ampere enclosed flame arc lamp. (Internal
auto-transformer gives 10 amperes at arc.)
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
ILLUMINATING ENGINEERING PRACTICE
10 Amp. A. C. Series Enclosed Flame Carbon
Arc Lamp with Clear Inner, Clear
Outer Globe and White Flame Carbons
Fig. 1 6. Alternating current series enclosed flame arc lamp (9.5 amperes at arc).
(Data furnished by Westinghouse Elec. & Mfg. Co.)
10 Amp., A. C. Series Enclosed Flame
Arc Lamp with. Clear Inner
Alba Outer Globe and White
Tlame Carbons
Fig. 17. Alternating current series ornamental^enclosed flame arc lamp.
(Data furnished by Westinghouse Elec. & Mfg. Co.)
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
tensive use. 26 Changes in the composition and form of electrodes
have been responsible for the increased efficiency and steadiness.
This type of lamp can be operated only on direct-current circuits.
The copper or positive electrode consumes slowly by erosion; the
negative or magnetite electrode furnishes the arc stream material.
Pendent Type
Ornamental Type
I 4 Amp. D. C. Series Luminous Arc Lamp
/ with High Efficiency Electrode
A Ornamental Type Equipped with
Light Alba Globe
B Pendent Type Equipped with
Carrara Globe and Internal
Concentric Reflector
C Pendent Type Equipped with
Clear Globe and Internal Concentric
Reflector
D - Pendent Type Equipped with
Clear Globe and Prismatic Glass
Reflector
Fig. 1 8. Candle-power distribution obtained with different equipments, 4 ampere luminous
arc lamp.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
In the lamp as made by the General Electric Company, a large
massive positive electrode (which is not replaced at each trimming)
is above the arc. On 4 amperes its operating life is from 6000 to
8000 hours; and on 6.6 amperes from 2000 to 4000 hours.
The magnetite electrodes are made in two types, designated as
" long-life" and "high-efficiency." The operating life of the former
152
ILLUMINATING ENGINEERING PRACTICE
is nearly double that of the latter, both varying inversely with the
amperage. The high-efficiency type is usually used on 4-ampere
lamps, giving about 175 hours; while the long-life type is used on the
6.6 ampere lamp, giving 100 hours or over.
The lamp made by the Westinghouse Electric & Manufacturing
Company employs what is known as the "down-draft" principle: 29
A small inexpensive positive electrode is located below the arc and is
renewed at each trimming.
While lamps for multiple operation have been made and used for
4 Amp. D. C. Series Metallic Flame Arc Lamp
with. Clear Globe
Fig. 19. Four ampere metallic flame arc lamp.
(Data furnished by Westinghouse Electric & Mfg. Co.)
industrial lighting, the large amount of ballast resistance necessary
to insure steady operation, makes them relatively inefficient and they
are no longer exploited. The series lamp, on the other hand, is quite
economical, having a low maintenance cost. The light approxi-
mates daylight in color and the operation is quite reliable. The
series direct current is usually secured from combination constant-
current transformers and mercury arc rectifiers, which in turn are
supplied with power from alternating-current multiple circuits.
One of the most interesting developments in connection with the
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
153
magnetite lamp is the variety of reflectors and of diffusing and re-
fracting globes by which the light distribution is modified to meet
the various requirements of street lighting.
A " 4 Amp., Long Life Electrode
B - 4 Amp., Higrh Efficiency Electrode
C 6 Amp., Long Life Electrode
D 5 Amp., High Efficiency Electrode
E 6.6 Amp., Long Life Electrode
Fig. 20. Luminous arc lamp, clear globe, concentric reflector.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
A = 4 Amp., Long Life Electrode
5-4 Amp., High Efficiency Electrode
C 5 Amp., Long Life Electrode
D 5 Amp., High Efficiency Electrode
JE7 6.6 Amp., Long Life Electrode
Fig. 21. Luminous arc lamp, clear globe, refractor.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
The photometric characteristics of the 4-amp. luminous lamps,
with the principal types of equipments are shown in Fig. 18. Those
154
ILLUMINATING ENGINEERING PRACTICE
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STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
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ILLUMINATING ENGINEERING PRACTICE
A-* 4 Amp., Long Life Electrode
B 4 Amp., High Efficiency Electrode
C 5 Amp., Long Life Electrode
D 5 Amp., High Efficiency Electrode ( Calculated )
E =- 6.6 Amp., Long Life Electrode
Fig. 22. Luminous arc lamp, opal globe.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
A =4 Amp., Light Density Alba Globe and
Long Life Electrode
.B = 4 Amp., Light Alba Globe and High
Efficiency Electrode
C 5 Amp., Light Alba Globe and Long
Life Electrode
D 5 Amp., Medium Density Diffusing Globe
and High Efficiency Electrode
E - 6.6 Amp., Medium Density Diffusing Globe
and Long Life Electrode
Fig. 23. Ornamental luminous arc lamp, opal globe.
(Data furnished by Ilium. Eng. Laboratory, G. E. Co., Schenectady, N. Y.)
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 157
of the 5 and 6.6-amp. lamps correspond approximately in form. The
actual candle-power performance of the various standard-equipments
is shown in Figs. 19, 20, 21, 22, and 23. These give average initial
values taken from several tests on separate lamps and electrodes.
For general data see Table III.
The ornamental type of lamp represents one of the important
developments, which is receiving wide use in "white way" lighting. 28
It is an inverted lamp only in the sense that the regulating mechan-
ism is located below the arc, so as to be concealed in the pole. Sev-
eral special types of globes have been furnished to conform with par-
ticular artistic requirements. Such equipments have different
candle-power characteristics due to variations in shape, light ab-
sorption and diffusion.
MERCURY VAPOR LAMPS
Two principal types of mercury vapor lamps are made in this
country; namely, low (vapor) pressure glass tube lamps and high
pressure, quartz tube lamps.
Glass Tube Lamps. There has been relatively little change in
this type of lamp since 1910. Some improvements have been intro-
duced in the alternating-current lamp, making it a little more efficient
and reliable in operation.
A fluorescent reflector 30 has been developed with a view to color cor-
rection, supplying some of the missing red rays. While considerable
color modification is obtained by this means, it is at some sacrifice in
efficiency, and the fluorescent reflector is not used to any consider-
able extent.
In order to provide a more convenient arrangement for photo-
graphic lighting, where a large flood of light is necessary, as in a mov-
ing picture studio, special supporting frames have been devised for
banking tubes from high power units. These are arranged to pro-
ject the light in one general direction (Fig. 26).
The usual line of lamps for industrial lighting, 31 is illustrated in
Fig. 24, which gives candle-power distribution curves. The curves
show the initial candle-power. Fig. 25 shows the variety of standard
tubes. The operating life of tubes is stated by the manufacturer as
4000 hours. Published data indicates that the candle-power falls
to 80 per cent, of the initial in about 2000 hours. General data
are given in Table IV.
158
ILLUMINATING ENGINEERING PRACTICE
TABLE IV. CANDLE-POWER CHARACTERISTICS OF COOPER HEWITT
MERCURY VAPOR LAMPS
Lamps for Alternating-current Circuits
Rating
of lamp
in aver-
age watts
Voltage
Type
Length
of tube
in inches
Mean
lower
hemi-
spherical
c.p.
Watts per
mean lower
hemi-
spherical
c.p.
Total
lumens
Lumens
per watt
2IO
380
IQ2
385
385
22O
385
726
100-125
100-125
E
F
35
50
400
800
0-53
0.48
3,179
6,283
I5-I4
16.53
For Direct-current Circuits
Series on
100-125
100-125
100-125
100-125
100-125
H
HH
K
L
P
21
21
45
35
50
300
600
700
4OO
800
0.64
0.64
0-55
0-55
0.48
2,388
4,712
5,529
3,142
6,283
12.43
12.23
I4-36
14.28
16.31
Quartz Lamps for Direct-current Circuits
200-240
Z
4
2400
3
18,839
25.96
Data furnished by Cooper Hewitt Electric Co.
Quartz Tube Mercury Arc. The high pressure mercury arc was
developed in Europe. Its commercial exploitation in this country,
dates from about 1913. 32 The tube is much shorter than that of the
low pressure arc and the current density greater. The high tem-
perature of operation necessitates the use of quartz glass as a tube
material. Most of its characteristics are similar to those of the low
pressure arc, but the appearance of the unit is more like that of a
flame arc lamp. In starting, an electro-magnet mechanism tilts the
tube to draw the arc. In starting with the lamp cold, about five
minutes is required to attain the operating condition. During this
period the current and watts are above normal and the candle-power
below normal.
Practically all the lamps in use are operated on 2 20- volt direct
current circuits. Lamps for other wattages and alternating current
have been made.
The light is essentially similar in color to that of the low pressure
mercury arc. Use is made of an outer globe of glass which cuts off
D. C. 55 Volt 3.5 Amp. Two in Series on
100-125 Volt Circuit
A. C. 100-125 Volt 4.1 Amp.
2 & 3
Curve A Represents the Candle-Power Distribution in a
Plane Perpendicular to the Axis of the Tube
Curve B Represents the Candle-Power Distribution in a
Plane Parallel to the Axis of the Tube
Curve C Represents the Mean of Curves A & B
Fig. 24. Glass tube mercury vapor lamps Cooper Hewitt.
(Data furnished by Cooper Hewitt Co.)
(Facing page 158.)
Fig. 25. Standard tubes now manufactured by Cooper Hewitt El. Co.
Fig. 26. Cooper Hewitt mercury vapor lamps banked for moving-picture photography.
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS
the ultra-violet light and tends to diffuse the light. A metal reflector
confines practically all the light to the lower hemisphere.
The unit is essentially one of high power, so that in industrial
plants it is usually installed where it can be hung 20 ft. or more from
the floor.
This lamp is illustrated in Fig. 27, which also shows candle-power
distribution, while the general data are given in Table IV.
Hemispherical Candle-
Power Distribution with
Reflector and Clear
Glass Globe
Hemispherical Candle-
Power Distribution with
Reflector and Diffusing
Globe
Curve A Represents the Candle -Power Distribution in a
Plane Perpendicular to the Axis of the Burner
Curve B Represents the Candle - Power Distribution in a
Plane Parallel to the Axis of the Burner
Curve C Represents the Mean of Curves A & B
Fig. 27. Quartz tube mercury vapor lamp Cooper Hewitt.
(Data furnished by Cooper Hewitt Co.)
The light from both forms of mercury arc lamp is steady and fairly
diffuse. Its most prominent characteristic is its blue-green color,
there being no red rays. This precludes its use for decorative light-
ing, except for special effects. The appearance of faces under the
light is not at all pleasing. On the other hand, the light is highly
actinic and of such character as to reveal detail to advantage, where
visual acuity is an important factor.
The light from the quartz tube lamp (without glass globe) is
destructive to certain forms of germ life, and hence, valuable for
sterilization.
Both the low-pressure and the high-pressure lamps are used exten-
l6o ILLUMINATING ENGINEERING PRACTICE
sively for photographic lighting, for which the actinicity of the light
renders them quite effective.
These lamps also find considerable application in industrial
lighting. 31
CARE OF LAMPS
The performance of any lamp depends upon its receiving a reason-
able amount of care. While some types of lamps require more at-
tention than others, no lamp will give its best service if entirely
neglected.
The glassware, whether of lamps or windows, will, if allowed to
become coated with dirt or dust, absorb an excessive amount of
light. The same is true, though usually in a lesser degree, of reflect-
ing surfaces. It is economical to keep globes and reflectors clean,
especially those which are so turned as to facilitate the accumulation
of dust. Moreover, the good appearance, both lighted and un-
lighted, often depends very much on cleanliness.
Beside the depreciation due to external accumulation, all illumi-
nants are subject to what is sometimes designated as inherent de-
preciation; that is, decrease in light due to accumulation or changes
inherent in the light source itself. For example, incandescent lamps
are subject during their operating life to a gradual decrease in can-
dle-power, due to bulb blackening and the filament shrinking. At
the end of the rated life, this depreciation amounts to from 10 to 20
per cent. With nearly all other illuminants the losses are fully as
great or greater. With arc lamps losses are principally due to the
accumulation of electrode material on the globes. With some arc
lamps, the washing of the globe at the time of trimming returns the
lamp to initial efficiency, while with others the material fuses into the
globes so that it cannot be readily removed. In the case of the
incandescent lamp and mercury-vapor lamp, the lamp should be
replaced when the loss exceeds certain economic limits, 20 per cent,
loss having been generally assumed as the " smashing point" for
the incandescent lamp. For arc lamps, the cleaning of the globes at
each trimming and the replacing of the globes when badly pitted
are the recommended practices.
Unfortunately for best economy the above-mentioned losses
accumulate so slowly that their magnitude is not generally recog-
nized, and many lamps are operated at unnecessarily low economies.
In a large installation, it is profitable to provide for regular periodic
inspection, cleaning and replacement.
In trimming arc lamps it is important to use electrodes of the cor-
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 161
rect length and proper diameter, and to make sure that they are
in alignment, making good electrical contact with holders.
Mechanisms should be kept clean and in adjustment. Care
should be taken on installing to insure that the adjustment is proper
for the line current and frequency. Certain forms of arc lamps have
suffered more in popularity from careless maintenance than through
inherent inferiority.
Incandescent lamps should be specified to correspond to the actual
socket voltage (series lamps to amperage). All lamps operate best
on steady voltage. Excessive change in voltage means unsteadiness
of light, and in some cases objectionable jumping and flickering.
SELECTION OF LAMPS
For most classes of lighting, practice has indicated some one type
of lamp which is better suited than others, so that there is not so
keen a competition between types as formerly. The problem now is
more the selection of lamps of proper power. This subject is so
broad and involves spacing and height, as well as candle-power
distribution characteristics to such an extent as to render a full
discussion at this point impracticable. It does seem desirable,
however, to warn against giving too much weight to abstract com-
parisons of candle-power or lumen output, or efficiency, or even of
operating cost, especially where the differentials are relatively small.
Reliable and accurate comparisons can only be made by taking
into account many factors with reference to the conditions to be met
in installation. It often happens that the higher efficiency of a
high power lamp is counteracted by waste of light, or objectionable
shadows, accompanying wide spacing. Again, an efficient lamp
may have a high investment or maintenance cost.
Cost comparisons are of value and should be made where large
numbers of lamps are involved. Such an estimate should include
the following items:
1 . Cost of energy.
2. Material of maintenance.
3. Labor of maintenance.
4. Depreciation (which will refund the investment when the lamps are worn
out or become obsolete, but not include material of maintenance).
5. Interest on investment (including installation cost).
6. Any other overhead charges, such as insurance.
On the other hand, there are important factors which do not lend
themselves to expression in figures.
1 62 ILLUMINATING ENGINEERING PRACTICE
The following are some of the desirable qualities which should be
considered in lamp selection: 33
(a) Intensity or light flux suited for condition, allowance being made for
depreciation.
(6) Diffusion of a degree depending upon requirements.
(c) Distribution characteristic such as to insure economical utilization.
(d) Color to meet the demands of utility and pleasing appearance.
(e) Steadiness slight animation not being necessarily objectionable; per-
ceptible flicker almost invariably objectionable.
(/) Reliability insuring continuity of illumination, also safety.
(g) Economy as previously noted, should be judged by concrete rather than
abstract estimate of costs.
(A) Artistic features involves the appearance of lamp fixtures, lighted and
unlighted, as well as the lighting effect itself. Deserves more attention in
ordinary installations than it usually receives.
(j) Adaptability this is important in large installations where it is desirable
to meet a variety of conditions with a minimum number of types and renewal
parts to be kept in stock.
(K) Construction quality. Practically all the established illuminants are
well made. For severe service, special constructions are sometimes necessary.
(/) Convenience ease of handling by unskilled persons.
(m) Congruity. This applies to the general suitability of the illuminant to
its surroundings.
While no accurate method of applying these considerations is
here suggested, a common-sense consideration of these points will
facilitate forming a true evaluation of a lighting unit for particular
service.
CONCLUSION
Much of the foregoing is necessarily suggestive, but definite in-
formation is given where practicable. It must be remembered
that, with the rapid advance in the art of lamp manufacture, the
performance of illuminants is likely to be bettered in the near future.
In conclusion, the writer desires to express appreciation for the
data and information furnished by lamp and electrode manufacturers.
References .
1 C. P. STEINMETZ. "Electric Illuminants." Lectures on Illuminating
Engineering, Johns Hopkins University, 1910, page 109.
2 E. P. HYDE. "The Physical Characteristics of Light Sources." Lectures on
Illuminating Engineering, Johns Hopkins University, 1910, page 25.
3 W. R. WHITNEY. "The Chemistry of Luminous Sources." Lectures on
Illuminating Engineering, Johns Hopkins University, 1910, page 93.
4 W. F. LITTLE . "Lighting Accessories" (See lecture in this series).
6 F. W. SMITH, Chairman, Lamp Committee N. E. L. A. "Report of Lamp
Committee." Proceedings of National Elec. Light Assn., 1916.
STICKNEY: DEVELOPMENTS IN ELECTRIC LAMPS 163
6 C. G. FINK. "Ductile Tungsten and Molybdenum." Trans. American
Electro- Chemical Society, Vol. XVII (1910), page 229. General Electric Review,
Vol. XII (1910) page 323.
7 W. D. COOLIDGE. "Wrought Tungsten." Trans. American Inst. of
Electrical Engineers. Vol. XXIX (1910), page 961.
8 J. W. HOWELL "The Manufacture of Drawn Wire Tungsten Lamps."
G. E. Review, Vol. XVII (1914), page 276.
'WARD HARRISON and E. J. EDWARDS. "Recent Improvements in Incan-
descent Lamp Manufacture." Trans. 111. Eng. Society. Vol. VIII (1913),
page 533-
10 E. J. EDWARDS and H. H. MAGDSICK. "Light Projection" (See lecture in
this series).
11 IRVING LANGMUIR. "The Blackening of Tungsten Lamps and Methods of
Preventing It." Trans. American Inst. of Electrical Engrs., Vol. XXXII (1913),
page 1913.
12 IRVING LANGMUIR and J. A. ORANGE. "Nitrogen Filled Lamps." Trans.
Amer. Inst. of Elect. Engrs., Vol. XXXII (1913), page 1935.
13 G. M. J. MACKAY. "The Characteristics of Gas-filled Lamps." Trans.
111. Eng. Society, Vol. IX (1914), page 775.
14 F. W. SMITH (Chairman, Lamp Committee N. E. L. A.). "Report of Lamp
Committee." Proceedings National Elec. Light Assn., 1915.
G. F. MORRISON. Review of Lamp Committee Report. G. E. Review,
Vol. XVIII (1915), page 925.
16 D. B. RUSHMORE. "Frequency." Trans. American Inst. of Electrical
Engrs., Vol. XXXI (1912), pages 970 and 978.
16 D. MCFARLAN MOORE. "Gaseous Conductor Lamps for Color Matching."
Trans. 111. Eng. Society, Vol. XI (1916), page 162.
17 GEORGES CLAUDE. "Neon Tube Lighting." Trans. 111. Eng. Society,
Vol. VIII (1913), page 371.
18 W. D. COOLIDGE. "A Powerful Rontgen Ray Tube with Pure Electron
Discharge." Physical Review, Dec., 1913. G. E. Review, Vol. XVII (1914),
page 104.
19 C. S. MCDOWELL. "Illumination in the Navy." Trans. 111. Eng. Society,
Vol. XI (1916), page 574.
20 S. H. BLAKE. "Flame Arc Lamps." G. E. Review, Vol. XIV (1911),
page 595-
21 G. N. CHAMBERLIN." Enclosed Flame Arc Lamp." G. E. Review, Vol.
XV (1912), page 706.
22 R. B. CHILLAS. "The Development of the Flame Carbon." Trans. 111.
Eng. Society, Vol. IX (1914), page 710.
" V. A. CLARK. " Present Status of Arc Lamp Carbons." Electrical Review
and Western Electrician, Vol. LXVII (1915), page 406.
24 C. E. STEPHENS. "Modern Arc Lamps." Electrical Review and Western
Electrician, Vol. LXVII (1915), page 409.
25 A. T. BALDWIN. "The Flaming Arc in the Iron and Steel Industry."
Proceedings Assn. Iron & Steel Elect. Engrs. (1914), page 491.
26 C. A. B. HELVORSON, JR. "New Types of Ornamental Luminous Arc
Lamps." G. E. Review, Vol. XV (1912), page 710.
164 ILLUMINATING ENGINEERING PRACTICE
27 C. A. B. HALVORSON, JR. "Improvements in the Magnetite Lamp."
G. E. Review, Vol. XVII (1914), page 283.
28 C. A. B. HALVORSON, S. C. ROGERS and R. B. HUSSEY. "Arc Lamps for
Street Lighting." Trans. 111. Eng. Society, Vol. XI (1916), page 251.
29 F. CONRAD and W. A. D ARRAH. " The History of the Arc Lamp." Electric
Journal, 1916, pages 103 and 140.
30 H. E. IVES. "Study of the Light from the Mercury Arc." Electrical
World, Vol. LX (1912), page 304.
31 W. A. D. EVANS. "Industrial Lighting with Mercury Vapor Lamps."
Trans. 111. Eng. Society, Vol. X (1915), page 883.
32 W. A. D. EVANS. "The Mercury Vapor Quartz Lamp." Trans. 111. Eng.
Society, Vol. IX (1914), page i.
33 P. S. MILLAR. "The Status of the Lighting Art." Trans. 111. Eng. Society,
Vol. VIII (1913), page 652 (See "Categories of Illumination," page 654).
RECENT DEVELOPMENTS IN GAS LIGHTING
BY ROBERT FFRENCH PIERCE
For the purpose of this lecture the term "recent developments,"
will be applied to changes and improvements in gas lighting appli-
ances effected and reduced to commercial practice since 1910,
progress prior to that year having been set forth in the lectures at
Johns Hopkins University.
The economic position of the gas industry has tended to restrict
development to the refinement and elaboration of existing types
rather than to encourage increasing diversity in the application of
gas to lighting.
Gas was the first central station illuminant and until 1880 the
only one. At the present time, in the older communities of the
East there are from four to seven times as many gas meters as elec-
tric meters in use, while even in the newer communities of the
West, where cheap hydro-electric power and dear coal place the
gas industry under a severe handicap, the number of gas meters
usually exceeds that of electric meters in use. Following the line
of least resistance the gas industry has directed such of its energies
as have been devoted to lighting toward those improvements which
would best protect its existing lighting business, while the commercial
exigencies of electrical development have favored the creation of
new uses and excursions into new fields.
During the past five years the principal developments in gas
lighting have had for their objects increased economy in light pro-
duction through more efficient utilization of the gas and decreased
maintenance expense, and the elimination of inconvenience in the
use and maintenance of gas lighting units, with the purpose of fore-
stalling, overcoming or reducing the users' inclination toward
providing facilities for the use of competing illuminants.
The gas lamp is composed of two essential parts the burner and
the mantle, the former usually being fitted with a glass chimney to
secure satisfactory and efficient operation.
Possibilities of increased economy of light production lie in ob-
taining higher temperatures through improved burner design; in
165
1 66 ILLUMINATING ENGINEERING PRACTICE
securing a larger proportion of luminous radiation through the selec-
tion of mantle materials having a more favorable selective radia-
tion characteristics; in prolonging the useful life of the mantle by
the utilization of less fragile base fabrics; and in eliminating such
accessories as chimneys the maintenance of which is an item of
expense.
Opportunities for securing added convenience in the use of gas
lamps lie in such of the above developments as reduce the number
of parts requiring attention and the frequency with which essential
parts need replacement, and in the provision of simple, inexpensive
and reliable means of ignition and distance control.
THE MANTLE
The physical character of the mantle is determined by the two
essential substances which enter into its manufacture, (i) the organic
fabric which is impregnated with solutions of salts of the (2) rare
earths (ceria and thoria) that form the ultimate mantle structure,
the organic matter being burned out in the process of manufacture.
The character of the fabric used determines the mechanical strength
of the mantle, its shrinkage under the continued heat of the flame,
and to a small extent the luminosity of the mantle. The rare earths
employed determine the radiant efficiency of the mantle, and the
color of the light emitted.
No significant change in the proportions of ceria and thoria em-
ployed has taken place in the past twenty years, and although a
theoretical consideration of the physics of rare earths radiation
indicates the possibility of greatly increased efficiency through the
employment of hitherto unused elements, no promising experimental
results have as yet been recorded.
The utilization of " artificial silk" as a base fabric was noted by
Whittaker in his Johns Hopkins lecture, but this material, had not
at that time been brought to such a commercial stage as would war-
rant specific quantitative statements as to its performance, and the
employment of this substance may for the purposes of this lecture
be regarded as a subsequent development. Mantles made upon this
base have been used in large quantities during the past three years
and exhibit a great superiority over previous types in tensile
strength, flexibility, permanence of form and maintenance of
luminosity. The artificial silk mantle of the upright type after
several hundred hours service will support a suspended weight of
s
PIERCE: DEVELOPMENTS IN GAS LIGHTING
i6 7
one ounce, may with care and skill be folded and crumpled upon itself
and restored to its original form without apparent damage and will
maintain its initial candle-power practically unimpaired for an
indefinite period 5000 hours actual service producing a deprecia-
tion of less than 10 per cent. These facts while exemplifying no
practical condition, are highly significant as indicating most desir-
able and important physical properties. It should be understood,
of course, that the rather theatrical demonstrations of desirable
physical qualities referred to are not to be attempted by the user
unless he wishes to purchase a new mantle.
New
Old
\
2345678
% Ceria
Fig. 6. Influence of ceria content on candle-power of mantle.
10
The desirable qualities of artificial silk are due to the fact that the
fibers are solid and continuous, instead of cellular and comparatively
short.
Figs. 3, 4 and 5 showing magnified sections of different mantle
fabrics illustrate the steel-cable-like structure of the artificial silk
mantles compared to that of mantles based upon vegetable fibers
more resembling a hempen rope. The cellular structure is largely
responsible for the shrinkage during burning which characterizes
cotton mantles.
Due to causes not altogether apparent the luminosity of a mantle
is considerably influenced by proportioning of the rare earth contents
with relation to the physical structure of the mantle fabric, and re-
finements in manufacturing processes have resulted not only in
1 68 ILLUMINATING ENGINEERING PRACTICE
increasing efficiencies with the same fabrics, but in altering the re-
lation between ceria content and luminosity. Fig. 6 shows two
curves of mantles, made upon the same fabric, the one designated
"old" being that reproduced by Whittaker in the Johns Hopkins
lectures. Since the yellowness of the light emitted varies with the
ceria content, it is apparent that the later mantles appreciably widen
the range of color-values which may be economically obtained in
the gas mantle.
Other interesting developments involving departures from
previous methods of mantle construction have occurred, but, since
they are more directly related to modifications in the burner, they
will be introduced later.
BURNERS
Since the efficiency of an incandescent gas lamp is directly related
to the flame temperature, and the latter depends largely upon the
proportion of primary air entrained, it is desirable that the latter
be as large as practicable. But since the speed of flame propaga-
tion is also increased with the proportion of primary air, the latter
is practically limited by the velocity of the outflowing mixture at
the nozzle, because the speed of flame propagation and velocity of
outflow must be equal in order to avoid " flashing back" of the
flame on the one hand, or, "blowing off" on the other the latter
difficulty, however, never being experienced at ordinary pressures.
The highest velocity of outflow is secured by means of proper
design of the bunsen tube and freedom from bends or obstructions
in the burner. Such a burner, however, fails to secure thorough
mixture of the gas and air with the result that the more highly
aerated "streaks" permit the flashing back of the flame even though
the average speed of flame propagation is far below that in the more
highly aerated portions. Since thorough mixture of the gas with
the entrained air involves some loss in the velocity of outflow,
burner design is resolved into the elimination of all obstructing and
retarding influences except those required for mixing the gas and
air in the most efficient manner.
The sole source of energy for the entrainment of air, mixing it
with the gas and the propulsion of the mixture into the flame is the
kinetic energy of the gas issuing from the orifice under a pressure of
(ordinarily) less than 2 ounces per square inch, and it is the conserva-
tion of this small amount of energy that presents the greatest problem
to the designer of incandescent gas lamps.
PIERCE: DEVELOPMENTS IN GAS LIGHTING 169
Within the last three years a greater appreciation of the im-
portance of this feature has led to the development of a type of
burner having not only improved efficiency, but simpler construc-
tion and fewer parts than have characterized previous types. These
results are direct consequences of greater air entrainment, more
thorough mixing of the gas and air, and higher nozzle velocities.
In the previous types larger proportions of secondary air were
required. To bring this secondary air into the flame with sufficient
speed to localize the combustion area most effectively in the mantle
surface and secure satisfactory efficiencies, various devices were
employed notably air-hole cylinders and " stacks" to produce
strong upward drafts. These accessories complicated design, in-
creased maintenance expense and often interfered with adaptation
of the lamps in fixture design. In the recent lamps it has been
found practical to eliminate chimneys, lamp housings, stacks, etc.,
with no loss of efficiency. The elimination of the chimney Or
cylinder removes one of the most troublesome sources of candle-
power depreciation in gas lamps. Reduction of illumination of
from 10 to 20 per cent, in 1000 hours' active service commonly
results from the dust deposits on chimneys.
Relieved from the necessity of accommodating these accessories,
the designer has employed greater freedom in the development of a
range of sizes, and in their application and these lamps are now made
in sizes from one to six mantles and in upright, inverted and hori-
zontal forms. The mantle generally used with these burners is
ij in. in diameter by ij^ in. long, mounted on the common open
top ring. It has been found however that with this type of burner
closed top mantles 5^ X i.in., consuming about i cu. ft. of gas per
hour may be used, there being no necessity for leaving a space at
the top of the mantle for the egress of combustion products in excess
of those which pass through the mantle mesh. This permits
the use of the so-called rag or soft mantle a mantle from which
the organic fabric has not been burned out, this operation, which
is usually performed in the factory, being done by the purchaser.
In order for the mantle to fill out properly an appreciable pressure
inside the mantle is necessary. This is obtained by the use of com-
pressed ah- at the factory, but on the customers' premises only the
ordinary pressure within the mantle is available, and in order
for this to be effective, the top of the mantle must be closed, forcing
all the products through the meshes of the fabric. With the
existing pressures on the customers premises, it is not practicable to
170 ILLUMINATING ENGINEERING PRACTICE
burn off and properly harden a mantle larger than % X i in. on
the customers' burners.
The rag mantle has many advantages. It is as soft and pliable
as any other knitted fabric. It cannot be injured by handling and
may be packed in a small space and transported with impunity.
The lamp shown in Figs. 7 and 8 is equipped with three of these
small size rag mantles and is particularly adapted to fixtures with
upright outlets, as for example, those ordinarily fitted with open
flame tips.
The inverted lamp (that is, that in which the bunsen type projects
downward from the gas orifice) requires a housing of some sort to
which the shade may be attached and in which means for conducting
the combustion products away from the air ports may be provided.
Fig. 7. New upright burner with Fig. 8. Installation of lamp shown in
inverted mantles. (Cut about one- Fig. 7.
third actual size.)
Until recently, the discoloration of the lamp housing and support-
ing fixture arm or pendant by heat and combustion products was a
serious drawback in the use of inverted gas lamps, particularly in
residences and in mercantile establishments of the better class.
In the latest designs this trouble has been eliminated by providing
an air space between on the inner and the outer shell, and a deflector
which ejects the combustion products with sufficient velocity to
carry them several inches out from the top of the lamp. Figs, ga
and gb show distributions of temperatures about two lamps of this
type, the center of the uprising column of products being shown by
the heavy line connecting the points of maximum temperature at
each level. Protracted tests indicate that the elimination of fixture
discoloration by this method is complete.
An interesting development in the design of inverted burners is
PIERCE: DEVELOPMENTS IN GAS LIGHTING
171
TEST 3761
Welsbach Testing Laboratories
3-3-'/6 J.RA.
Fig. pa. Distribution of temperature about side-vent lamp consuming 2% cubic feet of
gas per hour.
172
ILLUMINATING ENGINEERING PRACTICE
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TEMPERATURE I
72* F
TEST 2341
ing aboratorifs
Fig. 96. Distribution of temperature about side-vent lamp consuming 9}-^ cubic feet of
gas per hour.
PIERCE: DEVELOPMENTS IN GAS LIGHTING 173
shown in Fig. 10. In this design both the air intakes and the vents
are concealed from ordinary view, and the parts are so arranged as
to permit the design of a burner exterior of unobtrusive form and
attractive lines. This design has been applied to sizes ranging from
85 to 250 candle-power, arid thus accomplishes a standardization of
appearance approaching that obtained by means of the standardized
socket construction in incandescent electric lamps. The gas cock
is operated by a single pull chain, and the complete unit possesses
many features which appeal to the fixture designer as well as to the
customer who wishes to avoid the use of lamps which too strongly
announce by their appearance the nature of the illuminant supplied
to them.
These lamps together with the small upright lamp shown in Fig.
7 comprise the leading products of the most important American
manufacturers of gas lamps, and it is interesting to note that in
both types the tendency has been to eliminate features which
emphasize to the eye the burner itself.
IGNITION
During the past five years several means of ignition have been
attempted, the most general being electrical in the form of either
a jump spark or an electrically heated platinum wire. The accessory
apparatus required dry batteries, accumulators, etc., and the
comparatively high cost of the ignition devices, have limited the
application of even the most satisfactory of electrical systems to
special conditions in which ignition of this character is particularly
desirable. For several years the jump-spark system of ignition has
been utilized for gas ignition. This system usually consists of a
dry battery, induction coil and spark gaps, one for each lamp, ar-
ranged in series. The drawbacks to this system have been the
difficulty of securing proper insulation for the secondary or high-
tension circuit, the high first cost of the installation, and the neces-
sity for providing a separate system for distant control when the
latter is required which is usually the case. A recent development,
originating in and at present confined to Germany, but of sufficient
interest to warrant description here, involves the use of a special
form of switch which, when operated, sets in motion a vibrating
contractor in the primary circuit, the vibrations persisting for a
period sufficient to permit the gas, which is turned in by a magnet
valve in the same circuit, to reach the lamp before the high-tension
spark induced by the making and breaking of the primary circuit,
174 ILLUMINATING ENGINEERING PRACTICE
dies out. The induction coil is placed in a canopy above the lamp
which also contains the magnet valve. In this system the high-
tension circuit is confined to the lamp fixture. This device is
absolutely positive and reliable in action, its only drawback being the
high cost, a separate induction coil and magnet valve being required
for each fixture (see Figs, n and 12).
Many attempts have been made to utilize the catalytic action of
platinum for gas ignition. In the finely divided form known as
platinum black this element possesses the property of condensing
oxygen upon its surface and initiating combination with hydrogen
in the presence of the latter. The self-lighting mantles which
sporadically appear upon the market rely upon a "pill" of platinum
black upon the mantle surface to secure ignition. The catalytic
action is, however, so rapidly decreased by the agglomeration of the
particles of heat and other unavoidable influences, and the conse-
quent reduction of catalyzing surface presented, that this expedient
has never come into extended commercial application.
It has been found, however, that platinum wire heated to about
5ooC. is capable of initiating the combination of hydrogen and
oxygen and this fact has been utilized in the "hot-wire" ignition
system (Fig. 13), in which electric current from a small dry battery
or accumulator provides the heating energy. When this system
was first applied a dry battery was placed in the shell, a switch being
actuated by the operation of turning the gas cock. As long as the
battery voltage is regulated within narrow limits the results are very
satisfactory. A device of similar principle in which the heated
platinum filament is used to ignite a pilot flame which in turn ignites
the gas at the lamp burner, has been on the market for sometime but
apparently without radically affecting the current practice in gas
ignition, which is by means of a continuously burning pilot flame.
The pilot-flame method is too commonly used and known to re-
quire explanation. The greatest drawbacks of the earlier and in
fact all but the most recent types were the cost of the gas consumed,
which, though negligible in a frequently used installation, is com-
paratively great in the case of lamps in active service for only a
few hours per week; and the liability to outage from draughts, de-
posits of pipe-scale, tar, etc. In well-operated gas works the gas
is freed from the tar at the works. Where practice is poor in this
particular a small filter-box is placed in the gas supply to the pilot.
The asbestos packing in this filter-box which retains dust, scale, tar,
etc., and can easily be removed and renewed when fouled.
Fig. 10. Recent types of inverted lamps.
To Switch-On .ds*. To Switch-Off
Magnet Valve
Canopy
From Induction Coil
To Spark-Plug
Fig. II. Installation of magnet-valve and induction coil for distant control and jump-
spark ignition.
(Facing page 174.)
\ Primary
.Secondary /
i
On Magnet /
Off Magnet
.Ground
N "Armature
[JF^ Gas Cock
Ground
'Lamp
Around /Spark Gap
Fig. 12. Wiring connections for electro-magnetic distance control and ignition of gas
lamps.
Fig. 13. Self-contained fixture operating by "hot-wire" ignition.
PIERCE: DEVELOPMENTS IN GAS LIGHTING
Pilot flames may be protected against draughts to some extent by
a shield (Fig. 14) but this expedient is not sufficiently effective to
render the ordinary pilot an altogether reliable means
of ignition.
During the past four or five years the pilot flame has
been utilized to some extent as a low-intensity illumi-
nant. When a small Bunsen flame is directed against
the outside of a gas mantle the mantle area affected
becomes to all intents and purposes a small incandescent
mantle. A pilot flame of the Bunsen type consuming
Lg cu. ft. per hour will if directed against a mantle, pro-
duce about % horizontal candle-power, as against J
candle-power for a luminous or open flame consuming
gas at an equal rate. This is sufficient to enable the
occupant of a room to see his way about, to find keys
or pull-chains controlling the lamps, and measurably to Fig- 14- Pro-
discourage those adventurers into high finance who
operate at night and specialize in second-story operations. A lamp
equipped with such a pilot becomes a " high-low"
unit operating at "low" continuously. Such a unit
has a considerable field of application but the con-
sumption of 90 cu. ft. of gas per month per lamp
costing 9 cents per lamp per month with gas at $1.00
per 1000 cu. ft. constitutes in many cases an obstacle
to the general use of this system.
In 1916 a radical development appeared in the form
of a pilot (Fig. 15) consuming but J^ 6 cu. ft. per
hour and of a very simple and inexpensive construc-
tion. This pilot consists of a tip surrounded by a
small bundle of mantle fabric saturated with salts
of rare earths which have been found effective in re-
taining the flame. This device possesses the remark-
able property of being unaffected by breezes of 12
miles per hour, sufficient to blow a mantle of ordi-
Fig. 15. Section nary size from its supporting ring. A unit contain-
wit^Tame-retaS in g tnis device, combines an unfailing means of igni-
ing fabric of rare tion and a continuous small intensity of illumination,
24 hours per day, at a cost of only 4.5 cents per month
per lamp with gas at $1.00 per 1000 cu. ft. or 45 cents per month for
10 lamps about the number usually required in a y-room dwelling.
A tabulation of pilot consumptions follows:
i 7 6
ILLUMINATING ENGINEERING PRACTICE
Lamp
Normal
pilot
cons, per
hour
(cu. ft.)
Approx.
length
of flame
(inches)
Pilot
cons, per
year
(cu. ft.)
Normal
lamp
cons, per
hour,
(cu. ft.)
Lamp
cons, per
year 4
hrs. daily
(cu. ft.)
Pilot
cons, per
cent, of
total
cons.
I Burner inv. indoor Bunsen
pilot
O I2O
y.
t en
i Burner upr. indoor luminous
VA
A fie
6 780
3 Burner inv. indoor arc, semi-
*A
8 i
5 Burner inv. outdoor arc, semi-
Bunsen pilot
o 213
y~
l86e o
6 8
i Burner inv. indoor luminous
pilot
o 152
H
1331 s
I AC
5 037
20 9
"Glower" pilot
o 04
547 5
3 o 10*
Depending on the size of lamp.
It may be frankly stated that prior to the development of this
device the use of gas lighting imposed a certain unavoidable sacrifice
of convenience due mainly to the faultiness of existing ignition sys-
tems, which may now be regarded as eliminated.
DISTANT CONTROL
The difficulty of controlling gas lamps from a distant point lies
mainly in the necessity for controlling the flow of gas at a point near
the lamp. If considerable pipe capacity is placed between the gas
cock and the lamp the admission of the air in the pipe with the gas
entering when the cock is turned on, may be sufficient to cause the
flame to "flash back" to the orifice, and in any case the nearer the
cock to the lamp the less violent the ignition of the gas. Distant
control therefore necessitates means of operating a gas cock at or
very near to the lamp. Usually a very small amount of energy
must suffice for the actuating of the gas cock. Unfortunately, the
most satisfactory type of cock is the "plug" type in which a tapered
plug containing a gas-way is ground into a tapered seat, in which it
turns. On account of the large bearing surfaces the friction is con-
siderable, and though it may be much reduced by proper lubrica-
tion, the grease used is soluble in some of the gas constituents
(notably benzol), which liquefy at the temperatures occasionally
met in practice and dissolve the lubricant, thereby making a con-
siderable increase in the energy required to actuate the cock.
Another form of valve consists of an annular knife edge making
contact with a flat seat. Such a valve is easily actuated and requires
PIERCE: DEVELOPMENTS IN GAS LIGHTING 177
no lubricant, but may be kept from operating by small particles of
scale falling between the knife edge and its seat, thereby preventing
the closing of the valve and resulting in leakage. The probability
of failure through this cause may be greatly reduced by proper design,
and many very satisfactory valves have been constructed upon this
principle.
Gas valves for remote or distant control may be actuated by air
pressure, by gas pressure or by electricity.
One of the 'simplest examples of the application of the former
method is the pneumatic cock, consisting of a cylindrical plug with
gas- way which moving axially in a cylindrical seat controls the flow
of gas to the lamp.
A small hand pump having a bore of about % in. and a stroke of
from i to 3 in. furnishes the impulse, transmitted through a small
tube of Jf 2 m - inside diameter, which moves the cock, a single im-
pulse of compression or rarefaction sufficing to open and close the
gas way respectively. This device is simple, inexpensive and when
carefully designed, constructed and installed, reliable. Unfor-
tunately most of the commercial types which have been offered,
lacked the first two qualifications, and were so designed as to render
the accomplishment of the third difficult.
Another form of gas-pressure-actuated valve consists of an
inverted bell over mercury, the bell serving as the valve proper
and the mercury as the "seat." The bell is weighted so as to be
lifted and sustained clear of the mercury by the gas-pressure re-
quired to operate the lamp, sinking and cutting off the gas supply
when the pressure is reduced below a predetermined point. The
controlling valve is fitted with a by-pass which admits enough gas
to supply the pilot flame at the lower pressure when the main gas
supply is turned off. Valves of this type must be located at a
sufficient distance from the lamp to avoid evaporation of the mercury
by heat.
A simple and reliable automatic shut-off for extinguishing the
lamp-flame at a predetermined time consists of a clock incorporated
into the gas-cock arm, the latter being in a horizontal position for
turning the gas on. At the predetermined time the clock disen-
gages the chain which maintains the arm horizontal, the weight of
the clock and arm then closing the cock.
In another type of gas-pressure-actuated valve the valve proper
is a flexible metal diaphragm seating against an annular knife-
edge. The space opposite the seat is connected with the main gas
178 ILLUMINATING ENGINEERING PRACTICE
supply pipe by a small controlling pipe. At any convenient point
in the small controlling pipe a three-way cock is installed, which in
one position, connects the main gas supply with the diaphragm cham-
ber opposite the valve-seat, and in another connects the diaphragm
chamber with the outer air. In the first position the pressures on
either side of the diaphragm are equalized and the valve is closed.
In the second position the pressure in the chamber opposite the seat
is reduced to that of the atmosphere and the gas pressure on the seat
side of the diaphragm opens the valve.
ELECTROMAGNETIC VALVES
Two forms of electrically operated valves are in commercial use
in this country. In one the armatures of two electromagnets
actuate a tapered plug gas cock of the ordinary type, one turning the
gas on and the other off. On account of the energy required to
operate a cock of this type, it is desirable that the magnet be of
efficient design in order satisfactorily to utilize the limited amount
of energy available from small dry batteries. Most of the com-
mercial types fail to realize the possibilities of this system in this
direction and these valves are principally used in interior installa-
tions. They are comparatively expensive and do not enjoy exten-
sive commercial use. Four ordinary dry batteries are required for
one valve.
In a recent valve of the electromagnet type use is made of a polar-
ized core in a solenoid controlled by a reversing switch. The valve
itself consists of a diaphragm seating upon an annular knife edge.
The normal position of the diaphragm is in the open position, seating
being accomplished by the weight of the solenoid core, assisted by
a spring; current in one direction lifts the solenoid core and the
diaphragm, the residual magnetism retaining the core in its upper
position after the current is turned off. Current in the opposite
direction overcomes the influence of the residual magnetism and
permits the core to fall, closing the diaphragm against the seat.
One dry cell is sufficient to operate this valve and extremely satis-
factory operation has followed its commercial application. It is
somewhat more expensive than the previously described type of
electromagnet valve, and is limited in commercial application to the
larger units with which the cost of the valve is a relatively unim-
portant feature.
A recently developed magnet valve is shown in Figs. i6a and i6b.
PIERCE: DEVELOPMENTS IN GAS LIGHTING
179
The valve proper consists of a disc secured to the solenoid plunger
by means of a ball and socket joint, ensuring accurate seating against
the annular knife edge seat. A small spring secured in the plunger
and bearing against the bore of the magnet spool prevents the un-
Fig. i6a. Electro-magnetic gas valve, off.
seating of the valve from shock or vibration. This device is always
installed with the annular knife-edge in a vertical plane so as to
eliminate fouling from particles of pipe scale, and the seat is further
protected by a small drip in the upper part of the valve.
Fig. i6&. Electro-magnetic gas valve, on.
GENERAL DESIGN OF LAMP AND FIXTURES
Store Lighting. The development of means for securing highly
efficient incandescence of the gas mantle without chimneys, cylinders
or stacks has made possible a freedom and variety in design unattain-
able with the older types. As long as each mantle required enclosure
l8o ILLUMINATING ENGINEERING PRACTICE
in a chimney or cylinder, or each small group of mantles in a globe,
the output of individual lighting units was limited to about 300
c.p. and the design of attractive fixtures was difficult on ac-
count of the obtrusion of awkward mechanical features and the
limitations imposed thereby. Figs. 17 and 18 show the burner
arrangement and general appearance of new types of fixtures exem-
plifying the importance of recent developments in modifying and
improving semi-indirect fixture design. Figs. 19 and 20 show other
semi-indirect feature designs recently produced by leading Ameri-
can manufacturers.
Fig. 22 shows the plan of a 20oo-c.p. semi-indirect fixture the
largest modern low-pressure unit thus far constructed a type of
Fig. 17. Arrangement of horizontal burners in semi-indirect fixtures.
design altogether impossible with the older lamps. Several of
these units are in commercial service and have given excellent
satisfaction. The fixtures as installed are shown in Fig. 21.
RESIDENCE-LIGHTING FIXTURES
It is generally recognized that the upright mounting of lighting
units is preferable for the illumination of the more conventional
interiors. The inherited sense of appropriateness by the satisfaction
of which aesthetic requirements are governed, is based upon the
almost universal use of the flame as a light source in the past. The
tendency toward the inversion of the lighting unit notable of recent
years had its impulse in consideration of economy, which have in a
large measure been counterbalanced by improved efficiency in the
use of illuminants, reduced cost of energy, and by the increasing
Fig. 1 8. Recent type of semi-indirect gas fixture.
Fig. 19. Recent type of semi-indirect gas fixtures.
(Facing page 180.)
Fig. 20. A novel design in semi-indirect fixture.
Fig. 21. 2000-c.p. fixtures installed. (The small fixtures belong to the previous
installation.)
3 *
fe
Fig. 24. Incandescent gas-lamps arranged for lighting a photographic studio. Each
lamp consumes 4^ cubic feet of gas per hour, and is fitted with a special mantle giving
increased radiation at the shorter wave-lengths.
PIERCE: DEVELOPMENTS IN GAS LIGHTING
181
appreciation of the semi-indirect system of illumination. In the
older upright gas lamps use was made of a mantle suspended from
the top and open at the bottom. The mantle, freely swinging from
its support suffered mechanically from the repeated striking of the
lower portion against the burner head and the life was much shorter
than that of the. inverted mantle. Furthermore, the chimney re-
quired was an item of expense, and a source of annoyance by reason
of the cleaning required. The development of the inverted mantle
upright burner before mentioned (Fig. 7) has made possible the satis-
Fig. 22. Arrangement of six s-mantle burners with total candle-power of 2000 in
42-inch bowl.
factory and convenient use of gas in fixtures of the type shown in
Figs. 230 and 236.
The foregoing have been cited merely to emphasize the important
influence upon fixture design of the mechanical simplification of the
lamp. Though there is nothing unusual about any of these designs
they really exemplify considerable progress for they represent the
removal of great handicaps.
SPECIAL APPLICATIONS
Although the economic position of gas and the traditional con-
servatism of the industry have directed the principal developments
13
1 82 ILLUMINATING ENGINEERING PRACTICE
along the line of enhancing the value of gas lighting in existing uses
and to existing customers, some interesting excursions into new fields
have been conducted.
A great deal of shop-window lighting has been creditably done.
In one city where some efforts has been directed toward developing
this use, many of the leading exclusive stores in this city use the
incandescent gas lamp for show-window illumination. The greatest
obstacle to the use of gas for show-window lighting has been the
expense of and the space occupied by the installation, the mainte-
nance of clean glassware and the liability to pilot outage, since in
most cases a distributed system requiring a large number of units is
preferred.
PHOTOGRAPHY
Through the development of a mantle of low ceria content having
a large energy radiation in the violet end of the spectrum, very
creditable studio-lighting has been accomplished. Fig. 24 shows a
studio lighting fixture consuming about 100 cu. ft. per hour by which
portraits may be taken with shutter-drop (J^ second) exposures on
Orthonon plates. Although no effort has been made to exploit this
system a considerable commercial demand has spontaneously arisen.
The foregoing are indicative of the fact that gas lighting in its
most recent development is susceptible of a much more extended and
diversified use than it has enjoyed in the past, and waits only upon
the expenditure of energy on commercial activity in the less fre-
quently exploited fields.
MODERN LIGHTING ACCESSORIES
BY W. F. LITTLE
The term "accessories" is here used not in its general sense, but
rather according to the definition of the Committee on Nomen-
clature and Standards as employed to designate reflectors, shades,
globes and other devices for modifying and controlling the light
produced by lamps. The usual functions of such devices are to re-
direct the light; to diffuse the light; to interrupt the light in certain
directions; to modify the hue of the light, or to protect the light
source. Accessories in this sense are the tools with which the illumi-
nating engineer works.
Since 1910 development in accessories has followed principally
the lines of reduced brightness and improved appearance. The in-
creased brightness of light sources has led to the development of
accessories which partially or entirely conceal the source. These
form in themselves a secondary light source and are capable of decora-
tive treatment to a degree not offered by earlier forms of accessories.
Prior to 1910 control of light flux was considered to mean very largely
the re-direction of the light as desired. The progress of the past
six years lies in the broader definition of control which now is con-
sidered to include not only control of the direction of light but also
control of brightness and control of color. This extension of the
functions of lighting accessories has not involved the abandonment
of the most effective and flexible means of controlling direction of
light, such as prismatic glass and mirror glass, but has brought about
a more subtle and pleasing use of these means in combination with
other means for softening and tinting the light.
The development of more efficient illuminants in this interim has
brought not only the necessity for concealment of sources by light-
ing accessories, but also the opportunity to apply more effectively
the less expensive illumination which they make possible. Acces-
sories development is thus definitely involved with the improvement
in light sources, without which such development would probably
have been neither essential nor possible.
The increased brightness of the lamps, particularly in the smaller
183
184 ILLUMINATING ENGINEERING PRACTICE
sizes, makes some protecting substance advisable. Central station
interests, as well as lamp manufacturers, are experimenting with
certain bulb coatings which diffuse, and others which both diffuse
and tint the light. A demand is beginning to manifest itself for a
glass bulb to accomplish this same purpose.
Lamp accessories as above described may be made of a variety of
materials and of innumerable shapes, and may still fulfil the require-
ments of the foregoing.
The material from which accessories are made may be classified
in general as: Metal, enameled metal, glass, fabric, stone and
pottery; and the shapes as: Flat, cone, bowl and miscellaneous.
From the standpoint of tabulation it is rather unfortunate that
so many of the accessories fall in the miscellaneous class.
MATERIALS
The material should be selected with the proper weighting of the
several optical properties of lighting accessories, namely reflection,
transmission, and diffusion.
Metal. Metal accessories applied as reflecting media have many
advantages, such as durability and rigidity. However, few metal
surfaces retain their high reflecting power unless the reflecting
surface is protected. A few surfaces, such as polished or matte
satin finish aluminum, have been used with some success. Alumi-
num bronze lacquer on metal is largely used, and has been found very
satisfactory, particularly when properly protected from dust and
moisture by a transparent coating. Aluminum finished reflectors
without a protective surface have been known to depreciate 15 to
20 per cent, within a very short time, and once the surface lustre is
gone the reflection coefficient is permanently impaired. The metal
reflector with a porcelain or glass enameled surface has more than
held its own during recent years for purely utilitarian purposes.
The metal gives durability and rigidity and the enamel gives per-
manency of surface. The enamel surface is made so tough that it
will withstand much abuse without cracking. Metal reflectors
coated with paint, or baked enamel surfaces, make a reasonably
satisfactory substitute, where they are not subjected to too much
moisture or great changes in temperature. However, the reflecting
power deteriorates rapidly and the surface becomes yellow with age.
Glass. The best all round material for lighting accessories is
glass, which although brittle is beyond question the most permanent
LITTLE: MODERN LIGHTING ACCESSORIES 185
available for this purpose. On account of its reflection, transmission
and diffusion it is far in the lead. Clear glass, with mirror backing,
furnishes a combination of excellent qualities, such as permanency
and efficiency.
Fabrics. Silks, satins, chintz, etc., are much used for decorative
effects where efficiency and permanency are not of importance or
where they can be protected against depreciation.
Stone. Marble, alabaster and several other minerals have been
used to some extent where richness and distinction are sought.
Pottery. Pottery is used where decorative effects and not
efficiency are desired. Mirror reflectors are sometimes used in such
accessories, thus greatly increasing their efficiency.
USES
The uses to which lighting accessories are put may be divided into
four main classes: (i) utilitarian, (2) utilitarian and semi-decorative,
(3) semi-utilitarian and decorative and (4) purely decorative.
Utilitarian. The utilitarian accessory may be designated as one
whose main functions are efficiency, light control, and in some
cases color value, without serious regard to the appearance of the
unit. Usually the accessory is a reflector, and in a few cases a shade
or globe. Under this classification will be found a wide variation of
materials and types such as: Enamel, aluminum, aluminum bronze,
white glass, mirror glass and clear glass. Among the most practical
is the white enameled steel reflector. Its permanency and durability
of surface and practical indestructibility, coupled with its high
coefficient of reflection and diffusion, have caused it to be very widely
used.
Aluminum and aluminum bronze reflectors fulfill most of the
functions of the enameled reflector, with slightly better light control,
though the permanency of surface even when protected is not so
good. White diffusing glass, where protected from breakage, is
efficient and durable. Mirror and prismatic reflectors, by reason of
their flexibility of light control have a field of usefulness. Clear
blue glass units for color matching also fall in this class.
Utilitarian and Semi-Decorative. The utilitarian and semi-decora-
tive lighting accessories must be reasonably efficient, accurate in
light control, and present an appearance which is unobjectionable.
Of this class the majority of accessories are reflectors, a few are bowls,
and a few globes, and as a rule these are made of white, clear and mir-
rored glass. For this purpose the white diffusing glass is perhaps
1 86 ILLUMINATING ENGINEERING PRACTICE
most, used, though prismatic and mirrored glass are also employed
and in some cases the "crystal roughed inside" globe is still retained.
Decorative and Semi- Utilitarian. In the decorative and semi-utili-
tarian class the principal functions necessary are, a thoroughly satis-
factory appearance and a reasonable degree of efficiency and effec-
tiveness. The effectiveness must not only be measured by the ratio
of output to input, but also in terms of light control and satis-
faction. Light control in this connection is not necessarily light re-
direction, but is the securing of the proper balance or weighting of
reflection, transmission, and diffusion. This class embraces the
reflector, the transparent, translucent and opaque bowl, and the
transparent and translucent globe. It is therefore essential that a
wide range in these qualities be available. The materials from
which these are usually made are: white, clear and mirrored glass,
tinted or colored glass and fabrics. In this class may be placed the
white diffusing glass where transmission and diffusion are important;
the prismatic and mirrored glass where control and efficiency are im-
portant; tinted or colored glass, and fabrics where colored light and
decorations are required.
Decorative. The decorative accessory may be of such varied con-
struction, design or material that it may include anything from the
bare light source to the most inefficient and highly absorbing media.
It includes the reflector, the shade, the bowl, the globe and other
forms which cannot be classified. In many cases the decorative
feature is all-important and the illuminating value is a secondary
consideration. The materials from which these accessories are
made are: white, tinted or colored glass, iridescent and art glass,
fabrics, stone and pottery. The white glass, tinted with a superficial
coating of enamel, paint, or iridescent glass, is much used. This
superficial coating may be etched away, making innumerable possi-
bilities for ornamentation; or the white glass may be employed in its
usual form with the walls of the accessory varied in thickness, in
order to bring out the decoration in relief. The use of colored glass,
iridescent glass, art glass, fabrics and pottery is extensive in this
type of accessory, and glass has supplanted to a considerable extent
the metal work formerly utilized.
STRUCTURAL CHARACTERISTICS
The glass used for lighting accessories may be divided into four
structural types: Clear glass, opal glass, cased glass and suspension
glass. The clear or crystal glass is used in the manufacture of prism,
LITTLE: MODERN LIGHTING ACCESSORIES 187
" ground" and "daylight" accessories; the white, "opal" type of
glass for accessories in which the complete mix is homogeneous; the
cased glass, in which is a combination of the crystal or colored glass
and refined opal, and "diffusing" glass which may be described as
crystal glass with small reflecting particles held in suspension (ala-
baster type).
With these four types of glassware the manufacturers make prac-
tically all of the more popular grades of accessories. To be sure each
manufacturer has his own way of treating his product, and his own
slight variation of the mix and firing in order to give some character-
istic finish.
"The crystal glass is the ordinary clear glass when applied to illuminat-
ing glassware. This must not be confused with other crystal glass which
is used for cut glass, tableware, etc. The former is a common flint glass
with no particular brilliancy, and is of a more or less inferior quality in so
far as the glass itself is concerned. In the latter case, the glass is a highly
refined decolored prismatic glass, having unusual brilliancy. As the
commoner type of crystal glass meets all the requirements for illuminating
purposes, it is generally adopted for this class of work." 1
The opal type of glass is the basis of a large portion of all diffusing
glass, and is made in a number of degrees of refinement, from the
cheapest muddy white glass, as used in the earliest type of flat-
shades, to the refined opal used for casing purposes. Very different
and varied effects can be secured by surface treatment, and by vary-
ing the thickness of different portions of the glass. The thin por-
tions show in many cases a fiery red; the thicker ones a pure white
transmission. This characteristic is frequently taken advantage of
in working the design in the glassware. On the other hand opal glass
may be so made that it gives almost a pure white light transmission.
With the refinement comes a more nearly perfect diffusion, and the
glass usually becomes more dense. With the increased density the
flashing or cased process is usually employed. The casing may be
either on the inside, outside, or on both sides of the crystal glass, and
the layer of casing may be as thin or thick as desired, thus giving a
large range in transmission and diffusion.
The surface treatment may be an acid etch (wax or satin finish),
a sand blast, or a superficial tinting applied with an air brush or other
means, and fired in, making a fairly permanent surface. The tint-
ing may be shaded off by spraying the surface at an angle so that
shades and shadows are produced. The glass may also be covered
i Contributed by Mr. A. Douglas Nash.
1 88 ILLUMINATING ENGINEERING PRACTICE
with an enamelled tinted surface which is frequently etched away in
patterns. Other methods of treating the surface, such as chipped
glass, make very effective finishes. The chipping process is accom-
plished by covering the surface of the roughed glass with a specially
prepared mucilage or glue, and then placing the glass in a furnace and
allowing the paste to shrink away, pulling small particles of the
glass with it. The opal glass is suitable for all classes of manufac-
ture, while the cased glass is made only by the blown process.
Tinted glass may be made having the same structure and char-
acteristics as the white opal, the tinting being in the glass and making
a homogeneous mass. This glass is of course selective in reflection
and transmission, and therefore not highly efficient. However, it
has possibilities as a practical lighting glassware, where color effects
are desirable.
The alabaster type (sometimes called phosphate or alumina glass),
or crystal glass holding small reflecting particles in suspension, forms
the basis of many diffusing accessories. This glass may be made
in varying degrees of density, from almost transparent to almost
opaque. It may be either blown or pressed. The formulas and
methods of. working this glass are varied so that each manufacturer
may secure his characteristic types. Glass is produced with particles
so fine that the mass appears homogeneous, or the particles may be
sufficiently large to be readily seen. The texture may present a
pure white appearance, or a watery appearance. It may be left
as it is taken from the iron mold, or it may be finished with a high
gloss or fire polish. When blown this glass is usually thin, highly
translucent, and in many cases poor in diffusing qualities. When
pressed it is usually more dense, and better in diffusing qualities.
A tinted glass of this same character has been produced, but up to
the present time there has been but little on the market.
Some effort has been made to secure perfect diffusion from clear
crystal glass. This, of course, would show a very high transmission
value with very little absorption. However, it will probably have
the disadvantage of appearing as bright as the light source in numer-
ous small spots. This same phenomenon manifests itself to a con-
siderable extent in prismatic glass, particularly where the prisms are
not sharply cut.
MANUFACTURE
Metal. The processes of manufacture of metal accessories need
but little explanation. However, they may be classified as follows :
LITTLE: MODERN LIGHTING ACCESSORIES 189
Process. Spinning and pressing.
Finishing. Polishing, etching, spraying, and enameling.
The metal reflectors are either spun on a form or pressed in a
die. The finishes consist of polishing the metal, scratch brushing
or acid etching the aluminum surfaces, or spraying aluminum bronze
on the surface and enameling with porcelain or paint. A consider-
able increase in the life of the aluminum surface is secured by the
use of a transparent coating which prevents the removal of the alum-
inum when cleaning and there are no roughened surfaces to accu-
mulate dust. The porcelain enamel is applied as a liquid, and fired
in the furnace, each reflecting surface receiving at least five coats,
each coat individually fired.
Glass. The glass accessories are manufactured from the different
types of glass already described, by the following processes: blown,
pressed, pressed-blown, cased, bent and offhand.
Blown Process. The blown process consists of blowing a bubble
of the glass in an iron or paste mould. The paste mould process is
used whenever the accessory may be rotated in the mould; namely
when smooth and without design. This mould is made of iron lined
with paste. The rotating not only eliminates the seam in the glass
but produces a highly polished surface. Where a pattern or design
is traced on the accessory, an iron mould without a paste lining is
used. In this case a seam corresponding to the parting of the mould
will usually be found on the glass, and where a high polish is desired
the accessory must be fire polished. The fire polishing is accom-
plished by re-heating the glass almost to the point of fusion and cool-
ing it slowly.
The pressed process consists of placing the glass in the mould and
pressing it by means of various shapes and types of plungers. The
blown process is one of expansion or stretching, while the pressed
process is one of compression. The blown accessory has its two sur-
faces, inside and out, parallel, and the glass is of approximately the
same thickness throughout, while the inside surface of a pressed
accessory does not necessarily conform to the outside surface, thus
giving a wall of varied thickness.
Casing or flashing of glass consists of superimposing upon a core
two or more layers of glass of different kind or structure. The cas-
ing is done while the glass is on the blow tube.
The very nature of this glass means that each layer has its own
coefficient of expansion, which may differ from the adjacent layers.
Therefore, the annealing process is more difficult, and after installa-
1 90 ILLUMINATING ENGINEERING PRACTICE
tion the ordinary cased glass may not be subjected to changes in
temperature as great as in a glass of a homogeneous structure. How-
ever, if it is possible to secure the cases of glass having the same
expansion characteristics the finished product compares favorably
with that from a homogeneous mix, even though subjected to exces-
sive temperature changes.
Many accessories are made of flat glass bent to the desired shape.
The bending process is accomplished by making metal moulds lined
with paste or chalk, laying the glass over the mould, and placing
it in an oven which brings the glass slowly to the proper temperature
so that it falls of its own weight, taking the shape of the mould.
This does not change the texture or structure of the glass. The
bent glass form may be cut and leaded to make a unit, or it may be
left as taken from the mould.
Another operation which has proven very satisfactory and a great
time-saver in the manufacture of globes is the pressed-blown process.
A mould is made cone-shaped with a rounded tip, the top having the
proper dimension for the opening in the globe. A blank is formed
in this mould and while soft and plastic, placed in a second mould
and the cone-shaped form is blown by compressed air to the desired
shape. This process insures a greater uniformity in the accessory
and retains the characteristics of blown glass.
"In the off-hand, or hand-made process, the glass is gathered in much
the same way as in the two previous methods. 1 So called moulds are some-
times used to produce characteristic designs or marks on the glass itself.
However, in this case, the piece of glass is dipped into these moulds before
blowing, so that the raised portions chill more rapidly and retain this
design during the process of making. In the case of opalescent glass, this
treatment is of manifest advantage, as it results in the chilling of the
raised portions. When the mass is re-heated, the chilled portions become
more opaque than the core, and when completely blown, the design in the
mould is shown on the piece by reason of this added opacity. This
method of dipping is also used in the case of mould blown glassware, and
has a tendency when blown into a paste-mould, of throwing the design or
corrugations to the inside, giving a very effective lens-like appearance to
the design. Hand-made glass lends itself to much more effective manipu-
lation than any other process. Venetian glass has always been made in
this way. Opportunities are offered for applying either to the core or
semi-finished product designs in various colored glasses. When applied
to the core, they produce designs which enlarge with the blowing of the
piece, and the ultimate effect is a flat design in color. When applied to
1 Contributed by Mr. A. Douglas Nash.
LITTLE: MODERN LIGHTING ACCESSORIES 191
the semi-finished product the design is in relief, this latter method is
elaborated upon by certain manufacturers by the use of pincers or some
other suitable tool to form the applied glass into various shapes, producing
very elaborate results. The well-known Salviati glass is the best example
of a production of this character. In the production of Favrile glass, both
methods are used. Owing to the fact that the coloring of glass has a
tendency to somewhat change its chemical characteristics, these processes
require unusual care in annealing, and in the production of some effects,
the loss on this account is very great.
The annealing of glass is very important, each piece requiring a
sufficient period of time to cool. The larger and thicker the piece
the slower the cooling process. The annealing of large pieces is most
important as they are usually thick and heavy and breakage after
installation may be serious, not only as to cost but also from a stand-
point of safety.
"This argument leads to the matter of annealing as applied to all
classes of glassware used for lighting purposes. 1 The modern use of large
units has led many manufacturers to adopt special means of annealing.
In normal glassware, the annealing process should take not less than
twenty-four hours, during which time the article should be very gradually
reduced from its working temperature to atmospheric temperature, but
additional time should be given to this when the weight or size of the
article varies as in the case of pressed glass. The imperfectly annealed
article may break from no apparent cause.
OPTICAL PROPERTIES
Reflection. The coefficient of reflection as defined by the Committee on
Nomenclature and Standards of the Illuminating Engineering Society is:
"the ratio of total luminous flux reflected by a surface to the total lumin-
ous flux incident upon it. ... The reflection from a surface may be
regular, diffuse or mixed. In perfect regular reflection all of the flux is
reflected from the surface at an angle of reflection equal to the angle of
incidence. In perfect diffuse reflection the flux is reflected from the sur-
face in all directions in accordance with Lambert's cosine law. In most
practical cases there is a superposition of regular and diffuse reflection.
"Coefficient of regular reflection is the ratio of the luminous flux
reflected regularly to the total incident flux.
" Coefficient of diffuse reflection is the ratio of the luminous flux re-
flected diffusely to the total incident flux."
Polished metal, mirror, clear and prismatic glass in fact any highly
polished surface follow the law of regular reflection and the coefficient
varies with the perfection of the surface and angle of incidence.
1 Contributed by Mr. A. Douglas Nash.
I Q2 ILLUMINATING ENGINEERING PRACTICE
Enamel and white glass with polished surface follow both laws of
reflection, while matte surfaces such as aluminum bronze, depolished
or rough glass, normally tend to follow the law of diffuse reflection.
No surface will produce perfect diffusion for the reason that all
surfaces reflect regularly to some extent. The quantity of diffuse
reflection will vary to some extent according to the perfection of the
surface.
Transmission. The light transmission through glass will depend
upon its density, its surface and index of refraction. Referring to the
Fresnal formula 1 for light transmission through glass, it is seen that
through the ordinary sheet of glass whose index of refraction is 1.5,
there can be only 92 per cent, of light transmission, neglecting
absorption. This is for the reason that approximately 4 per cent,
of the incident flux is reflected from each surface. Some recent
experiments in the oxidation of glass surfaces show an apparent re-
duction in the index of refraction of the outer surface which has
reduced this reflection from 8 per cent, to approximately 3 per cent.
This apparent change in refractive index when applied to a lens does
not in any way change its focal length.
Light may be transmitted through glass in several different ways, as
1. Transmission without redirection.
2. Transmission with redirection without diffusion.
3. Transmission with redirection and diffusion.
Transmission without redirection is the transmission of light
through clear glass having both sides parallel.
Transmission with redirection without diffusion is the phenomenon
secured with the use of totally reflecting prisms and mirror.
Transmission with redirection and diffusion is that which is secured
when light is passed through roughed or ground crystal glass or
through white diffusing glass. The degree of redirection and dif-
fusion in white glass is dependent upon the quality of the glass and
character of the surface.
The absorption of light in glass is a difficult property to measure.
It has been stated that the absorption of light in clear optical glass is
approximately 3 per cent, per inch. Light absorption in glass is the
difference between total flux of light on the glass and reflected light
plus transmitted light.
Table I-IV shows the per cent, of light reflected, transmitted
and absorbed for various flat and nearly flat samples of clear
1 Fresnal formula, "Light Transmission through Telescopes" F. Kollmorgan, paper
read before the New York Section of the Illuminating Engineering Society, Jan. 13, 1916.
LITTLE: MODERN LIGHTING ACCESSORIES
193
and diffusing glass and the per cent, reflected for opaque surfaces.
These values are indicative only of the range in reflection and trans-
mission for the several classes of surfaces and materials, for the
reason that the perfection of the surfaces and the thickness of the
glass may not, and in some cases do not, represent average condi-
tions. Further, the values of absorption represent the per cent,
absorbed of the total flux falling upon the glass, and not the per cent,
absorption of the light which enters the glass; for instance, the Cal-
cite sample reflects 80 per cent., therefore 20 per cent, enters the
glass and 7 per cent, is transmitted, or 65 per cent, of the light
entering the glass is absorbed.
TABLE I. PER CENT. REFLECTED, OPAQUE MATERIAL; LIGHT INCIDENT AT 20
Per cent,
reflection
New Aluminum Bronze (unprotected)
CA
Corrugated Mirror .
So
Polished Brass
60
Polished nickel plate
64
Polished silver plate
QO
Polished Aluminum
67
Baked White Enamel (Paint)
72
High Gloss Porcelain Enamel
78
Mat Surface Porcelain Enamel, Sample No. i
70
Mat Surface Porcelain Enamel, Sample No. 2
76
Regular Surface Porcelain Enamel, Sample No i
77
Regular Surface Porcelain Enamel, Sample No. 2
75
* Silvered Mirror
83
* Uranium Glass Silvered Mirror
70
Mirrors supplied by C. A. Matisse.
TABLE II. PER CENT. REFLECTED, TRANSMITTED, AND ABSORBED,
CRYSTAL GLASS; LIGHT INCIDENT AT 20
Thickness
(mm.)
Per cent,
reflected
Per cent,
trans-
mitted
Per cent,
absorbed
4
M
"Pebbled" smooth side
rough side
" Roughed" smooth side
rough side
18
13
25
17
81
69
I
6
3*
7
"Cathedral" Glass smooth side.,
rough side
"Clear"
25
20
II
74
88
i
i
13
194
ILLUMINATING ENGINEERING PRACTICE
TABLE III. PER CENT. REFLECTED, TRANSMITTED AND ABSORBED; LIGHT
INCIDENT AT 20
Thick-
ness
(mm.)
Sample
Dense
Medium
Light
Per cent,
ref.
Per cent.
trans.
1j)
;r cent,
ref.
,r cent,
rans.
tH 03
;r cent,
ref.
;r cent,
rans.
c
PH
PH
*
PH
PH
PH
2
2
2
2
2
4
4'
3
3
4
4
4
3
2
4
3
9
4
2
2
7
6
6
6
3
Opal glass
Blanco R.O.*.
25
39
39
49
48
49
48
50
43
54
50
66
69
56
5
8
5
9
4
2
7
Blanco*
Acmelite*
Monex No. i
. j ..
46
44
46
Monex No. 2
Sudan: *
Polished side
59
57
59
56
29
29
12
12
Depolished side. .
Magnolia R. O.:*
Polished side
Depolished side
Radiant*
Calcite No. i:
Polished side
Iridescent side
Calcite No. 2:
Polished side
Iridescent side
80
78
79
76
7
12
13
19
74
70
12
14
Milk glass:
Polished side. . . .
Roughed side . .
Veluria*
Equalite:
Polished side
66
64
26
26
36
36
8
10
48
Semi-polished side
Cased glass
Polycase *
Camia*.
Acme Cased*
Sheet Glass
Celestialite (Three Layers)
Suspension glass
Parian Treated (R.I.) *
54
26
20
Parian Pressed*
Carrara*
33
4
6
9
30
29
37
Blown Alba No i
2
48
48
45
43
43
48
46
4 8
Blown Alba No. 2
Pressed Alba No. i
Pressed Alba No. 2
AlbaR. O. No. i:
Smooth side,
56
42
Rough side. .
Alba R. 0. No. 2:
Smooth side
Rough side
Druid*
* Samples slightly curved. Values therefore questionable.
LITTLE: MODERN LIGHTING ACCESSORIES ic
TABLE IV. PER CENT. REFLECTED, TRANSMITTED AND ABSORBED IRIDES-
CENT ART GLASS; LIGHT INCIDENT AT 20
^7
|i|
Sample
Per cent,
reflected
Percent,
transmitted
Per cent,
absorbed
Gold.. .
13
43
50
29
4
21
10
2
66
6l
94
Silver on Opal
Gold on Opal
Pink on Opal
Deep Blue
PER CENT. REFLECTED, TRANSMITTED AND ABSORBED, ROUGH ART
GLASS; LIGHT INCIDENT AT 20
M
Art Fire Opal: polished side
smooth side
17
29
15
14
68
57
ANALYSIS OF LIGHT LOSSES IN ENCLOSING FIXTURES
The light lost in the several parts of a fixture is shown in the following tabula-
tion. The tests have been made on two street lighting fixtures.
(a) Loss of light in housing 16 per cent.
Loss of light in glassware 16 per cent.
Loss of light in complete fixture 36 per cent.
(b) Loss of light in housing 18 per cent.
Loss of light in glassware 15 per cent.
Loss of light in complete fixture 37 per cent.
It will be noted that the sum of the losses in the glassware and
housing is less than the loss in the complete unit. This is occasioned
by the fact that the globe reflects additional light into the housing,
thus increasing the loss in the housing.
SKYLIGHT GLASS 1
The glass in plate form used in ceiling windows or the so-called
"skylights" backed by lamps is receiving more attention than here-
tofore. Crystal glass with various diffusing surfaces is available in
sufficient characteristic forms to enable the engineer to secure al-
most any light distribution required, from the slightly diffusing to
the widely distributing. Also the surface may be covered with
prisms to bend and redirect the rays of light.
1 1. E. S. Transactions, Vol. 9, page ion, "Lighting of Rooms through Translucent Glass
Ceilings." by Evan J. Edwards.
196 ILLUMINATING ENGINEERING PRACTICE
REFLECTOR DESIGN
With the more concentrated light sources as found in the gas-
filled tungsten or "Mazda C" lamps conies more accurate light
control from reflectors. Also the problem of reflector design is
simplified. Remembering that the angle of reflection is equal to
the angle of incidence wherever the surface follows the law of regu-
lar reflection, it is obvious that the widely distributing light dis-
tribution may be secured in either of two ways, viz:
The rays may cross, or diverge. If they are to cross, a deep
reflector must be used and a large percentage of the light impinges
upon the reflecting surfaces. Conversely if the rays diverge little
light falls upon the reflecting surface. Thus less control is secured.
Obviously, therefore, the crossed rays allow a more accurate light
control and at the same time tend toward a better concealment of
the bright source.
The concentrating reflector must produce more or less parallel
rays, and therefore, must approach the parabolic in shape.
The majority of types of light distribution range between these
two. Therefore, the surfaces need but slight modification to
secure the required results. As a rule when properly designed
the deeper the reflector the better the control and greater the light
loss.
To produce a predetermined light distribution with a diffusely
reflecting surface is more difficult and sometimes impossible. How-
ever, the same principle is followed.
No symmetrical reflector, or one whose surface is a surface of
revolution, will increase to any marked degree the light in a hori-
zontal direction about a lamp. The so-called deflector was designed
with this idea in view, with a surface parabolic in shape and the
source in the focus. But in order that a fair percentage of the light
should fall upon it its diameter would have to be so great as to make
it impracticable.
An unnecessary loss is experienced in many accessories by
trapping the light. This is very likely to be serious with the
ventilated units for Mazda C lamps. The top of the accessory is
usually closer to the filament than any other portion and subtends
a larger solid angle of light, and therefore should be most active
and valuable in light reflection. If this surface is not of the proper
contour to throw the light out, the light loss in the unit may be
excessive.
LITTLE: MODERN LIGHTING ACCESSORIES 197
PHOTOMETRIC PROPERTIES
Metal Accessories. The metal accessories have kept pace with
the change in lamp design and construction. With practically
each change in filament dimension, shape or location it has been
necessary to re-design the reflector. With the advent of the Mazda
C lamp many changes were necessary.
Aluminum Finished Reflector. The aluminum finished reflectors
are essentially indoor accessories of the utilitarian type. They are
made in deep and shallow cones and bowls, angle, trough or show-
Fig, i. Aluminum finished
reflectors.
case reflectors, and produce a complete range in distribution char-
acteristics from the widely distributing to the moderately concentrat-
ing. They are designed for practically all types of electric lamps
from the lo-watt Mazda B to the large sizes of Mazda C.
In Fig. i are shown characteristic candle-power distribution
curves for bowl type accessories; the light loss to be expected in this
type of reflector varies from 20 to 40 per cent.
Porcelain Enameled Steel. The enameled steel accessory is some-
what similar to the aluminum finished with a slightly increased re-
flector coefficient. It has a wider application, as it may be used in
or out of doors. It is made in all of the conventional reflector
I g8 ILLUMINATING ENGINEERING PRACTICE
shapes, and in addition, is designed for numerous asymmetric
distributions, where large flat vertical surfaces are to be evenly
illuminated. The light control is not as accurate as with the alumi-
num surface due to the diffusing qualities of the enamel.
Many of the reflectors, particularly of the deep bowl type, which
are used with the large sizes of Mazda C lamps are constructed with
ventilating hoods, and as they are frequently used with enclosing
glassware the ventilation feature is doubly important. However,
this ventilating feature is regarded by the manufacturer as becom-
ing less important as the lamps are now constructed. Where re-
Fig. 2. Porcelain enamel reflectors.
quired, enameled accessories without ventilators may be used with
enclosing gas or vapor-proof glass envelopes.
Some of the types of deep bowls have been constructed with fluted
surfaces for the purpose of eliminating bright streaks. These flut-
ings or corrugations also add to the rigidity of the reflector. Charac-
teristic candle-power distribution curves for these reflectors are
shown in Fig. 2. The loss of light for enameled accessory will vary
from 15 to 35 per cent.
Painted Enameled Reflectors. The painted enameled reflectors are
made in shapes similar to the more common types of porcelain
enameled reflectors. However, their chief quality is cheapness.
The connecting link between the metal and glass accessories is the
Figs. 3 and 4. Connecting link between metal and glass accessories.
Fig. 5. Prismatic semi-indirect unit.
(Facing page 198.)
Fig. 6. Typical modern fixtures
Fig. 7. Fixture to which may be attached a choice from a number of interchangeable bowls.
LITTLE: MODERN LIGHTING ACCESSORIES 199
metal hood and enclosing or semi-enclosing glassware (Figs. 3
and 4). Many decorative and semi-decorative units have been
designed embodying a hood or holder to which is attached the socket
and glassware.
GLASS ACCESSORIES
The glass accessories lend themselves to practically all lighting
purposes and are made up in innumerable designs. Unfortunately,
however, with few exceptions the accessory is made to meet the ideas
or tastes of the designer with little or no consideration for the light
distribution. Among the exceptions, may be cited most prismatic
and mirrored reflectors.
Clear Glass. Clear glass is used in the manufacture of a number of
types of accessories, namely: Clear; ground or etched; cut; pris-
matic, and mirrored.
Clear accessories are usually globes, the principal function of
which is the protection of the light source.
Ground or etched accessories in some cases lend themselves to
decoration, but it is regrettable that they must be classed as lighting
accessories, as their diffusion is poor and light-redirecting qualities
practically nil.
The only excuse for the existence of the cut accessory is to serve
as a medium of decoration, though in a few units it produces
some sparkle and life. Its redirecting qualities are, usually of little
importance.
The so-called "daylight" unit is properly an accessory, which,
when used with an artificial illuminant, will produce a light equiva-
lent in color to daylight (north sky or sunlight). This corrective
process usually consists in the use of the subtractive method of color
correction or the reduction of all of the light in proportion to the ratio
of the blue in the artificial light to the blue in daylight. The blue in
most artificial illuminants is approximately 10 per cent, of north sky
or 20 per cent, of sunlight. Therefore, the maximum theoretical
efficiency obtainable is 10 per cent, for north sky, and 20 per cent,
for sunlight.
However, where a whiter light is required than that produced by
the bare lamp, it has been found satisfactory to employ an accessory
which absorbs not over 50 per cent. This unit, of course, must not
be regarded as a color-matching unit. A slight reduction in the red
component frequently produces a very noticeable change in the
apparent color of fabrics, particularly where the blues predominate.
200
ILLUMINATING ENGINEERING PRACTICE
Glass manufacturers have taken advantage of this fact and have
made accessories with a thin casing of blue glass, usually on the
inside, thus not changing the appearance of the unit during the day-
light hours, -but at night producing a somewhat whiter light than
would otherwise be secured.
One claim for these modified units is that the light apparently
mixes to better advantage with daylight or twilight than the light
from the unmodified unit.
When a unit producing true daylight is to be installed where it
can be contrasted with the unmodified artificial light, it appears very
blue and observers do not believe it to produce light of real daylight
quality.
From tests made at the Electrical Testing Laboratories there is
an indication that transparent colored glass and gelatine increases
in absorption toward the blue end of the spectrum. The following
table shows the transmission values for red, green and blue light
through corresponding colors in glass and gelatine.
Color of
substance
Color of
light
Per cent, transmitted
Jena glass
Wratten filter
Red .
Red.
92
55
30
90
36
23
Green
Green
Blue
Blue
Both the glass and the gelatine filters were supposedly designed
for monochromatic light transmission, and the difference in trans-
mission values between the two substances is probably accounted for
in that the color in one case is purer than in the other ; therefore, more
nearly monochromatic. This absorption of light militates further
against the efficient production of an accurate daylight unit by the
subtractive method.
Prismatic Accessories. Unlike the majority of accessories, the
prismatic units are usually designed according to carefully worked
out prototype curves. Light control is accomplished by the use of
totally reflecting prisms which follow the general contour of the glass.
Furthermore, prisms may be refracting as well as reflecting. In
some units results have been secured by the use of both kinds of
prisms. If the contour of the glass and shape of the prisms are
properly formed, almost any light distribution may be secured.
LITTLE: MODERN LIGHTING ACCESSORIES 201
Corrugations have been placed in glass for the purpose of diffusing
light. These corrugations have been frequently called diffusing
prisms.
Prismatic accessories are made in numerous designs, each having
its characteristic distribution and function to fulfill. The conven-
tional forms of prismatic reflectors are well known; therefore, only
the newer types are here discussed. Totally enclosing prismatic
units are made to control the light quite as accurately as the pris-
matic reflector with but slightly increased loss. In this way the
light source can be entirely enclosed and still secure the desired light
distribution. Combinations of prismatic glass with white diffusing
glass make possible light control and elimination of glare such as
would be impossible with either one alone (Fig. 5) . A so-called semi-
direct unit has recently been developed consisting of a prismatic
reflector designed after a prototype curve using a clear glass or
"velvet" finish glass envelope conforming closely to the contour of
the reflector. Between these two is placed any fabric or paper to
correspond with the surrounding decorations. With this unit it is
possible to secure almost any desired ratio between the direct and
indirect components of light unit brightness and at the same time
secure decoration and color effects from the transmitted light.
Asymmetric prismatic reflectors have a large field of usefulness.
The refractor unit is notable in that it will to a marked degree re-
direct a large portion of the light at or near the horizontal. This
accessory has also been made in the form of a band refractor which
intercepts only the light above the horizontal, and this light may be
redirected wherever required, adding considerably to the light in the
lower hemisphere. The band carrying the refracting prisms is sur-
rounded by a second band carrying corrugations or ribbings which
diffuse the light in a plane normal to the surface, this producing a
nearly uniform brightness over the entire band rather than a bright
spot at its center.
An enclosing prismatic accessory is made with a standard reflector
for the upper portion and refracting prisms for the lower portion.
The refracting prisms break up and redirect the light falling upon
them, thus helping to eliminate excessive glare. The same general
function is performed by the reflector-refractor, which with
its combination of reflecting and refracting prisms breaks up and
redirects the light as desired.
A range in candle-power distribution curves which may be secured
from prismatic accessories is shown in Fig. 8. To the left is an
202
ILLUMINATING ENGINEERING PRACTICE
Fig. 8. Prismatic reflectors.
Fig. 9. Prismatic accessories.
LITTLE: MODERN LIGHTING ACCESSORIES
203
asymmetric reflector, the center a concentrating type, the right a
distributing type. The losses to be expected in these reflectors
range -from 12 to 14 per cent.
In Fig. 9 to the right will be found a reflector-refractor showing
a loss of light of about 20 per cent, thus showing large redirection
from an enclosing medium with a relatively small loss. To the left
will be found the candle-power distribution characteristic of a semi-
indirect prismatic unit (see Fig. 5).
Mirrored Accessories. The mirrored reflector, as in the case of
the prismatic, is usually designed to produce a predetermined light
Fig. 10. Mirrored reflectors.
distribution. Its efficiency is high and light control excellent. The
problem, however, is to retain a permanent reflecting surface. This
is a comparatively simple matter where not subjected to excessive
heat or moisture. Deterioration from the former cause has proven
a very formidable obstacle since the widespread use of the Mazda
C lamp. The mirror reflector must consistently follow the changes
in lamp construction, filament location and design, for the reason
that it functions by the principle of regular reflection. Therefore
its efficiency is closely related to its contour and location of light
center. With the introduction of the concentrated filament it was
204 ILLUMINATING ENGINEERING PRACTICE
found that the corrugations in the reflectors made for the Mazda B
lamps were not sufficiently fine or numerous to prevent bright
streaks. This led to the development of a new series of reflectors
with very fine waves or corrugations.
The flat corrugated mirror strips in trough reflectors are still
much used. With this type of reflector extremely accurate light
control in a plane normal to the axis of the reflector can be secured,
as the strips can be made as wide or as narrow as desired and each
installation may have a particular reflector designed for it.
In Fig. 10 is shown characteristic distributions of mirrored acces-
sories. The light loss in these units is from 15 to 20 per cent.
Diffusing Glassware. Diffusing glassware as used in lighting ac-
cessories furnishes a wide range in reflection, transmission and diffu-
sion, and this range may be varied to a considerable extent in any
one type of glassware by changing its density, its thickness, its sur-
face and its contour. Added flexibility is frequently secured by
coating the glass with a white enamel. The enameled surface has
a high reflecting power and low transmission.
Opal glass is used in the manufacture of reflectors, bowls and
globes. By the proper selection of thickness and densities varied
effects may be secured. The dense opal accessory when properly
shaped may produce an excellent reflector so far as light control is
concerned.
When used in bowls it can be thin with high transmission or dense
with little transmission. The diffusing qualities are very good,
particularly in all cases when the surfaces are roughed.
Per cent,
reflected
Per cent.
transmitted
Dense (4 samples)
80 to 76
7 to 12
Medium (2 samples)
74 to 70
12
Light (9 samples)
CO to 4^
2O to 40
The characteristic candle-power distributions to be expected from
bowl reflectors of the opal type are shown in Fig. n. To the left is
the pressed reflector; to the right is the blown. The light loss in
these reflectors ranges from 12 to 20 per cent.
Accessories made of cased glass are usually of the totally enclosing
type as its principal purpose is high transmission coupled with good
diffusion. Some so-called reflectors are made of cased glass but they
LITTLE: MODERN LIGHTING ACCESSORIES
205
should rightly be classed among the shades. Occasionally the cas-
ings are made sufficiently thick to reduce the transmission to a com-
paratively low figure.
Bowls have been made of cased glass but the units are usually un-
satisfactory, resulting in a very high brightness and small reflection.
Fig. ii. Opal reflectors.
Per cent,
reflected
Per cent,
transmitted
Dense (i sample, 3-layer glass)
T4
26
Medium (2 samples)
36
Light (2 samples)
48
43 to 50
Fig. 12 shows a characteristic candle-power distribution for a
cased glass bowl reflector. It will be noticed that the transmission
is high and reflection low, the loss in this type being approximately
8 per cent.
The active interest in diffusing glassware found its beginning in
the suspension type. Other diffusing accessories were made in opal,
cased and roughed crystal glass, but not until the development of
2O6
ILLUMINATING ENGINEERING PRACTICE
the alabaster type did the use of diffusing glassware receive its full
impetus. The flexibility of this glassware makes it particularly
valuable.
Fig. 12. Cased glass reflector.
Fig. 13. Suspension glass accessories.
LITTLE: MODERN LIGHTING ACCESSORIES 207
Per cent,
reflected
Per cent,
transmitted
Dense (2 samples) ..
<;6
32 to 4.2
Medium (5 samples).
4.8 to 4.1
46 to 48
Light (5 samples)
37 to 29
69 to 50
In Fig. 12 will be seen candle-power distributions for suspension
glass accessories. To the left are two types of distribution curves
both showing considerable transmission and comparatively little re-
flection. The loss in these accessories varies from 8 to 12 per cent.
To the right is a suspension glass dish. The loss in this accessory
is ii per cent.
Suspension glass lends itself readily to either blown or pressed
accessories made in either iron or paste molds. As its -density
varies from the almost transparent to the almost opaque, so also do
its diffusive qualities. In the dense glassware fairly accurate light
control may be secured. Therefore, it is used to good advantage
in reflectors and bowls and the less dense glass is used in globes.
FIXTURES
The problem of good fixture design is complex and few designers
approach it from the same standpoint. The introduction of bowls of
diffusing glassware has to a very considerable extent curtailed the
demand for the conventional (old-fashioned) fixture. This curtail-
ment has been obviously caused by the entrance into the field of the
glass manufacturer. In many cases the glass superseded the metal
work in fixture. Had the fixture houses been as active in pushing
the glass bowl type of unit as they were in pushing older, more con-
ventional types, it probably would not have been necessary for the
glass manufacturer to enter the field, and the types of fixtures might
have followed a different style of design.
The fixture business may be divided into two principal classifica-
tions, the stock fixture and the special fixture. The stock fixture
follows to some extent a definite period design. The special fixture
is supposedly made to harmonize with a particular environment. As
examples of the type of recent stock fixtures might be cited the can-
delabra wall bracket, usually using frosted round bulb electric lamps;
the center candelabra chandelier, similarly equipped; the chandelier
in the ring or bracket form using the same round bulb frosted lamps;
208 ILLUMINATING ENGINEERING PRACTICE
the dome in art glass or silk; the table lamp with glass or silk shades,
Fig. 6. Possibly the only characteristic shape on the table lamp
shade of silk is that of the frustum of a cone with the top diameter
only slightly less than the bottom.
The officers of a much imitated fixture house which boasts of the
pick of the trade and carries no stock fixtures assert that there has
been no advance or characteristic change in fixture design for the past
twenty years other than a tendency toward a larger number of lamps
of lower intensity. They state that the crystal fixture is more
popular than ever. The side-wall bracket is coming into great favor,
in the majority of cases using the bare frosted lamp, and in some cases
a silk shade or eye shield. In almost no case is glassware of any
description used. They do state, however, that the table lamp is used
to supplement the wall brackets and chandeliers. Further, appar-
ently no effort is made to redirect or control in any way the light
from the small round bulb frosted lamps. The light distribution
characteristics of these fixtures is of no consequence whatever to
these fixture designers. Even in the case of enclosing glassware the
tendency is toward cluster rather than single unit lamps. When the
diffusing bowl type of unit is employed it is frequently supplemented
by candle brackets.
One of the representative manufacturers stated that the resultant
illumination produced was almost never considered as part of his
work, or part of the artistic and aesthetic feature of the installation
as a whole. This state of affairs should be looked upon with con-
siderable alarm by the illuminating engineers, particularly at this
time when light source brightnesses are so decidedly on the increase.
An exception to this practice was found in one of the largest and
best fixture houses in New York where the quality of light, light dis-
tribution and light control are the first conditions, and the design
is worked around these. This house would not consent to showing
designs or photographs of any fixtures, as such, without knowing
where and under what conditions the fixtures were to be used.
Here at least good taste and quality of light are paramount, and the
purpose of the fixture in many cases is disguised in the design. This
does not mean, however, that unnatural and disconcerting conditions
are tolerated.
A very popular and satisfactory diffusing bowl unit is found in the
alabaster accessory. The stone as it is taken from the Italian quarry
is quite translucent and in some places almost transparent, thus
producing a beautiful effect of fire and life without excessive bright-
LITTLE: MODERN LIGHTING ACCESSORIES 209
ness. Unfortunately it must be used with discretion as it cracks
readily and will blacken if not properly protected from the lamp.
Here again little attention is given to shapes or designs which might
produce an advantageous light distribution but as they are usually
employed to produce a generally diffused illumination and supple-
mented with localized lighting, their distribution characteristics are
of little importance. Beautiful designs have been worked out on
these bowls, in some cases the depressions are colored with a sepia
stain giving them a rich day as well as night value. The density of
the stone is quite sufficient to keep the surface brightness down to a
satisfactory value.
The total disregard of quality, fitness and distribution of light is
not so prevalent among the glass- and reflector-manufacturers who
also make fixtures. In many cases they are attempting to secure
the desired weighting of transmission and reflection, and the tend-
ency is toward a consideration for quality by tinting the glass.
On the other hand, glass manufacturers are prone to consider their
one or two types of glassware the panacea for all lighting ills,
whereas a slight modification in the mix or density of the ware
would make success of failure. Such a step in this direction has
been taken by a fixture producer, in the design of a single stem
from which are mounted either gas or electric lamps and to which
may be readily attached any one of a number of bowls having
different shapes, densities and colors, Fig. 7.
An improvement in the resultant illumination from semi-direct
fixture is being accomplished by placing a thin diffusing glass plate
over the bowl. This eliminates chain shadows and simplifies the
cleaning problems.
Indirect fixtures are now made utilizing the accurately designed
mirror reflector inside a diffusing glass bowl, and by means of an
auxiliary lamp or a diffusing cup in the bottom of a mirror reflector
the bowl is illuminated to the desired brightness. This arrangement
to a very marked degree, eliminates the argument against indirect
fixtures, namely, that the fixture appears dark against the illuminated
ceiling.
Table lamps are also designed with some conception of the result-
ing light distribution. The lamps, using silk shades as above de-
scribed, are frequently equipped with real reflectors which control the
light produced. This shape will allow of a considerable upward
component for a semi-indirect unit or may be equipped with a
reflector throwing a large portion of the light downward. Fre-
14
2IO ILLUMINATING ENGINEERING PRACTICE
quently the silk alone is used as a reflecting surface. Much flexi-
bility can be secured as the silk may be left highly transluent or
diffusing and additional layers may be added to secure the desired
transmission and different colors for different effects.
In an effort to make semi-indirect units more universal and allow
them to be used even where highly reflecting ceilings are not avail-
able or in those locations where the ceilings are too high above the
logical locations of the unit the fixture manufacturer has designed
a small portion of ceiling to go with the bowl. The fixture therefore
consists of a diffusing bowl and a reflecting surface a short distance
above. This method serves to enlarge slightly the light giving area,
and thus to decrease correspondingly the fixture brightness. The
upper reflecting surface has been changed in size, location and shape
by the several manufacturers, but always serves the same purpose.
A survey of the field indicates that excellent accessories of a wide
variety are available. As a rule, however, these follow conventional
lines according to well recognized concepts of design and use. Only
to a slight extent are designers undertaking to provide accessories
which represent adaptation of simple means of directing, diffusing
and tinting the light along unconventional lines.
References
E. B. ROWE. "Some tendencies in the design of illuminating glassware."
Electrical Engineering, Sepember, 1914.
JAMES R. CRAVATH. "Glass globes for street lamps." Municipal Journal,
August 27, 1914.
RENE CHASSERIAUD. "The art of logical lighting (French)." Societe
Beiges des Electriciens (Brussells), May, 1914.
GUIDO PERI. "Present status and tendencies in electric illumination
(Italian)." L'Industria (Milan), November i, 1914.
H. B. WHEELER. "Lighting of show windows." Illuminating Engineering
Society Transactions, September, 1913.
W. W. COBLENTZ. "The diffuse reflecting power of various substances."
Bulletin of Bureau of Standards, April i, 1913.
H. J. TAITE and T. W. ROLPH "Notes on metal reflector design."
General Electric Review, May, 1914.
A. L. POWELL. "An investigation of reflectors for tungsten lamps."
General Electric Review, November, 1912.
M. LUCKIESH." Investigation of diffusing glassware." Electrical World,
November 16, 1912.
W. W. COBLENTZ. "Diffuse reflecting power of various substances."
Journal Franklin Institute, November, 1912.
L. BLOCK. "Reflectors and accessories for lighting inner rooms with metal
LITTLE: MODERN LIGHTING ACCESSORIES 211
filament lamps (German)." Elektrotechnik Und Maschinenbau, October
13, 1912.
DR. L. BLOCK. "Reflectors for metal-filament lamps." London Elec-
trician, March 21, 1913.
A. L. POWELL and G. H. STICKNEY. " Data concerning incandescent
reflectors." Electrical World, September 6, 1913.
VAN RENSSELAER LANSINGH. "Characteristics of enclosing glassware."
Illuminating Engineering Society Transactions, September, 1913.
W. T. MACCALL. "Half frosted lamps in reflectors." London Electrician,
October 3, 1913.
A. L. POWELL. "Reflectors for tungsten lamps in industrial and office
lighting." Electrical Engineering, October, 1913.
DR. L. BLOCH. "Choice of reflectors for street lighting." London Elec-
trician, May 31, 1912.
L. BLOCH. "Choice of reflectors and proper heights for metal filament
street lamps." Elektrotechnik und Maschinenbau, December 3, 1911.
R. HORATIO WRIGHT. "The mazda lamps with a few common types of
reflectors." Sibley Journal of Engineering, November, 1910.
C. TOONE. "Globes, shades and reflectors." London Electrical Review,
June 16, 1911.
P. G. NUTTING, L. A. JONES and F. A. ELLIOTT. "Tests of some possible
reflecting power standards." Illuminating Engineering Society Transactions,
Volume 9, No. 7, 1914.
LEONARD MURPHY and H. L. MORGAN. "Distribution and efficiency tests
on lamp shades and reflectors." London Electrical Review, July 7, 1911.
GEO. H. MCCORMACK, ALBERT JACKSON MARSHALL, L. W. YOUNG; Intro-
ductory remarks by BASSETT JONES, JR. "Symposium on illuminating glass-
ware." Illuminating Engineering Society Transactions, September, 1911.
THOMAS W. ROLPH. "Reflectors for incandescent lamps." Electric
Journal, May, 1910.
J. R. CRAVATH. "Show window illumination." Central Stations, May
1910.
C. E. FERREE and G. RAND. "Some experiments on the eye with inverted
ieflectors of different densities." Illuminating Engineering Society Transac-
trons, December 20, 1915.
FRANK A. BENFORD. "The parabolic mirror." Illuminating Engineering
Society Transactions, December 20, 1915.
HAYDEN T. HARRISON. "Efficiency of projectors and reflectors." Ab-
stract of a paper read before the Liverpool Engineering Society.
LIGHT PROJECTION: ITS APPLICATIONS
BY E. J. EDWARDS AND H. H. MAGDSICK
Light projection, as the term is commonly employed, covers the
redirection of light flux from artificial sources by means of suitable
optical systems so that it may be utilized within solid angles which
are small as compared with those encountered in equipment for gen-
eral illumination purposes. It was in connection with such applica-
tions in a few restricted fields that some of the more important prin-
ciples of optics and illuminating engineering were long since devel-
oped and applied. During the past few years these applications
have multiplied rapidly, occupying the attention of many illuminat-
ing engineers and giving rise to numerous papers in the Transactions
of the Illuminating Engineering Society and articles in the technical
press dealing with the principles of optics, searchlighting for military
and navigation purposes, flood-light projectors for displaying sur-
faces at a distance, headlighting for vehicles, orientation lighting
for the navigator, light signals, and apparatus for the projection
of enlargements of transparencies.
Two general classes of apparatus are used to direct the flux from
a source into the desired small angle: Opaque reflector systems con-
trolling the light by the principle of specular reflection, and lens
systems depending upon the refractive properties of glass. Fre-
quently the two forms of control are combined in the same device.
In Fig. i, A, is illustrated the action of a simple convex lens. A
light ray emerging from the focus, F, is refracted in passing through
the lens so as to be projected parallel with the axis, while from a
larger source as shown at the focus, a cone of light is projected with
an angle of divergence, 26, depending upon the size of the source,
the focal length of the lens and the angle, a, at which it is emitted.
The greatest angle of divergence is that of the cone issuing at the
axis of the lens. These statements apply to lenses intercepting the
flux in a relatively small solid angle. As the diameter of a lens in-
creases relative to the focal length, the thickness, and hence the
absorption, increase rapidly and the control of the emerging rays is
limited by the increasing spherical and chromatic aberration. To
213
214
ILLUMINATING ENGINEERING PRACTICE
reduce these elements of inefficiency, Fresnel nearly one hundred
years ago built a lens of concentric rings, Fig. i, B; in effect a large
convex lens with sections of the glass removed. He also added con-
centric prism rings to direct additional light into the beam by total
reflection. Later these prisms were given a curved surface and re-
Axis
B
Fig. i. Light projection with lenses.
fraction was combined with reflection to produce the desired results.
It will be noted that the sections give rise to a series of dark rings
when viewed within the beam, since the light striking the risers is
deflected at a large angle from the axis. In Fresnel lenses of reason-
Axis
Axis
Fig. 2. Light projection with opaque reflectors.
ably effective angle, the solid angle subtended by the lens at the focus,
the contour of the surface may be so corrected as to secure very ac-
curate control of light. They are frequently referred to as stepped
or as corrugated lenses.
Rays emerging from a source at the center of a sphere are reflected
from the polished surface as shown in Fig. 2, A. Used in this manner
EDWARDS AND MAGDSICK: LIGHT PROJECTION 215
as an accessory with a lens on the other side of the source, the mirror
increases the amount of light intercepted by the lens, providing the
source is at least partially transparent. With the source placed on
the axis of a spherical mirror at half the radius, rays are returned
with only a small divergence from the parallel when the effective
angle is not large. Mangin devised a spherical mirror of silvered
glass with the radius of the inner surface less than that of the outer,
Fig.. 2, B. The varying degree of refraction introduced by this con-
cavo-convex lens is utilized to keep the divergence of the beam
within narrow limits for effective angles up to as much as 120.
The greatest efficiency and accuracy in concentrating light with
an opaque reflector is secured with a parabolic contour, since all
rays from the focus are reflected parallel with the axis no matter
o
Axis
B
Fig. 3. Light projection with opaque reflectors.
how large the effective angle is made. The divergence from a source
as in Fig. 3, A, is greatest at the axis and decreases with increasing
angles. Only within the angle of the cone showing the smallest
divergence, that is the cone emanating from the edge of the mirror,
does the beam contain light from all parts of the surface, and hence
only in this region does the measured candle-power obey the inverse-
square law. Beyond this limiting cone, light is received from a de-
creasing zone of the reflector until at the edge of the cone only the
point at the axis is effective. Fig. 3, J5, shows one combination of
reflecting surfaces and lens among several that may be employed to
meet various requirements.
In all of the projection devices described above a part of the beam
receives light from the entire surface. In some cases this is at the
axis only; in others, over a wider angle. The brightness of the sur-
2l6 ILLUMINATING ENGINEERING PRACTICE
face is in every case the brightness of the source at the respective
angle multiplied by the coefficient of reflection or transmission of
the system. The intensity of the beam within this range is, there-
fore, the product of the brightness and the projected area of the sur-
face; variations in the focal length and the effective angle do not
change the result. The multiplying factor of the system is then
approximately the ratio of the squares of the diameter of the mirror
and the diameter of the source. Table I, giving the brightness of
the various sources used in projection apparatus, indicates their
relative value so far as the production of the maximum beam in-
tensities is concerned.
In most applications a beam can advantageously be utilized with
a divergence so great that the total amount of flux in the beam is of
equal or greater importance than the central density. The effective
angle of the system, the size of the source and the focal length are
important factors in determining the width of the beam, the total flux
and its distribution. Table II gives the solid angles subtended at
the focus by parabolic reflectors and lenses of various proportions.
The latter are most often applied where accuracy of control is re-
quired; the former where it is desired to intercept the flux in a rela-
tively large solid angle. The average opaque projector system
directs from 30 to 60 per cent, of the available light into the beam ;
with lens systems, typical effective angles are so small that only
5 to 10 per cent, is transmitted. The cost of the respective types
of apparatus for different sizes is, of course, often the determining
factor in their adoption; in general the cost of lenses increases the
more rapidly with larger size.
TABLE I. INTRINSIC BRILLIANCY OF COMMON PROJECTION SOURCES
Source
Candle-power per sq. inch
Flame Arc for search lighting
250 000350 ooo
Carbon Arc " " "
80,000 90,000
Magnetite Arc
4 ooo~- 6 ooo
Mazda C Projection Type
o ooo~ 1 8 ooo
Mazda C Regular.
3 coo
Mazda B Concentrated
Mazda B Regular . .
1,200
7 ^O
Kerosene Mantle..
2OO~'5OO
Acetylene.
60
Gas Mantle
7O CO
Kerosene Flame
S TO
EDWARDS AND MAGDSICK: LIGHT PROJECTION
217
TABLE II. PERCENTAGE OF TOTAL SOLID ANGLE SUBTENDED BY PARABOLIC
REFLECTORS AND CONDENSING LENSES
Parabolic reflectors
Condensing lenses
Ratio of diameter
of opening to focal
length (R)
Percentage of total
solid angle
Ratio of diameter to
focal length
(R)
Percentage of total
solid angle
2
2O. O
0-3
0.6
3
36.0
0.4
I.O
4
50.0
0-5
1.5
5
61 .0
0.6
2 . I
6
69.2
0.7
2.8
7
75-4
0.8
3-6
8
80.0
0.9
4-4
9
83-5
.0
S-3
10
86.2
.1
6.2
.2
7-i
Percentage of Total Solid Angle
3
8.1
cos - + i
4
9.1
2 V inn
5
IO.O
Ps loo
2
2.0
14.6
J?2
./Y
2 -5
10. I
~ # 2 +i6 X I0
Percentage of Total Solid Angle
/i - cos 6 \
= (-S ~
/ />S 1<J<J
/ I X 100
There are four principal surfaces employed in opaque projectors.
Those of mirrored glass and silvered metal have a coefficient of
reflection of the order of 85 per cent. Polished aluminum reflects
slightly more than 60 per cent, of the incident light, and a nickel-
plated brass surface has an efficiency of less than 55 per cent. All of
the metal surfaces tarnish and require repolishing or replating from
time to time. Silvered metal deteriorates rapidly where air circu-
lates over it, particularly in a salt atmosphere and where fumes from
stacks are present. The nickeled and aluminum surfaces depreciate
less rapidly. The aluminum has the further advantage that re-
polishing does not also in time involve replating as with the other
metal units. Silvered glass is usually found the most desirable and
economical in the long run, although where there is no intense heating
and the reflectors may be tightly enclosed, silvered metal is found
very satisfactory. The light absorption by lenses varies with the
thickness of the glass and the nature of the construction; 10 to 15 per
2l8
ILLUMINATING ENGINEERING PRACTICE
cent, may be taken as typical values. With Fresnel lenses there is a
further loss due to the f "gs produced by the risers.
The large proportion oi t. projection field served by the parabolic
reflector makes a further analysis of its properties with different
sources desirable. The following curves, Figs. 4, 5, and 6, and
accompanying formulae are taken from Benford's paper 1 on this
subject. In Fig. 4 are shown the beam characteristics that are
approached as the source approaches a point radiating equally in all
directions. The rays are parallel and the apparent candle-power is,
Fig. 4. Parabolic mirror and point source beam characteristics.
of course, different at each distance measured. The density of the
flux at any radius is given 2 by the formula, D = ^
results are shown for three reflectors of equal diameter but of differ-
ent focal length and effective angle.
In Fig. 5 a similar analysis is made for a spherical source of 0.5 in.
diameter and a brilliancy of 1000 c.p. per sq. in. In this case the
equation for the axial density of the beam becomes
sL*
tan 2
Hence
/ = -jrR 2 Bm.
1 Frank A. Benford, Jr., "The Parabolic Mirror," Trans. I. E. S., Vol. X, page 905.
2 The following symbols are employed: E = illumination on a plane normal to beam, in
foot-candles; I a = intensity of source at angle a from axis of mirror, in candles; IB = in-
tensity of beam, in candles; B = brilliancy of light source, in candles per sq. in.; m =
coefficient of reflection of mirror; F = focal length of mirror, in inches; R = radius of mirror,
in inches; L = distance from focal point to point in beam, in feet; r radius of source, in
inches; 5 *= area of light source, in sq. in.; a angle measured about focus, in degrees.
EDWARDS AND MAGDSICK: LIGHT PROJECTION
219
The intensity varies, for fixed focal length, with the square of the
tangent of one-fourth the effective angle; for fixed angle, as the
square of the focal length. Also, the axial intensity is seen to depend
upon the brightness of the source but is not affected by its size; it is
Fig. 5. Parabolic mirror and spherical source beam characteristics.
S
equal for all parabolic mirrors having the same diameter. The same
intensity will be directed at all angles within which light is received
from the entire surface of the reflector. This angular spread is
determined by the size of the source and its angular radius viewed
Fig. 6. Parabolic mirror and disk source beam characteristics.
from the edge of the reflector. The intensity at other angles is pro-
portional to the area of the mirror contributing light.
These characteristics of the beam apply at distances beyond the
point at which the rays from the extreme edge of the reflector cross
22O ILLUMINATING ENGINEERING PRACTICE
the axis. This point of maximum density from which the inverse
square law takes effect is found from the equation
R(
I2T
For a disk source the characteristics are given in Fig. 6. Here
again IB = irR^Bm.
With a disk source a wider angular opening than 180 is not effect-
ive, since the projected area becomes zero at 90 from the axis.
The effective diameter of reflector C is therefore reduced to 2 A.
The distance from the focus at which the inverse square region begins
is in this case
L
Lo =
i2r cos a
EQUIPMENTS FOR VEHICLE HEADLIGHTING
The opaque projectors find by far their greatest application on
vehicles; the number of automobiles, street and interurban railway
cars, electric and steam locomotives equipped with projectors is, no
doubt, in excess of 3,000,000 in the United States alone.
The first object in equipping automobiles with projectors was, of
course, to light the road ahead for the driver. It is desirable that the
driver of an automobile be able to see his way for several hundred
feet in advance, and since he must provide his own lamps and direct
the light unfavorably for lighting the roadway, it becomes necessary to
project a high intensity. It was the effort to accomplish this, as well
as to give ease of control, that brought about the rapid change from
oil and acetylene units to the electric system employing closely coiled
low-voltage filaments in deep projectors, giving both accurate con-
trol and high efficiency. The intensities now vary throughout a
wide range up to hundreds of thousands of candle-power with an
average of about 25,000. If there were but one automobile and a
lonely road, the headlighting problem might be considered solved.
But the higher intensity lighting equipments have, in solving the
problem of lighting the road, introduced a new and serious problem
in that they temporarily blind the driver or pedestrian who happens
to come within their angle of action.
"Glare" is the one word most used in referring to the blinding
EDWARDS AND MAGDSICK: LIGHT PROJECTION 221
effect of high candle-power units. This question probably commands
more widespread interest at the present time than any other prob-
lem within the scope of the illuminating engineer. A few states and
many cities have enacted legislation designed to regulate the use of
projector lamps to eliminate dangerous glare. Other states and
cities have such laws in contemplation. In Table III is found a sum-
mary of the automobile laws of the various states as obtained in re-
sponse to a general letter sent to the Secretaries of State under date
of June, 1916. It is seen that a small percentage have laws per-
taining to glare, and that there seems to be lack of definiteness in the
laws which do exist. It should be possible with more general knowl-
edge as to the causes of glare and with a more widespread under-
standing of the methods of measuring light, to create laws which will
be definite, consistent with a consideration of the factors involved,
and stated in terms which permit of verification by measurement.
It is generally agreed that the main factors involved in producing
glare are included in the following:
1. Luminosity of background.
2. Solid angle subtended by source projected area at eye of observer; in other
words, source size and distance.
3. Luminous intensity of source in direction in question.
Automobile headlighting units are limited by cost and appearance
considerations to sizes under i ft. in diameter, and the size can
be considered as a constant in a consideration of the glare problem.
The luminosity of the background under worst conditions is zero, the
complete darkness of the country road, and it is likely to be for some
time, until all roads are artificially illuminated at night. Therefore,
the third factor, the luminosity of the background, is also a constant
so far as the present problem is concerned. There remains then only
one controllable factor, the luminous intensity.
There is likely to be a great difference of opinion as to the limit of
intensity which would be fairly safe and yet endurable. An in-
tensity of 100,000 candle-power is unquestionably bad, and there is
hardly an appreciable reduction in the glare in cutting down to 10,000
candle-power, assuming, of course, the worst background conditions.
It is unfortunate that no consistent method has been devised for the
measurement of interference with vision, making it possible to de-
termine the relation between glare effect and candle-power for some
fixed road condition. Observations have indicated that such a curve
taken on a dark country road would be of the general form of Fig. 7.
222
ILLUMINATING ENGINEERING PRACTICE
TABLE IIL4
ftuTOMQBiLC HEAOLIS-HT IA/Q- REG-ULATIO/VS
STATE
ALABAMA
4LASKA TEXX/TDKY
ARIZONA.
ARKANSAS
CALIFORNIA.
COLOT4ATJQ
CONNECTICUT
\\
it
1911
Voia.*
1913
1913
1915
1913
I9IS
!!
*8
2
2
2
2
2
2
"DISTANCE. VISIBLE.
7?ASOMBi DISTANCf.
3OO FEET
SOO FEET
NOT STATED
SOO fT
|
-j
5
YfS
ys
yes
ys
YS
1
NO
1
/
1
IH TEKMS Of HOURS
AfTfH SUNSET AND
BEFORE SUNRISE.
i AfTEn- / Bffone
StlMSET- 1 BEfORE.
'/sAFTER-'/tSEfOKE
SVIVSET - 3 U At It IS E
'/sAerEir-j& BffoxE.
NON-GLA^E REGULATIONS
/VO/VE.
AfOME
/VO/VE.
MOM E-
MONE-
HONE.
DE.LE.WA-RE.
VIST OF COLUMBIA.
FLORIDA.
&eofie-/A .
raAHo
TiL/MO/S
TMZ3/ANA
ZTOWA
KAS/SA8
wr-ib
I9l<o
1916
/9/f-/6
1916
ft/3
I9IS
1913
/
2
2
2
2
2
2
/
SOO FET
POO Frj-FKON^SIbes, KEAK
A/07- STATD
XZASOHAZiE. D/S7XVC
BOO f.T
SOO FEET
SOO FEET
SJDO FET
XEASO//A3i J>IS7Af/C.
ys
yes
MO
ys
~ys
/S
VS
YS
ys
I
1
2."
1
1
1
1
lAFTEff- 1 BEFORE.
'/sAfrEff- % BEFORE
SVNie.r- SUNHI3E.
/ A F TEK- 1 BEFORE
St/t/ST - / BEfO/tf.
'/ x AFrK-fc. BEFORE
'/lAfrEff-ftsefofiE.
'/LA f re ft -/. seroKE
WST HOT BLIND TERSQNS INSTKEtTS WALKS
HONE
NONE.
NONE.
SSOME.
NONE.
S/ONE.
NONE-
NONE-.
KENTUCKY
1914
Z
SOO FEET
/
/
ZU#SEr- IBEfOKE,
MONE.
LOU/SI AW A
1915*
MA/ WE.
1915
z
NOT STATED
YS
1
'/lAFrf* - % BEfORE-
WOHE.
NIA-RYt-AND
19/6
a
ZOO FEET
*ES
1
'/zAfrEX- 'A.BEFORE.
rtAX/m/fl C.f>. OF L/GHT= 3O C.7?
MASSACHUSETTS
1916
2
SOO FEET
/ES
/
/AFTER- / BEFORE
4T3OFr.*'OGl.AmHI6HET1THAN3J[ABOVEC,TH>W/L
MICH t& AM
19/6
/
SOO FEET
YES
1
IAFTEX- /BEFORE.
*tUfr//OTBi//VDDRlVEl? OF APPfOAtMl 'N6 VHlCLt
MINNESOTA
19/S
2
SOO FEET
yss
/Arrert -/ BEFORE.
.NONE.
H/SS/SS/-PT>I
1916
SOO FEET
ys
2
'/m-ArreR-/!, BEFORE.
NON ',
MISOUTi/
2,
00 FEET
ys
2"
'/^.AFTeJt-^BEFOffE
f/O/VE.
MOrtTAKA
1913
2
ZOO FEET
VS
2
/ AFTR- IBEFORC.
NONE.
/VEBKASKA
I9IS
/
XASOMABL DISTANCE.
Y5
/
/AFTfK- /BEFORE
NONE.
MEVATJA.
19/6
??ASOf/AMLS. 3>tSTAfiK..
y5
/
/ AFTEX - J BCFOfte.
NONE.
HEW HAHPSH/RE-
1916
2
soo FEET
YS
RAFTER - fi BEFORE
D/H WHttf AVPKOAGHIM& AUTOS OR STTfecrCAK
*fW J-XSEY
1916
2
2SO FEET
ys
/
RAFTER- fa BEFORE
/TO JLAJt H/Mf.R7HAN P/IKALifL4^frAVtfCTnn
VEWflEKlCO Tf?R
1913
a
f*OT STATED
y5
/AFTEK- /BEFORE
NONE.
/Vw yort/f
/9I6
2
SOO FEET
/s
2"
'/I AFTER ~'/i BEFOUL
NONE.
A/0. CAROLINA
19/3
2
MOT STATED
VS
/
RAFTER -'/^BEFORE
NONE.
WO. DAKOTA
19 IS
2
NOT STATED
/V<3
/
A/07- STATED
HONE.
OH/0
I9IS
2
200 FEET
Y5
/
%.AF7CR -frXEfORE.
NONE.
OKLAHOMA
I91&
OKE6QN
I9IS
Z
SOO FEET
YS
/
/AF-Tfff -/BEFORE
NON EL
"PENNSY*. VAN! A
1913
2
ZOO FEET
/i/O
/
tAFrE.lt - /BEFORE
"RHODE ISLAND
1916
2
SOO FEET
YES
tft
NOT STATES
NONE.
SO. CA7JOLI/VA
SO. T3AKOTA
1913
2
NOT -STATED
YS
/
faAFTER-fe BEFORE.
NON EL.
TfSt/S.
19/6*
TfX/14
NOLfttt
UTAH
19/S
2
TtEASOt/ABLB. TllSTAftCl
YS
2.
1 AFTER - / 8EFORE-
NONE.
VEKMOHT
19/6
2
ZOO FEET
YES
1
% A FTER- %XCFOHl.
NONE
VIK&INIA,
1916
/
/OO FEET
YES
1
1 AFTER - IgEfORE
NONE.
WASHlM&TON
19/S
Z.
200 FEET
YES
1
HOVRi OF &ARKNC3S
NONE.
WEST VI-RG-INIA
19/S
2
REASONABLE DISTANCE.
YES
Z'
1 AFTER- /SEfORE.
NON E,
WS CO MS IN
1915
/
1ASO/*A8LY BK/&HT
YS
1
/ Z AFTEf?-%8FORE.
NOt/E-
WV0/y//V6-
1913
/
YES
1
/ AFTER- /BEFORE^
NONE-
# REGISTRATION AND OPERAT/ON J.AW& ONLY
LAW ASS IS MSB TO APPLY TO BOTH AUTOS ANTj MOTOKC.VCLE&
If the background is entirely dark, as often occurs in country
driving, there seems to be interference with vision even with the
lowest intensity dimmed light sources. In reducing the intensity,
EDWARDS AND MAGDSICK: LIGHT PROJECTION
TABLE IllB
223
LOCOMOTIVE HLADLlCrHTlN$ Re.Cr(jLf\ T IONf>
STATE
\\
H qDLIG-HTIH(r LAyy
ring, of U*NIN+
in Tcxns if Hjuns
Ifre* SUfSSET AND
BCfOXf SUHKISE-.
Mi
fort
VIOLATION
UASKA TRR/rD7*Y
OL *
ARHANSAS
1913*
/SOO C.7>.
CALIFORNIA,
f9l3^
Suff,e.lf.HT TO VIS.T,t,VISHDAKH O&TC.LT S'2t of
COLORADO
/9/4^
/SOOCP MtASuKED WITHOUT AID Of FIEfiECTOX
CON/HECTIC UT
VOLAV/
DELEWATffL
M3 4AW
VIST OF COLUMBIA.
FLORIDA
/9/3*
/SO C..7*.
&EOKQ-/A
MUST COMSUHE. 300 WATTS ATAKC.-S3 fffL.
TDAHO
1913
** tvr c* T H temerov TO *****
END/AN A
/SOO C..T 3 .
7TOWA
1913^
iy'/Vfr PKONt 0." rfTAC.f AT BISTAKCf- f/OO fCET
H A MSA 3
OBSftrSIZf OF HAN AT DH.TA.VCf. BOO FT
KE./VTUCKY
e
LOU/3IA/VA
HA//VE.
MA -RYL A NX>
MASSACHUSETTS
MlCHt&AN
I9IA-
fffwDen VISIBLE careers 3SO FT AHEAD
NOT STATES
YS
^100
MIWESOTA
1913
Y3
*S&Z */oo
ff/SS/SS/ff/
N
riusTcoJvsune 300 WATTS AT AHC ,- /e'ffffi.
MIS cunt
19/4*
MO/VTAKA
1914*
/SOO C-P rtASVf?D WITHOUT AID OF H FLC.TC>9
/VE377ASKA
19/4*
AT6,'oo e rr T By '?>'/^2S'^ i o'c M /y f>f''*AL *'isfof* "*"
ME VA DA
/9/4^
/Soo c-T ffASf/rt> WITHOUT AID offffnEcTox
HEW HAflPSH/RE.
HEW TEKSE.Y
rvf-WfiEnico reffK
/VW YORK
8
ASO. CAROLINA
/S00Cr/1ASl/eD WITHOUT A/D Of XfiC.roX
/va DAKOTA
/9/4
/zoo cp neAsuxED WITHOUT AID ofJtft.CTorr
&UNSCT - SUNK IS t.
YS
* 100
OHIO
OKLAHOMA
19/6
^^J~
too -*iooo
OKE60N
PEMNSYL VAN/A
19/4*
\ ewer- a ,* f *A~ AT P r A,m mrtir
RHODE ISLAND
SO CA-HOLI/VA
*
101 oa ET *a. "o^^AM^T^'rAvyj^} FT*
SO. X3AKOTA
1911
tSOO CP MEASURED WITHOUT AIP OF RfFLcCTOH
ffOT STATED
ys
*/00- */006
TEWf/ESEE.
TfXA-S
1907
YS
r/00- /OOO
UTA H
VEKMOHT
V/J^&I ' N 1 A
'%7
YS
IzfJtoo
WASHIfS&TOM
west vmcriftiA
190^
Yt/SCOHSlM
1913^
SUFflClCNT CP WtTH XEFLee.T O * To alJCA
OBJCtr 3IZ.E Of MAf AT Dt3T-Afft. BOO FT
A T MI&HTT/PIE.
Y-S
*/00 - J-QO
\VYOM /A/6-
VOLAH
* REG
iAW
//O REPLY TO INQUIRIES RECEIVED
^ DATA XIYCOM'PLE.TE- TULT i,ian
the glare vanishes completely only when the candle-power reaches
zero. When the background is not dark, there can, of course, be
considerable intensity without marked interference.
22 4
ILLUMINATING ENGINEERING PRACTICE
There are many devices on the market which reduce the glare
by cutting down the intensity of the beam. They are diffusing doors
of various forms and degree. As a rule, they slightly reduce inter-
ference at the maximum glare angle by diminishing beam candle-
power to a small fraction of the previous value, and increase hundreds
of times the solid angle in which glare is experienced. A well-focused,
accurately made parabolic headlighting unit may produce blinding
glare in the angle of the beam, but it has the one inherent virtue that
except for the filament itself, its field of action is limited within a
small angle. The approaching driver may face the beam at a
considerable distance, but it likely to escape it when within, say,
Conditic
100*
on of To
ally
Blinding
Gla
10,000 20,000 30,000
40,000 80,000 60,000
Beam Candle-Power
70,000 80.000 90,000 100,000
Fig. 7. Nature of relation between beam candle-power and visibility of objects viewed
against beam where the background is totally dark.
ioo ft. of the approaching car. There is no escape from the
diffusing equipment. One is like the small-pox, serious when en-
countered but not difficult to avoid; the other like the measles, not
so serious, but unavoidable, it seems. If one of these diseases could
be eliminated, many would vote that the measles should go. Fig. 8
illustrates light distribution from three typical classes of equipment;
an unmodified parabola with covers of clear glass, partially frosted
"lens" and all-frosted diffusing glass.
If the one object in regulation were to eliminate glare, the answer
would be simple : Eliminate the concentrating headlights. Limits to
the glare effect mainly protect the approaching driver; the problem
EDWARDS AND MAGDSICK: LIGHT PROJECTION
22;
must be considered from the point of view of the driver behind the
headlights as well. There are also pedestrians and the occupants of
unlighted vehicles, whose safety depends upon the ability of the auto-
mobile driver to see them in sufficient time to avoid running them
down. A just regulation should do more than place upper limits of
permissible intensity; there should be lower limits in so far as road
illumination is concerned. If the beams from automobile lamps are
to be at all times capable of good road illumination and at the same
56.000
20" 16
8 4 4 8 C
Angle from Axis
12 16 20
Fig. 8. Beam candle-power of parabolic automobile projector with 6-8 volt, 3.0 ampere
Mazda C lamp.
time incapable of causing glare under average conditions, there seems
to be but one solution, and that is to greatly reduce or entirely
eliminate the light from the angles above, say, 4 ft. from the ground,
and retain the light at the lower angles.
Many devices have been designed for reducing or eliminating the
upward light, redirecting the intercepted light in downward di-
rections. The simplest method of eliminating strong upward light
with accurately made headlamps is to tilt them downward by an
angle equal to half the angle of spread of beam; many headlamps,
is
226 ILLUMINATING ENGINEERING PRACTICE
however, are not made sufficiently accurate to have any well-defined
beam. Another method commonly used is to set the light source
back of the focal point of the reflector and to cover the upper half of
the door with an obscuring material. Obviously, this method is
inefficient. The Patent Office records show a wide variety of
devices for diverting the light from directions above the horizontal.
One is a cup-shaped spherical reflector placed over the lamp bulb to
return the upward light back along its initial path. When placed
over the bulb, it is assumed that the filament is placed back of the
focus. These devices are frequently seen placed on the lower side of
the bulb, thus utilizing the upper instead of the lower half of the
parabolic reflector, and when so used the filament must be forward
of the focus in order to be effective. It not infrequently happens
that the filament is in focus as adjusted by the owner, in which case
the device has no effect except to reduce the efficiency of the lamp.
In another class of devices use is made of compound curvatures in
the reflector. There is the offset parabola where the upper half has a
focal point back of that of the lower half, so that the filament may be
placed back of the focus of the lower half at the same time that it is
placed forward of the focus of the upper half. A tilted upper half,
where the upper surface has been revolved downward about the
focus as a center, is another device described. Still another is a
combination consisting of a parabolic lower part and an ellipsoidal
upper part. This device if perfectly made would give no light above
the horizontal, not even the direct light from the filament. Proper
adjustment requires that the filament be placed a little more than
half its axial length back of the focus of the parabolic part. The
ellipsoid is arranged to have one of its foci at the proper position of
the filament and the other, through which the intercepted rays are
directed, at a point on the axis of the lamp within the plane of the
front glass. There are also a number of prismatic glass covers that
reduce the upward light, bending the reflected rays downward and
to the side of the road. These devices seem to be limited in the de-
gree to which they can cut down upward intensities, because in
being designed to take care of the light coming from the reflector,
they are sure to throw some of the direct light from the filament
upward in narrow high-intensity beams, although this may be
obviated by screening the tip of the bulb. Figs. 9, 10 and n are
photographs by C. A. B. Halvorson, Jr., of a screen illuminated at a
distance of 10 ft. by three types of equipment, star frosted, pris-
matic and paraboloid-ellipsoid.
Fig. 9. Screen il-
luminated at 10 ft. by
parabolic reflector with
star frosted "lens."
Fig. 10. Screen il-
luminated at 10 ft. by
parabolic reflector with
prismatic cover.
Fig. ii. Screen il-
luminated at 10 ft. by
paraboloid - ellipsoidal
reflector with clear
glass cover.
(Facing page 226.)
EDWARDS AND MAGDSICK: LIGHT PROJECTION 227
No one of the so-called non-glare devices that are now in general
use can be said to be a complete solution of the problem. Each
may have its favorable point or points, just as the unmodified
parabolic lamp has its advantage. An equipment which gives no
light above the horizontal, may give good road surface illumination
at the same time that it is incapable of glare on a dark road, but it
can blind the approaching driver coming up into view on a convexity
in the road, and has the further disadvantage that it ordinarily must
show up vehicles and other objects by their lower extremities only
and may miss entirely the near objects when approaching the foot of
a hill. A lamp with no light above the horizontal is sure, on account
of the varying curvatures in road profile, to have a widely varying
range of throw. From the driver's standpoint it has great advan-
tage in a fog since there is none of the usual luminous haze between
the driver's eyes and the road.
The details of the various devices which have appeared are
interesting but they are not as important at the present time as
the study of the underlying principles. If the best solution to the
problem were a matter of general agreement, a device which would
accomplish the result would probably soon appear. As a matter of
fact, some of those already on the market give results which the
inventors believe to be the best solution. Some of the states have
evidently assumed that it is necessary only to place upper limits of
intensity above 3 or 4 ft. from a level road; presumably it is
considered unnecessary to place lower limits of intensity for lower
angles. Assuming that the best answer to the glare problem is the
elimination of light above the horizontal, it should be possible to
draught a regulation which would be definite and in terms capable of
measurement. It would be necessary to use only one technical
term. Such a law might specify that the head lighting beam shall
not have an intensity at any angle above the horizontal exceeding a
certain amount, say, 20 candle-power, and that it shall have not
less than an average of, say, 10,000 candle-power measured at equal
vertical angular increments from the axis down to the road, at a
distance of 100 ft. If desirable, lower limits of intensity at the
lower angles to the side might be specified in order to insure that the
driver can at all times see the curb or other sidewise limits to the
road. The point to be emphasized is that once general agreement
is obtained as to the best solution of the problem, the necessary regu-
lations can be stated in simple terms involving only luminous in-
tensity measurements in addition to simple measurements of length.
228 ILLUMINATING ENGINEERING PRACTICE
It may safely be said that the present tendency is toward cutting
down or entirely eliminating the upward light, but it appears that
this method in itself can never be entirely satisfactory to the motor-
ist. Much of the pleasure and sense of security in night driving come
from bringing into view the overhanging foliage and other high
objects along the road, as is done with the usual parabolic units of
high power. Evidently there is no harm done as long as there
are no eyes ahead to be blinded. This feature may have
prompted the recommendation of the glare committee of the
Illuminating Engineering Society to the effect that unmodified
parabolic equipments could be used if they were always ex-
tinguished when meeting another driver. This plan seems to
have disadvantages, for on a dark road the sudden change is
likely to interfere with the vision of both drivers due to the length
of time required for adaptation, and, to an extent, greater than
would be obtained with the glare of the undimmed lamps.
Perhaps the best solution is a regulation such as outlined above,
but taken to apply only when meeting other vehicles on unlighted
roads. Such a regulation would require on every automobile
used at night an equipment giving no upward light, but would
allow of any kind of additional equipment that might be
desired. It would seem that an equipment consisting of one or
two high-powered parabolic units with two lower-powered non-glare
(no upward light) lamps would be satisfactory. The glareless lamps
would be used for all night driving, both city and country, and the
additional equipment of parabolic lamps could be employed at times
when no harm would result to others.
Modification of the color quality of the light emitted from a head-
lamp is sometimes secured by means of yellow glass in the reflector
or cover. Two advantages are sought by the suppression of the
shorter wave lengths; increased acuity and decreased scattering of
light. The former is seldom realized since the better acuity usually
fails to compensate for the lower intensity. The latter effect is
more often of importance since the rays scattered by a haze or fog
produce a luminous veil that may seriously interfere with vision;
an appreciable reduction of this veiling is obtained with the yellow
glasses. The same purpose is served by the use of ordinary head-
lamps and a yellow disc on the wind-shield or yellow glasses worn
by the driver. A further step in this direction has been attempted
by making the reflector of glass with fluorescent properties and thus
converting the shorter wave lengths instead of absorbing them.
EDWARDS AND MAGDSICK: LIGHT PROJECTION
229
The plan possesses little utility, however, since the transformed
light is not projected into the beam by the reflector but issues as
from a diffusely emitting medium.
EQUIPMENTS FOR RAILWAY HEADLIGHTING
On street cars for ordinary city service, the head lamps need
serve only as markers. For suburban and interurban runs with the
higher speeds and dark roads, a higher intensity is required both
16,000
14,000
12,000
* 10,000
| 8,000
6
| 6.000
4,000
2,000
110- Volt, 94-Watt
' Mazda B
/
\
/
\
110-Volt. 72-Watt
MazdaB
/
\
/
/
^
^s
7
/
/
3
\
z
/
\
\
//
/
N
\\
/
7
[ /
/
X
\1
/
z
^
ff
^.n""
>.
"^
s\
\
/
c
'/
/"
V
\
t
i
110- Volt, 23-Watt
Mazda B
1
J
110,
Volt, 46-Watt
Mazda B
5 4 3 2 1 1 2 3 4 5
Angle from Axia
Fig. 12. Beam candle-power of typical electric street railway head-lamp. Parabolic re-
flector of 1 54 in. focus and %>% in. diameter.
as a warning at greater distances of the approach of a car, and to
illuminate objects on the track at a sufficient distance to allow the
car to be stopped before reaching them. Fig. 12 gives photometric
data for several lamps for headlighting typical of those used in this
service. The advantage of the more concentrated low- voltage source
in increasing the beam candle-power is apparent, but this greater
230
ILLUMINATING ENGINEERING PRACTICE
concentration is not always required. Since high-voltage direct
current is available, the magnetite arc has been found to be particu-
larly useful in this field when a high intensity beam is wanted. The
large amount of steadying resistance stabilizes the arc, and when the
equipment includes a lens cover, good control is secured, with the
results shown in Fig. 13.
400,000
4-Ampere
High -Efficiency
Electrode
4 3 2
Angle from Axis
Fig- 13- Beam candle-power of luminous arc interurban head-lamp with 12 in.
semaphore lens.
The proper headlighting equipment for steam and electric loco-
motives has been exhaustively studied by railway associations,
individual roads and utility commissions. The headlamp in this
case may be made to serve as a marker for the head end of a train and
as a warning of the approach of a train, for the illumination of way-
side objects, for displaying numbers in the case and to enable the
engineman to see objects on the track at a distance so great that he
may stop the train before reaching them. All of these requirements
EDWARDS AND. MAGDSICK : LIGHT PROJECTION 231
isooo
40 800 IZOO 1600 7009
DISTANCE IN FEET DISTANCE IN FEET
Fig. 14. Locomotive beam intensities required to render dummies visible.
SECONDS
5 10 IS tO
60
40
Z 20
CURVE I
\
400 800 IZOO
DISTANCE IN FEET
SECONDS
1500 2000
60
40
^Q
J 1
1 1
I 1 1
1 1 1
1
^S
"V
X,
x
Cl
JRVE
\
\
\
\
400 800 1200 1600 1000
DISTANCE IN FEET
Fig. 15. Deceleration curves for heavy express train with older and modern braking systems.
232
ILLUMINATING ENGINEERING PRACTICE
can probably be met with the best apparatus available at present,
providing the brakes are applied immediately whenever any indica-
tion of an object on the track is seen. However, its use may lead to
some difficulty in temporarily blinding a person passing through the
beam and thus introducing an element of confusion at multiple track
crossings, and in interfering with the visibility and correct reading
15000
200 400 600 600 1000
DISTANCE IN FEET
Fig. 16. Visibility of signals and objects with various beam intensities.
of color and position of semaphore and hand signals, classification
lights, etc.
From Minick's admirable summary 3 and presentation of the
findings of the Headlight Committee of the Railway Master Mechan-
ics Association and other investigators, covering the several classes
of oil, acetylene, incandescent electric and arc lamps, are taken
Figs. 14 to 17. Fig. 14 shows the beam intensity required to see
J. L. Minick, "The Locomotive Headlight;" Trans. I. E. S., Vol. 9, page 909.
EDWARDS AND MAGDSICK: LIGHT PROJECTION
233
at different distances dummies of the size of a man dressed in light,
medium and dark clothing. The curves at the left refer to the
tests on the oil, acetylene and incandescent electric lamps; those on
the right to the arc tests. There is a marked advantage in favor of
the more yellow light sources due, no doubt, in part to the lack of
steadiness in an arc and to the fact that there is a considerable pro-
portion of blue rays for which the eye does not focus accurately;
thus the visibility of a distant object is reduced with a given inten-
sity of illumination. Fig. 15 shows deceleration curves for a heavy
express train running at 60 miles per hour with both the older and
more modern braking systems. It is evident that to stop the train
3000
PWREES
Fig. 17. Candle-power specifications for locomotive head lamps recommended by Com-
mittee of Railway Master Mechanics Assn.
before reaching a detected object requires the use of exceedingly
high beam candle-powers. From Fig. 16, recording the test data
indicating the range within which the various signals may be
identified without danger of error for different beam intensities,
it would appear that only relatively low values of beam candle-
power would meet the requirements from this standpoint with the
prevailing signal sources. It is possible that the substitution of
sources of higher intensity or greater concentration in the signals
would make the use of high candle-power lamps satisfactory in
every respect. The conclusion of the Master Mechanics Com-
mittee was that the intensity of locomotive headlights should fall
234
ILLUMINATING ENGINEERING PRACTICE
within the limits given in Fig. 17. These limits cover the range
in which fall the oil and acetylene lamps with which most loco-
motives are still operated.
It would appear that the estimation of the relative importance of
the several factors involved must determine the choice of headlighting
characteristics. For multiple tracks and winding roads this will
lead to different conclusions than for single-track roads without
block signals. The Interstate Commerce Commission, which re-
cently undertook the supervision of these devices used in interstate
1,200.000
10 8 6 4 2 ' 2 4
Angle from Axis
Fig. 1 8. Distribution of light from incandescent headlight. Parabolic reflector of 2% -in.
focal length and 20 in. diameter.
traffic, ruled that after October i, 1916, all new locomotives for road
service and those given a general overhauling must be so equipped
that a person of normal vision at the engine may be able to see a
dark object of the size of a man at a distance of 1000 feet or more,
under normal weather conditions. Furthermore, all locomotives
must be so equipped before January i, 1920. This is, of course,
very different from the recommendations of the report above cited.
As a result of the new ruling it would seem that electric units will
be utilized in order to obtain the necessary intensity, and that since
Fig. 19. Locomotive type incandescent head lamp.
Fig. 20. Hand-controlled commercial searchlighting equipments.
(Facing page 234.)
Fig. 2 i .
Fig. 22.
Fig. 23.
EDWARDS AND MAGDSICK: LIGHT PROJECTION 235
arc lamps have been found to possess less suitable characteristics
and to be not so well adapted to the desirable electric systems in this
service, incandescent lamps will be favored. The demand for mir-
rored glass reflectors may be expected to increase since the silvered
metal parabolas which have been employed most in the past can-
not so easily be maintained in the condition required.
It appears that the rulings of the commission can be met by the
36, 72 and io8-watt 6- volt gas-filled tungsten-filament lamps
and the 150 and 25o-watt 32-volt lamps. The io8-watt, 6-volt
and 2 50- watt 32-volt provide a good factor of safety and will prob-
ably be most often employed. Fig. 19 illustrates one of the larger
incandescent headlighting reflectors. In Fig. 18 are given the pho-
tometric results with three different lamps in this reflector. The
folly of the headlighting legislation of a number of states (see Table
III) requiring the use of a source of 1500 unreflected candle-power is
apparent, since equal beam intensities may be secured with con-
siderably reduced wattages. Requirements as to diameter, visual
tests, etc., all are unnecessarily indefinite and lead to needless con-
fusion. The entrance of the Interstate Commerce Commission
into the field promises to relieve the chaotic condition resulting from
the legislation of the individual states, but it would seem entirely
feasible that its requirements be stated in the form of a specifica-
tion of the beam characteristics and the method of measurement.
SEARCHLIGHTING EQUIPMENTS
Searchlighting equipments were developed principally for the mili-
tary service. They have been employed by the army and navy for
more than 50 years as one of the most effective means of defense
against night attack, for locating enemy vessels and fortifications
as well as for signaling purposes. About 30 years ago the first
accurately ground parabolic mirrors became available and these with
the direct-current carbon arc have been the standard equipment.
No radical improvements in either the light source or optical
system were made until recently, when the increasing range of
torpedoes made these developments particularly desirable.
Fig. 20 shows a number of small hand-controlled searchlighting
equipments such as are employed also in commercial work and in
navigation. It will be seen that the electrodes are in a horizontal
position, with the positive tip at the focus of the mirror inasmuch as
most of the light is radiated from this surface. Fig. 21 is a mili-
2 3 6
ILLUMINATING ENGINEERING PRACTICE
tary equipment provided with automatic control and feeding
mechanism. In addition to the clear glass front cover, there is an
iris shutter for the purpose of quickly shutting off the beam or
making it available at full candle-power; a considerable delay in
securing full intensity would be involved if the arc were extinguished.
In field operation the equipments are mounted on trucks with ele-
vating platforms as shown in Fig. 22 and provided with reels of cable
for connection with the energy supply. For rapid signaling Vene-
tian blinds or louvers are used in front of the cover glass, Fig. 23.
Some data for typical equipments are given in Table IV:
TABLE IV. TYPICAL CARBON-ARC SEARCHLIGHTING PROJECTORS
Nominal
diameter
of mirror
in inches
Amperes
Actual diame-
ter of mirror
in inches
Focal
length
in inches
Reflector
9
IO
Mangin Mirror
13
20
Mangin Mirror
18
35
19^6
7%
Mangin Mirror
24
50
25 H
IO
Parabolic Mirror
30
80
31 Me
H
Parabolic Mirror
36
no
37
14%
Parabolic Mirror
60
175
Parabolic Mirror
The 36-in. size has been standard in the navy, while the 6o-in.
size is used very generally in land fortifications. The beam intensity
of such units is of the order of 60,000,000 and 200,000,000 candle-
power, respectively. It will be noted that the focal length is in
each case about 40 per cent, of the diameter, corresponding to an
effective angle of 120 to 130. Within this angle is included a large
percentage of the light emitted by the arc; to increase the angle for
a given diameter would be to decrease the beam intensity on account
of the greater divergence resulting from the increased angle sub-
tended by the source.
Since it is especially necessary to maintain the arc steady and at the
focus of the reflector, very careful attention must be given to the
electrical characteristics, the feeding mechanism, the uniformity
of the electrodes f to secure constant rate of consumption, and a
proper selection of sizes of electrodes. High current density and
small crater result in high intrinsic brilliancy and beam intensity.
The efficiency is increased as the diameter of the negative electrode
is decreased and the arc lengthened, with the accompanying reduc-
EDWARDS AND MAGDSICK: LIGHT PROJECTION 237
tion in the angle of shadow. The small negative is also advantage-
ous in steadying the arc; but if the current density is carried too high,
the electrode spindles, that is, oxidizes near the tip and thus further
reduces the diameter.
Chillas has reported the results of an investigation 4 which showed
that by reducing the positive electrode size to the point where the
arc crater covers practically the entire tip and using a small negative
provided with a copper coating to increase the conductivity and pre-
vent spindling, equilibrium conditions are attained more rapidly
after starting, the arc is more steady, there is a higher average bright-
ness of the positive, a smaller dispersion of the beam and hence a
considerable increase in its intensity. These advantages are secured
at a sacrifice of electrode life and with the necessity for some adjust-
ment of the arc from time to time. The size of electrodes recom-
mended is about ij^ in. positive and % in. negative in the 36-in.
reflector, and for the 6o-in., i%-in. positive and ^-in. negative.
Heretofore a 2-in. positive has ordinarily been employed in the 6o-in.
lamp.
The recent developments in the application of flame electrodes
at high current densities have produced a notable advance in the
performance of searchlighting equipments. The electrode diame-
ters are only about % in. for the positive and %g m - f r the nega-
tive. The arc is somewhat longer than with pure carbons and the
negative electrode is inclined at an angle of about 20 below the axis.
At the high currents employed, the luminous flame is confined in the
deep crater of the positive, where the gases are superheated to an
exceedingly high temperature, producing a brightness of about
350,000 candles per sq. in. The positive electrode is continually
rotated and thus the crater kept symmetrical; the negative may
also, with advantage, be rotated. At the high temperature involved,
some provision must be made against spindling and for cooling the
electrodes. In one form of lamp this is done by bathing the tips
with burning alcohol vapor, which, with the radiating discs on the
holder, acts as a cooling agent and prevents oxidation of the carbon
shell. In another form, the holders are also provided with radiating
discs which are cooled by a blast of air; the positive electrode is fed
through a quartz tube to prevent spindling.
The Navy Department tests 5 have shown that in addition to its
* R. B. Chillas, Jr., "Operating Characteristics of Searchlight Carbons;" Journal of
the United States Artillery, page 191, March-April, 1916.
* Lieut. C. A. McDowell, "Searchlights;" Proc. A. I. E. E., Vol. 24. page 207.
238 ILLUMINATING ENGINEERING PRACTICE
higher efficiency of light production, the flame arc directs a greater
percentage of the flux into the effective angle of the mirror. The
small source results in a narrow angle of divergence only i to 2,
as compared with 2^ to 3 for the beam from standard carbon arcs.
In general, it is reported that these factors combine to produce with
the flame arc units beam intensities about five times as great as those
from standard carbon lamps.
The formula Ji = YI(I P) ZL K, giving the intensity reaching
the observer from the object illuminated is frequently referred to
as indicating that the range 6 of a beam is proportional to the fourth
root of the intensity. On the other hand, it is contended by some
that since the brightness of an object remains the same at all dis-
tances, that is, the luminous density on the retina is constant, visi-
bility is dependent only upon the illumination of the object and that
the range, therefore; varies with the square root of the beam intensity
rather than the fourth root. For an object subtending a large angle
this would doubtless be true, but it is still a moot question whether
for small angles visibility is determined by the total flux or by "the
flux density. The factor of acuity doubtless is of the greatest im-
portance. The dimensions of the object; the color, form and nature
of its surface; the degree of contrast with surroundings; the influence
of telescope, glasses or spectacles and the physiological peculiarities of
the observer's eye all enter into the range at which a beam is effective.
These factors have been analyzed by Blondel 7 who states that the
range increases even less rapidly than the fourth root of the intensity.
To multiply the range five fold under atmospheric conditions giving
70 per cent, transmission per kilometer, he estimates that the intens-
ity would have to be increased 42,000 fold for typical military work.
The impression prevails that blue light is particularly desirable
in the rays of a searchlighting beam since the surfaces observed are
often bluish gray and because of the Purkinje effect. Whenever a
preponderance of blue rays is reflected an advantage probably ex-
ists, but in the usual case it would seem to be detrimental since the
eye will not focus for the blue rays when the longer wave lengths
predominate, and vision is, therefore, impaired.
The present European war has brought about a number of inno-
6 In this formula Ji = intensity directed toward eye of observer; J = intensity of search-
light beam; L = distances of illuminated object, observer assumed near searchlight; P =
absorption of atmosphere; K = coefficient of reflection of object.
7 Prof. A. Blondel, "A Method for Determining the Range of Searchlights;" The
Illuminating Engineer (London), Vol. 8, page 85, 153.
Fig. 24.
Fig. 25.
Fig. 26. Fig. 27.
Figs. 24, 25, 26, 27. Commercial flood-lighting projectors.
(Facing page 238.)
Fig. 28. Representative flood-lighting installation.
EDWARDS AND MAGDSICK: LIGHT PROJECTION 239
vations in searchlighting equipments, such as the use of a Fresnel
lens above the arc with units directed upward in anti-aircraft
work, thus replacing a mirror below the arc, which would be subject
to cracking by the molten carbon. For short ranges incandescent
electric lamps with their steadier light, greater portability and ease
of control, have been employed to advantage. Oxy-acetylene equip-
ment has found similar application.
FLOOD LIGHTING
Flood lighting of the exteriors of structures with sources concealed
at a distance is more a problem of aesthetics than of optics.
Although arc projectors had been employed for temporary
lighting spectacles of this nature, the general application was not
found feasible until the concentrated tungsten-filament lamps of
high efficiency were developed. With these units of relatively
small size, the necessary flexibility in installation and control of
intensity and direction were attained, so that artistic results might
be secured. Flood lighting supplements the older forms of display
illumination; it lends itself particularly to the fields of sculpture,
monumental public buildings and commercial structures. It finds
application also in the illumination of large outdoor spaces devoted
to pageants or to sports, in the yards of industrial plants and
railroads.
Desirable distributions of light for the majority of installations
range from an angle of divergence of 6 to one of 30. This is
determined by the area of the surface, its distance from the units and
the angle at which the beam is incident. A small amount of scat-
tered light is usually not detrimental. The problem of reflector
design is, therefore, one of securing high over-all efficiency and
adjustment of beam spread rather than of narrow divergence and
accurate control. Short focal lengths are, then, desirable so as to
secure a large effective angle with a reasonable diameter and cost of
reflector. It would seem that the tendency has been too general
to secure spreading of the beam by placing the source out of focus
in a parabolic reflector, which results in marked lack of uniformity
in the spot. Except for the narrow beam units, the rational method
is to proceed with the design of the reflector from the desired
distribution curve and the limiting dimensions, just as with any other
specular surface equipment. In this manner units are secured which
not only produce a given spread with reasonable uniformity but,
240 ILLUMINATING ENGINEERING PRACTICE
by careful design, also permit a considerable adjustment of beam
divergence.
Typical commercial flood-lighting projectors are shown in Figs.
24-27. All of these have reflectors of mirrored glass protected in
various ways to withstand high temperatures and atmospheric
conditions to which they may be subjected. A reflecting surface
of this class is the only one to be recommended in the great majority
of installations.
The projectors of Figs. 24 and 25 are designed for use with 250-
watt flood-lighting lamps. In both cases the contour of the reflector
departs somewhat from a parabola to give greater uniformity of beam
with varying divergence as the position of the lamp is adjusted.
The back part of the reflectors is spherical to accommodate the lamp
bulb and direct the light back through the source, making possible
a unit of short focal length and considerable depth, hence of high
efficiency. The one unit is enclosed in a ventilated weatherproof
housing with heat-resisting glass cover. The other has a similar
cover but is tightly enclosed without a housing about the reflector;
the copper backing provides the necessary strength and the dull
black enameling facilitates radiation sufficiently to render ventila-
tion unnecessary. Both units combine compactness and low cost.
Fig. 26 shows a unit usually employed with the 5oo-watt lamp.
It is of parabolic contour, relatively more shallow but giving
a more concentrated beam. For extreme concentration a reflector
of this type is to be recommended with a 2 50- watt lamp. The pro-
jector of Fig. 27, designed for iise with looo-watt lamps, is a para-
bolic reflector that is shallow and hence inefficient in utilizing the
flux; it is a desirable unit for few applications.
In Fig. 28 is given the distribution of candle-power from a typical
2 50- watt projector with the lamp in two positions. With the proper
equipment it is usually possible to deliver from 30 to 50 per cent, of
the total flux from the lamp on the surface of the structure to
be illuminated. An experience of several years in flood lighting
has demonstrated that a unit of the general type of Fig. 24 or 25
can most often be used advantageously and efficiently. The higher
efficiency of the larger sizes of lamps favor their use, but the better
control of direction and intensity with the smaller units frequently
out-weighs this. It is desirable always to have every part of the
surface receive light from several projectors in order to eliminate
the striations (images of the filament) and to provide against ap-
parent lack of intensity at any point when individual lamps burn out.
EDWARDS AND MAGDSICK: LIGHT PROJECTION
241
The intensity need by no means be made equal for all parts of a
structure; rather, the brightness should be so distributed as to
display the structure as nearly as possible as the architect or sculptor
intended. Frequently certain features can be emphasized with
advantage over the results secured with daylight. In general,
desirable average intensities are dictated by the reflecting character-
istics of the building in both amount and direction, the bright-
ness of surroundings, the average distance from which it is to
be viewed and maximum radius of visibility desired, as well as
nature of the structure itself. There is seldom danger of overlight-
ing if the installation is properly made.
50,000
25 20
20 25
15 10 s 5" 5 U 10
Angle from Axis
Fig. 29. Beam candle-power of typical complete flood-lighting unit with 250- watt Mazda
C flood-lighting lamp in two positions.
The latitude in direction of light and intensities employed may
be indicated by reference to a few representative installations.
Fig. 28 is a structure of simple Doric form in light Bedford stone and
granite. Considerable choice is here offered both in the size of
units and their location. The projectors are placed on the roof of
a four-story building diagonally across the street and the electrical
power provided is slightly more than % watt per square foot of
building surface.
The granite building of Fig. 30 with its massive Corinthian
columns and decorations in relief, required particular attention
from the standpoint of direction. The light sources are placed
16
242 ILLUMINATING ENGINEERING PRACTICE
across the street and slightly higher than the bank building. About
i watt per square foot is provided, and 2 50- watt units are employed
in order to secure the necessary control of distribution to em-
phasize the architectural features.
The monument shown in Fig. 31 is 284 ft. high and stands in an
open circle. To light the narrow shaft most efficiently requires the
use of parabolic reflectors giving a concentrated distribution of light.
The projectors are placed in four groups on the surrounding build-
ings at a distance of 230 ft., and a total of 25 kw. is employed.
The Wool worth Tower, Fig. 32, receives its illumination from 550
projectors of the 2 50- watt size. The average power consumption
increases from 0.75 watt per sq. ft. at the lower section to four times
this value at the top. The use of small units with considerable lati-
tude of adjustment was here required because of the necessity for
mounting the equipment on the Tower itself and the desirability of
preserving the vertical lines which form the main architectural
feature. The glazed terra cotta surface of this Tower 8 complicated
the design of the system.
LIGHTHOUSES
Lighthouses differ from the projector applications discussed above
in that they exist for orientation purposes rather than for the illumi-
nation of other objects. Questions of visibility here pertain to a
point source, that is, one subtending an angle of less than 30 seconds,
the limit for the resolving power of the eye.
Metallic reflectors were at one time employed in this service but
are now found in only a few installations on lightships. Lens sys-
tems form the standard equipment, and their application in this field
is notable for the large effective angles and hence the high efficiencies
obtained. The careful correction of these lenses has led to a degree
of control surprising in view of the extended sources of relatively low
intrinsic brilliancy employed. Reliability, simplicity and low cost of
operation, rather than extreme intensities, are the primary requisites
in the majority of lighthouses. From the optical standpoint, electric
arc or concentrated incandescent lamps are most nearly ideal, but
since central electric service is seldom available, their application
requires an installation of high initial and operating cost with skilled
attendance. For these reasons, oil lamps, of both the wick and in-
candescent mantle type, are still generally employed. The former
is the most reliable of all sources; the latter excels it in brightness and
Electrical World, Vol. 68, page 412.
Fig. 30. Representative flood-lighting installation.
(Facing Pag* 242.)
Pig. 31. Representative flood-lighting installation.
EDWARDS AND MAGDSICK: LIGHT PROJECTION 243
has the lowest operating cost of any lamp used in the service. Elec-
tric lamps are installed in some of the more important lighthouses
where high intensity is necessary. They are also found on all the
larger light vessels.
The lens systems are divided into orders according to their focal
lengths, ranging from 150 mm. for the 6th order to 920 mm. for the
ist and 1330 mm. for the hyper-radial. For fixed beams, giving a
band of light continuous in a horizontal plane, the lenses are cylin-
drical in form about a vertical axis, Fig. 33. The light issues as a
belt of narrow vertical divergence; this angle and the intensity of the
beam vary directly with the focal length for a given light source.
The central part of a typical lens covers an angle at the source of
nearly 60 and contributes about 60 per cent, of the light. This
portion of the lens is dioptric, redirecting the light by refraction
only. The upper and lower parts of the lens system are catadioptric,
acting by both refraction and total reflection. The lower prisms
cover about 20 and furnish 10 per cent, of the beam; the upper, nearly
50 and 30 per cent, of the light. Frequently a dioptric belt of
about 80 effective angle is employed alone.
If lenses developed about a horizontal axis are used, both vertical
and horizontal concentration is secured and a very intense narrow
cone of light results, varying for a given source roughly as the square
of the focal length of the lens. Such a hemispherical lens, Fig. 34,
with a spherical mirror on the opposite side of the source gives a
powerful beam in one fixed direction, as for range lighting along
a channel. Two such hemispheres, known as the bi- valve lens, give
high intensity beams at 180 and are utilized rotating about the
source to produce the highest powered flashing effects. Another
lens giving four flashes per revolution is shown in Fig. 35. By vary-
ing the design, any desired sequence of flashing with controlled
period of flash and interval may be secured.
Variations from a fixed beam are introduced in part to differen-
tiate lighthouses from each other and from shore stations. Where
low intensity suffices, this is often accomplished by an occulting
device which covers the source at characteristic intervals, or by
rotating the lens after screening sections of it. If spherical mirrors
are used as screens, the beam intensity is thereby also increased.
The other important reason for the use of the flashing lens is the
enormous increase in beam intensity realized; this is practically in-
versely proportional to the ratio of period of flash to interval between
flashes.
244 ILLUMINATING ENGINEERING PRACTICE
The lenses shown in the illustration represent a recent develop-
ment in that they are ground by machinery; hence all sections are
interchangeable among different units of the same type. This
is not the case with the hand-made imported lenses previously used;
yet the new lenses, designed by Hower, are exceedingly accurate,
with a divergence, it is reported, of less than one degree in some sizes,
and with little scattered light.
Patterson and Budding 9 found that the visibility of a point source
is proportional to the candle-power and inversely to the square of the
distance; that visibility is independent of brightness for sources sub-
tending an arc of less than two minutes. Their investigation showed
values for the range of lights of different colors only slightly less
than the following reported by the German lighthouse Board of Ham-
burg as the results of their tests in 1894:
R = i.53\/I For white light in clear weather, where R represents
the range in miles and 7 the candle-power.
R = i.09\/i For white light in rainy weather.
R = 1.63^/1 For green light in clear weather.
It will be seen that for ordinary atmospheric conditions relatively
low intensities would suffice to be visible at the geographic limit.
Many of the larger incandescent mantle oil lanterns give intensities
of the order of several hundred thousand candle-power. Electric
units give beams that are measured in millions; the largest is the
Navesink equipment at the entrance to New York Harbor reported
variously as from 25,000,000 to 60,000,000 candle-power. In
many installations the duration of the flash is o.i second or even less.
This is probably shorter than the time required at low illuminations
to produce the same sensation as a steady beam of the same inten-
sity. The results produced by different durations of flash and inter-
vening periods are only partially known; nevertheless the work of
Blondel and Rey leads them to conclude that for maximum utiliza-
tion of a source at range limits short flashes are required.
There is a marked tendency toward using numbers of buoys in-
stead of erecting a few lighthouses of high intensity. With Pintsch
gas or acetylene these buoys frequently operate for periods as high as
nine months or a year without attention. They can be operated
with interrupted beams by means of mechanism actuated by the gas
pressure, which turns the main burner off and on. With the large
buoys it is also found economical to utilize valves which are kept
closed during the day by the daylight radiation.
Proc. Phys. Soc. London, 24, page 379, IQI3-
Fig. 32. Representative flood-lighting installation.
(Facing page 244.)
Fig. 33. Fourth order six-panel fixed lens.
Fig. 34. Fourth order range lens.
Fig. 35. Fourth order four-panel flashing lens.
Fig. 36. Signalling projector for aircraft
EDWARDS AND MAGDSICK: LIGHT PROJECTION
245
LIGHT SIGNALS '
Other applications of light signals are principally in the railway
and military fields. Table V, taken from a paper by Gage, 10 shows
the usual sizes and types of semaphore lenses with the axial candle-
power values and beam spread for both the long-time and one-day
kerosene burners, which flames give about one and two candle-power,
respectively. The optical lens is of the usual Fresnel type with the
edge of the prismatic rings toward the flame; the inverted has these
pointing outward and requires a cover glass. The inverted lens has
the advantage that none of the light is deflected by the risers of the
prisms. The values in the table are for clear lenses. In most
signals colored glasses 11 are employed. With the same sources, the
TABLE V. DATA FOR OPTICAL LENSES
With long-time Burner
With one-day burner
Diam.,
Focus,
Spread, ft. per 100
Spread, ft. per 100
inches
inches
r^ /^1
C* A\
A
S
power,
Of 50 per cent,
intensity,
D
Extreme,
E
power,
Of 50 per cent,
intensity,
G
Extreme
H
4
H
37-5
14.0
16.6
30.6
24-3
26.7
4
3/-6
40.5
12.2
14.4
32.8
21 .0
23-3
4H
2^
39-6
15.2
17-4
32.3
25 5
28.1
4*
sM
42.0
12-5
14-7
33 5
21.5
23-7
4%
3
44-5
12.9
15 3
36.3
22.3
24-7
4H
3H
48.0
II. 9
14.1
38.5
20. 6
22.7
5
3>
57-0
II. 7
13-8
46-5
2O. 2
22.3
sK
3H
69.0
11.75
14.0
56.2
20.4
22.6
6
3*i
82.0
10.6
12.6
67.1
18.4
20.3
6}i
3?i
90.5
10.5
12.4
74-2
18.1
20.0
8^
4
130.0
8.4
ii .7
106.5
14-6
20.2
SK
5
142 .0
7-4
8.7
116.0
12.7
14-0
I
DATA FOR INVERTED LENSES
4
3H
35-4
14-5
17-5
29.0
24.0
31 o
4K
2>i
42.0
17.0
21 .1
34.2 28.0
38.3
4H
3
51.8
16.1
19.8 42.3 26.4
35-7
S
3^
62.5
14.2
17.75
51 o 23.4
32.1
5^
2K
59
17-0
19 3
48.0 28.0 53-0
5^
3^
66.8
13-8
16.75
55 3 22.7 30.3
6 6
3?4
89.8
12.7
16.5
73-2 20.8
29.6
7^
3
94 5
13-5
23-7
77-1 22.3
42.8
8^
3H
I2O.O
ii. 8
19-8 97-8 19-5 35-7
10 H. P. Gage, "Types of Signal Lenses," TRANS, I. E. S.. Vol. 9. page 486.
11 For a resum6 of the subject of color and vision, see "Color and It Applications" by
M. Luckiesh.
246 ILLUMINATING ENGINEERING PRACTICE
effective range in miles for commercial colored lenses is reported
by the Railway Signal Association (1908) as:
Red 3 to 3. 5
Yellow i to i . 5
Green 2 . 5 to 3 . o
The range for a clear lens is estimated at from 8 to 1 2 miles. The
visibility is decreased when the field surrounding the lens is slightly
illuminated, as in a slight haze or when other sources are near by.
Small electric lamps are rapidly coming into use in semaphore
signals and, with the stronger intensities produced by these more
concentrated sources, they are found to be more satisfactory by
day than are the arms. In one type of equipment use is made of
rows of lenses in the three arm positions. On the C. M. & St.
P. R. R., three signals, red, green and white, are aligned vertically.
Behind each lens are two lamps, one operating at low efficiency, to
prevent failure of the signal. The normal daylight range is 3000
feet, and under the worst conditions when opposed to direct sun-
light the range is not less than 2000 feet. It is reported that they
are seen more easily than semaphore arms under all circumstances
and that they show two or three times as far as the latter in a
snowstorm.
Military searchlighting projectors have been used to transmit sig-
nals at night more than 50 miles by training the beam on a cloud.
They are also used in the navy directly for day signaling over con-
siderable distances, and have the advantage that the narrow beam
precludes observation by other vessels even though only a few degrees
removed. The type of shutter equipment used is illustrated in Fig.
23. Several small incandescent lamps mounted in the ring focus of
a cylindrical Fresnel lens are used with a Morse key for night signal-
ling in the navy at moderate distances, superseding the Ardois and
other devices. In the European war, extensive use is made among
the land forces of i.2-c.p. metal-filament lamps equipped with para-
bolic reflectors. 12 Morse signals are reported to have been read at
ii miles at the rate of 17 words per minute with this apparatus.
Fig. 36 illustrates a 150- watt signalling projector employed on
British aircraft. The properties of the spherical and parabolic
mirrors as well as the dioptric lens are utilized.
PROJECTION OF TRANSPARENCIES
One of the most familiar applications of lens systems in lighting
equipment is for the projection of lantern slides and motion picture
15 Illuminating Engineer, London, Vol. 8, page 62.
EDWARDS AND MAGDSICK : LIGHT PROJECTION
247
films. The scope of this lecture permits reference only to the
fundamental optical systems and the light sources for the most com-
mon classes of equipment. Numerous treatises of a more detailed
nature are available; some of the more recent ones are mentioned in
the appended bibliography.
The elements of the optical system for lantern-slide projection are
shown in Fig. 3 jA . The condenser intercepting the flux from the lamp
becomes a secondary source having a brightness differing from the
intrinsic brilliancy of the light source by only the percentage of
losses in the glass, and directs a converging beam through the slide
into the objective lens. The focal length of the latter is determined
by the distance to the screen and the size of the picture desired
Light Source
Condensing Lens
Slide Holder
Screen
Mirror
Screen
Fig. 37. A, Simple optical system for the projection of lantern slides. B, Simple optical
system for the projection of motion pictures.
Focusing for the different distances is accomplished by adjusting the
position of the objective with reference to the slide. If the objective
were limited to a very small aperture, the source of light would have
to be highly concentrated in order that the rays might be accurately
controlled and concentrated at this point. In practice, these may
be made of considerable size; hence it is possible to secure the re-
quired illumination from a somewhat extended source. Cost con-
siderations determine the best combination of source brightness and
objective diameter. To secure uniform results over the entire pic-
ture, it is necessary that from any point in it a view through the
objective and slide holder disclose condenser surface covering the
entire area. In order to keep the condenser diameter within reason-
248 ILLUMINATING ENGINEERING PRACTICE
able limits, it is important to place the slide holder close to it.
Mounting the light source near the condenser results in the utiliza-
tion of the flux in a relatively large solid angle, and, therefore, makes
for efficiency. The usual opening in the slide holder is 3 X 3/4 in.
To illuminate all parts and avoid spherical and chromatic aberration
requires a beam of a diameter even greater than the diagonal of
the opening; thus a considerable percentage of the light is lost.
In motion picture work, Fig. 37 B, the intensity requirements
are far more severe and the brightness of the light source is corre-
spondingly important. The aperture of the plate through which the
film is fed has an area of 0.680 X 0.906 in. It is, therefore, placed
well forward of the condenser in the narrower part of the beam.
Additional losses are encountered through the necessity for a shutter,
usually a sectored disc, to cut off the light during the period of
film shifting, which occurs, with the usual pictures, 16 times every
second. Since this frequency would be apparent as a distinct
flicker, a two-wing or three- wing shutter is provided so that the light
may be shut off 32 or 48 times per second.
Kerosene and acetylene flames, incandescent mantles and Nernst
glowers and oxy-hydrogen lime light sources, have all been em-
ployed in the projection of lantern slides. To-day electric arc and
incandescent lamps are used almost exclusively.
The positive crater of the direct-current arc is particularly de-
sirable as a source of light because of its high intrinsic brilliancy.
It is not practicable to utilize the maximum brightness since the
electrodes must be so arranged that the positive tip is at an angle
with the condenser or that the negative shades a part of it. In
order to keep the arc steady, it is desirable to have a small negative
electrode, and this is secured with the necessary current-carrying
capacity by coating the carbon with metal. For lantern slide pro-
jection, 13 currents of from 4 to 25 amperes are found ample, with
electrodes ranging from 6 to 13 mm. in diameter. For the ordi-
nary motion picture films, currents of from 40 to no amperes are
employed with positive electrodes ranging from 13 to 25 mm. in
diameter and negative electrodes of from 8 to 22 mm., depending
upon the current and the composition.
Alternating-current lamps of low amperage are operated with a
long arc. Since the arc is continuously reversing, there is no sharply
defined crater of high brilliancy on either electrode. Such lamps are
distinctly inferior in efficiency to the direct-current arcs, although
l * R. B. Chillas, Jr., "Projection Engineering;" Trans. I. E. S., Vol. u, page 1097.
EDWARDS AND MAGDSICK : LIGHT PROJECTION
249
ample for most lantern slide work. For motion picture projection,
the alternating-current electrodes are operated close together to
secure better craters. The electrodes are inclined to each other so
as to expose as much as possible of one of the tips to the condenser.
However, the brightness of the source is still lower than with direct-
current, and considerable shading results due to the interference of
the other electrode. Shutters employed with alternating-current
equipment are of the two- wing type; the three-wing shutter with a
frequency of 48 per second gives rise to stroboscopic effects with
6o-cycle current.
Incandescent lamps of special concentrated-filament construc-
tion are used for the projection of lantern slides under all condi-
tions, and take care of the requirements amply. Recently the gas-
filled tungsten-filament lamps have also been successfully applied
Condenser
Objective
Light Source
Per Cent -100 20.2 14.0
Lumens-23,600 4770 3300
Fig. 38. Typical efficiency chart for motion picture projection with mazda lamp; machine
operating without film.
to motion picture projection. It will be seen from Table I that the
brilliancy of such sources is still below that of the carbon arc;
nevertheless, their application is feasible because of other advan-
tages gained. Among these is a somewhat more efficient utiliza-
tion of the flux due to the fact that the source can be placed closer
to the condensing lens. When used with objectives of the larger
apertures the incandescent filament is found to be sufficiently con-
centrated, and the intensification of flicker and irregularities pro-
duced by such lenses with arc sources is obviated. The steadiness
of the light and the elimination of operating difficulties are quite as
important as the reduction in operating cost realized. In Fig. 38
are shown the utilization and losses of the flux in such apparatus.
Although the losses in the system may appear high, it should be
noted that the results for each part of the apparatus are superior
to those secured with most equipment in use to-day. The illumina-
250 ILLUMINATING ENGINEERING PRACTICE
tion intensity on a picture area of 150 square feet is seen to be in
excess of 5 foot-candles.
The question of the most desirable intensity for motion picture
projection is one on which a difference of opinion still exists. The
Committee on Glare of the Illuminating Engineering Society 14 has
recommended a brightness for the picture corresponding to a screen
illumination of 2.5 foot-candles with no film in the machine, with a
factor of 5 either way. A brightness which is too high causes not
only fatigue to the eye, but also makes the flicker, wandering of the
arc, etc., more pronounced. It appears that the present high-
current arc installations are operating in the upper range of desirable
intensities.
BIBLIOGRAPHY
GENERAL PRINCIPLES or LIGHT PROJECTION
BENFORD, F. A., JR. "The Parabolic Mirror;" Trans. I. E. S., Vol. 10, p. 905.
GAGE, S. H. and H. P. "Optic Projection," Comstock.
NATIONAL LAMP WORKS or G. E. Co. " Mazda Lamps for Projection
Purposes;" Eng. Dept. Bulletin No. 23.
ORANGE, J. A. "Photometric Methods in Connection with Magic Lantern
and Moving Picture Outfits, and a Simple Method of Studying the Intrinsic
Brilliancy of Projection Sources;" G. E. Review, Vol. 19, p. 404.
PORTER, L. C. "Photometric Measurements of Projectors;" Lighting Jour-
nal, Vol. 4, p. 7.
"New Developments in the Projection of Light;" Trans. I. E. S., Vol. 10,
p. 38.
AUTOMOBILE HEADLIGHTING
CLARK, EMERSON L. "Automobile Lighting from the Lighting Viewpoint;"
Bull. Soc. Auto. Engs., April, 1916, p. 45.
Discussion. "Headlight Glare;" Bull. Soc. Auto. Engs., Feb., 1916,
p. 296.
Symposium. "Glare-Preventing Devices for Headlights;" Trans. Soc.
Auto. Engs., Vol. 9, Part II, p. 284.
RAILWAY HEADLIGHTING
American Ry. Master Mechanics Ass'n. "Report of Headlight Committee,"
1914.
Ass'n. of Ry. Elec. Engineers. "Report of Committee on Locomotive
Headlights;" Ry. Elec. Eng., Vol. 5, p. 199. .
BABCOCK, A. H. "Southern Pacific Six- Volt Electric Headlight Equip-
ment;" Ry. Elec. Eng., Vol. 7, p. 233.
14 Committee on Glare, "Diffusing Media; Projection and Focusing Screens, "Trans.
I. E. S., Vol. ii, page 92.
EDWARDS AND MACDSICK: LIGHT PROJECTION 251
BAILEY, P. S. "Incandescent Headlights for Street Railway and Locomo-
tive Service;" G. E. Review, Vol. 19, p. 638.
HARDING, C. F., AND TOPPING, A. N. "Headlight Tests;" Trans. A. I. E. E.
Vol. 29, p. 1053.
MINICK, J. L. "The Locomotive Headlight;" Trans. I. E. S., Vol. 9, p. 909.
PORTER, L. C. "Meeting the Federal Headlight Requirements;" Ry.
Elec. Eng., Vol. 7, p. 468.
Ry. Elec. Engineer, Vol. 3. "Electric Headlights Wisconsin Railroad
Commission Tests."
SCRUGHAN, J. G. "Electric Headlight Tests;" Ry. Elec. Eng., Vol. 5,
P- 349-
Symposium (Succ, CHAS. R., DENNINGTON, A. R., PORTER, L. C.). "Theory,
Design and Operation of Head-Lamps;" Elec. World, Vol. 62, p. 741.
SEARCHLIGHTING
BLONDEL, A. "A Method for Determining the Range of Searchlights;"
Illuminating Eng. (London), Vol. 8, pp. 85, 153.
CHILLAS, R. B., JR. " Searchlight Carbons;" Journal of U. S. Artillery,
March-April, 1916, p. 191.
Electrical World. Vol. 64, p. 181; "Search Lamp with Vapor-cooled Elec-
trodes" (Beck). Vol. 68, p. 611; "High-Intensity Searchlight for Governmental
Purposes" (Sperry).
MCDOWELL, LIEUT, C. S. "Searchlights;" Proc. A. I. E. E., Vol. 34, p. 195.
"Illumination in the Navy;" Trans. I. E. S., Vol. n, p. 573.
NERZ, F. "Searchlights; Their Theory, Construction and Applications;"
Van Nostrand.
Symposium (LEDGER, P. G., AYRTON, MRS. HERTHA, TROTTER, A. P., etc.)
"Searchlights; Their Scientific Development and Practical Applications;"
Illuminating Eng. (London), Vol. 8, pp. 53-84.
WEDDING, W. "A New Searchlight" (Beck); Electrotechnische Zeit-
schrift, 1914, p. 901.
FLOOD LIGHTING
BAYLEY, G. L. "Illumination of Panama-Pacific Exposition;" Elec.
World, Vol. 65, p. 391.
Elec. Review and Western Electrician, Vol. 67, p. 1104; "Indianapolis
Bank Adopts Flood Lighting." Vol. 67, p. 724, "Flood Lighting of Building
Fronts from Ornamental Cluster Posts."
Electrical World, Vol. 67, p. 1173; "Flood Lighting a Flag;" Vol. 67,
p. 1462; "Adding Hours to Summer Days for Outdoor Recreations." Vol.
68, p. 453; "Niagara Falls Flood-Lighted."
HARRISON, WARD and EDWARDS, EVAN J. "Recent Improvements in In-
candescent Lamp Manufacture;" Trans. I. E. S., Vol. 8, p. 533.
Lighting Journal, Volume 4, p. 18; "Projectors for Flood Lighting."
MACGREGOR, R. A. "Lighting the Soldiers' and Sailors' Monument;"
Ltg. Journal, Vol. 4, p. 175.
MAGDSICK, H. H." Flood Lighting the World's Tallest Building;" Elec.
World, Vol. 68, p. 412.
252 ILLUMINATING ENGINEERING PRACTICE
PORTER, L. C. "Pageant Lighting;" Ltg. Journal, Vol. 3, p. 169.
RYAN, W. D'A. " Spectacular Illumination;" G. E. Review, Vol. 17, p. 329.
"Illumination of the Panama-Pacific International Exposition;" G. E.
Review, Vol. 18, p. 579.
SUMMERS, J. A. "Flood Lighting the State House at Boston;" Ltg. Journal,
Vol. 4, p. 2.
UHL, A. W. "Flood Lighting of a Great Outdoor Pageant;" Ltg. Journal,
Vol. 4, p. 172.
LIGHTHOUSES
Encyclopaedia Brittanica, nth Edition.
HASKELL, RAYMOND. "Lighthouse Illumination;" Trans. I. E. S., Vol.
10, p. 209.
MACBETH, GEO. A. "Lighthouse Lenses;" Proc. Engs. Soc. Western
Penn., Vol. 30, p. 231.
LIGHT SIGNALS
CHURCHILL, WM. "Red as a Danger Indication;" Trans. I. E. S., Vol. 9,
P- 37i.
GAGE, H. P. "Types of Signal Lenses;" Trans. I. E. S., Vol. 9, p. 486.
LUCKIESH, M. "Color and Its Applications," Van Nostrand.
MCDOWELL, LIEUT. C. S. "Illumination in the Navy;" Trans. I. E. S.,
Vol. ii, p. 573-
SAUNDERS, J. E. "Recent Developments in Light Signals for Control of
High-Speed Traffic;" Elec. Journal, Vol. 13, p. 443-
STEVENS, THOS. S. "Illumination of Signals;" Trans. I. E. S., Vol. 9, p. 387.
PROJECTION OF TRANSPARENCIES
CHILLAS, R. B., JR. "Projection Engineering;" Trans. I. E. S., Vol. n,
p. 1097.
GAGE, S. H. and H. P. "Optic Projection."
ORANGE, J. A. "Optic Projection as a Problem in Illumination;" Trans.
I. E. S., Vol. 11, p. 768.
TAYLOR, J. B. "The Projection Lantern;" Trans. I. E. S., Vol. n, p. 414.
THE ARCHITECTURAL AND DECORATIVE ASPECTS OF
LIGHTING
BY GUY LOWELL
There is surely no scientific profession, there is no branch of the
engineering fraternity for which a thorough artistic training is more
desirable than the profession of illuminating engineering. We can
see, however, by looking over the list of lectures in the usual engi-
neering courses that the technical knowledge which one should have
is so great there are so many scientific subjects to be discussed
that there can be but little time left in the curriculum for the study
of the fine arts. Yet after all the aims of the illuminating engineer
and of the artist are similar it is to reach the mind through the
eyes. The point of view of the engineer is, however, largely objec-
tive. He often seems to think that his mission is ended when he
has made it possible to convey to the mind the facts as they are.
The artist idealizes and wishes to state the facts as they might or
should be, or as we say, colloquially, he wants to show them in the
best possible light. These two methods of seeing the subjective
method and the objective method, are often not very different, and
I want to spend my time this morning considering the common aims
of the illuminating engineer and the artist, and show how close to-
gether the paths of the two really lie.
We have been taught that were it not for the reflected light that
comes from all the different objects on this earth of ours, our world
would appear to be in darkness, because we could not see the objects
around us. There might be sources of light, like the fire, the incan-
descent filament, or the electric arc which would be visible in them-
selves, but the light that comes from the heavens or from some man-
made source must be reflected from an object in greater or less in-
tensity for us to be able to see it. Furthermore, we all know that
the effect that an object makes on the retina and thereby on the
mind is dependent on the way the light is reflected from an object,
and partly therefore on the way the light falls on that object.
Since it is this pattern made by rays of varying intensity and of
varying color on the retina, calling up various reminiscences to our
mind, that enables us to see to understand what lies before us, it
253
254 ILLUMINATING ENGINEERING PRACTICE
follows that the type of lighting that sets in motion the most power-
ful train of associative ideas is the one that may have the greatest
emotional effect; but the intensity of the emotional effect is not
measured by the intensity of the light even though the intensity of
that light may affect the clearness with which we judge of the phys-
ical aspect of the object on which it falls. We are not always neces-
sarily interested, however, in the physical aspect in the intricate
details of the object at which we are looking. We are often more
interested in the memories it calls up. Let me illustrate what I
mean by an example.
When I realized some weeks ago that I was going to talk to the
members of this society on the aesthetic principles instead of the
scientific principles involved in some of the every-day problems of
lighting, it occurred to me to get a variety of opinions on the mental
reaction produced by such a simple source of light as one bright
star in the midnight sky. So I asked three people among my neigh-
bors one a distinguished astronomer, the next a young girl just
back from college, and the third an immigrant woman whose husband
worked as gardener on the place what their thoughts would be
were they to wake up in the middle of a wintry night, and as they
came back to consciousness were to see through the window a bright
star. The astronomer said he would begin to wonder which star it
was among all the myriads in the heavens; the young girl with a
mind full of classical poetry said she would think of the mytholog-
ical stories connected with the stars; the working woman said,
" Sure if it was a single star on a wintry night I would think of the Star of
Bethlehem."
But when I said to each one of my three friends, " Supposing you
were told that it was not a star after all but a distant electric light,
what would you do? " They all three made a similar answer, " We'd
turn over and go to sleep."
Now the interest in these answers lies here. No one of the three
was interested in the one little bright spot in the sky as a source of
light; so long as it was a star, it called up a whole series of associative
thoughts.
Whether it calls up with its suggestion of infinite distances and
infinite time a whole theory of cosmic philosophy; or whether it
suggests to the pagan mind the mythological intrigues of a Jupiter,
a Mars and a Venus; or whether the star recalls one of the most
touching stories of our Christian faith the story of the Star of
Bethlehem seen by the watching shepherds from the hillside nearly
LOWELL: ASPECTS OF LIGHTING 255
two thousand years ago, certain it is that most of us when we see
the brilliant star set in its wonderful background of midnight blue,
project into our thoughts the reminiscence of some earlier associated
idea, and thereby enjoy intellectual pleasures that we could not get
from the mere contrast alone of a brilliant light against a dark back-
ground. That one little spark of light in the sky is able to suggest
a whole train of speculative thought, and serves as a strong stimulus
to the imagination; in other words, fulfills the functions of a work of
art, for in stimulating the imagination it has called up thoughts of
beauty.
What I want to consider more particularly to-day is the artistic
function of lighting and show how the lighting scheme of the scene
at which we are looking may quite independently of its efficiency,
technical excellence or physiological advantages, control the emo-
tional reaction which it produces influences the aesthetic result
produced. Now instead of a single star, the scene at which we are
perhaps looking may be the harmonious grouping of the many differ-
ent objects in a natural landscape or inside a room, all reflecting
different kinds of light in different ways and combining to make up
the picture that is conveyed to the mind by the eye. I have already
said that the strength of the intellectual reaction made by the pic-
ture we see is in no sense dependent on the intensity of the lighting,
nor necessarily on the clearness of vision with which we see the ob-
jects in our scene.
Let me show you again that the artistic effect is quite as much
due to the train of associative ideas it calls up as to the clearness
with which we see that scene. For this purpose I am going to ust
as an illustration an outdoor scene, since we can select some beautiful
view and nature will kindly shift the light for us, so that in our out-
door laboratory we may judge of the changing thoughts produced
by the same or similar scenes but under different conditions of light-
ing. In order to show the difference between a scene clearly defined
because of its uniform lighting, and a similar scene where only the
important elements are brought out by the artist, I would ask you
to compare a photograph of some familiar object with a painting
by an artist of that same object.
Photography is of use because it provides an illustration of the way
we really see things in that it gives a record full of detail of what we
see. The image permanently produced on the photographic plates
after chemical development is monochromatic it is true and cannot
by black and white present all the different colors nor are the light
256 ILLUMINATING ENGINEERING PRACTICE
values in the photograph always relatively right, but the direct
photograph being what one might call a mechanical record of the
scene before us provides us with an interesting way of comparing
actuality with the way an artist would treat a similar scene, for the
artist first looks, then apprehends, and then selects from all that he
sees only that which he desires to record.
The painter with his easel set up about to paint a landscape or a
portrait waits till the lighting on his subject is just right, of the proper
concentration or diffusion, from the right direction, of the right color,
and is, therefore, dependent on the vagaries of nature. And to him
the proper lighting of his subject is of tremendous artistic importance.
Artificial light in the hands of the illuminating engineer can be con-
trolled and arranged as the artist wishes, and the architect in the
planning of his lighting scheme considers the same rules of compo-
sition, studies the same effects of contrasts, produces by the position
of his sources of light the same harmonies of line that the painter
patiently waits often day after day for nature to produce.
Right here we must emphasize once more the fact that uniform
visibility and great distinctness of vision are not necessarily desir-
able; it is wrong to assume that because, for instance, much time,
thought and money have been spent on some decorative detail, or
even on some art object among a collector's treasures, that it must be
clearly brought out in the picture as a whole that what is costly
and of value should, to use a naval expression, have high visibility.
The artist does not want you to see everything with equal dis-
tinctness. In his composition as in a symphonic poem some of the
most beautiful passages, though full of suggestion, are low in tone,
thus bring out in greater contrast the general theme throwing a
high light on some other beautiful part. The musical composer only
puts into his composition what he believes to be of importance to the
creating of a proper impression of the whole; the artist or the worker
in black or white leaves out what he does not want. The artist who
arranges the light sources, who provides the illumination of a building
must do the same, and the elimination, in the picture that presents
itself to the eye, of the undesired elements by one method or another,
should be an important part of his artistic result.
The illumination of work shops, clerical offices, manufacturing
plants, mercantile buildings as well as schools and buildings more
directly under governmental control has been carefully studied, and
we have been told at this convention here of the increased efficiency,
the better health and the greater freedom from accidents that have
LOWELL: ASPECTS OF LIGHTING 257
been brought about by a proper and efficient system of lighting and
by the proper treatment of the wall surfaces and ceilings, so that they
will not absorb an undue amount of light. That is a practical
problem that you gentlemen are well qualified to solve; but the
architect is at times, when he is not building loft buildings, offices,
hospitals or industrial plants, but is designing buildings that are to
serve for rest and for recreation rather than for work and efficiency,
called upon to forget cost of operation and to neglect efficiency in
order to produce a greater emotional effect. I am making a plea for
the architect. You as engineers have not done your complete duty
when you have thrown enough light by some economical system that
does not require the paying of too large tribute to the electric light
company, to enable one to see clearly all part of some new building.
You may feel that I am talking too much about beauty, and too
little about lumens and amperes, but after all I am only telling you
how the trained artist with his surety of taste resulting from his long
study of composition must always study to make lighting right,
aesthetically, for that enables him to show the form and the color of
what he represents in the most artistically effective way
That from an architect's, as well as the artist's point of view is the
artistic function of artificial lighting.
The architect, however, is constantly trying to apply his artistic
ideals to the practical solution of his problem. He recognizes at
times that the utilitarian must prevail, but he also believes that there
are times when the aesthetic appearance is of paramount importance
and his resulting lighting scheme may be neither economical nor
physiologically correct.
We are often told that whatever is scientifically right must be good
artistically, and that whatever in our universe is functionally correct
and calculated to its needs with nicety is beautiful for that reason.
I do not entirely believe that myself, but I am going to concede to a
gathering of the scientific-minded like this that the scientific solution
is undoubtedly the best for most problems; but qualify it by saying
that the scientific mind often finds it hard to grasp what the artistic
problem really is, for science is dealing with facts, is interpreting them
and converting them to use, but is not interested in the emotional
effect, for that is dependent on the different reaction on different
individuals.
For the understanding of many of these problems where the artistic
and the scientific seem to come in conflict real powers of imagination
seem necessary. What is imagination? Imagination might be
17
258 ILLUMINATING ENGINEERING PRACTICE
defined as the power to realize that there are variations from the rule
and that such variations require a special treatment. If you agree
with that definition of imagination consider the artistic treatment of
the lighting problem as a possible variation from the rule and allow
your imagination full play.
So the lighting scheme laid out by an architect in connection with
a building may be for one or two purposes:
(a) Primarily for use and not for decoration.
(b) To produce a decorative effect without special care being taken
to have it economical and efficient from the engineering standpoint.
In a practical system of architectural lighting the usual object is
to reproduce in so far as economical utilitarian consideration will
allow a properly diffused light resembling daylight if possible, and
in sufficient quantity to enable one to do one's work or see about the
lighted rooms or spaces with absolute ease. The best way to ob-
tain such scientifically worked out lighting so that it shall be efficient
and economical is really a practical question and not an aesthetic one.
In what I consider an artistic scheme the sources of light 'screened
or unscreened are grouped in such a way as to produce not diffusion
but contrasts. The spots of strong reflected light and the spots of
deep shadow are composed much as artists would compose light and
dark spots in a drawing or painting. I have an admirable illustra-
tion in mind of two art museums with these two absolutely different
types of artificial lighting. They are the Art Museum in Boston
and the private collection of Mrs. Gardner near it. At the Museum
we tried to arrange the light so that it will as nearly as possible
reproduce in direction and color the daylight those are the condi-
tions that exist during the greater part of the time that the Museum
is open and there every object can be clearly seen and studied.
Mrs. Gardner lights her rooms with a few candles placed around so
that some one particularly interesting object can be seen, standing
out as it were from the surrounding shadow. No indirect lighting
system in her house could begin to have the same charm.
Even in an art museum the chosen method of lighting might
depend on whether it is for use or for artistic effect.
In my garage, in my kitchen, and in my work room, I try to
diffuse the electric light as much as possible by reflecting surfaces.
In my own dining room and parlor, however, though I have electric
light brackets on the walls, they are never turned on, and the room
is either lit entirely by candles, or by candles and portable lamps.
Let me illustrate these two different ways of looking at the same
LOWELL: ASPECTS OF LIGHTING 259
thing that is, the objective and the subjective. Consider, first of
all, a photograph of a bridge; every detail is clearly brought out in
the picture, the arches of the cement bridge, the trolley poles, the
roadway, the ugly buildings, all jumbled together. No artist com-
posed the picture it is just a record of homely facts. Now let us
see a bridge through the eye of an artist. Perhaps it is a little un-
fair to contrast with the photograph of a modern cement bridge,
say, one of Whistler's lithographs, but this shows in a simple drawing
the beauty the artist saw in what is really a very ugly bridge. He
tried to express only as much of the bridge as seemed to him in his
mood at the time as necessary to call up a certain impression. In
other words he threw the light on only the essentials and left the
unessentials undefined. To some this drawing calls up a long train
of associative ideas, to others it represents little more than a beauti-
ful pattern in black and white. I would have you consider the
Presentation in the Temple, by Rembrandt. Here we have the
strong lighting of the important figures, the background subdued,
and only half felt to be there, like the subdued accompaniment to
the principal melody in music. We might almost think that
Rembrandt had invented the modern theatrical spot light in his
desire to accent strongly the personages in his picture, and this
characteristic of strongly marked high light is produced in all his
paintings, because he knew that skilfully disposed lights, despite
the strong contrasts, produce an agreeable pattern of lights and
shadows. There is a simple way to study composition, by taking the
paintings of the acknowledged masters, and when we are sure that
we like a certain work try to analyze its composition, judge the com-
posing of light and try to express in ideas, in words, wherein its ex-
cellence lies.
Nature, too, has a lot to teach us. We suddenly come on an
opening in the woods, and the scene before us seems to make a
satisfying and inspiring picture. To what is the charm due? Is
it the color of the young green leaves with the sun shining through
them; is it the sweep of the tree trunks and the branches into a
smooth and flowing pattern; is it the distant vista of lake or moun-
tain? It may be one or all of these, but there is always the light
which above all is just right. Were it to come from a different
direction, the leaves would be in shadow, the dark lines of the
branches would make a different pattern, the high lights would be
differently placed. But to have just the right picture you must see
your scene as the artist would with its chosen lighting.
260 ILLUMINATING ENGINEERING PRACTICE
Now let us consider scenes by other painters. Gainsborough,
full of vigor with strongly marked lines in the composition; Turner
with a satisfying and harmonious sweep of line from one side of the
canvas to the other; Rembrandt with his high lights like the strong
blare of the trumpet in an orchestral piece.
You see efficiency and intensity of lighting are left far behind in
our minds when it comes to tracing harmonious patterns and pro-
ducing wonderful blendings of color and light and shade.
Think of the advantage you have as artists if you will so consider
yourselves. You hold in your hand brushes dipped in light; you
have a pallette set with all the colors that we find in nature. You
can make your high lights shimmer at will. You can throw the
confused detail into the mysterious and shadowy background.
Equipped with a sound technical knowledge, the effects you can
produce are only limited by your artistic training. But the road to
art is long. A short lecture like this can only show that there is
such a road; it cannot for a moment do more than that. For the
power to understand the artistic impulse, the power to create what
is artistically good, must come as the result of years of thought and
study.
We have given a hasty glance without attempting to classify
them at the lighting schemes of nature in outdoor landscapes. The
most direct copy of those effects of lighting we find on the stage of
the modern theatre.
There the aim is to produce illusions, to produce the illusion of
reality, not necessarily as we have ourselves experienced it, but as
we can conceive that it might exist, and there are no limits to-day
to what one can do. For that reason the conventions for lighting
of the stage of the last generation are disappearing. The strange
effect produced by footlighting, with the resulting prominent chins
and receding foreheads, is giving way to a flood of colored light pro-
ducing the effect of a shadowless stage where the whole company
is suffused in light. We are now looking at a pattern of color and
even the advent of solid properties on the stage has failed to give
us quite the real sense of solidity that comes with the feeling of mod-
elling one gets from natural unilateral lighting. The thoughtful
stage manager tells us that we have not solved his problem because
he has to work with color and has to give up attempts at modelling.
But since he states that the problem needs solving, you gentlemen
must help him.
I have purposely made this comparison of unilateral lighting out-
LOWELL: ASPECTS OF LIGHTING 261
doors, and diffused lighting on the stage because the stage manager
with all possible kinds of lighting at hand has to resort to the subter-
fuge of painted shadows in order to make objects on his stage appear
real, to appear solid. He paints a shadow under the cornice of a
building, he shades one side of a real round column with paint, he
even darkens the eye sockets and wrinkles of the actors. Though
he is working in a space suffused with light he must paint in shadows
to make his picture real and solid. And painting will not take the
place of real high lights and real shadows as nature produces them.
It is exactly the same thing in lighting our buildings, we must make
them seem real, and what is more, make them seem solid.
We thus see in our examples chosen from natural landscapes and
from the work of the landscape painters that the emotional reaction
of the scene before us when we are in the open looking at a natural
landscape that the mood produced in us by the artist's painting is
largely affected by the way the lighting is done. We also see how
the theatrical manager, with the wonderful power of creating im-
pressions, of inducing sensations, which is given him, is again abso-
lutely dependent on the varying effects of lighting for painting his
stage lights and his shadows. But the final effect produced on the
individual is dependent on the taste of the individual. And the
individual's taste is the result of experience, of education, of varying
reminiscences, and it therefore is impossible to dogmatize and say
that such and such a way is the best way to light a given subject.
Some one asked me which I preferred for the exterior of a monu-
mental building a row of incandescent lamps or flood lighting.
How can I say? I do know that the answer depends on what the
architect is trying to emphasize and on what he is trying to hide.
Consider flood lighting for a moment. It is rarely so done as to
do justice to the architecture. It was all right at the San Francisco
fair. It was wonderful in fact because like on the stage spoken of
a few minutes ago it served to bring out color and not form. But as
used on the occasional building it is hardly so successful, for instance,
the Boston State House last fall, or the new Technology buildings
last spring. In the case of the last two buildings the architect had
worked out all his contrasts of opening with wall space, all his con-
trasts of supporting column with its lintel, to be seen by daylight,
and by daylight the light comes from above. How can one expect
flood lighting to do anything bat invert the architectural effect if it
is thrown up from a lower building onto a higher one? And how can
you expect the interior of a building to be artistically satisfactory
262 ILLUMINATING ENGINEERING PRACTICE
if the light that comes from the windows by <day produces a composi-
tion that is entirely inverted by the change in direction of the arti-
ficial lighting at night.
How do these theories work out in our homes? There can be no
one accepted set of rules for the guidance of the man who lays out
the electric lighting in a private house. I personally like as few
light sources as possible, and I certainly only put in wall brackets,
because handsome, skilfully designed electric brackets are of value
decoratively on the walls, but as I have already said I never light
these in the living rooms in my own house.
I remember that when I first built a big room for the furniture which
I had collected while living abroad, we all gathered one evening
to discuss the position of the various pieces and to move them around
till we thought all were perfectly placed. The room was lighted by
electric wall brackets, some old French ones I had had wired, and
by lamps on the tables they were connected to floor receptacles.
Nothing around looked right! We almost broke our backs for two
hours, moving the furniture, but each change seemed only to make
the composition worse. I turned the switch which extinguished all
the wall brackets. The room suddenly felt " right." What I found
was successful in my own case is what I now try to apply for. my
clients. I get along without light from the wall brackets.
So when it comes to the varying treatment of the different rooms
of the house I don't for a minute concede that efficiency alone
should be the keynote; what we should have above all is a beautiful
effect. Yet, we should have enough light for the average person.
One's old Jacobean oak staircase should be light enough so that a
short-sighted person will not fall down and break his neck, but there
might be some of the dim mysteriousness of the ancient staircase
arranged for in its lighting.
I want to lay great emphasis here on the difference there is between
rooms in public buildings and similar rooms in private buildings.
Take for instance the reading rooms in a public library and the li-
brary (reading room) in a private house. It is obvious that the
public reading rooms should be so lighted that a maximum number of
workers should be provided for, every reader in fact should have the
light fall with the correct intensity and from the correct angle. But
the private library is quite different. Half the time the people sitting
in it may not be reading; at any rate it surely needs to accommo-
date not more than one or two actual readers at a time. There
hould be light enough to enable one to see the people around the room
LOWELL: ASPECTS OF LIGHTING 263
but in my opinion the rest of the room should be only so lit as to make
the most artistic background. In one corner you may see a lamp on
a table throwing its light onto the richly bound books in the bookcase
and there is nothing more decorative for a background than hand-
some bindings; in another you may see a well placed picture lamp or
the worker's desk lamp. But there should be no attempt to make
the room look like a picture gallery. The use of picture lighting in
a private home is a matter of personal taste. So many of the large
homes to-day make claims to have valuable collections of paintings
stored in them that the owners feel that they should be shown off
to the greatest advantage at all times; hence, that abomination of the
architect the modern picture lamps, with their reflectors bracketed
out from the wall or ceiling about the picture.
Again let me say that there must be a difference between the
lighting of the room of a public building and a private one. Put
your fine pictures, if you must show them off and not enjoy them
quietly and separately, in a gallery with all the advantages of a care-
fully, skilfully worked out lighting scheme, shown where they can be
seen to the best advantage, but don't force your guests to eat in a-
picture gallery because the dining room has some of the owner's
choicest paintings in it, brilliantly lighted by reflectors. I have in
mind one beautiful walnut panelled room where each picture on the
wall at night is so brilliantly lighted that every picture with its frame,
seems like a window cut in the panelling looking into the landscape
beyond, and there is no sense of support in the wall surface.
Let us make another comparison the dining room in a public
restaurant and the private dining room. When I go to a public
restaurant I find in America that the desire is* to have a brilliantly
lighted room, profusely lighted by different sources. My own dining
room however is a Jacobean oak room taken from an old house in
England. Near the pantry door there is a hidden receptacle which is
used with a lamp and a reflector when the table is being set. When
all is ready for the guests the lamp is removed and the room is lighted
only by candles. No system of lighting that I know makes a more
becoming light for the guests, or sets off the linen, the china, and the
silver to better advantage. Each source of light too is so low in
intensity that its contrast with the dark background is not un-
pleasant. I like to see a similar method of lighting applied to the
restaurant dining room because I personally, though I go to a public
dining room, like to confine my attention to the people at the same
table with me. I like the sense of privacy which the French like
264 ILLUMINATING ENGINEERING PRACTICE
even in a public restaurant. Sherry's in New York is arranged that
way. There are, however, many people who go to a restaurant to see
or to be seen throughout the room ; for them some system of indirect
lighting may seem more practical.
The private house, should as you pass from one room to another,
provide a series of pictures, where the furniture, the light sources, the
people are all so combined as to make an artistic and picturesque
composition. Like the landscape painting we already considered
it will have individuality in which the lighting will be the controlling
factor. You can perhaps now understand my personal objection
to indirect lighting. It makes it so hard to compose the picture.
You must not assume, however, that indirect lighting according to
my opinion has no place in a well arranged home. For there are
many cases where it is advantageous to obtain our light from a large
surface with a low intensity rather than single sources of concen-
trated light. I have the evil habit for instance of reading in bed.
My bedroom is sanitarily painted in white. I throw the light with
two strong reflectors onto the wall behind me and obtain a wonderful
'light on my book or over the large pages of a newspaper. Scientif-
ically correct but artistically vile. But then I don't receive visitors
in my bedroom.
The transition from the private dwelling to the art museum is
direct.
It has always been my contention that the ideal public art museum
should have many of the characteristics of a private house. Most of
the objects in it were originally made for everyday use. Pictures
were to be hung on the walls of the living rooms. China, silver, fur-
niture, fabrics, carvings were all used in rooms that were human in
scale. There are a few colossal paintings, a few heroic statues to be
provided for, but surely the keenest artistic pleasures come from
seeing works of art in the surroundings for which they were intended.
Modern tendencies in museums have been in the other direction,
however. The tendency was to group all the paintings together, to
put all the furniture in another group, and the objects d'art off by
themselves. This has been, I think, largely due to the fact that the
administrative staff of many museums have thought too much of the
executive part of their work and too little of the artistic part.
The art museum is for the student, but it is quite as much for the
occasional visitor as for those who frequent the galleries day after
day. Now the constant passing through the galleries, the constant
work in the exhibition rooms of the museum staff is apt to make them
LOWELL: ASPECTS OF LIGHTING 265
consider the visual comfort of the constant worker even more than
the aesthetic satisfaction to the occasional visitor who comes to get
the artistic stimulus of seing beautiful things skilfully shown in pleas-
ant groupings.
This all creates a two-fold problem, for the objects must be clearly
lighted with a considerable intensity of light and yet it is of para-
mount importance that the ultimate effect of the galleries as a
whole be harmonious. It is at once a practical and an artistic prob-
lem and the method adopted in arranging for the lighting of such a
building offers a good example of the way the artist must always
attempt to solve the problem of lighting both by daylight and by
artificial light.
COLOR IN LIGHTING
BY M. LUCKIESH
INTRODUCTION
It appears desirable from an analytical viewpoint to divide the
problem of lighting into two parts, namely, that which involves light
and shade or brightness distribution, and that which involves color.
In dealing with the first part, the lighting expert is concerned with
the distribution of light and with the second part, with the quality
or spectral character of th illuminant. Sometimes these two prob-
lems are intricately interwoven but there is much advantage in gen-
eral in considering the problems separately especially if it be granted
that lighting should be considered from the standpoint of the appear-
ances of objects either singly or as a group.
The subject of color in lighting is complicated by the fact that
the eye is not analytic but synthetic in its operation. For instance,
a color sensation in general is the result of the integral effects of
radiant energy of many wave-lengths or frequencies. Often it is
necessary to know the luminous or energy intensities of these various
components yet sometimes merely the subjective or resultant color
is of interest. Spectrum analysis yields the desired data in the
former cases while such instruments as the monochromatic color-
imeter furnish satisfactory data in the latter cases depending upon
the problem at hand. One of these two viewpoints must be chosen
by the lighting expert for a given problem.
It is not the intention to ask the lighting expert to become a color
specialist and it is impossible to present a complete treatment of
the subject of color in lighting in a single lecture. However, it will
be the aim to present a sufficient amount of the science of color to
enable the lighting expert to diagnose his problems and a sufficiently
varied number of applications of color in lighting to show the trend
of progress and to impress him with the extent of the field if neces-
sary. Notwithstanding the brevity with which the theory of color
will be presented it may appear to some that it is unnecessary to be
acquainted with some of the aspects discussed. However, it cannot
267
268 ILLUMINATING ENGINEERING PRACTICE
be too strongly emphasized that an art is an applied science; that
is, science is the tool.
In the practice of color science it is desirable that definite termi-
nology be used consistently. The measurement of color is necessary
for specifying installations and for recording results. Many appli-
cations of color in lighting are directly dependent, for successful
results, upon a knowledge of the principles of color mixture. Obvi-
ously color and vision are closely related in the applications of color
in lighting but unfortunately many of these relations cannot be
discussed here. The psychology of color is of extreme importance
but many of the questions raised by the lighting expert are at pres-
ent unanswered. The color of surroundings is of much greater im-
portance than has been generally recognized in practice. The sur-
roundings influence the visual impression and also the color of the
useful light. It should be recognized that the color exhibited by
the lighting unit very often plays a dominating part in the impression
of a lighting condition even in those cases when the color of the useful
light is far different from the color of the visible portion of the unit.
Wrong conclusions have been arrived at by failing to recognize such
facts as the foregoing or, in other words, by not applying a searching
analysis to the conditions. Artificial daylight has been demanded
for many places and it is now practicable owing to the relative
high efficiencies of modern illuminants. Many arts have been
standardized in daylight and the eye has been evolved under day-
light conditions. For these reasons, and others, artificial daylight
finds many fields of application. Artificial daylight units have been
available for some time and it is now possible to record some experi-
ences gained from a great many installations. On the other hand,
aesthetic taste sometimes demands that the early illuminants be simu-
lated in color. This provides an interesting aspect although the
problems are not difficult because only the subjective color is usually
of interest. The means for obtaining various color effects are gradu-
ally being developed although the lighting expert must yet provide
colored media for many special applications. It has been thought
desirable to conclude this lecture with brief descriptions of a number
of applications of the science of color in lighting and the appended
bibliography will be depended upon to cover much that cannot be
incorporated in a single lecture. It appears best not to attempt to
incorporate much numerical data obtained from experiments in the
various fields treated in this lecture because these data are available
elsewhere (see Bibliography). The method of treatment, therefore,
LUCKIESH: COLOR IN LIGHTING 269
will be general, the more important viewpoints will be discussed and
an attempt will be made to present a broad discussion of an extensive
subject.
COLOR TERMINOLOGY
There is a great need for the standardization of color terminology
and for the development of a practicable system of color notation.
Many terms are in use for describing a few color qualities and there
is great confusion owing to the fact that the same term is used by
different persons to describe different factors or various terms are
applied to the same factor. It appears unnecessary to recount this
confused state but it is advisable to propose terminology to be stand-
ardized by this society. The proposals which follow appear to the
author to be the most satisfactory and the most consistently used.
Color can be considered from two broad standpoints. Spectrum
analysis provides data which are the most generally useful in the
science of color. On the other hand, interest in color is often merely
in regard to its appearance. The two viewpoints are, therefore,
objective and subjective and, while both must necessarily be inter-
woven into a complete system of terminology, the latter dominates
in the present consideration. Nevertheless it must be understood
that analytical data, which will be discussed later, supply the solid
foundation of color science and art.
Hue. This is the visual quality of a color which is correlated
on the physical side with the length or frequency of the predominat-
ing lightwaves with the exception of a large class of colors, called
purple, which includes also such colors as pink and rose. The
purples are mixtures of red with blue or violet and have no spectral
match in hue. It is customary to designate the spectral hue of the
complementary to the purple. In many cases the hue is directly
apparent in the name of a color; however, there are a great many
color names in daily use which are burdensome owing to the lack
of any suggestion of the hue. The hue of a color is determined by
comparing the color directly with spectral colors. If a match in
hue be made between a given color and a spectral hue at equal
brightnesses, in general it will be found that the two colors do not
yet appear alike. The difference is accounted for by the next
quality to be considered.
Saturation. In general, in order to make the foregoing match
perfect, it will be necessary to add a certain amount of white light
270 ILLUMINATING ENGINEERING PRACTICE
to the comparison spectral hue. The percentage of light of spectral
hue in the total mixture of white and spectral hue is a measure of
the saturation or purity. The term "purity" is misleading to many.
For example, if a perfect black be added to a given pigment, the
saturation or purity is unaltered; however, black naturally suggests
impurity to many. A spectral hue represents complete saturation
(zero "per cent, white") or a color of highest purity as considered
from the physical side. The usual procedure of using the term,
saturation or purity, in discussions and in measurements of determin-
ing the "per cent, white" is confusing to many persons. It is sug-
gested that the term, saturation or purity, be always used instead
of "per cent, white" and denoted by 100 per cent, minus the per
cent, white. Spectral colors would then be represented physically
by unity or 100 per cent. A color, which is matched by a mixture
of two luminous intensities corresponding to 8 parts of white light
and 2 parts of a certain spectral hue, would then have a saturation
of 0.2 or 20 per cent.
In making determinations of saturation two chief difficulties are
encountered, namely, a standard white light and a standard method
of color photometry. Average daylight, which is considered by some
to be clear noon sunlight, corresponding in spectral distribution of
energy to that of a black body at a temperature of 5oooC., can be
accepted as a standard white. This can be accurately matched by
means of an artificial light-source equipped with a proper colored
screen. However, a true physiological white is considered by some
to meet certain requirements which are not necessarily met by clear
noon sunlight. The flicker photometer provides a method of color
photometry which at least gives consistent results although the
method has yet to receive the approval of standardizing laboratories
for the photometry of extreme color differences.
Brightness. The third quality of a color is defined by the term
brightness. The measurement of this requires a standard method
of color photometry as discussed in the preceding paragraph. This
quality of a color is of interest both as relative and absolute bright-
ness. In general the relative brightness of a color, that is, its re-
flection or transmission factor, is of chief interest. However, it
must be noted that the reflection or transmission factors of colors
are not constant as in the case of neutral colors but depend upon
the spectral character of the illuminant.
From the standpoint of describing, and recording appearances of
colors the three factors hue, saturation and brightness are sufficient;
LUCKIESH: COLOR IN LIGHTING 271
however, the two following terms are quite useful and complete a
terminology of large descriptive power.
Tint. If the hue and brightness of a color be maintained constant
and the saturation be changed throughout a complete range from
zero to unity, a series of tints of constant hue and brightness will
result. If to a given color of high saturation, white light be added in
gradually increasing amounts a series of tints of constant hue and
increasing brightness will result. Tints, then, are colors of partial
saturation.
Shade. If the hue and saturation of a color be maintained con-
stant and the brightness be varied by varying the intensity of illumi-
nation, a series of shades will result. In the case of pigments the
addition of various quantities of perfect black results in the pro-
duction of different shades of a color of constant hue and saturation.
A system of notation is one of the great needs in the art and science
of color but the problem is too complicated to discuss extensively
here, especially inasmuch as there are more vital problems to deal
with. It is unlikely that a single system of color notation will be
developed to satisfy all the requirements of the applications of color
but the most promising system appears to be one which includes an
accurate description of the three qualities, hue, saturation, and
brightness. The scientist can supply himself with the more analyt-
ical data necessary for his purposes.
COLOR MEASUREMENTS
It is quite outside the scope of this lecture to enter deeply into a
discussion of color measurements, for such information can be found
elsewhere; however, it appears advisable to describe the various
methods briefly and to point out a few applications and limitations
of the results.
Photometry. Only two methods are of sufficient importance here to
be treated. These are the direct comparison and flicker methods of
photometry. For color photometry, the latter method appears to
be the more desirable owing to the greater consistency of the results.
It has not yet been proved that this method provides a true measure
of brightness in the case of great differences in color, nevertheless it
measures, with a high degree of consistency, a factor which very
likely is the brightness quality of color. It does not eliminate
differences due to normal variations in color vision among various
persons.
272 ILLUMINATING ENGINEERING PRACTICE
Spectro photometry. The spectrophotometer, by means of which
are obtained the relative luminous or energy intensities throughout
the spectrum of an illuminant or of the light reflected or transmitted
by a colored medium, furnishes the most analytical data. The ap-
plications of color in lighting are often dependent for success upon the
spectral character of the illuminants and of the colored media used.
Unfortunately such data cannot be expressed simply and cannot be
readily interpreted without considerable experience, nevertheless,
the lighting expert is working blindly in many cases without the
aid of such data. The eye is incapable of determining the spectral
character of light without the aid of proper instruments and many
instances are encountered where the lighting expert has stumbled
into pitfalls owing to the absence of the information provided by
analytical data in such cases.
Colorinketry. There are available many types of colorimeters
which provide data varying considerably in analytical nature, but
each has fields of application in which it is quite satisfactory. The
simplest forms might be termed tintometers. These instruments
are generally used for such purposes as maintaining a product within
certain limits or in classifying a product according to color. A
series of colors of the same hue but varying in saturation or slightly
varying in hue or brightness, are provided as a series of comparison
standards. Such instruments have few applications in lighting
practice. Other instruments employ a mixture of two colors for
limited ranges of color comparison. The resulting data are of a
slightly greater analytical nature.
It is well-known that any color can be matched by a mixture of
three colors, red, green, and blue, in proper proportions. By means
of this tri-chromatic instrument an illuminant or colored medium
can be analyzed in terms of the three arbitrary colors. There being
an infinite number of sets of three colors which fulfill the foregoing
requirements for most practical purposes, it is seen that the data
which are obtained are restricted to the three colors used unless
properly reduced to a standard system. These can be transformed
into other systems but in the present state of knowledge the author
has little confidence in the value of data after being subjected to
such transformations. Extreme caution must be exercised in
interpreting such data because the color-matches are merely sub-
jective and therefore furnish little information regarding the spec-
tral character of the colors examined. For instance, with such an
instrument the color of the light from a quartz-tube mercury-arc is
LUCKIESH: COLOR IN LIGHTING 273
specified in practically the same numerical terms as average day-
light, although the spectral character of the two illuminants are
very different.
The monochromatic colorimeter is a very satisfactory colorimeter
because by means of it the three qualities of a color, namely, hue,
saturation, and brightness, are determined. It has a further
advantage in referring color measurements to a reproducible stand-
ard the spectrum although the lack of a standard and constant
white causes difficulty. It should be borne in mind that those
measurements are made by subjective color-matches so that the
data do not provide a spectral analysis of the color which is examined.
Spectrophotography. Photography of the spectrum provides a
means of analyzing colors spectrally. The application of spectrum
photography is usually confined to those requirements which are
not so exacting although by careful procedure fairly accurate analyses
may be consummated. Many variables enter, such as exposure,
development, and non-uniformity of emulsion but of the greatest
importance is the non-uniform spectral sensibility of photographic
emulsions. Various means can be resorted to in order to eliminate
the effects of non-uniform dispersion in the case of a prism spectro-
graph and the non-uniform spectral sensibility of the emulsion.
COLOR MIXTURE
In many applications of color in lighting the principles of color
mixture may be used. The greatest difficulties have been encoun-
tered perhaps through the confusion of the primary colors. There
are three general methods of color mixture, namely, the additive, the
subtractive, and the juxtapositional, although the first two are of
chief importance here. Many applications of color mixture involve
both methods.
As previously stated any color can be matched in hue by a proper
mixture of three primary colors, namely, red, green, and blue. This
method is termed additive. Many sets of primary colors can be
used and a satisfactory set can be determined by experiment. In
order to obtain these primary colors it is generally necessary in
practice to subtract certain colored rays from the illuminant usually
by colored screens. The latter is an example of the subtractive
method and is the one employed in the mixture of pigments. The
subtractive primaries are usually considered to be red, yellow and
blue. The specification of three primaries depends upon the object
18
274 ILLUMINATING ENGINEERING PRACTICE
to be attained but it does not appear that there is sufficient justi-
fication for considering red, yellow and blue to be the true sub-
tractive primaries. Purple, yellow, and blue-green appear to have
a greater claim as the sub tractive primaries because by mixture of
these a greater range of hues is obtainable. For instance, from the
former set a purple color cannot be obtained, yet in using the latter
primaries nothing is sacrificed because purple is available and red
can be obtained by a mixture of yellow and purple. As a matter of
fact, the red used as a primary in the mixture of pigments is in reality
a purple so that the confusion appears to arise from a misnomer
applied to this pigment.
As an illustration of the difference between the two methods the
mixture of yellow and blue provides an excellent example. On
mixing these additively in proper proportions, white light is obtained
but on mixing them subtractively green is obtained. In producing
colored screens for lighting purposes, the spectral characters of the
illuminant to be used and of the colored media available are invalu-
able guides in obtaining the desired results. Likewise this is true
in many cases of the additive mixture of colored light. The juxta-
positional method is well exemplified in some processes of color
photography where minute colored filters, red, green and blue in
color, are used in the form of rulings or starch granules. This method
is of little importance in the general practice of color in lighting al-
though there are instances where it can be used to great advantage.
COLOR AND VISION
The visual phenomena of color have been very extensively studied
yet there remains a vast unexplored unknown. Many of the prob-
lems pertaining to color which arise in lighting practice can be solved,
or at least can be better understood, by applying present knowledge
pertaining to color and vision. A few of the most important phe-
nomena are briefly described below.
Simultaneous Contrast. Colors mutually affect each other when
viewed simultaneously, the magnitude of the influence being great-
est when the colors are in juxtaposition. The phenomena may be
divided into two general parts, namely, hue contrast and brightness
contrast. These two influences are usually at work simultaneously
so that it requires keen analysis to diagnose a particular case. This
phenomenon is perhaps the most important in the viewing of colored
objects and must be credited with supplying a great deal of beauty
to all vari-colored objects.
LUCKIESH: COLOR IN LIGHTING 275
Growth and Decay of Color Sensation. The various color sensations
do not rise to full value immediately upon presentation of the stimuli
and likewise they do not decay to zero immediately upon cessation
of the stimuli. Further, the different color sensations rise and fall
at different rates. Of the red, green, and blue sensations the green
is the most sluggish and the blue the most active.
After-images. After a stimulus of a color sensation is removed the
sensation persists for some time depending upon the color. This
persistence of the sensation is termed an after-image. During its
decay its appearance continually changes. If immediately after
the stimulus has ceased, the retina be stimulated with a moderate
intensity of white light the after-image due to the first stimulus will
usually be approximately complementary in color to the original
sensation. Obviously the results will usually be very complicated.
No attempt will be made to explain these here except by the indefi-
nite fatigue, because the leading color theories seriously differ in
their explanations.
Retinal Color Sensitivity. The retina varies over its surface in its
sensitivity to color. The central region is relatively less sensi-
tive to light of short wave-lengths due perhaps to the yellowish
pigmentation which has resulted in defining this region as the
" yellow-spot." The extreme peripheral retina is relatively in-
sensitive to color. The sensitive area varies for different colors and
also is dependent upon the size and brightness of the colored patch
which is viewed. The minimum perceptible brightness-difference is
approximately constant for all colors at high intensities but differs
considerably at low intensities. In general it decreases as the wave-
length decreases.
Purkmje Effect. The eye is relatively more sensitive to short-
wave energy than to long-wave energy at low intensities than it is
at high intensities. In other words, if blue and red colors appear of
the same brightness under ordinary intensities of illumination, the
blue will appear much brighter than the red when the intensity of
illumination is reduced to a very low value. The intensity at
which the effect begins to be noticeable depends upon many condi-
tions but an approximate average is at a brightness of a white sur-
face illuminated to an intensity of one-tenth foot-candle or of one
meter-candle. This effect is best described in a few words by stating
that, in general, at low illuminations the spectral sensibility curve of
the eye shifts toward the shorter wave-lengths.
Visual Acuity. It has been proved that visual acuity, or the
276 ILLUMINATING ENGINEERING PRACTICE
ability to distinguish fine detail, is better in monochromatic light
than in light of extended spectral character. The effect is not as
marked for ordinary seeing yet details, such as letters on an ordi-
nary printed page, do appear better defined under monochromatic
light. In other words, for equal discrimination or clearness of a
page of type, lower intensities of illumination are required with light
approaching monochromatism than with light having a more
extended spectral character. Results obtained by the author using a
yellow light whose spectral character could be so altered as to
approach more and more toward monochromatism indicate that the
increase in defining power in this case approximately offsets the
opposite effect due to the attendant decreasing illumination.
Color Vision. No hypothesis of color vision at the present time is
in complete accord with the experimental data available. The fault
obviously may lie with either the hypothesis or with the data.
However, the fact is mentioned because of the tendency of those not
fully informed on the subject to accept one hypothesis and to attempt
to explain everything pertaining to color-vision on this basis. There
are two hypotheses whose proponents have been serious opponents
to each other. One of these is called the Young-Helmholtz theory.
It was enunciated by Young and given considerable experimental
foundation by the great work of Helmholtz. This hypothesis was
builded largely from the side of physics and is, therefore, based largely
upon the observed facts of color mixture. Three substances or sets
of nerves which are responsible for three color sensations, respec-
tively, red, green, and blue, have been., assumed to exist, and to
account for all color sensations by their varying degrees of response
to different stimuli. Anatomical research has not verified the exis-
tence of those assumed substances.
The other hypothesis which ranks with the foregoing in importance
is known as the Hering theory. This hypothesis has been builded
largely from the side of psychology on the basis that four distinc-
tive colors are seen in the visible spectrum, namely, red, yellow,
green, and blue. Three substances are assumed to exist, the break-
ing down of a substance being effected by one color of a pair and the
building up of the same substance being attributed to the other color
of the pair. Black and white are assumed to be distinct sensations
unconnected with color sensations with the result that the three pairs
are considered to be red and green, blue and yellow, black and white.
There is no anatomical evidence of the existence of these three sub-
stances or processes.
LUCKIESH: COLOR IN LIGHTING 277
Von Kries is largely responsible for injecting the "rod and cone"
hypothesis in color-vision theory. The cones, which exist practi-
cally alone in the center of the retina (the fovea) and become less
dense toward the periphery, are assumed to be responsible for both
achromatic and chromatic sensation. The rods, which predominate
in the peripheral regions and are absent in the central region, are
assumed to be responsible for achromatic sensations. The rods are
supposed to be largely responsible for light sensation at low inten-
sities and are in general more responsive to rays of shorter wave-
lengths. The cones are supposed not to be rendered very much more
sensitive be dark adaptation. The rods and cones actually exist in
the retina as revealed by anatomical research. Many experimental
facts have been beautifully woven into this ''duplicity" hypothesis.
Many interesting modifications of these hypotheses have been
made and hypotheses based upon other principles are also worthy of
attention. It is quite beyond the scope of this lecture to discuss the
many proposed hypotheses; however, adequate treatments are avail-
able elsewhere.
PSYCHOLOGY OF COLOR
The great unknowns in lighting are chiefly those involving psy-
chology which, as an experimental science is in a primary stage of
development. The foregoing applies equally to the subject of color in
lighting. The definite data on the psychology of color are so meager
that it is difficult to treat the subject briefly, therefore, in order not
to stray too far afield, only a few general statements will be incor-
porated here. It appears quite probable that at some future time
the language of color will be understood. Occasionally glimmerings
of understanding appear among the chaos of color experience yet,
on the whole, there is no great amount of data to aid the lighting
expert.
There is general agreement in classifying colors into warm, neutral
and cold groups. Spectrally these attributes are found to lie in
regular succession. Yellow, orange, and red are the regions to
which the attribute of warmth is given. The cold colors are found
at and near the blue region. The neutral colors are found in the
central region, namely, the greens and adjacent colors and neu-
trality is again approached at the very extremes of the spectrum.
Fairly neutral colors also usually result from an additive mixture of
the colors near the extreme limits of the spectrum. Intelligent use
of this knowledge can be applied to many lighting problems. How-
278 ILLUMINATING ENGINEERING PRACTICE
ever, it is necessary at this point to insert a word of caution. The
lighting expert should carefully discriminate between that portion
of a given condition which is predominantly responsible for the
impression arising from color. In general, if the light source is
visible (it may be either a primary or a secondary light-source) its
color plays a dominating part in the impression upon the ordinary
observer. If the primary light-sources are concealed the color of the
surroundings are more effective in producing the impression than
the actual, color of the important surface such as a book which the
observer may be reading or goods on display in a show-window.
Specific examples may make the point clear. If a semi-indirect light-
source bowl be of a warm color, such as orange-yellow, the observer
whose aesthetic sense demands the warm color will often neglect to
inquire further. In other words, the lighting will usually be satis-
factory to him notwithstanding the light which constitutes the pre-
dominant part of the useful illumination may be the much whiter
light emitted by a gas mantle or tungsten filament located in the
semi-indirect bowl. Another example can be drawn from many
installations of artificial daylight which have recently been made.
Notwithstanding that a quality of daylight closely approaching day-
light is, in many cases, not only desirable, but proper, tradition or
habit requires that the artificial light must be of a yellowish color.
If the surroundings, such as the background in a show-window or the
walls and ceiling of a paintings gallery, be covered with warm colors,
the white light from the artificial daylight units can be directed upon
the objects to be displayed and yet the warm appearance of the
whole will be largely maintained.
A room with southern exposure, which in this zone of latitude
receives much direct sunlight can be " cooled" to some extent by
the employment of cool colors in the furnishings. Conversely a
room with northern exposure can be " warmed" considerably by the
employment of warm colors in 'the surroundings. It is true that
the light is somewhat altered by selective reflection from the colored
surroundings but the major portion of the effect is often apparently
purely psychological.
At this point it is well to emphasize the apparent existence of
two distinct mental attitudes in regard to color in lighting. Rooms
are generally decorated for daylight conditions and are presumably
satisfactory when completed. However, notwithstanding all illu-
minants ordinarily used for general interior lighting are quite yellow
in integral color in comparison with daylight, complaint is sometimes
LUCKIESH: COLOR IN LIGHTING 279
heard of the garish whiteness of the unaltered light emitted by
modern gas and electric filament lamps. The correction resorted
to is usually the application of yellow screens of glass, gelatine, or
silk fabric. Why, if the daylight condition is satisfactory, is the
artificial lighting too cold? Obviously the question is answered by
admitting the existence of day and night criteria which are widely
different. The reason for the existence of these two very different
criteria possibly may be traced to phenomena of vision but probably
may be correctly attributed to tradition. Artificial light for ages
was quite yellow and only recently have the illuminants become con-
siderably whiter. Perhaps the demand for yellow artificial light
arising from some aesthetic senses is largely due to the insistence of
habit. It is difficult to account for the foregoing in any other man-
ner considering the tremendous difference in color still existing
between artificial illuminants and natural daylight. That the
double standard can be partially eliminated at least, the author can
testify from experience. It is not the desire here to condemn this
double requirement but to diagnose it. It is a condition which the
lighting expert must meet and one which involves many of the facts
and applications of color science.
Colors have been characterized according to their emotional
effect by such words as exciting, soothing, gay, somber, serene, and
many others. In studying the attributes applied to colors by poets
and painters it is found that there is apparently a general agreement
in usage. However, a treatment of the subject is beyond the scope
of this lecture. The emotional value of colors has been mentioned
in passing with the hope that the lighting expert will avail himself,
by study and observation, of the possibilities of expression through
the language of color.
There are available some data on color preference, but such data
must be carefully interpreted or difficulties will be encountered.
In obtaining data on color preference the observer is concerned with
nothing except the colors being compared. Other considerations
enter into lighting problems which call for a modification of data on
color preference before it can be applied. For instance, pure colors
are more frequently preferred than tints and shades, a fact estab-
lished by various investigators, yet this does not apply to the decora-
tion and lighting of an interior. Of the pure colors the reds and
blues are the more often preferred of a group of pigments representing
the entire range of spectral colors as well as the purples. Yellow
usually ranks quite low in the preference order. Strangely enough,
280 ILLUMINATING ENGINEERING PRACTICE
the colors more commonly encountered in interior decoration
(cream, yellow, orange, buff, brown) generally rank low in such
color-preference investigations. Perhaps, in such investigations,
the momentary delight in the less common color sways the judg-
ment oppositely to that resulting from prolonged association with
the color. Certainly the warmer tints and shades predominate in
interiors and usually these correspond in hue to the yellow-orange
region of the spectrum.
The distribution of light, shade, and color in an interior deter-
mines the mood of the setting as a whole and often should be con-
sidered the chief factor to be studied; however, too often it is not
pre-visualized but incidentally results from a certain arrangement
of outlets equipped with a unit that is merely popular. Great
opportunities are open to the lighting expert who learns to apply the
language of light, shade, and color.
SURROUNDINGS
As previously stated the surroundings are very important in
molding the mental impression of a lighting condition. The distribu-
tion of light and shade is largely controlled by the reflection coeffi-
cients of the surroundings. Color is intricately interwoven into the
whole, but, inasmuch as the psychological importance of the color
of the surroundings has been touched upon, the discussion here will
be confined to the modification of light by selective reflection from
the colored surroundings.
A colored surface appears colored by reflected light because it has
the property of reflecting light of certain wave-lengths and of absorb-
ing others, thereby altering the incident light. A yellow wall
paper reflects the blue rays only slightly, the result of subtracting
blue rays from white light being a yellow light. A red fabric appears
red under daylight because it reflects only the red rays in daylight.
It appears a relatively brighter red under tungsten or gas light than
under daylight for equal illuminations owing to the relatively
greater amount of red rays present in the light from the artificial
illuminants per unit of light flux. Under the light from a mercury
arc lamp the red fabric appears almost black because there are
present in the light from the mercury arc practically no rays which
the red fabric is able to reflect. This shows that the relative bright-
nesses of colored objects varies with the spectral character of the
illuminant and that selective reflection from the surroundings is
LUCKIESH: COLOR IN LIGHTING 281
responsible for a change in the color of the incident light. Daylight
entering interiors usually has been altered by reflection from many
colored objects, such as buildings, foliage, pavements, lawns, and
earth, with the result that daylight in interiors is quite variable in
quality. This variation causes difficulty in accurate color work from
day to day and from season to season. Skylight is much more
bluish in color than sunlight so that tremendous variations in
quality are apparent as the relative amounts of sunlight and sky-
light vary. Moreover, the variation in the relative amounts of
skylight and sunlight entering windows or other openings is generally
continuous.
The magnitude of the change due to reflection from colored sur-
roundings has been measured in a miniature interior for various color
combinations of the walls and ceiling and for different systems of
lighting. Obviously the influence of the surroundings upon the color
of the useful light at a given point such as a desk-top, depends upon
the relative amounts of light reaching the point directly and indi-
rectly. For ordinary direct-lighting systems the alteration due to
colored surroundings is usually appreciable although not as great as
for indirect-lighting systems. In a representative case it was found
that the light from tungsten lamps in an indirect lighting fixture was
altered to a color far yellower than the old carbon lamps when the
colors of the cream-tinted ceiling and brownish yellow walls were
of a very common combination. The effect is of considerable mag-
nitude in semi-indirect installations depending, of course, upon the
relative values of the direct and indirect components.
If in a given case of indirect lighting the artificial illuminant is
too cold, it is possible to obtain the identical results by two expedi-
ents. In one case the walls and ceiling would be refinished with
coverings of a warmer or yellower tint, in the other case a yellowish
screen would be placed over the lighting unit so as to alter the light
by selective absorption. If artificial illuminants have become too
cold in color to suit the aesthetic sense, why not in many cases, resort
to the use of warmer colors for the surroundings such as walls and
ceiling? This method would also tend to warm up daylight in color
which is very much colder than the common artificial illuminants.
But here the question of the double standard enters again.
In an installation of artificial daylight it was desirable to have
the lighting units appear as yellowish as possible to harmonize with
the general color scheme of the room and it was also essential to
have the lamps enclosed by a glass ball. This was achieved by
282 ILLUMINATING ENGINEERING PRACTICE
etching the ball inside and tinting with a very unsaturated yellow
with the result that the ball appeared quite yellowish in color while
the light which was approximately directly transmitted was only
slightly altered in color. There was a slight loss of light due to the
coloring but this was negligible in comparison with the satisfactori-
ness of the results. There are many important and interesting con-
siderations which are beyond the scope of this treatment. An
analysis of a given condition will reveal them.
In closing this discussion it appears profitable to enunciate a few
simple but pertinent facts. A yellowish surface under daylight
illumination may appear exactly like a neutral surface under an
ordinary yellowish artificial illuminant. Surroundings consisting
chiefly of such colors as brown, buff, yellow or orange shades, which
are neutral or warm in appearance under daylight appear relatively
much warmer by ordinary artificial light. In indirect and many
semi-indirect systems of lighting the alteration of the light by colored
surroundings is so great as to produce in many cases an effect with a
modern illuminant similar to that obtained with the old illuminants
in ordinary direct fixtures.
ARTIFICIAL DAYLIGHT
For the production and appreciation of colored objects, daylight
is the generally accepted standard. The arts as well as the eye have
been evolved under natural light with the result that the demand for
light approaching daylight in quality for many purposes is deeply
and permanently rooted. Daylight varies tremendously in spectral
character so that it is necessary to determine the standards. Meas-
urements of intensity and quality of north skylight on a clear day
reveal a fair constancy which doubtless accounts for the dependence
upon north skylight for accurate color-discrimination. However,
north skylight varies from clear to cloudy days but not as much as
the light from other points of the compass in northern latitudes.
Clear noon sunlight is quite constant and although not always
available represents a fair average daylight outdoors. Noon sun-
light and north skylight have, therefore, been accepted as two
distinct standard daylights.
There are three possible methods of producing artificial daylight:
namely, (i) directly from the light source, (2) by adding comple-
mentary light in proper proportions, (3) by altering the light from
an illuminant by means of a selective screen. The only available
illuminant which fills directly the requirements of accurate color
LUCKIESH: COLOR IN LIGHTING 283
work is the Moore carbon-dioxide tube lamp. Some arcs emit light
roughly approximating sunlight in color but the variations in in-
tensity, and usually in quality, have discouraged their use for refined
color work. The Moore tube lamp emits light approximating sky-
light in quality closely enough for the most exacting color-matching.
Some years ago the light from the tungsten lamp was combined
with that from the mercury arc in such proportions as to give a
subjective white light. This combination met some requirements
but could not possibly approximate daylight in spectral character
owing to the discontinuous spectrum of the mercury arc. The
spectrum of the light from the Moore carbon-dioxide tube is dis-
continuous but only for small intervals. On various occasions
colored lights have been combined with the light from ordinary
artificial illuminants to produce an approximate daylight effect.
However, the only method of producing artificial daylight which up
to the present has been extensively applied is that which involves
the use of colored screens. These have included the use of gas
mantles, arc lamps, and tungsten filament lamps.
The historical development of such units, which has been treated
elsewhere, will not be repeated here in order to preserve the limited
space for a discussion of the more practical aspects of the subject.
Inasmuch as the author is unaware of any other extensive and diver-
sified installations of artificial daylight, the remaining discussion of
this subject will be largely drawn from records of hundreds of in-
stallations in which many tungsten filament '" daylight" units of
various types have been used. Excepting for the accurate north
skylight units, practical considerations and requirements have
played an important part in determining the final units developed.
The colored screens have been made for several years entirely of
glass, to comply with the requirement of a satisfactory unit.
In imitating north skylight it has proved most satisfactory to
press the colored glass in the form of a plate or shallow dish in order
to insure uniformity. In such cases where accurate color-matching
is required, efficiency should be a minor consideration and experi-
ence has proved this to be very generally true. Using modern gas-
filled tungsten lamps, north skylight of satisfactory quality is
reproduced by this subtract! ve method at losses of from 75 to 85
per cent, of the original light. It has been found that the colored
screens can be produced inexpensively and with sufficient accuracy
to meet the requirements. A brief resume of the fields in which such
units are operating at the present time is presented later.
284 ILLUMINATING ENGINEERING PRACTICE
Experience has shown that, for the less refined color work and for
the layman's eye, untrained in accurate color discrimination, little
or no advantage is gained in correcting the light further than to an
approximation to clear noon sunlight. For this reason practical
artificial sunlight units have been developed. These units, whose
important part consists of an enclosing colored glass envelope, have
been installed for general lighting purposes in many different fields.
The absorption losses of these units, using gas-filled lamps operating
in the neighborhood of 18 lumens per watt, is approximately 50
per cent. A brief resume of the fields in which sun units are operat-
ing at the present time is presented later.
Besides the preceding considerations other reasons have led to
the development of a gas-filled "daylight" lamp. In the general
practice of lighting a daylight lamp has usually been preferred to a
daylight unit whose design conformed to the ideas of the manu-
facturer and whose types were limited by manufacturing expediency.
There has been a constant demand for an illuminant approaching
daylight in quality but of sufficiently high efficiency for general
lighting purposes. For some time the daylight efficiency of arti-
ficial illuminants has been rapidly increasing. Obviously the time
has been approaching when this demand could be supplied and the
experiment has been tried with successful results. Other considera-
tions hastened the culmination of this event. For example, artificial
daylight is cold in appearance, although there are many applications
where this "coldness" is a delightful part of the illusion. However,
experience appeared to indicate at the present time a limit to the
"coldness" which would be acceptable for general lighting at night
in many fields. Furthermore, luminous efficiency is of some im-
portance when illuminants are used for general lighting purposes.
Therefore, there has been developed a gas-filled tungsten "daylight "
lamp which corrects the light well toward average daylight, the
resulting light approximating black-body radiation at a temperature
somewhat below the apparent black-body temperature of the sun.
It appears quite legitimate and desirable to increase the apparent
temperature of the tungsten filament hundreds of degrees above its
melting point by means of a proper colored-glass bulb. The color
of the resulting light blends well with daylight entering interiors
and has proved satisfactory in hundreds of installations. A brief
summary of the fields in which this quality of light is at present used
is presented later.
Various developments of artificial daylight units have been dis-
LUCKIESH: COLOR IN LIGHTING 285
cussed herewith to illustrate the practical requirements. The dis-
cussion would apply in general as well to other illuminants besides
the tungsten lamp but it has been necessary to confine the discus-
sion to developments in connection with the tungsten lamp because
these represent the first, and at present, the only developments on
a large commercial scale. For the same reason the installations
described later must be largely confined to the tungsten lamp.
Various other light sources have been used on a small scale and
usually for a single kind of artificial daylight unit. In such de-
velopments the demands, opinions, and tastes of consumers are very
influential, and hence experiences have been incorporated with the
hope that they will aid in future developments.
Data on the light absorption of daylight illuminants are available
elsewhere. There is a lack of agreement in the results by different
investigators due doubtless chiefly to the variation of the daylight
standard. The absorptions presented by the author in the fore-
going are those obtained by actual measurement upon units which
satisfactorily fulfilled their missions.
SIMULATING OLD ILLUMINANTS
The development of artificial daylight units has been for the
purpose of satisfying a requirement which is not generally or promi-
nently influenced by individual taste. It has had for its object the
extension of day; that is, by its use many arts can be pursued and
appreciated at night as well as during the day. It provides in-
surance against the failure of natural daylight even during day-
light working hours which occurs often especially in the crowded
cities. Esthetic taste has been neither a factor for nor against this
development; however, the aesthetic taste does enter prominently
into many problems of lighting. A director of an art museum in
fairness to the artist and to the general public should make use of
artificial daylight if economical considerations are favorable, but,
an individual in his home may satisfy his taste without being justly
criticized. Some of these tastes demand, for many purposes in
the home, a light of a warm quality a quality simulating that of
the old illuminants. It is not a question here whether this demand
is a result of tradition or the insistence of habit. The problem of
the lighting expert is to appease the desire if only the individual
taste is important. The increasing efficiency in light production
makes it possible to alter artificial light to meet any requirements.
286 ILLUMINATING ENGINEERING PRACTICE
The problem of simulating old illuminants is relatively simple
compared with the exacting requirements in the production of arti-
ficial daylight: In the former case only an approximate subjective
color-match is necessary while in the latter case a close approxima-
tion in spectral character is required. Usually yellow fabric, dyes,
or amber glass are used to produce the warmer color. However,
the available colors if used singly are usually greenish yellow when
sufficiently unsaturated as in the case of amber glass.
If a kerosene flame, or carbon incandescent lamp be concealed in
a diffusing glass accessory, the color is seldom noticed. The un-
saturated yellow of these old illuminants has been termed an
''aesthetic yellow" in order to distinguish it from other yellows in
the vocabulary of the lighting expert. Data have been obtained by
many observers with various units of this character especially one
unit containing tungsten lamps tinted with ordinary amber and
another containing lamps colored with the "aesthetic yellow" color
which produced a close match to the kerosene flame. In general
the amber color was considered obtrusive while the other color was
apparently unnoticed. This point is worthy of consideration in any
extensive application of the foregoing.
Unfortunately no single dye is available having the desirable
characteristics of high transparency, but, it is a simple matter to
correct any of the common yellows of greenish tinge. This is readily
eliminated by the use of a slight amount of pink coloring. Many of
the so-called red coloring materials, when of light density, afford a
satisfactory pink. The ordinary lamp dyes can be readily mixed
to provide the proper color but these are usually more or less fugitive.
If possible it is well to color a glass plate and place this at sufficient
distance from the light unit. Coloring media which are very fugitive
when subjected to Considerable heat are often quite permanent to
light when kept reasonably cool. A metal screen placed in contact
with the colored screen, will often keep the latter sufficiently cool.
Coloring media are available for incorporating into glass, which
produce a proper unsaturated yellow color which simulates the
yellow of the older illuminants, but the difficulties in obtaining a
glass of the proper color and high transparency are very great.
Amber glass when very dense loses its greenish tinge but then the
color is too saturated for the present purpose. However, a satis-
factory approximation to a subjective match with the old illuminants
can readily be obtained by combining a small percentage of the light
passing through a dense amber glass with a large amount of unaltered
LUCKIESH: COJ.OR IN LIGHTING 287
light. Such a procedure is not always practicable but is a highly
satisfactory solution in suitable cases. A satisfactory coloring
material, disregarding its opacity, for use where the temperature
is high as on the bulb of a gas-filled tungsten lamp is a yellow-orange
pigment used in ordinary oil painting. This can be incorporated
in a suitable binder after being thoroughly ground and sifted.
Good results have been obtained with yellow shellac in alcohol in
which the yellow-orange pigment is thoroughly stirred although
some other binders are more satisfactory. On standing, the pig-
ment settles out but can be brought readily into complete suspen-
sion by slight stirring or shaking. Gas-filled tungsten lamps covered
with this pigment have been in continuous service for many days
without showing any sign of discoloring. The lamps can be dipped
although it has also been found satisfactory to apply the coloring
on a rotating lamp by means of a camel-hair brush.
It may be of interest to know the theoretical efficiencies of modern
illuminants when screened to simulate the old illuminants exactly
in spectral character. Results of computations have been pub-
lished elsewhere for the vacuum and gas-filled tungsten lamps oper-
ating at various efficiencies. From these data an idea of the amount
of light absorbed can be obtained. The resulting specific outputs
of the vacuum tungsten lamp operating at 7.9 lumens per watt
when screened to match a kerosene flame and a carbon incandescent
lamp in color are respectively 4.5 and 6.3 lumens per watt. Similar
specific outputs for the gas-filled tungsten lamp operating at 16
lumens per watt are respectively 7.4 and n lumens per watt. For
the gas-filled lamp operating at 12 lumens per watt the corresponding
outputs are 6.3 and 9.3 lumens per watt.
COLORED MEDIA
Essential tools in applying color in lighting are colored media and
a knowledge of the fundamental principles of the science of color.
The latter have been briefly discussed in preceding paragraphs and
a few suggestions regarding colored media are presented below.
Illuminants differing in color have been harmoniously blended in
many instances but the greater possibilities of such applications
naturally are found in installations of great magnitude. In the
general practice of color in lighting an acquaintance with colored
media is essential. Among the chief colored media are glasses, silk
fabrics, gelatines, lacquers, pigments, aniline dyes, and chemical
288 ILLUMINATING ENGINEERING PRACTICE
salts. Often a problem can be solved very readily through an ac-
quaintance with the availability of colored media.
Colored glasses can be obtained from a number of jobbing houses
as well as glass factories. Fairly pure colors can be obtained
from manufacturers of signal glasses. With little or no correction,
these often afford excellent primary colors for applications of color
mixture.
Colored lacquers can be obtained very readily. These are usually
of two classes, one for indoor applications and the other for outdoor
uses. The latter are usually colored varnishes which resist the
action of moisture. Unfortunately colored lacquers are not, in
general, very permanent under the combined effects of light, heat,
moisture, and gases. To insure permanency it is well to flow these
lacquers upon plane sheets of glass and, after drying, to protect them
with glass covers. If these are installed in such a manner as to
prevent undue heating, many ordinary lacquers will be fairly per-
manent. Ventilation is quite essential and sometimes by placing
a metal screen of coarse mesh in contact with the colored screen
the life of the latter can be prolonged. Lacquers can be colored
with aniline dyes and other materials providing a proper solvent is
employed.
Often an insoluble pigment or dye can be suspended in a binding
medium to a sufficient degree to enable lamps or glassware or other
media to be colored by immersion. An air brush can be used success-
fully in such work with the advantage that it is necessary to prepare
only small amounts of the colored solution. Colored gelatines can
be obtained from theatrical supply houses, a wide range of colors
being available. For special purposes, and in cases of emergency,
gelatine can be dyed and flowed upon sheets of glass. These are
not permanent but their life can be prolonged considerably if kept
reasonably cool.
Colored fabrics such as silk lend themselves to many applications
of interior lighting. Colored solutions find uses especially in tem-
porary lighting installations and in demonstrations.
The method of using these materials obviously varies with the
problem at hand. If colored glasses of proper spectral characteris-
tics are available they can be placed in such a position as to inter-
cept the light emitted by the illuminant. However, if the correct
tint is not at hand, it is often possible to obtain the desired result
by combining colors according to the various methods of color-
mixture. For instance if a pink glass be not available, a pink tinge
LUCKIESH: COLOR IN LIGHTING 289
can be obtained by adding red light and a slight amount of blue or
violet to the unaltered light" emitted by an ordinary illuminant.
This combination may be obtained by using three light sources or
by using a single one. In the latter case a checkerboard pattern
can be built up by means of red, blue, and clear glass; however, the
light must be emitted from an extended area so that the colors are
well blended. Lacquers can be used in quite the same manner.
If only one lighting unit is to be used, the screen can be made by
daubing on a clear glass various spots of the proper colors. How-
ever, lacquers can be readily mixed or diluted to obtain the desired
color. This can be done by following the principles of color mixture.
In general it should be noted that the eye cannot always be trusted
to judge the satisfactoriness of a color for a specific purpose because
it operates synthetically in regard to light-waves.
APPLICATIONS OF COLOR IN LIGHTING
It has appeared advisable to supplement the broad general treat-
ment of color in lighting with brief descriptions of applications.
Some of these will be representative of many similar cases but all
have been chosen as useful illustrations of the great possibilities of
color in aiding and pleasing mankind. This aspect of lighting is
endless in extent and its ramifications are numberless. No strict
classification will be adhered to but in general those having a more
scientific basis for existence will be treated first while those involving
chiefly aesthetic taste will be discussed later.
Artificial North Skylight. This quality of light is the most generally
acceptable for accurate color work, such as in matching and in
inspecting colors. There is no need to discuss it further than to
record the classes of work for which it is being used at present.
It is not used for lighting large areas in the sense of general lighting
although there are some rather extensive installations in existence.
Artificial north skylight is at present used in the following places
and occupations and perhaps in others: department stores, color
printing, textile mills, dye houses, laboratories, cigar factories
and stores, haberdasheries, chiropody, hair dressing, sugar testing,
dentistry, painting, paint and wall paper stores, millinery shops,
button factories, diamond and jewelry shops, medical examinations,
surgical operations, microscopy, chemical analysis, flour mills, paper
mills, garment factories, cotton mills.
Artificial Noon Sunlight. This quality of light is at present quite
19
2 QO ILLUMINATING ENGINEERING PRACTICE
extensively used for general lighting in many cases where the require-
ments are not as refined as in the previous cases, and where eyes un-
trained in refined color-discrimination are involved. Among the
places and occupations in which this quality of daylight is at present
being used are the following: lithographing, paint shops, and stores,
tailor shops, wall-paper stores, in green houses, artists' studios, art
galleries, operating rooms in hospitals, paper mills, flour mills,
garment factories, shoe stores, textile mills, florist shops, dry clean-
ing, laundries, furniture stores, undertaking, millinery shops,
haberdasheries, art schools, and in illuminating color photographs.
1 Approximate Artificial Daylight. Under this head will be discussed
briefly the applications that have been made of "artificial daylight''
which is only an approximation to average daylight being in reality
a compromise between quality and efficiency. This discussion of
the application of this quality of light, which in these particular cases
is obtained from a tungsten lamp corrected by means of a colored
glass bulb so that its visible spectrum closely approximates that of a
black body operating at a temperature midway between the melting
point of tungsten and the apparent temperature of the sun, applies
to any other artificial light-source similarly corrected. The effi-
ciency of light production has reached a point where it has proved
expedient to obtain a better quality of light by sacrificing some of the
light. Data are available from installations involving many thou-
sands of lamps but only a few points will be discussed for the purpose
of showing the trend in this aspect of artificial daylighting. A record
of the applications of this approximate daylight includes all of the
fields included under the preceding two paragraphs with the excep-
tion of the cases where very accurate color discrimination is required.
Investigation shows that such a quality of light is in use for general
lighting in the following places and occupations and perhaps in
others: department stores, haberdasheries, cigar stores, art galleries,
clothing stores, millinery shops, tailor shops, shoe stores, jewelry
shops, paint and wall paper stores, furniture stores, undertaking,
laundries, dry cleaning, medicine and surgery, hospitals, color print-
ing, hardware stores, libraries, grist mills, florist shops, automobile
display rooms, textile plants, illumination of color photographs,
photographic studios, offices, drug stores, hair goods shops, stationary
stores, barbor shops, laboratories, microscopy, grocery stores,
confectionary stores, upholstering shops, breweries, hair dressing,
show windows, fur stores, and in a number of isolated places. The
application of this illuminant to art museums is especially worthy
LUCKIESH: COLOR IN LIGHTING 291
of attention owing to the exacting requirements. A number of
museums are at present equipped with this illuminant, notably the
Cleveland Museum of Art. One interesting result has been the
popularity of the museum at night. These applications of artificial
daylight are sufficiently numerous and diversified to indicate that
consumers are not universally satisfied to accept the accidental
quality of light emitted by various artificial illuminants providing a
much better quality of light can be obtained without a prohibitive
loss in luminous efficiency. This is a natural result of the education
of the public resulting from the activities of this society.
No further discussion of the applications of artificial daylight
appears necessary in those fields which prominently involve the
appearance of colors; however, artificial daylight has found its way
into fields not generally expected. For instance, there has always
existed a feeling of unsatisfactoriness in the lighting during the period
of the day when daylight must be reinforced by artificial light.
This is perhaps partially due to a difference in the distribution of
light in the two cases. However, the difficulty is also partially, if
not largely, due to the difference in color. Experiments with arti-
ficial daylight for desk-lighting have been quite convincing to many
persons. A number of installations of approximate artificial day-
light units have indicated that this is probably a large field for
future development. Physiological and psychological research has
yet to explore this field. Many other unique applications could be
discussed to advantage but it is believed that sufficient space has been
given to this subject at present. However, it has been considered
profitable in this lecture to devote considerable space to this develop-
ment in lighting because it represents perhaps the stride of greatest
magnitude and portend in the application of the science of color in
lighting that has been made recently.
Applications of Color Mixture. Many diversified applications of
the principles of color mixture are open to the lighting expert. The
stage offers the greatest possibilities although ordinary specifications
of stage-lighting often provide only clear, red, and blue lamps. It
is obvious that the range of colors resulting from mixtures of these is
quite limited. When it is considered that the lighting effects are
valuable tools in the hands of the stage director it is wondered why
facilities are npt provided for using at least the three primary colors,
red, green, and blue, and also clear lamps. If space permits it would
be desirable to add yellow lamps. Of course, yellow could be ob-
tained by mixing red and green but inasmuch as it is an important
2Q2 ILLUMINATING ENGINEERING PRACTICE
stage-lighting color it appears undesirable to sacrifice it in obtaining
the red and green originally and then to produce it again by mixture
at a greatly reduced efficiency.
The primary colors have been used in show windows and for many
special effects. One unique installation is found in a pretentious
residence. Red, green, and blue lamps are installed above a large
oval panel of opal glass set in the ceiling of a dining room. Any
quality of light could be obtained by controlling various lamps
by means of three rheostats located in a cabinet in the wall. A
number of installations on a larger scale have been placed in ball-
rooms and restaurants. Such applications should be more numerous
considering the pleasure obtainable. A few cases have been noted
where colored lights have been mixed for the general illumination of
theatres, bill boards, special displays, ball rooms, etc. Flashers have
usually been used but rheostats can be readily designed to be mechan-
ically operated so as to vary the intensity of the various components
by imperceptible increments. Beautiful effects have been obtained
by illuminating clothing models with mixtures of the primary colors,
accentuating the effects occasionally by directed unaltered light.
The latter effect is intensely beautified by the colored shadows which
remain due to a flood of colored light of a lower intensity than the
clear directed light. Incidentally this brings out the point that
colored shadows can be used in many lighting effects with wonderful
success. Many possibilities of the use of color in lighting are found
in interiors. Colored lights obtained by mixture provide pleasing
variety and deal harshly with the monotony of ordinary lighting
installations. In ordinary lighting tints are more satisfying to the
aesthetic sense than saturated colors and these tints are readily
obtained by adding lights, fairly saturated in color, to the ordinary
unaltered light. In general it is necessary to conceal the sources.
In the home the tint can easily be adapte'd to fit the place, the occa-
sion, or the mood. Various possibilities can be provided in different
rooms or in the same room. Moonlight, sunlight, candle-light,
fire-light, etc., can be provided with ease.
Recently a moving picture theatre has been provided with a
yellowish light of low intensity for use ordinarily during the projec-
tion of pictures and a bluish light for use when night scenes are on
the screen. This is an example of the many possibilities of using
colored light in illusory presentations.
Colored light has been used successfully in the flood-lighting of
monuments, buildings, and pageants.
LUCKIESH: COLOR IN LIGHTING 293
Special Color Effects. In a few rare instances colored light has been
applied to billboards and other displays and doubtless this field for
colored light will be developed eventually. The play of colored light
on properly painted displays is attractive and when the efficiency of
light production has sufficiently increased these applications should
increase in number. Special color effects have been proposed in
which complete changes are produced by properly associating the
colored pigments used in painting the scene, or advertising material,
with the colored illuminants. These should eventually find a wide
field on the stage and in displays. A few applications have been
made but the difficulty at present lies in the necessity of a complete
grasp of color science in order to accomplish the desired results.
Notable Installations of Colored Light. A notable installation of
luminants of different color and brilliancy is found in the Allegheny
County Soldier's Memorial Building, Pittsburgh, in which mercury
arc, Moore tube, flame arc, and tungsten lamps are woven into
harmonious effect.
The applications of light and color at the Panama-Pacific Exposi-
tion are well known. This installation represents one of the greatest
undertakings in lighting ever attempted and also stands as an
example of the achievements that can be attained by the lighting
expert who has the hearty cooperation of architects and other
responsible authorities.
Simulating Old Illuminants. A few applications of this char-
acter have been made but it is difficult to discuss this subject
analytically because the requirements are not sufficiently exacting
to demand uniformity in the developments. The results have
been obtained by the use of color in ornamental glassware, of colored
screens over the aperture of indirect units, of colored fabrics, and
of colored lamps. Many interior lighting units approach this
result by the unconscious application of warm tints to the lighting
accessories. In those cases where the aim has been specifically to
simulate older illuminants a common error has been made in em-
ploying an amber color instead of an unsaturated yellow as discussed
earlier in this text.
Modifying Daylight. A few installations of this character have
been noted, the object usually being to eliminate the cold appear-
ance of daylight by using ceiling or side windows glazed with an
unsaturated yellow glass. Several notable installations are found
in pretentious buildings. A satisfactory glass has been obtainable
in the market. Such applications have their best field in open-
2Q4 ILLUMINATING ENGINEERING PRACTICE
ings where only skylight enters. A specific instance was observed
in an elaborate hotel where a ceiling window at the bottom of a
lighting court was glazed with a yellowish glass. Other instances
have been found in residences. In one case the windows in the
dining room received little sunlight and the windows were glazed
with a transparent yellow glass. The effect of the unobtrusive,
unsaturated yellow glass was always pleasing and extremely so on
dismal rainy days. Stained glass windows are colored chiefly for
decorative effect but the modification of the light which passes
through them often adds variety and interest to the interior.
Bibliography
In this general lecture it has been thought best to exclude references to various
investigators and practitioners who have contributed to the progress of the art
because historical treatment would lead the discussion far afield; however, a
bibliography of the representative work on the subject has been appended.
No pretense to completeness in the bibliography is entertained, although the
following references have been selected with this lecture in mind. Preference
has been given to published work of recent years, to those works which include
extensive bibliographies of the available material, to discussions treated from
practical viewpoints, and to the availability of the publication. As a result of
such a procedure and of the desire to be concise, many worthy papers have not
been directly mentioned. However, by referring to the various bibliographies
found in the publications actually referred to, a comprehensive view of the
various subjects can be obtained.
J. W. BAIRD. "Color Sensitivity of the Peripheral Retina." Carnegie Inst.
Pub. 1905, p. 80.
PAUL F. BAUDER. " Reflection Coefficients." Trans. I. E. S., 6, 1911, p. 85.
Louis BELL. "Monochromatic Light and Visual Acuity." Elec. World, 57,
1911, p. 1163.
E. J. G. BRADFORD. "Color Appreciation." Amer. Jour, of Psych. 24,
1913, p. 545.
E. J. BRADY. "Daylight Glass." Trans. I. E. S., 9, 1914, p. 937.
BROCA and SULZER. "Growth and Decay of Color Sensations." Comp.
Fend. 2, 1902, p. 977, p. 1046.
M. E. CHEVREAL. "Harmony and Contrast of Colors," 1835.
J. COHN. " Gef uhlston und Sattigung der Farben." Phil. Stud. 15, 1900,
p. 279.
E. C. CRITTENDEN and F. K. RICHTMYER. "Color Photometry." Trans
I. E. S. n, 1916, p. 331.
GEORGE CLAUDE. " Neon Tube Lighting." Trans. I. E. S., 8, 1913, p. 371.
G. W. CASSIDY. "Art and Science in Home Lighting." Trans. I. E. S.,
10, 1915, p. 55.
JEAN ESCARD. "Mercury arc Modified to Give White Light." La Lum.
Elec. 15, 1911, p. 236.
LUCKIESH:- COLOR IN LIGHTING 295
C. H. FABRY. " Color Photometry." Trans. I. E. S., 8, 1913, p. 302.
R. B. HUSSEY. "Arc Lamp for Artificial Daylight." Trans. I. E. S., 7,
1912, p. 73-
E. P. HYDE and J. E. WOODWELL. "Test of Moore Tube Installation in
New York Post Ofl&ce." Trans. I. E. S., 4, 1909, p. 871.
F. E. IVES. " Tri-chromatic Colorimetry." Jour. Franklin Inst., July, Dec.,
1907.
H. E. IVES. " Color Photometry." Phil. Mag., 1912. Trans. I. E. S., 5,
1910, p. 711; 7, 1912, p. 376.
"Color Measurements of Uluminants." Trans. I. E. S., 5, 1910, p. 189.
"Relation of the Color of the Illuminant to the Color of the Object."
Trans. I. E. S., 7, 1912, p. 62.
"Transformation of Color-mixture Equations." Jour. Frank. Inst., 1915,
P- 673-
"Mercury Arc Modified to Give White Light." Elec. World, 60, 1912, p.
304; Bull. Bur. Stds. 6, 1909, p. 265.
H. E. IVES and E. J. BRADY. "A Gas Artificial Daylight." Light, Jour.
i, 1913, p. 131.
H. E. IVES and E. F. KINGSBURY. " Color Photometry." Trans. I. E. S.,
10, 1915, pp. 203, 253, 259, 716.
H. E. IVES and M. LUCKIESH. "Subtractive Production of Artificial Day-
light." Elec. World, May 4, 1911; Lond. Blum. Engr. 4, 1911, p. 394.
BASSETT JONES. "Lighting of Allegheny County Soldier's Memorial."
Trans. I. E. S., 6, 1911, p. 9.
"Mobile Color and Stage Lighting." Elec. World, 66, 1915, pp. 245, 295,
346, 407, 454.
L. A. JONES. "Color of Illuminants." Trans. I. E. S., 9, 1914, p. 687.
F. PARK LEWIS. "Psychic Value of Light, Shade, and Color." Trans.
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M. LUCKIESH. "Light and Art." Light. Jour., Mar., 1913, April, 1914;
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April, 1914; Amer. Gas Inst., 8, 1913, p. 783; Good Lighting, May, 1912.
" Color and Its Applications." New York, 1915.
"Light and Shade and Their Applications." New York, 1916.
"The Language of Color" (in preparation).
" Color Photometry." Elec. World, May 16, 1914; April 19, 1913; Mar. 22,
" Monochromatic Light and Visual Acuity." Elec. World, Aug. 19, 1911;
Nov. 18, 1911; Dec. 6, 1913; Trans. I. E. S., April, 1912.
"Growth and Decay of Color Sensations." Phys. Rev., July, 1914.
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" Influence of Colored Surroundings on the Color of the Useful Light." Trans
I. E. S., 8, 1913, p. 61.
"Yellow Light." Trans. I. E. S., 10, 1915, p. 1015.
"Color Effects for the Stage and Displays." Elec. World, Apr. 4, 1914.
"The Art of Mobile Color." Sci. Amer. Sup., June 26, 1915.
"Artificial Moonlight Window." Light. Jour., Aug., 1915.
296 ILLUMINATING ENGINEERING PRACTICE
M. LUCKIESH and F. E. CADY. "Artificial Daylight Its Production and
Use." Trans. I. E. S., 9, 1914, p. 839.
L. W. McOMBER. " Moving Picture Theater Lighting." Elec. World, 68
1916, p. 122.
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G. S. MERRILL. "Tungsten Lamps." Proc. A. I. E. E., 1910, p. 1709.
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CHURCH LIGHTING REQUIREMENTS
9 BY EMILE G. PERROT
From the very beginning light has played a most important part
in the life of the world; shut out light from any living thing plant,
brute or man, and part of life itself is taken away. As light is
necessary to the fullness of physical life, in like manner the spiritual
life of man craves as its perfection, spiritual light.
The old law prescribed a seven-branch candlestick as part of the
sacred treasures to be kept before the eyes of the people; when
Christ came he voiced the need of men's souls when he proclaimed:
"I am the Light of the World." As a symbol of Him, the Light of
the World, the early Christians lit candles in the dark chambers of
the catacombs; symbols these lights were indeed, but they served
the added purpose of illumination.
So then, the architect, whether designer of lofty cathedral or
lowly church, must consider light both symbolic and illuminant.
In the early centuries of Christianity, the use of a multitude of
candles and lamps was undoubtedly a prominent feature of the cele-
bration of the Easter vigil, dating, we may believe, almost from
Apostolic times. Eusebius speaks of the "pillars of wax" with
which Constantine transformed night into day, and other authors
have left eloquent descriptions of the brilliance within the churches.
The number of lamps which Constantine destined for the Lateran
Basilica has been estimated at 8730. The practice of providing
immense hanging coronoe to be lighted on the great festivals seems
to have lasted throughout the Middle Ages and to have extended
to every part of Christendom.
We, in these days of brilliant artificial light, cannot easily realize
what unwonted splendor such displays imparted to worship in a
comparatively rude and barbarous age. To these magnificent
chandeliers various names are given, for example, cantharus, corona,
stantareum, pharus, etc. Such works of art were often presented
by emperors or royal personages to the basilicas of Rome.
Much more remarkable, however, are the remains of some mag-
nificent metal work on a vast scale. The great candelabrum of
297
298 ILLUMINATING ENGINEERING PRACTICE
Reims was preserved until the French Revolution. It was no doubt
meant to stand before the high altar in imitation of the great seven-
branch candlestick of the temple of Jerusalem. Its height was
over eighteen feet and its width fifteen.
No less wonderful, and happily still entire, is the great candela-
brum of Milan, commonly known as "The Virgin Tree." This
chef-d'oeuvre of twelfth-century art is also a seven-branch candle-
stick and over eighteen feet in height. With such great standing
candelabra as those of Reims and Milan, we may associate certain
large chandeliers still preserved from the eleventh, twelfth, and
thirteenth centuries. Those of Reims and Toul perished in the
French Revolution. But at Hildesheim we have a circular corona
of gilt copper suspended from the roof and dating from 1050, twenty
feet in circumference and bearing seventy-two candles. That at
Aix-la-Chapelle is still larger and still more remarkable for the
artistic beauty of its details. While as a splendid specimen of later
medieval work is that still preserved in the church of Aerschot,
Belgium, at least until recently.
As an example of a beautiful and at the same time unique candela-
brum, that in the church of Leau, Belgium, is extremely interesting,
combining a lectern with the candelabrum. The chapel of the
Hotel des Invalides, Paris, represents a very fine type of candle
lighting, with two rows of chandeliers. The Madeleine at Paris is
also a specimen of the same method of lighting.
In the early days the candle was the only illuminant, and in
Ronian Catholic Churches it is still required by the rubrics to be
burned on the altar during Mass and other ceremonies. In the
advance of science, however, religion caught the benefit and flooded
its churches with the imprisoned sunlight let free from oil and coal.
When later electricity was employed, religion seized the new light
to serve its purpose.
That we may understand the "raison d'etre" so to speak, of sym-
bolism in the church, it will be well to consider briefly the subject
of symbolism in art and the principles which underlie it, and which
give it the importance it deserves. Art does not produce the real;
it merely implies or suggests the real by the use of certain signs and
symbols which have been recognized as equivalent. If, for example,
we wish to bring to the mind of another the thought of water, we
do not bring a glassful and place it before the person; we simply use
the word "water," a word of five letters, which bears no resemblance
or likeness to the real article, yet brings the original to mind at once.
Fig. 3. Evangelical church. An excellent example of concealed chancel illumination.
Fig. 4. Evangelical church. Good example of semi-concealed lighting. Lamps, with
proper reflectors, are placed on the chancel side of the hammer beams and in the rear of the
PERROT: CHURCH LIGHTING REQUIREMENTS 299
This is the linguistic sign for water. The chemical sign for it, H 2 O,
is quite as arbitrary, but to the chemist represents the original as
clearly as the word does to the mind of another. And only a little
less arbitrary are the artistic signs for it. The old Egyptians con-
veyed their meaning by drawing a zigzag line up and down the wall.
Turner, in England, often made a few horizontal scratches from a
lead pencil to do duty for it, and in modern painting we have some
blue or green paint touched with high lights to represent the same
thing. None of these symbols attempt to reproduce the original,
or have any other meaning than to suggest it. They are signs which
have meaning because we agree beforehand thus to understand
them.
Now, the agreement to understand the sign is what might be
called the recognition of the convention. All art is in a measure
conventional, arbitrary, unreal, if you please. Everyone knows
that Hamlet in real life would not talk blank verse in his latest
breath.
The drama, and all poetry, for that matter, is an absurdity if one
insists upon asking, " Is it natural? " It is not natural; it is artificial,
and unless the artificial be accepted as symbolizing the natural,
unless the convention of metre and rhyme be recognized, one is not in
a position to appreciate verse. This is equally true of music. The
opera is a most palpable convention, and the flow of music which so
beautifully suggests the depths of passion and the heights of romance,
is merely an arbitrary symbol of reality. Recognize this and you
have taken the first step forward toward the understanding of art;
fail to recognize this, and art must remain a closed book to you.
Furthermore, the principle of indirectly representing by a sign the
Godhead or the truths which He came to establish, had its sanction
in the Divine Master Himself, for in His own public life He con-
tinually makes use of parables and indirect means to convey to
His followers the divine lessons He wished to teach.
It is for this reason that we find so much use of signs, emblems, and
symbolic expressions in the churches of centuries ago as well as in
the ritual of the religion taught. Much might be said on this sub-
ject, but a few examples will suffice to present more clearly this phase
of the subject.
In the tenth chapter of St. John we find Christ speaking in pro-
verbs and referring to Himself as the "door."
In the third chapter of St. Matthew, at the baptism of Christ by
St. John we read of the "Spirit of God descending as a dove and
3OO ILLUMINATING ENGINEERING PRACTICE
coming upon Him." Thus we have Christ symbolized as a door, and
the Holy Ghost as a dove. Many other examples occur in holy writ
which could be mentioned in this connection, but the foregoing rep-
resent in a very striking manner the direct use of symbols to
represent the persons of the Deity.
Among the symbols employed by the primitive Christians that of
the fish ranks probably first in importance. The symbol itself may
have been suggested by the miraculous multiplication of the loaves
and fishes, but its popularity among Christians was due principally,
it would seem, to the famous acrostic consisting of the initial letters
of five Greek words forming the word for fish ('Txflus) which words
briefly but clearly described the character of Christ and His claim to
the worship of believers, that is, Jesus Christ, Son of God, Savior.
The word then, as well as the representation of a fish, held for
Christians a meaning of the highest significance. After the fourth
century the symbolism of the fish gradually disappeared.
Referring now to the specific subject of illumination, we can con-
sider the candle as the symbolical representation of Christ, "The
Light of the World" (the wax typifying the flesh of Christ born of
a Virgin mother, because of the supposed virginity of bees. The
wick symbolizes more particularly the Soul of Christ, and the
flame the Divinity which absorbs and dominates both).
The Christian religion, as we know it in the twentieth century,
has formed itself into two great bodies, which we may term the
evangelical and the ritualistic. To light a church so that the lamps
may serve the practical purpose as illuminant, and at the same time
keep the religious symbolism in the spirit of each of these great
divisions, is the problem of church lighting that I propose to discuss.
THE EVANGELICAL CHURCH
The evangelical church holds specially to the Scriptures, and the
keynote of its service is the spoken word of the expounder of the
Holy Book. So light must fill the auditorium, must center on the
preacher, as symbol of the Heavenly Light that he teaches, filling
men's souls.
In the other great division, a subdued light must envelop the
congregation as befits those attending on great mysteries, and the
light must center on the altar, shining against the darkness of the
background, appearing above all else in the church, as symbol of
the Light of Heaven resting on the mysteries.
Fig. 7. Evangelical church. Excellent example of direct lighting for dark-ceilinged
church.
Fig. 8. Ritualistic church. A good example of indirect lighting by means of lamps con-
cealed on top of cornice.
PERROT: CHURCH LIGHTING REQUIREMENTS 301
Thus we have, in general, the thought underlying the scheme of
lighting for churches of both divisions.
While it may not be possible to show practical examples of lighting
that exactly illustrate the principles enumerated above, yet in the
main, we will find these principles carried out to a greater or less
degree in all well-arranged churches. Of course, the architectural
treatment of the design will influence the scheme of lighting, but the
architectural scheme should follow the above principles, just as the
lighting is intended to do; for instance, the plan of evangelical
churches naturally takes a form best calculated to permit everyone
to see and hear the speaker, hence there are large auditoriums so
designed as to meet these requirements. On the other hand, the
plan of ritualistic churches aims not so much to make a perfect
auditorium as a place first for the altar, about which the people
may gather to take part in the solemn sacrifice which is offered
thereon, the part played by the speaker being second in importance
to the great mysteries of the sacrifice.
Further, the ritualistic ceremony naturally begets symbolical
forms in the architectural treatment, so that there are depicted
throughout representations of the great mysteries of religion, both
in the structural parts and in the minutest details.
As the problem of lighting evangelical churches resolves itself into
that of general illumination, the treatment of such buildings can best
be made to follow the general rules recognized as a standard for the
lighting of auditoriums.
THE RITUALISTIC CHURCH
The problem of lighting ritualistic churches, particularly Roman
Catholic churches, is one that requires more study since the predomi-
nance of the symbolical over the practical is very marked. There
is an added problem in these churches of decorative lighting in addi-
tion to the practical and symbolic lighting. This of late years, has
become very marked, due to the ease of obtaining decorative effects
with the use of the many sizes and styles of electric lamps. A scheme
of lighting for a Catholic Church which does not include facilities
for decorative lighting around the sanctuary where the altars are
placed is incomplete. While the use of candles on the altars is
required by the rubrics of the church, and they must be used, the
added use of electric and gas candelabra makes it possible to
obtain decorative effects in light for celebrations far surpassing the
effect of the candle light.
302 ILLUMINATING ENGINEERING PRACTICE
The principal reason why electric decorative lighting has come into
play in this church is due to the fact that as the church proper was
lit by electricity, the insignificance of the illumination of the altar
by candles alone became very apparent, and as the altar is the ob-
ject for which the church exists, and in its symbolical sense, should
be the richest part of the church, it was necessary to add electric
illumination to this part of the edifice also.
To come now to the actual working out of these principles to
concrete problems, it would be well to endeavor to establish rules
for guidance which can be used in most cases. The method of
lighting can generally be included under one of the three systems:
"Direct general illumination," "semi-indirect" and "indirect," or
a combination of any two of them. In examining the general form
of evangelical churches, it is found that in plan they may be grouped
as follows: Square or rectangular plan, and Greek Cross plan, all
usually consisting of one clear span. The church may or may not
have a gallery, but as a rule, the floor area in the center must be
illuminated from the high ceiling above. Usually it is preferable
to hang chandeliers from points each side of the center of the build-
ing. The use of central chandeliers is, as a rule, an unhappy solu-
tion, and should be avoided unless the architectural treatment of
the ceiling is such as not to permit of the use of two rows of fixtures;
then the use of one row or one central fixture must be resorted to.
The lighting of the chancel should be such that ample light
falls on the preacher. Should there be a chancel arch, concealed
lamps around the arch produce a very impressive effect.
Should a gallery be used, the part of the church under the gallery
can best be lit by ceiling lamps under the gallery, or lamps can be
arranged around the columns near the caps. If there is no gallery,
side lamps on the walls are sometimes necessary to supplement the
light from the ceiling. There is no reason, though, why ample light
cannot be arranged for in the ceiling. The one point to bear in
mind is to avoid the use of naked lamps in line with the vision of the
congregation. The use of brackets with naked lamps on the wall
back of the chancel is injurious to the eyes of the people, and should
be avoided.
Should there be a dome or window in the center of the ceiling,
rows of lamps arranged to suit the architectural motives can be
used instead of pendants. Daylight effect can be produced by
placing lamps with suitable reflectors back of the glass.
When open truss work occurs the fixtures can be suspended from
PERROT: CHURCH LIGHTING REQUIREMENTS 303
the trusses, or else the lamps can be concealed from the congregation
by being put on the chancel side of the trusses or hammer-beams.
Very effective and satisfactory results are obtained by using the
indirect method of lighting. This can be accomplished in either
of two ways: one by concealing the lamps on top of a cornice, or
in recesses on the tops of column caps and projecting the rays upward,
depending on the reflected light from the ceiling for the general
illuminating effect; the other by using indirect pendant fixtures so
placed as to harmonize with the architectural treatment of the
ceiling, and projecting the light rays upward.
Many fine examples of this latter solution of the lighting of
evangelical churches can be seen. When the artificial lighting
is carefully worked out it follows closely the effects of the natural
light in the daytime.
Semi-direct lighting has been developed to a point where high
lighting efficiency coupled with artistic treatment have made this
method very popular, since it combines in a great measure the eye
comfort feature of indirect lighting, and at the same time possesses
the artistic effect attendant upon the use of subdued visible light
sources, for it must not be forgotten that when light is present, the
eye unconsciously seeks to determine its source, and when this ceases
to be a part of the decorative scheme the mind fails to get full sat-
isfaction from the illumination.
Turning next to the lighting of ritualistic churches, the problem
is more complex. As outlined above, symbolism plays an im-
portant part in the design of such churches, so much so as very fre-
quently to determine the shape of the floor plan. The cruciform
plan is the one most generally used for large churches, consisting
of a nave and two side aisles across the church, and the nave tran-
septs and apse for the three divisions of the length of the church.
Of course, all churches do hot have side aisles, nor do they all have
transepts, but this form of floor plan is symbolically correct, as it
represents the emblem of salvation, the cross.
Formerly, the common method of lighting was to arrange pendants
from the apex of the main nave arches, thus making a row of chande-
liers in the middle of the church. The side lamps were usually ar-
ranged around the columns or piers, sometimes in the form of a
corona, and sometimes as brackets.
With the advent of electric light, greater freedom of arrange-
ment of the lamps became apparent, hence marked progress was
made by arranging rows of electric lamps in cornices or other archi-
304 ILLUMINATING ENGINEERING PRACTICE
tectural features, doing away with the need of chandeliers. How-
ever, a combination of chandeliers and cornice lamps has become
very common, due to the marked decorative effect of outlining the
main architectural motives by means of lamps. This arrange-
ment was even attempted in former days with gas lighting.
A very remarkable example of semi-indirect lighting, using gas
as the illuminant, is that of the Roman Catholic Cathedral of Phila-
delphia. The burners are of the kinetic horizontal type which makes
possible the use of gas as the illuminant in bowls for semi-indirect
lighting, and represent a step far in advance in the progress of gas
lighting. Much study was given to the location and design of the
fixtures and the results are highly satisfactory.
The arrangement of lamps about the sanctuary where the altar is
placed requires the utmost care. In addition to the local lighting for
the altar, it is well to arrange concealed lamps back of the sanctuary
arch and pilasters supporting the same which can be lighted up at
certain parts of the service to flood the altar with light; moreover,
provision must be made for the decorative lighting, which changes
with the seasons of the ecclesiastical year. For instance, in Catholic
churches, there are certain services and parts of the service which
require special lighting effects, due to the nature of the service, and
whether the Blessed Sacrament is exposed or not. For grand cele-
brations, as at Easter or Christmas, special decorative effects in
lighting and decoration with plants and flowers are resorted to:
In one service of the church on Good Friday, there is a part where
total darkness reigns for a few seconds, and then instantly a flow
of light fills the church. While it is not the desire of the Church
in any way to attempt theatrical effects, it is the intention to make
the exterior signs an expression of the interior feeling one should
possess in attending the service. As all of these services are to
be performed in a strictly liturgical manner, it is a very delicate
matter to introduce effects in lighting which will not destroy the real
meaning of the service.
Before closing, I wish to call your attention to a very successful
scheme of day-lighting which serves to illustrate very forcibly the
proper method of using light, by suggesting to the beholder the
sanctity of the place and the feeling of awe which should possess him.
I refer to the tomb of Napoleon in the Hotel des Invalides, Paris.
The altar is in the apse and the tomb in the rotunda. The window
in the rear of the altar consists of tinted glass of a golden hue, so
that the light filling this part of the interior is always bright, no
> . Ritualistic church. Semi-indirect lighting; two rows of fixtures. Concealed light-
ing back of sanctuary arch.
Fig. 10. R. C. cathedral of Philadelphia. Semi-indirect lighting illustrating the diffusion
obtained by the improved method of installation.
(Facing page 304.)
Fig. ii. R. C. cathedral of Philadelphia. Semi-indirect gas fixture designed to conform
to the interior decorations.
Fig. 12. Ritualistic church. Decorative lighting for Christmas celebration. Floral and
electrical effects.
PERROT: CHURCH LIGHTING REQUIREMENTS 305
matter whether the sun shines or not, thus symbolizing the living
presence of God on the altar. While in the rotunda, casting its rays
upon the tomb, the light no longer suggests the brightness of heaven,
but on the contrary, is subdued by the blue tinting of the windows
of the transepts and dome, thus reminding one that he is walking
in the shadow of death.
The effect has been so thoroughly accomplished that anyone, even
though claiming no pretext to the understanding of art, can readily
feel the effect of the lighting as soon as the building is entered.
In conclusion it may be stated that that scheme of lighting a
church is best which considers illumination in its two-fold aspect:
First, eye-comfort illumination; secondly, the aesthetic, which em-
bodies those qualities which conduce to harmony in the general
architectural and symbolical treatment of the edifice.
LIGHTING OF SCHOOLS, LIBRARIES AND
AUDITORIUMS
BY F. A. VAUGHN
INTRODUCTION
The Committee on Lectures has suggested that this lecture course
should be supplemental in character to the Johns-Hopkins Univer-
sity course of six years ago, at which time the establishment of the
basic principles on which the science and art of illumination rests,
was the main object. In the f ollowihg discourse, therefore, particular
stesss will of necessity be placed on the progress, during the interim,
of the practical application of those basic principles and on the
utilization of the results and investigations and findings of the
various committees of the society appointed to coordinate and apply
those principles to the various practical problems, especially those
involving public comfort and welfare.
Since, also, the other lecturers in the present course have been
assigned subjects dealing with general details of design and applica-
tion, covering the entire field of the art, the following lecture will
necessarily be largely confined to an analysis of the specific problems
covered by the title, the application of already established principles
to these problems and a passing review of typical and notable in-
stallations exemplifying these applications.
The subject is extremely broad and inclusive, since the illumina-
tion of the various departments and divisions of schools, audito-
riums and libraries, embraces practically every known type of illumi-
nation, for all classes, ages and sexes of people. This discourse must
therefore be confined within limits, lest it overlap the subjects of
the other lecturers; therefore it will treat fully only those require-
ments which are more or less uniquely applicable to the narrower
interpretation of the subject, leaving the applications to the more
general functions to others, or to casual mention. For instance,
in the modern schools and colleges there are offices, catalogue files,
dining rooms, gymnasiums, swimming pools, shops, kitchens, hos-
pitals, grounds and architectural exteriors, which require practi-
307
308 ILLUMINATING ENGINEERING PRACTICE
cally the same treatment as the same character of spaces in other
establishments.
In the formation of this somewhat narrower viewpoint it will be
assumed that the more unique functions of the three divisions
of the subject will be largely confined to education, edification and
amusement, especially studying, reading, observing and listening.
For the purpose of this lecture, the principal function of the schools
will therefore be considered to be a public place for study; that of the
library, a public place for reading, and that of the auditorium a
public place for seeing and hearing. In each case large masses of
the public are involved, and each is a factor in the education, edifica-
tion and amusement of the public, with a preponderance toward the
educational or amusement functions in the order given. All are
fundamentally of general public benefit, welfare and uplift, if properly
used, and all deal with comparatively large spaces, accommodating
numerous people.
Surely, it is tremendously important to pursue the study of the
proper application of the art of good illumination to these spaces as
a means of augmenting the beneficial functions of these institutions,
and of avoiding the counteracting influence of the serious ill effects
of poor illumination on the physical, mental and moral develop-
ment and well-being of the public.
Besides being useful and efficient, it is demanded of the lighting
installation in each of the divisions that it be artistic and harmo-
nious. This requirement is at present perhaps most rigid in the case
of auditoriums; less so in the case of libraries, and still less in the
case of schools, with a growing tendency at the present time to intro-
duce and extend this influence more and more into the schools, not
in the form of elaboration or ornateness, perhaps, which is not
necessarily art; nor in an expensive form which does not always
indicate good taste but by the harmonious application of the in-
stallation to the architectural features and the functions of the room.
In each division of this subject, the importance of the physiological
effects as obtained through vision and the use of the eye, is tremen-
dous, and more and more widespread in its scope as we pass backward
toward the schools, where the development and conservation of the
eyesight of our children and youths for the future is paramount and
of greatest economic importance to the nation and the world. In
the other two divisions, however, the conservation of the remaining
eyesight of older persons is only relatively unimportant.
In each division it is desirable, for the successful carrying on of
VAUGHN: LIGHTING OF SCHOOLS, ETC. 309
the functions of that institution, to produce certain psychological
effects, exemplified by the quietude of the library, the attitude
of concentration in the schoolroom, and the inviting, pleasant and
comfortable atmosphere of the theatre. The steady, daily repeti-
tion of concentrated study in the schools is particularly conducive
to eye fatigue and strain, and makes the application of proper, com-
fortable illumination in schools more important than does the casual
frequenting of the library or amusement auditorium, which are still
respectively important. The problems, therefore, while similar in
many respects, are quite different in others, the similarities and
differences, from the standpoint of the following basic requirements
set forth by L. B. Marks in the 1910 Lectures, and their different
gradations, being more or less apparent.
These fundamental characteristics of good lighting then established
are:
1. Sufficient illumination.
2. Low specific brightness.
3. Freedom from glare.
4. Diffusion.
5. Cost of installation.
6. Economy of operation.
7. Simplicity and convenience.
8. Esthetic design.
In schools, the business of concentrated study for the attainment
of mental development and education has tended to minimize the
aesthetic side. In the library, a place for study and reading, for all
classes of readers of various ages and conditions of eyesight, and
developed and undeveloped aesthetic attitudes, particular attention
must be paid to the difficult problems affecting eye efficiency and
comfort in connection with the use of books, new and old, in all
types and on all grades and conditions of paper. Less of the element
of slavish work, more of the element of recreation and entertain-
ment enters into the use of libraries, where reading is done for edu-
cation, information and amusement, and there is more inclination
and opportunity for the observation of the aesthetic factors in the
installation. Auditoriums which the dictionary defines as "The
part of a public building, as a theatre, occupied by the audience;
hence any space so occupied" are principally used for purposes in
which the recreation and amusement of the public piedominate,
with a still greater inclination and tendency to observe and benefit
by the aesthetic factors. No study and very little reading is pursued
310 ILLUMINATING ENGINEERING PRACTICE
here; the main function being that of listening with the eyes open,
and therefore still subjected to the good or bad effects of the illumina-
tion. Auditoriums used as concert halls, churches, lodges or even
expositions, still retain a large educational element, although in
varying degrees, and it is thus apparent how these divisions of the
subject are inter-related.
The illumination requirements may be, for good and sufficient
reasons, lively or subdued; brilliantly glittering or dull; intensity
high or low; diffusion more or less; installation simple or elaborate;
expenditure spare or lavish; operation economical or expensive; but
the architect and engineer must produce a harmonious solution of
the specific problem, which is compatible with the results desired.
Schools, auditoriums and libraries all perform very important
general public service in the education and uplift of the common-
wealth. Notwithstanding the differences in the classes and ages
of the users; in the educational or amusement features; in artistic,
aesthetic or psychological aspects the physiological aspects are
the same in all, in so far as conservation of vision to the greatest
extent and for the longest period is of main importance, beginning
with preventive measures in the young and ending with preserva-
tive measures in the older. Even in schools for the blind, where
at least some of the inmates, including the teachers, still retain a
precious remnant of their vision, it is perfectly apropos to give most
careful consideration to the planning of the illumination.
Different intensities are, of course, required for sufficient illumina-
tion of the different classes of interiors and for the different purposes
for which they are used. During the daylight hours, the intensities
are relatively high, although very fluctuating, and if carefully planned,
the natural illumination is well diffused, though ofttimes definitely
directed. Different intensities may be best for young eyes or old
eyes. In theatres and lodges, the intensity is under the control of
an operator and may be adjusted according to the effects desired by
the stage director or required by the lodge services. In churches,
there may be differences in intensity desired by different denomina-
tions, as well as for different congregations and services, the reading
services of the Christian Science Church having set a relatively high
standard of intensity for church illumination, for instance. The
more subdued interiors of some of the Gothic churches require a
more subdued religious atmosphere. An example of a peculiar
requirement in a church during war times is of passing interest,
where, because of the Zeppelin air raids, a London church is required
VAUGHN: LIGHTING OF SCHOOLS, ETC. 311
to darken the top of its auditorium entirely by means of opaque
downward directing reflectors.
The selection of fixtures for various interiors and requirements is
a most important function, best performed through the cooperation
of the building committee, the architect and the illuminating
engineer. Cooperation with the architect, which, since the year
1910, has made great progress in our profession, cannot be empha-
sized too forcibly, for where an able architect is retained, the best
general effects can be obtained with his cooperation; for the prob-
lem of the illuminating engineer is not so much the selection of the
proper fixture design as the selection of the proper system and type,
quality and quantity of illumination, although every engineer who
assumes to deal with these problems should conscientiously train
his latent artistic faculties, so that when loaded with the responsi-
bility for the aesthetic features of an installation he can perform this
function in a manner beyond the ridicule of the architect and with
credit to hiis profession, and so that his engineering ability will not
be driven into oblivion in the shadow of artistic criticism.
Given by the architect certain architectural and aesthetic features,
such as Gothic design; dark decorations; dull colored ceilings; cer-
tain artistic effects and color schemes, the illuminating engineer
must first select the system of lighting and type of installation best
suited to these conditions, and the two collaborators may then work
out a combination of art and science in the form of the fixtures, the
artistic outer garment being designed by the architect with sufficient
dimensional and spacial features to contain properly the utilitarian
apparatus, such as the reflectors, lamps, etc. The selection of
fixtures or their design will not be specifically treated in this lec-
ture, as the illustrations will be sufficiently indicative and in the
majority of cases, the reason for this selection, often confined to
designs obtainable generally on the market, will be apparent.
PART I. SCHOOL ILLUMINATION
Because of the present general appreciation of the hygienic im-
portance of sufficient and proper illumination in schools, it hardly
seems necessary, except possibly to add emphasis, to make any state-
ment regarding the tremendous service which the illuminating en-
gineer can be to the present as well as future generations, by his un-
stinted activities in this phase of the art of illumination. Because
of its preponderant importance, the major part of the discussion in
this lecture will be devoted to a review of accomplishments in this
312 ILLUMINATING ENGINEERING PRACTICE
direction and of suggestions for future accomplishments. Within
the past five years, not only has the interest of the Illuminating
Engineering Society been awakened through its individuals and
groups of members, but the active and productive interest of many
school committees having charge of hygiene, sanitation, conserva-
tion of vision, selection of textbooks, and like important matters,
has been aroused, with the practical results of the recommendation
and adoption of several codes and rules tending toward the elimina-
tion of the ill effects of bad lighting in our schools through the im-
provements in illumination and otherwise. A committee of the
society, having in charge the preparation of a "Code of School
Lighting," is practically ready to submit for general adoption a
splendid compilation of scientific and common-sense suggestions,
the general adoption and enforcement of which would be of inestim-
able benefit to mankind. Mr. M. Luckiesh, the chairman of this
committee, last year discussed these matters in his paper on " Safe-
guarding the Eyesight of School Children," and much of the text and
some illustrations which follow in this division are excerpted from
his presentation, and the tentative compilation of a code prepared
by the committee.
As exemplifying the present active interest in this subject, outside
of the society, two typical references may be made, one from " Sani-
tary School 'Surveys as a Health Protective Measure," by J. H.
Berkowitz, published by the New York Association for Improving
the Condition of Poor;" and one from "Recommendations Adopted
by the Board of Superintendents of New York Schools," both dated
this year. The first reference is as follows:
"In no other respect is the teacher's responsibility for the physical well-
being of the pupils better defined than with reference to the protection of
eyesight. Posture is important, of course, and the proper adjustment of
desks and seats is a controlling factor in maintaining it. Eye-strain is
closely associated with incorrect posture, and likewise caused by poor
seating arrangements.
"Height of desk and seat, distance from each other, distance from
blackboard, etc., are some of the factors to be considered in relation to
eyesight. The prevention of glare from excessive light and reflective sur-
faces-is of the utmost importance, and yet perhaps the easiest to attain.
The proper means being provided, it rests entirely with the teacher.
Perhaps the function of window shades and their usefulness are not fully
appreciated, but teachers should know that glare and intense direct light
cause eye fatigue. This is particularly harmful to the immature and
highly susceptible eyes of children.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 313
"Unfortunately, in most of the classrooms surveyed the testimony
found is against teachers and others responsible for the welfare of the
pupils. Torn, unworkable window shades, particularly in classrooms,
which give an unobstructed exposure to the rays of the sun, are a menace
to children. Aside from the necessity of proper manipulation of shades
for the regulation of light, the simple obligation is imposed on the teacher
to report promptly any damage which may effectively interfere with the
proper use of such shades.
"The school authorities can be expected to correct defects only if they
are brought to their attention."
The second reference cites rules prepared by Dr. I. H. Gold-
berger, Assistant Director of Educational Hygiene, for the Board of
Superintendents.
"There are scores of classrooms in the public schools that are poorly
lighted. The women principals have been working for years to have these
rooms closed, but without result. Usually they are in buildings which are
congested and all available space must be used. The conditions are
recognized as serious and to counteract the ill-effects of studying and
reciting in such rooms the board of superintendents has had prepared the
following recommendations for the guidance of teachers in such rooms as
are insufficiently illuminated:
"i. Artificial illumination should be used whenever necessary. No
rule can be laid down to guide the teacher in this matter. She must use
her own discretion and judge when artificial light is necessary. It must
be used at once if pupils exhibit any difficulty in reading.
"2. Teachers should be alert to report to the principal if the windows,
walls, or prismatic glass reflectors are not clean.
"3. Dark-colored pictures should not be hung on the walls, and dark-
colored charts should be displayed only when necessary, for these diminish
the light in the classroom.
"4. Teachers should refrain from placing curtains or any other obstruc-
tions in the window.
"5. Window shades should be kept rolled up as much as possible.
Attention should be paid to the proper regulation of the shades, protecting
the children's eyes from insufficient or excessive light.
"6. To favor the maintenance of the proper reading and working dis-
tance, pupils should be seated so far as possible at dfcsks according to their
size. Janitors are under the by-laws required to make adjustment of
furniture upon instruction from the principal. Children having defective
vision should be seated as near as possible to the front of the room.
"7. The eyes should be raised occasionally from the work, and there
should not be two consecutive periods of close eye work."
If boards and individuals having the responsibility of the solution
of these problems are making such sincere and effectual efforts,
314
ILLUMINATING ENGINEERING PRACTICE
surely the Illuminating Engineering Society and its members who
are specializing in this work can be of great assistance through the
adoption of a code of school lighting, and by the further promulga-
tion of the correct basic principles through the Society, reciprocal
meetings with other societies, and by other cooperative means.
The problem exists to-day, in several thousand schoolrooms, affect-
ing millions of children. A remedy should be speedily applied. It
is a matter for most hearty congratulation that it is actually being
done.
A full consideration of the subject of illumination in schools must
include illumination by daylight, which, because of its close rela-
tionship to the design, location and position of the building, requires
the intimate cooperation of architect and engineer.
The orientation of the building, as well as the arrangement of the
ceiling and side windows, and other openings for admitting day-
light, should be so planned that direct sunlight, for at least a por-
tion of the day, enters the classrooms and other important portions
of the building. The order of preference of exposures is given by
the Code as easterly, southerly, westerly and northerly. There
should be sufficient light in the darkest working space and as good
uniformity as possible everywhere. The minimum daylight in-
tensities proposed are as follows:
Illumination i
n foot-candles
Minimum
Desirable
Storage, passageways, stairways, etc. .
o. 5
o. ^ =; .0
Classrooms, study rooms, libraries
Auditoriums
4.0
2 O
4-20
3 10
Sewing, drafting, etc
IO.O
10 30
Blackboards
40
420
On account of the difficulty of control and regulation of day-
light, actual or average working intensities cannot be readily es-
tablished, but the ceiling and side window shades and curtains
should be of such optical and mechanical characteristics and be so
maintained and operated as always to keep the illumination well
above the minimum set forth and to protect the children's eyes as
much as possible from glare. The daylight intensities will, of course,
generally be higher than by artificial illumination. Daylight
illumination should be unilateral. It should not enter the classroom
VAUGHN: LIGHTING OF SCHOOLS, ETC.
from the right, but should enter from the left, with a minimum win-
dow area, ordinarily, of not less than one-fifth of the floor area.
Windows in the rear may produce glare upon the blackboard, and
therefore should be regulated with care.
Natural lighting through wired glass windows in the ceiling may be
superior, but the architectural difficulties usually prohibit this
method from adoption in as many cases as would be desirable.
Lighting from side windows can be brought nearer to ceiling window
uniformity, by attention to their proper proportion to floor area and
effective visible sky area, and by the scientific use of prismatic and
other forms of diffusing and directing window glass, and by the con-
struction of relatively narrow schoolrooms, with high windows
and highly reflective surroundings.
With artificial illumination, as high intensities cannot be eco-
nomically obtained as with daylight, nor are they required, and the
following average intensities have been recommended.
Illumination i
n foot-candles
Minimum
Desirable
Storage corridors, stairways, etc
Rough shop work
0.25
I 2S
0.25- 5.0
1.2^ 2 . <
Fine shop work
Sewing, drafting, and the like
3-5
C o
3-5 ~ 6.0
5 .0 10.0
Auditorium
2.O
2.O - 4-O
Classrooms, study rooms and libraries. . . .
Gymnasium
3-o
I O
3-o - 5-o
i .0 <> .0
Laboratories
Blackboards
3-0
i o
3-o - S-o
? .0 .0
Sufficiency of illumination, which is the prime factor, can be
secured by any one of the three general arbitrary types of systems
of illumination known as direct, semi-direct, and indirect. Some of
the other important factors, however, may be best produced only
by one system or another. Greater diffusion can be obtained by
larger indirect component, while definiteness of control can be
obtained by the direct system with properly selected reflectors.
Through the exhaustive work of Drs. Ferree, Rand and Nutting, J. R.
Cravath, A. J. Sweet, T. W. Rolph, and others, the relative advantages
of the various systems with relation to eye fatigue and ocular com-
316 ILLUMINATING ENGINEERING PRACTICE
fort, have been so thoroughly investigated and discussed as to need
no repetition here. Since diffusion, as obtained by the indirect sys-
tem, tends to minimize the glare from the light sources, and from the
desks and furnishings, as well as from the pages of the books, the
indirect is preferable to the direct system. Since the semi-indirect
system is intended to be merely an intermediate between, or com-
bination of the direct and indirect, it may possess more or less of
the advantages or disadvantages of either system, according as
the particular installation possesses more or less of one compo-
nent or the other. Good results can be obtained by the use of
translucent bowls if the brightness of their surfaces which means
the direct component of illumination which passes through the
glass be kept down, thus producing really an indirect installa-
tion with relatively faintly luminous bowls. In this way the
semi-indirect is merged into the indirect.
Localized illumination has no place in the solution of this problem.
Approximate uniformity on the working plane, is highly desirable
and could not be obtained thereby. Light sources must be kept out
of the range of vision, and, especially in the direct system, should
be relatively small in size and large in number and should never
be visible against a dark background. Bare lamp filaments must
be shaded, screened or concealed. This is particularly necessary
when the modern extremely brilliant lamps are used. Luckiesh
states that the brightest permissible object visible from any normal
position of the observer should be not greater than 250 milli-lam-
berts with a maximum permissible brightness contrast in the
normal visual field not greater than 20 to i. The Committee on
Glare of the Illuminating Engineering Society, in an effort to
establish these and similar standards, compiled several reports
during the last year treating very thoroughly the subjects of
brightness and glare.
Other committees of the Illuminating Engineering Society have
established a contrast ratio of 100 to i, to apply over the whole
working space. The above ratio of 20 to i should be interpreted
as applicable to juxtaposed surfaces.
It should be stated that all wall areas, ceilings, and other reflect-
ing surfaces should be matte; the ceilings bright, especially for
indirect systems, and the walls only moderately so; the general
idea being to avoid great contrasts, mental depression and poor
efficiency.
A matter of almost as much importance as all that has thus far
VAUGHN: LIGHTING OF SCHOOLS, ETC. 317
been stated is the character of the surfaces at which the children
have to look and with which they have to work, especially that of
the paper used in the school books. A great amount of attention
has been given to the subject of glare from the pages of school books
and sufficient interest has been aroused to insure the future adop-
tion of paper with sufficiently matte surface to minimize specular
reflection and resultant glare and to assure the selection and use of
comfortable reading type, form of letter, spacing of lines and letters,
and the use of suitable qualities- of ink.
Well-diffused uniform illumination from and on matte surfaces,
and produced by screened or concealed sources, minimizes glare and
contrasts and is conducive to ocular efficiency and comfort.
Glare, as clearly denned and analyzed in the various reports of
the Committee on Glare of this year, is the omnipresent bugbear
particularly of school illumination. After its reduction by conceal-
ing or screening the light sources; properly locating and proportion-
ing the windows; using matte reflecting surfaces, unpolished desks,
and especially prepared book paper, it still lurks on the surfaces of
the blackboard. Mr. Luckiesh, who treats this subject simply and
plainly, states that:
"Glare due to specular reflection from glossy blackboards can be re-
duced or eliminated by lighting them by means of properly placed and well-
shielded artificial light sources. In Fig. 5 in Mr. Luckiesh's paper
on "Safeguarding the Eyesight of School Children" page 181 of the
Transactions of the Illuminating Engineering Society, Nov. 2, 1915 are
shown simple graphical considerations of blackboard lighting. In a is
shown a plain view of a room with windows on one side. Rays of light
are indicated by A, B, and C in a horizontal projection. These are sup-
posed to come from the bright sky. By the application of the simple
optical law of reflection the angle of incidence is equal to the angle of
reflection it is seen that the pupils seated in the shaded area will experi-
ence glare from the blackboard on the front wall. In b is shown the
vertical projection of the foregoing condition. It is seen from this graph-
ical illustration that by tilting the blackboard away from the wall at the
top edge the pupils in the back part of the room will be freed from the
present glaring condition. Whether or not this tilting will remedy bad
conditions can be readily determined in a given case. In b the effect of
specular reflection of the image of an artificial light source is shown by D.
In c is shown a proper method of lighting blackboards by means of artifi-
cial light sources. This will often remedy bad daylight conditions whether
due to an insufficient illumination intensity of daylight or due to reflected
images of a patch of sky."
318 ILLUMINATING ENGINEERING PRACTICE
The psychological as well as the physiological relation of colored
illumination to the hygienic, as well as to the aesthetic, conditions in
schools and other places of intensified and concentrated visual opera-
tions is important and has been thoroughly investigated by Mr.
Luckiesh and referred to by Messrs. Black and Vaughn, and others.
The editor of the Illuminating Engineer once said:
" Lighting installations, like men, may be notable for defects as well as
virtues. They have this also in common with men, that they are seldom
hopelessly bad or perfectly good.
"Careful observation, retentive memory, and a habit of analytical
reasoning are the basis upon which experience builds the structure of
skill which is above rules and formulae.
"An impartial study of different lighting systems, with a view to frank
criticism and comparison is one of the best of all methods of acquiring
facility of judgment."
With the spirit of these remarks in mind, let us now analytically
review several illumination installations exemplifying the good and
bad features discussed above.
For instance, a school room is improperly illuminated by daylight
when the light is admitted mainly from the right-hand side, as
shadows of the hand and arm are thrown on the working plane.
This produces illumination which will result in eye-fatigue. A school
room with proper daylight illumination is one in which the light is
admitted from the left side and the rear, as this gives as nearly a
perfect arrangement for maximum high efficiency as it is practical
to obtain without ceiling windows.
In Fig. i (Fig. 7 of Mr. Luckiesh's paper, " Safeguarding the Eye-
sight of School Children," above referred to) is shown an extremely
bad condition, found in a schoolroom where drafting is taught.
With such localized type of lighting each pupil can adjust the unit
to suit himself, with the result that he is not only injuring his own
eyesight and doing poor work on account of misdirected light, but
at the same time is subjecting many other pupils to bad light condi-
tions, on account of the position of his lamp. Actual inspection in
this case, Mr. Luckiesh says, showed that most of the pupils were
using the light improperly and were often working in such a way as to
produce annoying shadows.
However, a condition which is even worse than that in the draft-
ing department just shown is one where the lamps are suspended from
cords, as, with the type of angle steel reflector in popular use, the
lamp is always exposed to the view of anyone on the opposite side
VAUGHN: LIGHTING OF SCHOOLS, ETC. 319
of the reflector. Due to the fact that cords are used, the lamps do
not stay in any fixed position, but have a tendency to swing back and
forth on account of air currents or when hit by the students, so that
the light proves very unsatisfactory, as is always the case when
lighting equipment is used in such a way as to permit of adjustment
by the individual.
In Fig. 2 is shown a drafting or drawing room in which use is made
of the indirect system of lighting. It is to be noted that all of the
table tops are brightly and yet uniformly lighted, and, due to the
large expanse of lighted ceiling, no shadows are encountered by the
draftsmen when working.
In the manual training department of a modern technical high
school (Fig. 3), localized lighting is used in the wood-working shop,
where angle reflectors with bare lamps are employed. It was noted
that one of the lamps had been tied up in order to obtain a more
desired (though not necessarily desirable) illumination, and that in
another cord a knot had been tied evidently for a like purpose. A
student working on any of the benches would have the glare from the
exposed lamps of all the various units. A view was taken in the day-
time, with the lamps used just long enough to show the location of
the bulbs. It would have been impossible to have taken a satisfac-
tory picture entirely by the light of the units, since nothing would
have shown except the lamps themselves and a small portion of the
bench directly in front of them.
In the wood-working machine shop (Fig.4), the illumination is
furnished by direct units with mirrored glass reflectors with opaque
green enamel backing. The effect is to give an even illumination
over all the benches; at the same time so screening the light
sources as practically to eliminate all glare unless a person de-
liberately looks up into the reflector. It may be pointed out
here that while it is perfectly plain and well known that by the
multiplicity of units per unit of area greater uniformity of inten-
sity of illumination and greater dispersion of shadows can be secured,
it may not be so apparent to all that the same tendency obtains with
the indirect system and that sometimes numerous indirect fixtures
may be used in a given area and sometimes few, according to the
results demanded and the funds available. The higher grade dis-
tribution can be secured with few fixtures, if proper selection of
reflector, lamp, position and hanging length is obtained, so as to dis-
tribute the light as uniformly as possible over the ceiling.
In the sewing room of a domestic science department in a public
320 ILLUMINATING ENGINEERING PRACTICE
school (see Fig. 12, Luckiesh's paper "Safeguarding the Eyesight of
School Children"), steel reflectors suspended on flexible cords have
been used with the result that bare lamps are constantly within the
direct range of vision of the pupils, giving the worst possible condi-
tions for working.
The domestic science room in a high school was, again, so furnished
with improper daylight lighting that those working on one side are in
their own light with the windows at their backs; those on the other
side of the table get the full glare of the windows in their eyes; while
those at the far end of the room have the light coming in from the
right-hand side, and only a few can secure whatever advantages there
may be in this position the whole arrangement being very unsatis-
factory. The tables should be so arranged at right angles to the
windows that at least the largest possible number receive the day-
light from the left. On the other hand, the interior of a model
economics class kitchen in a college was so illuminated artificially as
to result in absolutely uniform illumination on the stove, table
and kitchen cabinet, with entire absence of glare. This was accom-
plished by means of the indirect system of lighting which has the
advantage that, no matter where a person is at work in this room,
very little shadow will be occasioned by his or her position.
In contradistinction to the above, one usually finds in a university
chemical laboratory, for instance, old types of lighting in which the
daylight conditions are very nearly as poor as those of artificial illumi-
nation. What the effect in these rooms is at night is known to most
of us from experience. The usual very low mounting height and the
general type of reflector used, its design evidently being based on
uniqueness of outline rather than on the principles of reflection or eye
efficiency and comfort, make this usual plan almost unbearable.
A notable grateful exception to the above method of illuminating
school work rooms is found in the chemical laboratory in the Boy's
High School at New Orleans. The photograph of Fig. 5, taken at
night, shows conditions with artificial illumination. Being lighted
by the indirect system, there is uniformity of illuminatibn and
absence of glare and shadows which is desirable for a room of this
character.
It may be interesting, for the sake of emphasis, to inspect one or
two examples of typically bad schoolroom lighting as it exists to-day
in many of our larger as well as smaller cities, where the millions of
school children are subjected to the deleterious influence of eye-
fatiguing installations.
Fig. i. A poor arrangement with localized, adjustable lighting equipment in a drafting
room.
Fig. 2. A drawing room properly illuminated by a totally indirect system, with simple
opaque fixtures.
(Facing page 320.)
Fig. 3- Manual training department of a school with very poor arrangement of localized
lightirig.
Fig. 4. Good arrangement of general illumination by direct lighting units in wood-working
shop.
Fig. 5. Indirect lighting or chemical laboratory,
Fig. 6. Typical poor lighting in a highschool assembly room.
(Facing Figs. 3 and 4.")
Fig. 7. Good direct lighting installed in an old schoolroom.
Fig. 8. Classroom lighting with a few indirect units.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 321
The usual low-hanging height of units, as well as the too often total
absence of reflectors and shades, produces the worst possible lighting
conditions for a room which is constantly used by a large number of
students as a place of study. Nevertheless, this was found to be the
case in a large assembly room of a western high school and can
be duplicated almost anywhere. (Fig. 6.) (See also Luckiesh's
paper " Safeguarding the Eyesight of School Children, Fig. 16.)
Sometimes an attempt is made to improve the very bad conditions
produced by old-style gas fixture illumination, by the use of direct
prismatic reflectors fastened directly to the old fixtures; but this,
while efficient, was, in the case under the writer's observation, im-
properly installed as far as avoidance of glare was concerned be-
cause the old fixtures were, as they almost invariably are, too long
and the glare therefore bad. A much more satisfactory as well as
cheaper installation could have been made by eliminating many feet
of fxture stems and raising the units beyond the range of vision of
the tudents.
E\ en a newly planned school lighting system may be faulty, as
an architect's modern plan, which the writer has in mind, called for
one central direct lighting unit per classroom, whereas a vast im-
provement would be obtained by four units uniformly spaced on the
ceiling.
Fig. 7 illustrates an old building which has been equipped with an
improved system of direct lighting. As will be noted, the units are
hung 'high, and the light sources are shielded by means of deep
diffusing reflectors. The reflectors allow enough light to pass
through to light the ceiling to a low intensity, which adds to the
cheerful appearance of the room.
In Fig. 8 is shown a classroom in the Boy's High School of New
Orleans. This is an installation of indirect lighting, with few units,
giving quite uniform illumination and cutting down the objectionable
reflection from the varnished surfaces of the desks, glass or black-
boards.
A satisfactory and inexpensive indirect system of artificial lighting
may also be obtained with numerous units in a large schoolroom, as
in the study room of the high school at Lincoln, Nebraska, for in-
stance. There is an exceedingly good uniformity of illumination and
an entire absence of glare from the desks and seats in spite of the
fact that it is usually not easy to eliminate the undesirable high
brightnesses in a large room.
A schoolroom illuminated by means of a satisfactory and inexpen-
322 ILLUMINATING ENGINEERING PRACTICE
sive semi-direct system of artificial lighting is shown in Fig. 9, where
inverted opal glass reflectors of sufficient density are employed to
reduce the surface brightness of the lighting units themselves to a
comfortably low value.
In the lighting of the auditorium of the J. W. Scott High School, at
Toledo, Ohio, use is made of diffusing globes hung high on the ceiling.
The high mounting height would tend to give good distribution with
reduced glare in the eyes of the audience.
The study-room of Lincoln (Neb.) High School, Fig. 10, is illumi-
nated by a type of indirect system using numerous units. No bright
sources of light are visible as one faces the full length of room. In
other words, there is no glare from unconcealed light sources.
Compare this with Fig. 6.
The Temple Speech Room of Rugby School, Rugby, England, is
illuminated entirely by indirect lighting. The installation is espe-
cially interesting because of the use of standard American fixtures
installed in England, such installation being made on account of
the higher efficiency of the American product.
In Fig. ii is shown the newly equipped drafting room of the Mass.
Institute of Technology, where use is made of inverted translucent
reflectors. The picture was taken by daylight to show the simplicity
of detail of the units.
It may be of at least passing interest to note the illumination of
some of the special departments in a school.
The cafeteria in the Lincoln (Nebr.) High School (Fig. 12) is
lighted ,by the luminous bowl indirect system, the bowls being of
a low intrinsic brilliancy, which is particularly desirable in this
installation, as the tables used have highly glazed tops, so that any
spot of high brilliancy on or near the ceiling would cause annoyance
by specular reflection from the surface of the tables.
The Harrison Park Gymnasium, Chicago, is illuminated by a
direct lighting system, the light sources being screened from view by
deep mirrored glass reflectors with opaque green enamel backing,
placed high out of the range of vision.
In the Northwestern University Gymnasium use is made of a com-
bination lighting system. The main illumination is entirely taken
care of by direct lighting units, the lamp being set in a deep mirrored
glass reflector with opaque green enamel backing, so as not to be
visible under ordinary conditions. Each unit is also furnished with
two small reflectors furnished with 25 watt lamps, which throw the
light upward, illuminating the ceiling. Complete uniformity of
illumination and absence of glare characterize this installation.
Fig. 9. Simple semi-indirect lighting installation in schoolroom.
Pig. 10. Type of indirect system of lighting used in study room.
(Facing page 322.)
Fig. n. Simple semi-indirect lighting installation in a college drafting room.
Fig. 12. Luminous bowl, indirect illumination in school cafeteria.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 323
The Armory of the University of Illinois presents an example of
good lighting of such an area. Use is made of units of the direct
lighting type, suspended from the steel trusses supporting the roof,
equipped with deep mirrored glass reflectors with opaque green
enamel backing, so as to screen the lamps from the eye except at an
angle from which the reflector would not ordinarily be viewed, with a
resulting satisfactory clearness and uniformity of illumination of
the floor. Indirect lighting in buildings of this nature is entirely
infeasible due to the type of roof and the structural framework
beneath, although some upward light is secured from the arrange-
ment of units and the construction thereof.
PART H. LIBRARY ILLUMINATION
The actual lighting of a library reading room, as far as the produc-
tion of the required intensity is concerned, can, of course, be accom-
plished by any one of the three systems of illumination already
spoken of. Since the character of the work performed in a library,
namely, reading and studying, is the same as that performed in
schools, the arguments set forth above apply in the case of libraries,
with the exceptions already noted, with respect to the psychological
and aesthetic aspects. If localized direct lighting is used at all, in
the library, care should be used to avoid the production of conditions
of glare and specular reflection, such as are often obtained by direct
lighting on newspaper racks or reading desks, often used in libraries.
If localized lighting is used on the general reading tables, which is
sometimes quite a tempting type of installation from the standpoint
of economy of operation, certain general principles must be observed,
so that the light will be as diffuse as is possible from a direct lighting
equipment and the source of light entirely concealed from view and
the intensity of the illumination from this source only moderate.
Sometimes a combination of a moderate amount of well-engineered
localized illumination can be made with the general diffused illumi-
nation of an indirect system, as shown in Fig. 13, but it contains, as
usually installed, too large a component of direct light to be
comfortable.
In this reading room it was desired to continue to use the table
lamps which had been originally installed, so that a slightly lower
intensity of general illumination would be adequate. Hence, the
reflectors of the table units were replaced by spun metal casings,
lowered on chain supports to the proper screening height above the
table, and a proper deep type of aluminized steel reflector was so
324 ILLUMINATING ENGINEERING PRACTICE
installed as to completely hide the lamp from view. The effect
with the lamps in service, is shown in the accompanying illustra-
tion, from which it is seen that there is no glare from the direct
lighting units.
Where it is at all possible in reading rooms, the position of the
reader should be permanently determined and so arranged if direct
lighting is used, that the illumination will fall on the book from the
rear and one side, and not on his face, or on the book so as to reflect
directly into his eyes. In the above illustration the reader could sit
and read comfortably with his side to the table but practically no one
will do this, unless the chairs are permanently fastened in such a
position.
In this reference room of the above library, three central units,
together with the marginal units in the gallery, supply indirect illumi-
nation as well as the lighting for the gallery book stacks.
The fixtures in this installation are intended to harmonize with the
architectural features. Of course, a simpler installation can be and
is made in a less pretentious reading room in the same library, Fig.
14, with due regard to the utilitarianism and harmony of design.
In this room the illumination is augmented by certain sky-lighted
portions, with both natural and artificial light. The ceiling windows
are small, however. This view shows the manner in which the
book stacks on the upper mezzanine are illuminated by the same
opaque indirect lighting units as are used for general illumination,
and the manner of lighting the book stacks on the lower mezzanine
by indirect units hung directly under the upper mezzanine floor,
which, while made of marble, has been painted matte white to
produce the proper reflecting surfaces.
In this view also is seen, to the right and to the left of the picture,
an adaption of the indirect bracket unit illuminating passageways
into the room. The steel half bowls are made for utilitarian purposes
primarily, and are part of the steel book stacks, special bracket
type mirrored glass reflectors being used. At the present time,
newspaper racks have been installed along the sides of the passage-
way under the indirect brackets. Note the entire absence of desk
lamps.
In another portion of the same library book stacks are illuminated
from indirect units, consisting of mirrored glass reflectors with green
opaque enameled backing placed on top of the book stacks and
throwing the light directly on the ceiling above, no fixtures of any
kind being installed. (See Fig. 15.)
VAUGHN: LIGHTING OF SCHOOLS, ETC. 325
Fig. 1 6 shows a newspaper reading room with newspaper racks as
well as tables for magazine use, illuminated by a simple indirect
system.
Next to the school room, perhaps, conservation of children's
eyesight can be best accomplished in the Children's Rooms of the
public libraries, and such a room in the Milwaukee Public Library
has been indirectly illuminated, in which library the last four ex-
amples also exist.
Improvements have recently been made in the illumination of the
Congressional Library at Washington. Fig. 17 represents the illumi-
nation of some of the book stacks in this library by means of "scoop-
ette" direct mirrored glass reflectors. In this library, the room in
which the paper racks are located has also been recently illuminated
by the indirect systen.
An example of semi-indirect illumination exists in the Toronto
public library, where the art collection is illuminated by means of
medium density opal bowls.
The large reading room of the John Hay Memorial Library at
Providence, R. I., Fig. 18, is lighted entirely by the opaque bowl
indirect system, the fixtures being unique in their artistic design
and in the fact that a lower section, or sub-structure, has been
worked into the design so as to contain light sources to illuminate
the outside of the main bowl directly above.
One of the important departments of the public library is the
catalogue room,. and the facility of properly illuminating the card
files is of great value. In the Milwaukee public library the catalogue
card indexes are illuminated by an indirect system installed on a very
elaborate and deeply cut ceiling, and the diffuse illumination from
this system gives a most satisfactory vertical component for use on
the cards in the files.
One of the important and ever-growing activities of the public
library in large cities is the establishment of branches in the outer
portions of the city, where people can be reached by the library if the
down town library cannot, or will not, be reached by the people. An
example of the illumination of a branch of this character is illustrated
in Fig. 19, showing the Austin Branch Library in Chicago, lighted by
a large number of indirect fixtures, producing maximum diffusion and
uniformity of illumination.
Not all branches of a library can be as well equipped as this, how-
ever, for the primary object of such branches is to reach a certain
locality and class of people, and it is often more utilitarian and
326 ILLUMINATING ENGINEERING PRACTICE
successful to establish more branches of less pretentions in vacated
store buildings or other suitable quarters. An example of the latter
sort of branch library may be found in Milwaukee's East Side
Branch, located in a store building, with portable book stacks and
unpretentious, though properly equipped illumination system, con-
sisting of three steel bowl type of indirect units, spaced down the
center of the room.
PART HI. AUDITORIUM ILLUMINATION
In taking up the description of the third division of this subject,
namely, auditoriums, it is thought best to divide it into sections,
such as churches, lodges, theatres, concert and lecture halls. From
an engineering standpoint, many more large interiors for public
gatherings might be included, but a narrower scope has been arbi-
trarily selected as sufficiently illustrative.
CHURCHES
The present tendency toward liberality in religion seems to be con-
ducive to liberality in illumination, and well-lighted churches to-day
are so ordinary an institution as to make it difficult to restrict the
number of illustrations exemplifying this section.
Historically, Fig. 20 represents one of the first attempts to light the
auditorium of a large church by practically a single indirect fixture,
and shows the Eighth Church of Christ, Scientist, of Chicago, in
which, except for the alcoves, the entire auditorium is so illuminated.
Another historical indirect installation is the North Chicago
Hebrew Congregational Temple, where a combination of indirect
illumination from suspended fixtures and from reflectors concealed
in coves is utilized. In the coves are reflectors, lighting the arched
central portion of the ceiling, while suspended indirect units are hung
from the flat ceilings at the sides.
Fig. 2 1 shows another early indirect church installation, the fixture
design of which should be contrasted with some of these following.
The church as a structure lends itself, perhaps, more readily to
architectural development than any other of the edifices with which
we are dealing, and for that reason considerably more attention
should be paid to the harmonious design of the lighting equipment
installed than to that used for other purposes. In some of the
following figures, particular attention is directed to the efforts made
to select and design fixtures which will produce harmonious results.
Fig. 13. Indirect lighting in library reference room with specially designed direct
table lamps.
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Fig. 15. Indirect lighting of book stacks without the use of fixtures.
Fig. 16. Indirect illumination in newspaper and magazine room of a public library.
Fig. 17. Direct lighting of library book stacks.
Fig. 1 8. Library reading roc
ited by the indirect system.
(Facing Figs. 15 and 16.)
Fig. 19. Indirect illumination in a branch library in a large city.
Fig. 20. Church illuminated principally by means of large central lighting unit.
21. Early indirect church lighting installation.
Fig. 22. Opaque indirect church fixture designed to harmonize with central ceiling
ornament.
(Facing Figs. 19 and 20.)
Fig. 27. Cove indirect illumination in church.
Fig. 28. Installation of direct lighting to harmonize with Gothic architecture.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 327
Fig. 22 shows an indirect installation in the Everhardt Memorial
Church, Mishawaka, Ind., where the main fixture is hung from a cen-
tral ceiling ornament and with some regard for the harmony between
ornament and fixture; this installation differing radically from the
Christian Science Church installation, where the fixture is suspended
in the center of a large dome, as well as from that represented in
Fig. 21.
Fig. 23 shows a church auditorium (Timothy Eaton Memorial
Church of Toronto, Canada) illuminated by specially designed
luminous bowl indirect fixtures, the translucent portions of the
bowl being illuminated by a small frosted lamp, the light for this
purpose seeping through tan silk panels in the sides and bottom of
the bowl.
An example of the use of a semi-indirect installation in a religious
auditorium is shown in Fig. 24, where medium density opal bowls are
suspended from the ceiling in two rows down the auditorium. As
stated earlier, an installation of this character with bowls of suffi-
ciently moderate brightness and hung sufficiently high, may be
made successful.
In certain cases, the architect may feel that a large fixture, sus-
pended in the center of a dome, due to the eccentricity of view from
almost every point in the auditorium, is aesthetically wrong for his
structure, and yet a diffused type of illumination may be desirable.
A solution of such a problem is illustrated in Fig. 25 where a special
diffusing glass ceiling window is constructed in such a manner as
to give practically the same diffusion to the light passing through
from the lamps above as with an indirect system, and this view of
the Second Church of Christ, Scientist, in Milwaukee, illustrates how
the architect and engineers obtained an artistic luminous "sky-
lighted" dome with satisfactory illumination results. By the selec-
tion of the glass and placing of lamps, almost perfect diffusion com-
bined with high efficiency was obtained.
Fig. 26 shows the arrangement of the lamps above the ceiling win-
dow and is illustrative of the use of the bare lamps in a diffusely
reflecting enclosure, which in this case, is the attic structure of the
church. By painting the attic the proper white, this space is used as
a huge diffusing reflector impinging the light on the window in a
very diffused condition, which, with the high diffusive character-
istics and low absorption of the selected glass, is so distributed as to
produce practically perfect diffusion in the auditorium without the
presence of the light sources being visible from below, thus producing
328 ILLUMINATING ENGINEERING PRACTICE
very closely the same effect as would be obtained from the illumi-
nation of the ceiling of a dome beneath by an indirect fixture, but
avoiding the presence of the fixture.
"Sky-lighting" of this character is often done by means of the
lamps above the glass structure being placed in mirrored glass or
steel-enameled reflectors, which throw more or less concentrated
light upon the glass. This may cause more difficulty in securing
diffusion of the illumination beneath, and may also make the light
sources more visible, especially if placed too close to the glass.
Indirect illumination can also be obtained from lighting equipment
and reflectors placed in coves at the spring of the arched ceilings, as
illustrated in Fig. 27, representing St. Cyril's Roman Catholic
Church, Chicago, where mirrored glass reflectors are thus installed.
An example of the use of the combination of direct and cove light-
ing exists in the illumination of the St. Helena Cathedral, Helena,
Mont. Here are used "beehive" reflectors in the main ceiling
arches, "hood" reflectors in the upper column capitals, and "mid-
get" reflectors (all mirrored glass) in the lower column capitals.
As intimated earlier in the discussion, for architectural or artistic
reasons, the structure or the architect may require that the upper
portions of the auditorium remain subdued in tone, and direct light-
* ing fixtures may therefore have to be designed which will at once
comply with the aesthetic requirements and at the same time mini-
mize the glare which would be bound to be present, especially from
the galleries, if not thoroughly provided against in such an installa-
tion. Fig. 28 shows the Plymouth Church in Milwaukee, where
the use of thoroughly diffusing selected glass in a lantern type of
fixture designed by the architect to harmonize with the English
Gothic architecture seems to suit the conditions by the use of a direct
lighting system. In these lanterns, a downward directing prismatic
reflector is used, and the lantern is made luminous by an additional
small lamp.
Another installation of like character is that of the First M. E.
Church, Evanston, 111., with the addition of side-wall lanterns. In
the units in this case, three mirrored glass reflectors are used in
combination with three prismatic glass reflectors, the combination
producing the desired distribution as well as the moderate luminosity
of the sides of the unit.
If for any reason, architectural or otherwise, it is desirable to
illuminate a church from floor standards, it can be done by the in-
direct system, providing the ceilings are suitable, by the use of the
VAUGHN: LIGHTING OF SCHOOLS, ETC. 329
type of floor standard shown in Fig. 29, which indicates such an
installation in the entrance of the First Church of Oak Park, HI.
LODGE ROOMS
In the lodge room, a semi-religious function must be considered,
and this division of the subject therefore is closely allied with the
illumination of the church.
As an example of the lodge room recently lighted by the indirect
system, there is shown in Fig. 30 the Masonic Hall in the Auditorium
Hotel in Chicago. This hall is lighted by the opaque indirect units
on the ceiling, and also by lamps in the cove under the pictures and
back of the windows. Due to the ritual work done in the hall, the
lamps are so arranged that the light intensity may be lowered to a
very low degree, or a certain portion of the lamps may be cut out
entirely, as, for instance, the ceiling lamps, leaving a dim illumin-
ation from only the cove lamps.
Fig. 31 shows the Boulevard Masonic Hall of Chicago which has
been lighted entirely by means of side-wall indirect brackets, which
are in the form of decorative boxes with vines, etc., giving the effect
of growing plants. Each box contains a number of mirrored glass
reflectors so designed as to throw the maximum amount of light onto
the ceiling, with a very small amount of so-called "wall-splash."
This lighting has been so arranged that it is possible to obtain three
different intensities besides a very dim lighting for ritual work.
Fig. 32 shows the lodge room of the Knights of Columbus, Mil-
waukee, Wisconsin. The lighting of this room was accomplished
entirely by luminous bowl indirect lighting units, using bowls with
glass panels, enough light being permitted to pass through the glass
to give them sufficient illumination not to appear dark. For ritual
work only the two center units are used, these being so arranged as
to furnish any colored light desired for such services. Each unit
contains red, blue, green and white colored light sources, each
color being controlled by a separate dimmer. By means of this color
mixing facility, it is possible to form any combination desired, and
to allow for the changing of the mixture from shade to shade, grad-
ually or quickly, as desirable.
Fig. 33, as an example of semi-indirectly illuminated lodge rooms,
shows the lodge rooms of No. i Masonic Temple, Washington, D. C.
The photograph gives no accurate conception of the actual illumina-
tion, since it was made by daylight and not by the use of the lighting
units themselves.
330 ILLUMINATING ENGINEERING PRACTICE
THEATRES
Before inspecting some of the modernly illuminated theatre audi-
toriums, it may be of interest, again, to take a backward step of a
few hundred years, and review historically the development of this
problem and its solutions.
Of historical interest is the English theatre of 300 years ago,
where the acting was done in the open air and the theatre was a
U-shaped structure with the audience on the two sides in two or
more storied amphitheatre resembling primitive "bleachers." In
this type most of the acting took place under the open sky where
the actors were surrounded on two sides by the audience. These
performances all took place in broad daylight, and hence required
no artificial illumination.
In the drama of the days of Shakespeare's boyhood, the actors
made use of little scenery and not much costuming or make-up, the
performance being given in the courtyard of a village inn as after-
noon matinees. The courtyards with their galleries formed auto-
matically a theatre somewhat similar to that just described, per-
formances again being given by daylight.
Coming now to this country, and a later date, the old John Street
Theatre, New York, which was opened in 1767, is of interest. The
auditorium and stage were of most primitive type and the very
earliest type of lighting known for an auditorium of this character
was used. The view of the interior of this theatre, which is on
record, indicates that there were two four-arm chandeliers near the
stage, each arm apparently carrying a kerosene lamp or candle, the
whole design being most primitive and simple. Nevertheless, con-
siderable publicity was given at that time to the successful accom-
plishment of the illumination of this theatre.
The old theatres of Europe present a few interesting features with
respect to the illumination and their general architectural design.
Fig. 34 is typical of the old type of auditorium lighting, which is
typified by a large elaborate central fixture supporting bare incan-
descent lamps in conjunction with ceiling studded effects and a
tremendous number of bracket lamps lavishly distributed over all
balconies and in the direct range of vision.
Turning to our country, probably the best known and surely the
most deserving of fame, is the Metropolitan Opera House in New
York (built in 1883). The lighting of this opera house carries out
the same general scheme as those in Europe, namely, the large
VAUGHN: LIGHTING OF SCHOOLS, ETC. 331
center fixture with its semi-protected lamps and its rows of lamps
around the fronts of the balconies and boxes all of which gives a
very hurtful illumination for spectators, except for those seated in
the body of the house and far enough forward to probably escape
the glare of the side lamps.
Later on, a movement, perhaps unwittingly made in the right
direction, placed the light sources directly on the ceiling and beams
in studded effects, which, if the architecture happened to be appro-
priate, took the sources out of the. range of vision; but the more
or less ignorant use of this type of illumination produced in many
cases even worse effects than the central fixture type.
The Auditorium Theatre in Chicago, shown in Fig. 35, is typical
of the bad examples of this type. This needs very little description.
The bare lamps in the arches indicate for themselves the effect on
the eyes of the spectators.
Fig. 36 shows the Auditorium in Milwaukee taken by artificial
illumination. The lighting underneath the top balcony is accom-
plished by means of lo-in. sand-blasted globes, while the main portion
of the hall is lighted by rows of hundreds of bare lamps, around the
center ceiling window and sand-blasted shades between the various
arches. The effect of glare on the audience is very bad indeed,
as it is almost impossible for a person to sit in this long hall without
having glaring lamps in his direct vision, the worst effect being experi-
enced by those in the first and second balconies, and in the boxes,
in which cases the eye receives the full glare of the center lamps.
Recently some changes have been made by the installation of smaller
and denser globes under the balconies, which eliminates the units
from vision. It is also contemplated to modernize the entire light-
ing scheme in this auditorium in the near future.
One of the earliest scientific attempts to eliminate light sources
from the view of the audience was in the University of Wisconsin
lecture room, where the lamps are placed in reflecting troughs on the
stage side of the beams, with very satisfactory results as to glare
from the viewpoint of the audience, but, of course, with the maximum
of bad effect to those on the stage facing the audience.
In another theatre of national fame, the Hippodrome of New
York, the lighting is accomplished by lamps carried along various %
construction members of the ceiling and also over the proscenium
arch. Here again the glare is bad, but a large portion of the audi-
ence is protected on account of the immensity of the auditorium.
Sometimes in the theatrical audience there seems to be consider-
332 ILLUMINATING ENGINEERING PRACTICE
?
able desire and possibly some excuse for ignoring almost entirely
the engineering and economic side of this question and producing
artistic results primarily, if not entirely. As an appropriate example
of a theatre where the fixtures have been made highly artistic with
comparatively little regard for other factors in the problem, Fig.
37 is presented, showing the Little Theatre of New York, with its
beautifully artistic interior.
A somewhat similar, foreign example of the use of highly artistic
fixtures is the Audience Room of the Royal Palace, Madrid. Here,
however, the most interesting feature is the use of the Moore
vapor tube type of system on the ceiling.
In later years greater efforts have been made to conceal the
light sources from the view of the audience and to produce soft,
though sufficient, illumination effects, at least, in certain types
of theatrical auditoriums. This latter movement has been toward
the indirect system of illumination, and the following are some
examples of this type.
One of the first, if not the first, theatre to be totally illuminated by
the indirect system was the Pabst Theatre of Milwaukee. This
theatre was originally lighted by a large central fixture, bare studded
lamps around the domed ceiling, bare lamps along the fronts of the
balconies, and also, bare lamps around the proscenium arch. Due to
the annoyance on account of the large amount of glare from the bare
lamps, this installation was finally superseded by a complete in-
direct lighting system. Use was made of a large central unit hang-
ing in the center of the dome augmented, underneath the various
balconies, by smaller units, mainly for artistic influence. The
theatre walls being dark in color and the furnishings of the same
color, it was necessary, in order to make indirect lighting a success,
to provide a means of redirecting the light from the ceiling down on
the working plane. This was accomplished by means of false
ceiling, or reflecting discs, painted a very light ivory, which were
placed above the various units. At the same time the dome was
redecorated in a lighter shade. Fig. 38 shows the central fixture
and its reflecting disc.
Fig. 39 shows an auditorium lighted by means of opaque indirect
.fixtures, typical of hundreds similar now in existence, where there are
no bare lamps directly in the range of vision of the audience.
In the Germania Theatre in Chicago, an unconventional system
of indirect lighting is used by placing the lighting units in specially
designed boxes on the side-walls (Fig. 40), carrying out certain
Fig. 29. Indirect illumination of church lobby by means of floor standards.
Fig. 30. Lodge room lighted by opaque indirect units and auxiliary special cove units.
(Facing page 332.)
Fig. 31. Lodge room illuminated by special design of indirect wall brackets.
Fig. 32. Lodge room illuminated by luminous-bowl indirect units.
p; g> 33 . Lodge room with semi-indirect lighting units.
Fig. 34. Early foreign type of direct lighting of theatres.
(Facing Figs. 31 and 32.)
Fig. 35- Typical bare lamp studded effect.
Fig. 36 Glaring lamps in auditorium lighting.
Fig. 37- Artistic arrangement of theatre lighting installation.
Fig. 38. Early indirect lighting installation in theatre.
(Facing Figs. 35 and 36.)
Fig. 39. Auditorium illumination by opaque indirect units.
Fig. 40. Theatre illuminated from indirewall-bracket units.
> A ' -N IT' >CJ. -^V
AMd
Fig. 41. Theatre illuminated by cove-type of indirect installation.
Fig. 42. Theatre illuminated by luminous-bowl type of indirect unit.
(Facing Figs. 39 and 40.)
Fig. 43. Theatre illuminated by artificial "skylight."
Fig. 44. Theatre illuminated without fixtures; units situated on top of ventilating registers.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 333
artistic effects as well as eliminating the use of fixtures from the
ceiling.
Another variation of the indirect system is shown in Fig. 41 of the
Wilmette Theatre, which is an example of cove indirect lighting from
trough reflectors place in the cornices of the room.
Perhaps the latest example of the indirect illumination of a theatre
is the Palace Theatre in Milwaukee, which has just been opened,
where the main auditorium, Fig. 42, is illuminated from a central
unit which has the luminous bowl characteristics through decorative
panels in the fixture.
The Rialto Theatre in New York is perhaps the latest and most
elaborate example of indirect lighting effects in a theatre. Besides
the idea of lighting by means of indirect illumination, many colored
schemes, decorative and flooding, are utilized very effectively, and
it is to be regretted that an adequate illustration of these effects
cannot be presented.
Auditoriums of photoplay theatres present a condition differing
somewhat from that in the auditoriums of the legitimate theatre, in
that sufficient light has to be furnished to permit the audience to find
their way about, and yet at the same time, it must be of a low
enough intensity so as not to interfere with the picture being shown
upon the screen. Again, the intensities of different parts of the
house may be materially different, since the surface most vitally im-
portant is the screen at the front of the theatre, and it is usually
possible to raise the illumination in the rear or entrance of the the-
atre, away from the screen, to a much higher intensity of illumina-
tion than toward the screen. In this way, a person entering is not
at first subjected to as low an intensity of illumination as he is after
having passed down toward the front of the theatre, and the .few
moments' lapse between the time of entering and the time of reach-
ing an area of the lower illumination gives the eye a certain amount
of time in which it can accustom itself to the lower illumination. A
second requirement of this type of lighting is to have the greatest
amount of illumination thrown upon the horizontal plane namely,
the seats and aisles. It is poor practice to throw any amount of
light on the side-walls, due to the effect upon the screen, and such
light is practically wasted, since it serves no utilitarian purpose. A
third point which has to be considered is the absence of all sources of
light from the field of vision, such as bracket lamps along the side
walls, or lamps on either side of the screen. Such lamps only tend
to disturb the eye and cause a diversion which distracts the atten-
334 ILLUMINATING ENGINEERING PRACTICE
tion from the picture on the screen. Several of the illustrations
already shown were examples of this type of auditorium.
In Fig. 43 is shown the auditorium of the Delft Theatre, Lscanaba,
Mich. This auditorium is used for both the legitimate theatre and
for photo-play productions. The lighting is effected solely by means
of windows in the ceiling, as shown. Above these windows are long
boxes, approximately 18 in. in height, painted white inside, which act
as diffuse reflectors, throwing the light through the windows into
the auditorium. The glass used gives very good diffusion and
efficiency. The lamps are arranged on three separate circuits, allow-
ing the use of full intensity, a secondary intensity, or a very low in-
tensity for photoplay work. The last, or lowest intensity, has been
so graded by the use of different size lamps as to furnish a very luw
illumination near the front of the theatre, but a higher illumina
toward the rear. . This type of lighting directs the greater percent-
age of the light directly to the seats and aisles.
In the Butterfly photoplay theatre of Milwaukee was made one of
the first attempts at indirectly illuminating an entire photoplay
theatre. The light here, again, is of two intensities either a full
intensity which is used at the end of the program, or a very low
graded intensity, which is on during the presentation of the films.
The lighting is accomplished entirely by the opaque indirect
units.
In Fig. 44 is shown a third photoplay auditorium; this being the
Merrill Theatre of Milwaukee. The lighting system employed here
is different from the other two shown, in that there are no fixtures on
the ceiling, for the indirect lighting. The light is thrown on the
ceiling from recesses in the side-walls. Use is made of mir-
rored glass reflectors set at such an angle as to give approximately
uniform illumination on the ceiling. There are in this installation
three different lighting schemes the first gives a comparatively high
intensity of illumination, which is used at the end of the program; the
second intensity is a very low, graded intensity, which is used during
the presentation of the films; the third intensity is still lower, use
being made of a blue light in place of the ordinary light of the vacuum
lamps, when blue tinted films are used on the screen for night scenes.
The lighting in the rear of the auditorium, under the balcony, is
accomplished by ceiling windows of the same general type as those
of the Delft Theatre previously shown such lighting being con-
sidered preferable to the effect produced by hanging fixtures, which
would give a cluttered appearance under the balcony.
VAUGHN: LIGHTING OF SCHOOLS, ETC. 335
CONCLUSION
t
-As stated at the outset, this subject is so broad and so much good
exemplary work has been done, that one might go on indefinitely
describing interesting installations, but a sufficient number, it is
believed, have been shown to indicate the progress in this branch of
the arfe of illumination, and to exemplify the different types of in-
stallations which have been developed to meet the various con-
ditions surrounding the specific problems.
' f
THE LIGHTING OF FACTORIES, MILLS AND WORKSHOPS
BY C. E. CLEWELL
In May, 1910, Prof. Chas. F. Scott, Sheffield Scientific School of
Yale University, in an editorial in the Electric Journal, made an
analysis of the costs of factory lighting in terms of wages, thus
emphasizing a new point of view in the consideration of industrial
lighting. In the years following, it has become quite common to
evaluate factory lighting costs to an equivalent proportion of the
wages of the employees who use the light, as one of the best ways of
expressing the advantages of good light in factory work.
RELATIONS OF ADEQUATE LIGHTING TO FACTORY PRODUCTION
Any factory executive or manager should take an interest in those
factors which may influence, for good, the production rate of his
plant, provided the matter is presented to him in a convincing man-
ner; and he will be found, in many cases, to accept as a working
basis for the value of the best lighting to his plant the return in
quantity and quality in production resulting directly or indirectly
from the expenditure for a modern system of lighting to replace an
old and an inadequate system.
The value of adequate factory lighting may thus be reduced in a
simple manner to such items as the time it saves the employees in
the performance of their regular work, the improved accuracy it
makes possible in workmanship, the protection and safeguarding of
the eyes of the workmen, the beneficial e/ect of bright and cheerful
surroundings on the temperament of those affected, and the tendency
it has to reduce accident hazard.
If, therefore, in summarizing the advantages of good factory
lighting, in contrast to inferior lighting conditions, the cost of im-
proved light be evaluated to the equivalent time saved the employees
in the general run of their work, it will be found that the wages thus
saved are usually materially greater than the cost of the lighting,
and the net saving to the plant, either through reduced wages for
the same output, or in larger and better output for given wages,
due to improved lighting, is just as definite and important an asset
22 337
338 ILLUMINATING ENGINEERING PRACTICE
to the plant as is a new machine tool which, due to its higher efficiency
in contrast to an older machine, is capable of effecting a similar
economy.
As a starting point, therefore, it is desirable to assume toward
adequate factory lighting an attitude of such a nature as to class it
as one of the economies in industrial management; and, rather than
to place too much emphasis on the cost of the different available
types of lamps or on the various systems of lighting, to concentrate
the major part of the attention on the improved quality and quan-
tity of workmanship which may be expected to accompany better
lighting. In brief, it is well to think of lighting as an asset to the
plant, and, when deciding on the type of lamp to install, to consider
which type is best suited to the needs of the factory, rather than to
direct all attention, as is so often the case, on those relatively small
differences in first cost, which sometimes lead to a selection of the
cheapest rather than the best.
As a matter of fact, the past five or ten years have witnessed wide-
spread improvements in many factories where the prevailing former
conditions were very poor, and a typical factory manager of to-day,
whose sections are equipped with modern lighting, is able to take a
certain pride in the improved appearance of the surroundings, and
at the same time he has the assurance that the accompanying im-
proved workmanship and sentiment of his employees, represent
material returns in excess of any outlay he may have been called
upon to make for the improvements in question.
As obvious as these indirect advantages may seem to be, they are
not as satisfying, nor are they as useful in the practical advancement
of better lighting conditions in the industries, as would be the case
were there more definite examples of cash returns available due to
improved light, or were there on record actual numerical percentages
of increases in output due to the same cause. The need for such
definite information is made evident in a statement by Dean A. J.
Rowland, in a discussion on the subject of factory lighting several
years ago, part of which follows: 1
"There is one very important detail of industrial lighting which seems
to have been given but little attention by anyone; that is, the accumulation
of data which will give the answer to this question: Is it or is it not worth
while to light rooms and machinery correctly and well?
"Such questions are as important as any which can be considered in
connection with industrial lighting. The kind of lamps used, their
1 Trans. I. E. S., vol. VIII, No. 6, pp. 286 and 287.
CLEWELL: LIGHTING OF FACTORIES, ETC. 339
arrangement, the kind of shades put on them, are insignificant matters
compared with the money value of good light to the industries. This
will have to be determined somehow if industrial lighting is to come into
its own."
This quotation from Dean Rowland's discussion is merely given
as typical of the impression which prevails that such data are badly
needed, and while this need is generally recognized, the data desired
are very difficult to obtain, and several quotations from a number
of authorities must be taken at this time roughly to indicate the
available information on this general phase of the problem. 2 It will
be noted that some of these quotations refer to advantages of good
light and the disadvantages of bad light, based on features other than
those of economy. In a general way, however, they bear directly
on the important question as to "Why good light is a necessity?"
As an example, the first report of the Departmental Committee on
Lighting in Factories and Workshops (London, 1915) contains several
comments as follows : 3
"Complaints of eye strain, headache, etc., attributed to insufficient
lighting are common, and while an exhaustive medical inquiry would be
necessary to establish the connection between these defects and inadequate
lighting, there is a general impression that unsatisfactory lighting is, in
various ways, prejudicial to health. It is also recognized that insufficient
light adds to the difficulty of the proper supervision of work, and of the
maintenance of cleanliness and sanitary conditions generally.
"Witnesses gave specific instances of the effect of improved lighting in
increasing the output and improving the quality of work turned out."
Again, in the same report, the following statement appears con-
cerning the diminished output of work due to insufficient light: 4
"The effect of improved lighting in increasing both the quantity and
the quality of the work is generally admitted, and specific instances are
quoted in the evidence. In one instance the output was diminished 12 to
20 per cent, during the hours of artificial lighting, and in another the earn-
ings of the workers increased 11.4 per cent, after the installation of a
better system of lighting."
A clause from one of the Public Health Bulletins of the United
States Health Service 5 presents the case from a somewhat different
point of view, as follows:
2 An experimental investigation is under way at this time to secure definite information
concerning the advantages of good factory lighting, the work being planned by the Lighting
Committee of the Commonwealth Edison Company of Chicago.
3 Memorandum of British Report, p. 2.
4 Main part of British Report, p. xiii.
* No. 71, May, 1915. p. 105, J. W. Schereschewsky and D. H. Tuck.
340 ILLUMINATING ENGINEERING PRACTICE
"In view of the fact that a large part of the industrial operations in the
women's garment trades involve the close and continuous use of the eyes,
the illuminating conditions which prevail in the workshops of the industry
become highly important from the standpoint of industrial hygiene. The
necessity for adequate and correct illumination on the various working
planes becomes the more apparent from the consideration of the data in
relation to the vision of garment workers contained in the foregoing portion
of this report. These data show that only a little over 25 per cent, of the
workers whose visual acuity was tested had normal ision in both eyes."
Turning now to somewhat more tangible wage equivalents, several
good examples are found in a discussion on factory lighting by M. H.
Flexner and A. O. Dicker, 6 one of which may be summarized as in
Table I.
TABLE I
Under the assumption that good factory lighting requires a loo-watt tungsten
lamp for each 100 sq. ft. of working area, and that one workman occupies each
100 sq. ft., the following statements may be made:
Constants:
Working hours per annum (10 X 300) 3,ooo hours
Lighting hours per annum (3^ X 300) 1,000 hours
Labor cost per hour 35 cents
Labor cost:
3,000 hours at 35 cents $1,050.00
Lighting cost:
Lamp (free renewals) $0.00
Reflector i . oo
Wiring 4 . oo
First cost
Initial cost per outlet $5 . oo
f Interest at 6 per cent $o . 30
I Depreciation at 12^ per cent 0.63
Operation \ Annual cleaning at 3 cents 0.36
Lamp renewals o.oo
100 kw-hr. at 5 cents 5 . oo
Annual operation cost $6 . 29
Conclusions:
Cost of light in per cent, of wages o. 60
Cost of light per hour $o . 006
Cost of labor per hour ' 0.35
Cost of light per day 0.02
Cost of labor per day 3 . 50
These data show that the cost of good lighting is a very small proportion of
the value of a man's time; in fact, if good lighting effects a saving of five minutes
of a man's time per day, a material gain would be experienced.
Trans. I. E. S., vol. VIII, No. 8, pp. 477 and 478.
Fig. i. Tungsten direct lighting with opaque mirrored glass reflectors.
Fig. 2. Drafting room with a system of semi-indirect tungsten lighting.
(Facing page 340.)
Fig. 3. System of industrial indirect lighting with tungsten lamps.
4. Machine shop with a system of mercury vapor lamps.
CLEWELL: LIGHTING OF FACTORIES, ETC. 341
A similar example worked out in a slightly different manner is
given in the recent Code of Factory Lighting of the Illuminating
Engineering Society, as follows: 7
"From the manufacturer's point of view, the cost of the annual operation
and maintenance of the illumination of a typical factory bay of 640 sq. ft.
area, may be taken at $50.00 under certain assumptions as to energy cost,
cleaning, interest and depreciation. If five workmen are employed in
such a bay at an average of say 25 cents per hour, the gross wages of the
men in such a bay, plus the cost of superintendence and indirect factory
expense, may equal from $5000 to $7000 per annum.
"In a case of this kind, therefore, the lighting will cost from 0.7 to i.o per
cent, of the wages, or the equivalent of less than the wages for from 4 to 6
minutes per day. We -may roughly say that a poor lighting system will
cost at least one-half this amount (sometimes even more through the use
of inefficient types of and a poor arrangement of lamps), or the equivalent
of the wages for from 2 to 3 minutes per day. Nearly all factories and
mills have at least some artificial light, hence, in general, if good light
enables a man to do better or more work to the extent of from 2 to 3
minutes per day, the installation of good lighting will easily pay for the
difference between good and bad light, through the time saved for the
workmen."
The foregoing discussions and quotations are typical of the view-
points which have been assumed in recent years toward the field of
industrial lighting, on which the following advantages of good light
may be based:
1 . Increased production for the same labor cost.
2. Greater accuracy in workmanship.
3. Reduced accident hazard.
4. Avoidance or at least reduction of eye strain.
5. Surroundings made more cheerful.
6. Work performed with less fatigue.
7. Order, neatness and sanitation promoted.
8. Superintendence rendered more effective.
In other words, factory lighting is important to production; its cost is
small in comparison with its advantages; and when its cost is interpreted
or reduced into the equivalent wages saved through its adoption, the
expenditure for the best lighting is usually a very small item by contrast.
SUMMARY OF FACTORY LIGHTING LEGISLATION
As pointed out in a recent paper 8 by L. B. Marks, legislation on
lighting is so meagre and scattered and apparently so little called for
7 Issued by the I. E. S. in 1915. Quotation from pp. 14 and 15.
Trans. I. E. S., No. i, 1916.
342
ILLUMINATING ENGINEERING PRACTICE
in this country that no legislative bureau has found it worth while to
collate it., Mr. Marks points out that five years ago (1911), based on
an extended review of factory legislation under the direction of
Prof. John R. Commons, 9 and reported by E. L. Elliot, 10 there were
only eleven states that made any mention of the subject of light in
their general factory or labor laws, and in not one of these were
the provisions sufficiently specific to render them of practical value.
Since that time, factory lighting legislation has received attention
in several states, the most prominent cases being Wisconsin, New
York and Pennsylvania. Briefly stated, the legislation in both Wis-
consin and New York, while a step forward in each case, has been
rather indefinite, mainly because in neither case are the requirements
specific with regard to the illumination at the point of work.
Following the legislation in these two states, two important new
developments have been made in this direction. The one has been
the publication in 1915 of a Code 11 for the lighting of factories, mills
and other work places by the Illuminating Engineering Society, as a
TABLE II
(Intensity Values in Foot-candles)
Classification of space
Clewell'*
(1913)
6?
H*
O
Wisconsin 1 *
(1914)
~v
*d
8
c/5 gi
w"
British report 16
(I9IS)
Pennsyl-
vania 14 (1916)
General lighting of work
rooms, irrespective of the
actual light required by
the work
o 80
O 2 ^
Yards. . . .
o 05
One
Stairways, passages, stor-
age, and the like
Foundries
0.50
0.50
3 OO
I CQ
0.25
I 2"?
O. IO
O 4.O
0.25
I 2?
Rough manufacturing. ....
Fine manufacturing
3-oo
5.00
2.00
5 .OO
0-75
I . "?O
1.25
2 CQ
1.25
2 CQ
American Legislative Review, vol. I, No. 2, June, 1911.
Trans. I. E. S., 1911, p. 722.
"Code of Lighting, Factories, Mills and other Work Places," issued by the Illuminat-
ing Engineering Society, Trans. I. E. S., vol. X, Nov. 20, 1915, pp. 605-641.
"Factory Lighting," N. Y., McGraw-Hill Book Co., 1913.
"Handbook on Incandescent Lamp Illumination," General Electric Co., 1913.
Intensities estimated on basis of specifications of candle-power per sq. ft.
Minimum requirements.
No recommendation is made for the illumination required for the work. Intensities
here listed are specified as minimum values.
CLEWELL: LIGHTING OF FACTORIES, ETC. 343
result of the work of several of its committees; and the other, the first
report of the Departmental Committee on Lighting in Factories and
Workshops, issued also in 1915 in London.
Following the completion of the I. E. S. Code, representatives of
the labor departments in Pennsylvania and New Jersey met with a
committee of the I. E. S., and on June i, 1916, as a result of these
conferences, the department of labor and industry in Pennsylvania,
John Price Jackson, Commissioner, adopted the I. E. S. Code in
slightly modified form. Pennsylvania, by this action, becomes the
first state in this country on record to adopt a factory lighting code
based on definite intensities of illumination on the work.
As a basis for study and comparison, Table II has been compiled.
This Table shows at a glance the various illumination intensity
requirements of certain states; those of the I. E. S. Code and of the
British Report; and the recommended intensities of certain authori-
ties. It is to be noted that the intensities placed under Wisconsin
have been worked up as the probable intensities equivalent to the
requirements of that state corresponding to candle-power per sq. ft.
specifications, and under the assumption that an overhead tungsten
system of lighting with efficient reflectors is used. No actual illu-
mination intensities are specified in the Wisconsin orders.
TYPES OF LAMPS AVAILABLE
The types of lamps available for factory lighting at the present
time include the various electric filament lamps, namely, the carbon
incandescent, the metallized filament, the tungsten or "Mazda"
vacuum and the Mazda gas-filled units; the various types of mantle
gas lamps; the mercury vapor glass- tube and quartz- tube units; and
the various types of electric arc lamps, namely, the enclosed carbon,
metallic flame (or magnetite), and the flame units. 17
Of these, the most widely used types in modern lighting systems,
and, in general, the most practical types for industrial service, are the
Mazda and the mercury-vapor electric lamps, and the mantle gas
lamps. Due to the very wide range of sizes in the Mazda units, there
is practically no class of factory location for which one or another of
the Mazda lamps may not be selected and used with excellent results.
This fact has brought about a remarkably wide use of Mazda lamps
in the industries during the past six or eight years, especially since
17 It has been decided not to include in this classification, those cases where oil, acetylene,
or other similar fuels are used for illumination purposes.
344 ILLUMINATING ENGINEERING PRACTICE
their manufacture has been developed to a point making it possible
to use them under rough factory surroundings. In a general way,
the mantle gas lamps, likewise, have been developed in such a variety
of types and sizes during the past few years that they are also
suitable for a very wide range of factory locations.
Since its initial application to an industrial use in this country in
1903, the mercury- vapor lamp has found wide service in such typical
industries as metal working plants, textile mills, newspaper and
printing establishments, and shipping and storage houses. Instal-
lations of mercury vapor lamps are on record with as many as 2,500
units in a single plant.
The notable development in the interest taken in factory lighting
has been promoted very largely by the design and marketing of these
modern types of lamps, and, physically speaking, the possibilities in
this field are due almost entirely to the introduction of the many
new types of lamps and auxiliaries, which, in turn, have made it
possible to light factory spaces properly with electricity or gas,
whereas, formerly, these same spaces could not be lighted properly
because of a lack of suitable lamp types and sizes adapted to given
conditions, such as ceiling heights, clearance between cranes and
ceilings, and similar limitations.
REQUIREMENTS
The principal requirements which should be met fully in planning
factory lighting may be summarized as follows:
(a) Sufficient intensity of general illumination over the floor area
to prevent accidents and to make it possible to handle material and
to get around the machinery readily.
(b) Sufficient intensity of the illumination at the point of work,
usually a higher intensity than in (a) although it is practical in some
cases to make the intensity of the general illumination adequate for
both (a) and (b).
(c) The use of suitable shades and reflectors with the lamps
mounted in such positions as to avoid eye-strain.
(d) The electric circuits and gas mains of sufficient size to assure
normal working pressures of the supply at all times.
(e) In addition to (d) the supply should be adequately protected
against interruption of service.
(/) The size of the lamp should be in accord with the ceiling height
of the section where it is employed, particularly where the entire
CLEWELL: LIGHTING OF FACTORIES, ETC. 345
illumination is furnished from lamps overhead, that is to say, where
no individual lamps are used close to the work.
As a supplement to the foregoing list of requirements, certain
specifications concerning artificial lighting as made by one of the
Federal Government Departments 18 are of value in relation to this
phase of the subject. These specifications, as abridged from the
bulletin of this department, are as follows:
General Illumination. The entire shop should have a system of general
artificial lighting adequate to insure an illumination of not less than i
foot-candle over the entire floor area. 19
Local Illumination. At the points of work, additional local illumination
should be provided, or the general illumination increased, to meet the
specified intensities for given classes of operations.
Character of Lighting Units for General Illumination. Satisfactory units
for the general illumination of shops would consist of tungsten or gas-
mantle lamps provided with deep-bowl reflectors having extensive dis-
tributing characteristics. The units should be suspended as nearly as
possible to the ceiling in such relative positions as to insure a minimum
distribution of i effective lumen over each square foot of floor area, that
is, a minimum of i foot-candle at each point of the floor area.
Character of Lighting Units for Local Illumination. The additional
local illumination of such cases as machines and finishing table may be
advantageously secured by the use of tungsten or gas-mantle lamps and
opaque reflectors with intensive distributing characteristics of the deep-
bowl or cone type. Fixed suspension should be used. The height of sus-
pension will depend upon the distribution characteristics of the reflector
used.
For such cases as cutting, basting and pressing tables, the local lighting
units may be made up of tungsten or gas-mantle lamps with deep-bowl
prismatic reflectors of glass with intensive distributing characteristics, the
height of suspension and the spacing being such as to meet the desirable
intensity for the operation in question.
Glare Effects. It is important to avoid all glare effects, for not only do
these make seeing difficult but they are injurious to the eyes. Glare is
present from any light source, under ordinary working conditions, when
it is in the field of vision and is of greater intrinsic brilliance than the ob-
ject to be viewed. It follows that in the local illumination of workshops,
bare lamps or reflectors of the shallow-saucer type should never be used.
Prismatic reflectors should be of the deep-bowl type and suspended at
such heights as to cause the units to become practically concealed sources.
18 Public Health Bulletin No. 71, May, 1915, J. W. Schereschewsky and D. H. Tuck.
Treasury Dept., Washington, D. C., pp. 147 and 148.
19 These requirements apply 'specifically to the workshops of the woman's garment
industry.
346 ILLUMINATING ENGINEERING PRACTICE
Opaque deep-bowl or cone reflectors are always to be used for local illumi-
nation when the height of suspension is such that the unit will be within
the ordinary field of vision.
All reflectors are made for use with a particular size of lamp. This
specific size should always be used with the reflector. The use of larger
lamps produces glare from the projecting portions and alters the distribu-
tion characteristics of the combination; the use of lamps smaller than that
for which the reflector is designed constitutes an uneconomical unit, which
may produce inadequate illumination and alter the distribution character-
istics of the reflector.
SYSTEMS OF ILLUMINATION IN USE
In the earlier days, before the introduction of the mercury vapor
and Mazda lamps, the use of the small carbon filament units and
the large arc lamps, usually resulted in a low degree of general
illumination when some of the lamps were mounted overhead, thus
making it essential to employ an individual or localized lamp close
to the work of each employee.
With the introduction of medium-sized lamps, that is to say, the
mercury vapor and Mazda lamps, with their wide range in sizes,
there has been made possible a comparatively new system commonly
termed the overhead or general system of illumination, whereby a
large number of medium (or even relatively small) units are mounted
well overhead in such density of numbers as to furnish entirely ade-
quate illumination at the work without the addition of individual
hand or localized lamps mounted directly at the work.
Again, it is sometimes found advisable to carry out the scheme
of general illumination, but instead of a uniform spacing over the
entire ceiling area, to mount each lamp with respect to some given
piece of machinery or work, thus forming a system somewhat be-
tween the general and the strictly localized lighting systems.
Overhead lighting may be subdivided into three general classes,
namely, the direct, the semi-indirect and the indirect systems. For
manufacturing spaces, the direct system has been, and probably is
now most widely used, partly because of its higher efficiency, and
partly because it is usually better adapted to factory spaces and is
ordinarily cheaper in first cost than the other systems. Exceptional
cases arise, however, where the semi-indirect or even the indirect
systems may prove economical on account of their peculiar advan-
tages under certain circumstances. For example, the indirect sys-
tem is now used with very satisfactory results in a number of textile
mills.
CLEWELL: LIGHTING OF FACTORIES, ETC. 347
Furthermore, in the drafting rooms and offices connected with
factories and mills, the semi-indirect and indirect systems are often
used, and it seems reasonable to expect that the illumination ad-
vantages of these systems will cause them to be even more widely
used under certain industrial conditions, particularly as the efficiency
of the commercial light sources is further increased. Figs, i to 6
inclusive show three overhead systems of tungsten lighting, that is,
a direct, a semi-indirect and an indirect system, and three systems
where the mercury vapor lamp is employed.
METHODS OF CLASSIFYING LOCATIONS AND WORK
A classification of typical industrial locations and of the various
kinds of work involved, is of value when planning new lighting sys-
tems where it may be important to know what intensity to select
for a given factory space in terms of the experience of others under
similar or somewhat similar circumstances, or when the drafting or
enforcement of lighting legislation is involved.
For the reason that the industries cover an immense variety of
locations and kinds of work, it is impracticable to attempt a compre-
hensive list of all industries, and the following cases are therefore
given merely as typical of some of the efforts which have been made
to formulate classifications of this general nature. They are in-
tended, for the same reason, to serve rather as a guide, and the refer-
ences made to a number of books and pamphlets will aid the reader
to continue the study further if these more or less typical classifica-
tions are found to be inadequate.
The Code of Factory Lighting 20 issued by the Illuminating Engi-
neering Society attempts a broad classification under four headings,
as follows:
1. Storage, passageways, stairways, and the like.
2. Rough manufacturing and other operations.
3. Fine manufacturing and other operations.
4. Special cases of fine work.
It is obvious that in a general summary of this nature, many un-
certain cases will naturally arise in the inspection of, or dealings with,
different factory buildings. In the code of the Illuminating Engi-
neering Society the suggestion is made that this general classification
be followed and that uncertain cases be left to the judgment of a
lighting expert. The lighting expert, if thus called upon to make a
* Trans. I. E. S., vol. X, Nov. 20, 1915, pp. 605-641.
348 ILLUMINATING ENGINEERING PRACTICE
decision on such uncertain cases, may depend on a more detailed
subdivision with intensities specified for locations intermediate be-
tween the main headings just listed.
The Departmental Committee on Lighting in Factories and Work-
shops in Great Britain, in its first report, which was limited to in-
clude the engineering, textile and clothing trades, subdivided its
recommendations under a classification as follows:
1. "Working areas" of workrooms.
2. Foundries.
3. All parts of factories and workshops not included in i.
4. Open places in which persons are employed and dangerous parts of the
regular road or way over a yard or other area forming the approach to any
place of work.
In its handbook on shop lighting for superintendents and elec-
tricians, issued in 1914 by the Industrial Commission of Wisconsin,
the following subdivisions are listed:
1. Departments with ceilings n to 16 feet in height, where there is no gas or
smoke.
2. Departments with ceilings 9 to u feet in height, where there is no gas or
smoke.
3. Foundries and forge shops.
4. Stairways.
5. Platforms.
6. Driveways and passageways between buildings.
7. Yards.
8. Individual machines.
9. Benches.
10. Drafting tables.
As typical of some of the more complete classifications in use, the
following subdivisions 21 under the general heading of machine shops,
indicate one way in which some of the operations carried on in such
departments have been outlined:
1. Bench work 22 (fine).
2. Bench work (rough).
3. Lathes (fine work).
4. Lathes (automatic).
5. Millers and shapers.
6. Planers.
7. Drills.
8. Buffers and grinders.
9. Saws.
11 General Electric Company's "Handbook on Incandescent Lamp Illumination for
1916, p. 109.
22 Bench work is further classified as follows: (a) single benches along the wall; (b) single
benches away from the wall; and (c) double benches with operators on both sides.
CLEWELL: LIGHTING OF FACTORIES, ETC. 349
10. Assembling, erecting and inspecting,
n. Painting.
TABULATION OF INTENSITIES COMMONLY RECOMMENDED
Any attempt to tabulate illumination intensities commonly em-
ployed for various industrial conditions, is rendered impracticable,
partly because of lack of data on existing systems, and also because a
table of this kind would tend to mislead on account of the inade-
quate values of the illumination so commonly found in many existing
factory lighting systems. It is perhaps better, therefore, to make up
a tabulation of this nature in the form of recommended values, as
indicated in the following paragraphs. Reference, in this connection,
should be made also to Table II in Section 2, headed Summary
of Factory Lighting Legislation, where the intensities recommended
by various authorities are given for a very limited classification of
work.
General Illumination. The United States Health Service 23 recom-
mends for garment working establishments, i.o foot-candle over
the entire floor area. The British Report for 1915 recommends 0.25
foot-candle without regard to the needs of the work itself. The
General Electric Company 24 recommends i.o to 2.0 foot-candles for
general illumination when supplemented by localized light.
In all of these recommendations, an effort is apparent to make sure
of sufficient illumination over every factory area so that the operators
may carry on their work without risk of accident and without loss of
time. From the data available at this time, an average of about i.o
foot-candle would appear to be about the minimum with somewhat
higher values under particular circumstances.
Intensities for the Work. When the general illumination is not
supplemented by localized light, then the intensities from the over-
head lamps should closely approximate those required when localized
units are mounted close to the work. The recommended intensities
shown in Table III may, therefore, be interpreted as a measure of the
illumination which should be furnished to the work for the various
operations listed, whether from localized or from overhead lamps as
the case may be. 25 Care must obviously be taken in the use of a
2J Public Health Bulletin No. 71, May, 1915, p. 147.
24 "Handbook on Incandescent Lamp Illumination" for 1916, p. 147.
25 This table has been compiled from the following sources: I. E. S. "Code of Lighting
for Factories, Mills and other Work Places;" G. E. "Handbook on Incandescent Lamp
Illumination;" United States Public Health Bulletin No. 71; Clewell's "Factory Lighting;"
and the " Electrical Salesman's Handbook " issued by the National Electric Light Association.
350
ILLUMINATING ENGINEERING PRACTICE
TABLE III. INTENSITIES
Bakery
or ILLUMIIS
CLASSES
Foot-
candles
2 .O- 3.O
rATiON RECOMMENDED FOR
OF WORK
Packing and shipping:
VARIOUS
Foot-
candles
2 . -3.0
2.0 -5-0
2.0 -4.0
4.0 -8.0
0.25-0.5
4.0 -6 . o
I . -2.0
2 . O -4.0
0.8 -i .5
2.0 -3-5
2.0 -4.0
2.0 -3-0
3-0 -5-0
6.0 -8.0
2.0 -4.0
3.0 -6 . o
2.0 -5-0
5-0 -7-0
3-0 -5.0
4.0 -6 . o
2.0 -4.0
0.25-0.5
0.3 -0.5
0.3 -0.5
O.I -0.3
o.i -0.3
I .0 -2.0
2.0 -5.0
0.8 -2.0
I .O -2 .O
0.5 -i .0
o.i -0.3
0.5 -i .0
i . o -3.0
2 . O -4.0
0.25-0.5
2.0 -4.0
2.0 -5.0
4.0 -8 .0
2 . O -4.0
3-0 -5.0
2 . O -4.0
2 . -3-O
3.0 -5-0
4.0 -0.6
Bench work:
Rough
I 5 S
Fine work
Paint shop:
Fine
. 3-5-10.0
2 . 4 .
Box factory
Fine work (finishing)
Passageways.
Book binding:
Cutting, punching, stitching
Embossing. .
. 3-0- 5.0
4 o 6 o
Pattern shop (metal)
Pottery:
Grinding
Pressing
Folding, assembling, pasting
Candy factory
Canning plants:
Coffee roasting at tables. . . .
Filling tables. . .
. 2.0- 4.0
2 .O- 4-0
. 3-0- 4.0
.0- i .5
.0- 2.0
.5- 2.5
Power house:
Boiler room 27
Packing tables
Preserving plant:
Cleaning
Packing tables (dried fruits)
.0- 1.5
5 2 5
Cooking
Shipping room
Printing:
Presses
Cotton mill weaving 2 '
Dairy or milk depot
. 2.0- 4.0
2 O 4 O
Type-setters
Sheet metal shop:
Assembling
Punching
Drafting room .
7 o
Electrotyping
3.0- 6.0
. 4.0- 7.0
. 2.0- 4.0
. 3.0- 6.0
. 3.0- 5.0
. 1.25-3-0
Factory:
Shoe shops:
Bench work
Cutting
Drills. . .
Millers
Silk mill:
Finishing
Weaving
Planers. .
Rough manufacturing ......
Special cases of fine work. . .
Forge and blacksmithing:
Ordinary anvil work
Machine forging
Tempering
. 10.0-15.0
2 .O- 4-O
. 2.0- 3.0
2 o 4 o
Winding forms
Stairways
Steel work:
Blast furnace (cast house).. . .
Loading yards (inspection) . . .
Mould, skull cracker and ore
yards
Open hearth floors (soaking
pits and cast house)
Rolling mills
Stamping and punching sheet
metal '
Tool forging
Foundry:
Bench moulding
. 3.0- 5.0
I . 0- 3.0
I .O- 2 .O
S.o
7.0
. 5.0- 6.0
6 o 10 o
Floor moulding
Garment industry:
Dark goods
Stock room
Threading floor of pipe mills .
Transfer and storage bays. . . .
Unloading yards
Warehouse
Glove factory:
Cutting
Sorting
Hat factory:
BlocKing
4.0- 6 .0
. 3-0-5.0
2 .O- 4.0
-3-0- 8.0
3.0- 6 . o
3-0- 5.0
4.0- 6.0
6.0- 8.0
2.0- 3.0
2.0- 4.0
3 .O
Stock rooms:
Rough materials . .
Forming
Fine materials
Storage
Wire drawing:
Coarse . .
Stiffening
Jewelry manufacturing
Knitting mill
Fence machines
Fine ....
Laundry
Wood working:
Rough
Fine ....
Leather working:
Cutting
Grading
Woolen mill:
Picking table
Meat packing:
Cleaning
Packing
Offices.. .
Weaving. . .
24 See also G. E. "Handbook on Incandescent Lamp Illumination" for 1916, pp. 103-107.
27 Supplemented by individual lamps at the gauges.
Fig. 5. Insulated wire department with a system of mercury vapor lamps.
Fig. 6. Hand press room in bureau of engraving and printing showing use of mercury
vapor lamps.
(Facing page 350.)
Fig. ii. Auxiliary or emergency scheme of lighting.
CLEWELL: LIGHTING OF FACTORIES, ETC. 351
table of this kind, because the values of intensities listed .may some-
times be based on experience in a limited set of conditions. Each
new location should, therefore, receive sufficient study, at least, to
ascertain whether the conditions warrant the selection of an intensity
value as listed in this table.
TYPICAL PLANS
Perhaps the most apparent and important need in planning factory
lighting is to be familiar with the best methods for replacing a few
large units for a given floor area, with a relatively large number of
medium sized or even small units. Fig. 7 shows an excellent example
of an old installation where a single large lamp 28 is suspended near the
center of the aisle with large separation distances between units down
the aisle. The chief disadvantages in a scheme of this kind are: first,
the very poor distribution of the light, resulting in high intensity
values at certain parts of the floor area (usually near the lamps for
such low mounting) and very low intensity values at points relatively
not very remote from the lamps; second, the severe tax on the eye-
sight, produced by the glare and by the concentration of all of the
light furnished such a double bay, in one large unit at its center; and
third, the very objectionable flicker of such lamps if the voltage con-
ditions of the supply circuit happen to be poor.
In Fig. 8 one typical solution of the case shown in Fig. 7 is indi-
cated. Here nearly twenty smaller lamps 29 replace the single large
unit, since there is only one lamp in Fig. 7 for each two bays. The
results from an installation like that of Fig. 8 are much improved over
the older plan, the illumination being very uniform over the entire
floor area and the disadvantages of the single large lamp being almost
entirely eliminated.
In Fig. 9 the use of one flaming arc lamp for every two bays is
contrasted with the use of sixteen loo-watt vacuum tungsten lamps
for the same area. The first-mentioned case is more or less repre-
sentative of many older schemes of lighting which were dictated by
the lack of smaller or medium-sized lamps; while the latter case show-
ing the many smaller units is representative, in like manner, of the
application which has been made in many factory spaces of the
tungsten units. It is comparatively easy to see that the distribution
* Actually an inclined electrode, short burning, flame carbon arc lamp.
19 loo-watt vacuum tungsten lamps under the mezzanine floor and 250-watt lamps of the
same type on the 20 ft. ceiling line. Prismatic glass reflectors are used for both sizes of
lamps in Fig. 8.
352
ILLUMINATING ENGINEERING PRACTICE
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CLEWELL: LIGHTING OF FACTORIES, ETC.
353
will be vastly more uniform with the larger number of lamps, although
comparative illumination tests with portable photometers under two
systems like these, render the conclusions even more convincing.
As an example of different points of attack in two factory sections
with the same dimensions (floor area and ceiling height) Fig. 10 is
shown. Here the kinds of work are the important factors. To the
left, the large cylindrical tanks require considerable light on the in-
Fig. 9. View in which a comparison is shown between the use of one large lamp in every
other bay and sixteen relatively small tungsten units in the same area.
side surfaces and thus make it almost essential to use enough overhead
lamps to reduce the possibilities of dense shadows on the inside of the
tanks. Of course, the use of one large flaming arc lamp at the center
of every other bay as shown, results in dense shadows on the inside of
practically all the tanks unless a tank happens to be almost directly
beneath a lamp. The use of six 2 50- watt vacuum tungsten lamps
with prismatic glass reflectors spaced 12 feet apart, proved, under
trial, a very much more satisfactory scheme.
23
354
ILLUMINATING ENGINEERING PRACTICE
In contrast to the left-hand portion of Fig. 10, the right-hand por-
tion may be studied with profit. Here a single large naming arc
lamp was formerly used for the bench surface illumination, but,
because of the poor distribution, drop cords and localized lamps were
needed to supplement the general illumination produced by the
single arc lamp. The use here of 2 50- watt vacuum tungsten lamps
to replace the arc lamps was based largely on the desire to produce
an almost uniform horizontal illumination intensity over the entire
bench areas, whereas in the case of the tanks, reduced shadows prob-
ably formed the most important single factor. In the bench section,
the use of the 2 50- watt units eliminated the need for localized lamps,
giving the space a much more pleasing appearance and actually ren-
dering more bench area available for work.
CLEWELL: LIGHTING OF FACTORIES, ETC. 355
The foregoing cases are typical of several actual locations with the
corresponding solutions of the given problems, and it will be noted
that in each case the improved scheme was made possible by the
availability of a small or medium-sized type of lamp, and the economy
with which a relatively large number of such lamps can be used over-
head. To' demonstrate the practical nature of such an improved
installation as compared with the older practice of using very
large units sparsely scattered over the ceiling area, it is usually a
good plan first, to demonstrate that the actual yearly cost of such
an improved system is a very small percentage 30 of the actual annual
outlay for wages in the given section ; and second, to make a sample
installation in several bays, which contain active work.
This procedure will usually prove conclusively to the manager
or owner that the investment represents not merely an expense
in the ordinarily accepted meaning of the term, but that it is an
outlay which will be accompanied with more or less tangible re-
turns because of a better and larger product for the same wages as
were previously expended under the conditions of the older inferior
lighting conditions.
DISTRIBUTION CIRCUITS
In motor driven factories, there is sometimes a tendency to supply
energy to motors and lamps from the same circuits. The motor
load, however, will most likely be a much larger proportion of the
total than the lighting load, and, due to possible excessive variations
in the motor load, there is a likelihood that the voltage fluctuations
at the lamps will be unduly large.
Where tungsten lamps are employed, large variations in the
supply voltage do not materially affect the life of the lamps, 31
but the candle-power and hence the illumination will vary over
wide ranges. With some other types of lamps, excessive voltage
fluctuations may cause more serious difficulties. As a general rule,
therefore, motor and lighting circuits should be separate and the
circuits individually designed and the loads on each kept down to
such values as to insure approximate constancy in the lighting
circuit supply voltage at all times.
For tungsten lighting, no- volt mains are usually best, because of
the higher efficiency and lower first cost of no- volt lamps in con-
trast to those of the 2 20- volt class. If 2 20- volt service exists and
30 The percentage should be worked out numerically.
11 Bulletin 20, Engineering Dept., National Lamp Works, General Electric Co., p. 19.
356 ILLUMINATING ENGINEERING PRACTICE
tungsten lamps are to be used, arrangements may be made for
no- volt operation. On alternating-current circuits, a frequency
of 60 cycles per second is preferable to one of 25 cycles per second,
but tungsten lamps are operative on 25 cycle circuits with a very
fair degree of satisfaction.
Switch Control. In the switch control of a lighting system com-
posed of a large number of small lamps, it will usually be economical
to connect a small group of lamps so that it may be controlled from a
single switch. However, where the energy cost is low and the instal-
lation expense for switch circuits is very high, it is well not to go
to extremes in the subdivision of the circuits. As a general rule, the
lamps should be grouped in rows parallel to the side walls containing
windows. 32
Emergency Lighting. The Illuminating Engineering Society's
Code of Factory Lighting in Article XI, calls for auxiliary lighting
in all large work spaces, such lamps to be in operation simultaneously
with the regular lighting system, so as to be available in case the
latter should become temporarily deranged.
In Section XVI of the descriptive portion of this code (p. 44)
this point is emphasized as follows:
"The auxiliary system of lighting called for in Article XI of the code,
is a safety-first precaution, which is insisted upon in a large proportion oi
the 1 200 buildings coming under the control of the Bureau of Water
Supply, Gas and Electricity in New York City, particularly such buildings
as are occupied by large numbers of people. The same precaution is now
observed by the Bell Telephone Company's offices fairly generally through-
out the country, also by a large number of private manufacturers and by
local ordinances compelling all types of amusement places to take this
precaution."
The Code of Lighting for Eactories, Mills and Other Work Places,
adopted by the state of Pennsylvania on June i, 1916, contains
a clause, under the title " Emergency Lighting" which reads as
follows :
"Emergency lighting shall be provided in all work space, aisles, stair-
ways, passageways, and exits; such lights shall be so arranged as to insure
their reliable operation when through accident or other cause the regular
lighting is extinguished."
Eig. ii shows a space in which ther^e are two systems of lighting
(a) a number of direct units, and (b) several indirect fixtures.
82 These statements apply also, in a general way, to the supply mains of gas lighting
systems.
CLEWELL: LIGHTING OF FACTORIES, ETC.
357
Normally, the illumination is furnished by the direct units; if these
go out for any reason the indirect units are turned on automatically.
MAINTENANCE
The deterioration of tungsten lamps and reflectors due to accumu-
lations of dust on the lamp and reflector surfaces is shown graphically
12
42
48
54
18 24 30 36
Elapsed Time in Days
Fig. 12. Deterioration of the same kind of lighting equipment in two different kinds of
locations.
H
A- Dome Enameled Steel
S-Bowl Enameled Steel
C- Dense Opal Glass
D- Prismatic Glass
E- Light Density Opal Glass
B
n
12 16 20 24 28
Elapsed Time in Weeks
Fig. 13. Deterioration of' various kinds of lighting equipment in the same kind of
location. The deterioration in Figs. 12 and 13 are due almost entirely to accumulations of
dust and dirt on the lamps and reflectors.
in Figs. 12 and 13. Fig. 1 2 shows the rate of deterioration for a given
type of lamp and reflector in two different kinds of locations;
while Fig. 13 shows the deterioration rates for a number of different
358 ILLUMINATING ENGINEERING PRACTICE
types of reflectors used with tungsten lamps under one fixed kind of
location.
These curves illustrate the importance of systematic attention to
the cleaning of both shop windows and lighting units. With the
latter, it is also very important to renew all burned out lamps promptly
and to " re-carbon" the arc lamps regularly. To insure regularity
in such work, it may be desirable to detail someone from the lamp or
maintenance department to inspect each unit in the lighting systems
at fairly frequent intervals, it being his duty to report all burned out
or defective lamps as well as particular shop sections where lamps
and reflectors are in need of cleaning. The cost of reflector cleaning
is sometimes included as one of the fixed charges, instead of a
maintenance item.
COST DATA
Wiring and installation expense in factory buildings is exceedingly
variable due to the extreme variety of conditions met with, and
hence an idea of the ranges in cost may advantageously be given
at the outset.
The actual total expense for labor and material, including lamps,
reflectors and switch circuits in tungsten systems, may range from
about $3.00 per outlet (each composed of one loo-watt lamp) for
wood moulding on a wood ceiling of 10 to 14 ft. height; up to about
$7.00 per outlet for the same size of lamp, for iron conduit work
attached to iron trusses on a line 16 ft. above the floor. Extreme
cases of very high ceilings, or peculiar difficulties in the installation
of the circuits, may run this cost up to very much higher values.
Obviously, also, the cost per outlet complete, will be very much
larger if the first cost of the type of lamp used is very large.
In the Handbook on Shop Lighting issued by the Industrial Com-
mission of Wisconsin (prepared by F. Schwarze) it is stated that the
wire and conduit in a shop lighting installation cost 150 per cent,
more than the cost of the lamp, and that the cost of the wire and
knobs for open work is 125 per cent, of the cost of the lamp. The
labor for a conduit installation is approximately 45 per cent, of the
cost of lamp and wiring materials. The labor for an open-work
installation costs approximately 50 per cent, of the cost of lamp and
wiring materials. This does not include, however, the cost of the
mains and distribution centers. The following two cases are given
by the same\uthority:
CLEWELL: LIGHTING OF FACTORIES, ETC. 359
CONDUIT INSTALLATION
Tungsten lamp $o . 70
Conduit, wire, etc i . 75
Labor 1.12
Reflector i . 25
Total $4 . 82
OPEN-TYPE INSTALLATION
Tungsten lamp $o . 70
Wiring materials o . 70
Labor o. 70
Reflector 1.25
Total $3.35
The total operating cost of tungsten lighting systems is very well
discussed in bulletin 20, Engineering Department, National Lamp
Works of General Electric Co., pp. 42 to 45, on which the following
information is based:
In determining the total operating cost of any system of lighting, three
items must be considered: first, fixed charges, which include interest on
the investment, depreciation of permanent parts, and other expenses
which are independent of the number of hours of use. Frequently this
item forms the greater part of the total operating expense, yet it is only too
often omitted from cost tables; second, maintenance charges, which include
renewal of parts, repairs, labor, and all costs except the cost of energy,
which depend upon the hours of burning; and third, the cost of energy,
which depends upon the hours burning and the rate per kilowatt-hour.
The life of a lighting system depends not only upon the wearing put of
parts, but also upon obsolescence. There are no installations in this
country which have been in use for a period of seven or eight years which
are not already obsolete. Although the lamps may be in good operating
condition, economy demands that they be replaced by more efficient
illuminants. There is every indication that the next few years will see
even greater progress in the development of lamps and the use of light.
The rate of depreciation on all permanent parts is equal to at least i2 L
per cent. The investment required in the tungsten system is relatively
very low.
Table IV, compiled by the same authority, is based on a total invest-
ment including the cost of lamps, reflectors, holders and sockets only.
The investment in permanent parts is therefore the total investment less
the price of the lamps. No depreciation is charged against the lamps in-
asmuch as they are regularly renewed. The labor item under fixed
charges provides for the cleaning of all units once each month. For the
smaller units with Holophane steel reflectors, the cost of cleaning is taken
360 ILLUMINATING ENGINEERING PRACTICE
TABLE IV. ANALYSIS OF OPERATING COSTS 100 TO 130 VOLT MAZDA UNITS*
Size of lamp, rated watts
40
60
IOO
150
250
400
500
Cost of Lamp, List
$0.350
0.315
1. 155
i .470
$0.450
0.405
i .292
i .697
$0.800
0.720
1.566
2.286
$1 .200
I .O8O
1.653
2.733
$2 .000
I .800
1.653
3-453
$3.600
3.240
2.617
5.857
$4 . ooo
3.600
2 .617
6.217
Cost of Lamp, Std.-Pkg. Disc
Cost of Reflector, Std.-Pkg. Disc
Cost of Unit, Std.-Pkg. Disc
Annual Fixed Charges:
Interest on Total Invest., 6 per cent.
Deprec'n on Reflector, 12^2 per cent.
Labor, Monthly Cleaning
$0.088
0-144
0.240
$0.102
0.162
0.240
$0.137
0.196
0.240
$0.164
0.207
0.360
$0.207
0.207
0.360
$0.351
0.327
0.480
$0.373
0.327
0.480
Total
$0.472
$0.504
$0.573
$0.731
$0.774
$1.158
$1 .180
Maintenance Cost per 1000 Hours:
Lamp Renewals at Std.-Pkg. Dis-
count
Lamp Renewals at $iso-Contract
Discount
Lamp Renewals at $i2oo-Contract
Discount
$0.315
0.291
0.256
$0.405
0-374
0.329
$0.720
0.664
0.584
$1 .080
0.996
0.876
$1.800
1.660
1.460
$3.240
2.988
2.628
$3.600
3.320
2 .920
Energy Cost per 1000 Hours at ic.
per Kw-hr
$0.400
$0.600
$1 .000
$1 .500
$2 .500
$4 . ooo
$5.000
* The prices on lamps and reflectors upon which the calculations of this table are based
are subject to change without notice; they are used here solely for convenience in engineering
calculations.
as $0.02 per unit for each cleaning. Data obtained from installations
where accurate cost records are kept, show that this figure is conservative
for labor at $0.20 per hour. The cost of cleaning other reflectors is taken
in proportion to the amount of labor required. Some illuminants require
attendance at regular intervals; the cleaning is done at the same time and
is, therefore, included under the maintenance charge. For units which
require no regular attendance, the cleaning expense becomes a separate
charge. It will be noted that the fixed charges form only a small part of
the total operating cost for a lighting system. The folly of using cheap
reflectors, which impair the efficiency of the units, is evident.
The maintenance charge is given for a looo-hour period of burning.
To find the annual charge in any case, it is necessary to multiply by the
ratio of the total hours of burning to 1000 hours. Where lamps are sold
at other than the prices given, the proper correction should be applied.
The renewal of lamps is the only maintenance expense.
The energy cost is given for a icoo-hour period with energy at $0.0 1 per
kilowatt-hour. The energy cost per year is found by multiplying by the
cost per kilowatt-hour in cents and by the time of burning in thousands
of hours.
An example will illustrate the use of Table IV. It is required to find
the total operating expense per unit per year for lighting a mill with
CLEWELL: LIGHTING OF FACTORIES, ETC. 361
2 50- watt tungsten lamps. The lamps are burned a total of 4000 hours
and are purchased as the discount obtained on a $150 contract. The cost
of energy is $0.02 per kilowatt-hour. From the table, the following results
are obtained:
1. Fixed charges 33 $ o. 774
2. Maintenance 4.000 X $1.660 6 . 640
3. Energy 4.000 X 2 X $2.50 20.000
Total $27.414
It is important when making a study of such cost data to keep in
mind the underlying advantages of good light as outlined in the first
part of this lecture, and to remember that small differences in first
cost or in the total operating costs of two systems under consideration,
should be entirely overlooked, if one system, from the illuminating
standpoint, possesses any distinct advantage over the other.
Bibliography
Attention is called to the selected list of references pertaining to illumination
design, contained in the Fourth Edition of the Standard Handbook for Electrical
Engineers, p. 1162.
The sections on lamps, lighting and illumination contained in the following
handbooks may also be consulted with profit :
American Handbook for Electrical Engineers, John Wiley and Sons, Inc.,
432 Fourth Ave., N. Y.
Handbook of Machine Shop Electricity by C. E. Clewell, McGraw-Hill Book
Co., Inc., 239 West 39th St., N. Y.
Standard Handbook for Electrical Engineers, McGraw-Hill Book Co., N. Y.
Bulletins and Data of the National X-Ray Reflector Co.
Bulletin 71, issued by the Federal Health Service, by J. W. Schereschewsky
and D. H. Tuck, Treasury Dept., Washington.
Code of Lighting for Factories, Mills and Other Work Places, issued by the
Illuminating Engineering Society, 1915. Contained in the Transactions of the
Society.
Factory Lighting by C. E. Clewell, McGraw-Hill Book Co.
First Report, Departmental Committee on the Lighting of Factories and
Workshops, Great Britain, 1915.
Handbook on Incandescent Lamp Illumination, General Electric Company,
Harrison, N. J.
Handbook on Shop Lighting issued by the Industrial Commission of Wisconsin.
Industrial Lighting, Bulletin 20, Engineering Dept., National Lamp Works
of G. E. Co.
Lighting Code, Pennsylvania Dept. of Labor and Industry, Harrisburg, Pa.
Publications of the National Electric Light Association.
Transactions of the Illuminating Engineering Society.
11 The value is taken as in the table. It will, of course, be reduced by the difference in
interest on the lamp at the standard-package price and at the $150 contract price. The
difference is practically negligible.
OFFICE, STORE AND WINDOW LIGHTING
BY NORMAN MACBETH
The art of applying lighting units to the production of useful and
artistic illumination in offices and stores has not kept pace with the
development of new illuminants. This is partly due to the remark-
able rapidity of this development, and partly to the fact that the
later illuminants could not be effectively applied after the comple-
tion of the buildings.
A great advance in the use of artificial lighting has been made in
the past decade. Too often this result has been accompanied with
an unnecessary sacrifice in the beauty of the building through the
use of inappropriate fixtures or the improper distribution of the
light. In resisting this influence architects have sometimes neg-
lected to provide useful illumination in keeping with present-day de-
mands. In such cases the artistic work of the architect has often
been undone by the users in the endeavor to meet their practical
needs. There are plenty of examples where bare lamps of higher
power have been substituted for lower-intensity frosted lamps, to the
ruination of the artistic effect and a sacrifice of the physical effect.
The cure for this is to provide ample artistic lighting in such a way
that it cannot easily be spoiled by the inexperienced. For example,
it is often practicable to provide clear lamps concealed in diffus-
ing glassware, tinted if desired to secure a particular color effect.
Pressed and blown glass in keeping with the important periods of
architecture and decoration are now available, while art glass can
readily be made up into any character of design.
All modern and efficient illuminants are too brilliant for use
without some provision for screening and diffusion.
It is necessary to give particular attention to the proper shielding
of filaments and mantles of lamps for the protection of our eyes.
This shielding, whether with globes, shades, or reflectors, should
be done in a pleasing manner so far as the design and general arrange-
ment of the fixtures are concerned. To be able to see clearly and
easily is the first step toward efficiency and the amount of energy
necessary, while frequently the only previous consideration when
speaking of "efficiency" as related to lighting, is secondary.
363
364 ILLUMINATING ENGINEERING PRACTICE
Calculations by the experienced lighting man are largely for a
check on his judgment. This judgment is the result of experience
on lighting installations and particularly from the results secured
from investigations which he personally has made of previous in-
stallations. These investigations should always be accompanied
with illumination and brightness measurements.
While a few years there was very little difference in efficiency
between the various sizes of incandescent gas and electric lamps,
largely used for office and store lighting, it was a rather general
practice to check up these calculations on the basis of cubic feet of
gas or watts per square foot. With the wide variation in efficiency
of various sizes of lamps at this time, the more simple exact method
should be adopted of basing the calculations on the total light output
of the lamp in lumens or of lumens per cubic foot of gas per hour,
or per watt.
It is necessary for good illumination that there should be a suf-
ficiently high intensity, with attention to uniformity, diffusion,
eye protection, appearance, and efficiency. The importance of these
characteristics of good lighting will vary in different installations.
Efficiency has at times been given too much attention and promi-
nence. It is used here to refer in office, store or window lighting
to that proportion of the generated light effective on an assumed
plane. At times this consideration is extremely important, and at
other times of practically no importance. It is necessary, of course,
for the predetermining of results, to know the probable efficiency
of the installation, that is, what per cent, of the total light produced
by the lamps reaches the working plane. This is generally termed
"utilization efficiency" and may vary from 70 per cent, to 10 per
cent, or less of the total light from the lamps. It is possible to
design a lighting installation which from this single standard would
have a high value, but which, from the standpoint of assistance to
easy and clear vision, that is, from the standpoint of good illumina-
tion, would be an absolute failure.
Within the past few years there has grown up a strong appreciation
of the ill effects of bright sources on the eye and of extremes of
contrast between the average brightest and darkest portions of the
room. There is a tremendous difference nevertheless between
equipment which without photometric tests appears to be quite
similar but which, from the standpoint of distribution of light
and the contrast conditions set up, may vary in efficiency upward
to 50 per cent.
MACBETH: LIGHTING OF OFFICES 365
Indirect lighting or semi-indirect lighting in which no part of the
fixture is brighter than the ceiling is generally more satisfactory
than the average system of direct lighting where clerical work is
done, as tests have shown that the efficiency of the eye is reduced
very rapidly under any system of illumination in which light sources
of a brilliancy of those of our commercial types are within the ordi-
nary range of the eye.
Much of the so-called semi-indirect lighting is but slightly modi-
fied direct lighting. This kind of lighting with light density glass-
ware has been most general and has undoubtedly resulted in the
unjust condemnation of semi-indirect lighting as a whole. It has
been shown, 1 however, that of the glassware on the market used for
semi-indirect lighting fixtures, at least 90 per cent, of it has too high
a transmission. A worthy endeavor is being made to reduce this con-
trast in lighting installations to within the range of 100 to i, that is,
the brightest object within the range of view should not be more than
100 times brighter than the average lower intensity. The average
semi-indirect lighting installation with light density opal glass is
merely an inefficient system of direct lighting. In many locations
direct lighting, particularly where the ceilings are low, would better
meet the requirements. In all such cases, however, very deep bowl
glass reflectors having low transmission should be used. The lamp
filament should be covered down to the 65 point, and it is also de-
sirable that the lower edge of the reflector be flared out so that this
part of the reflector interior as ordinarily seen be not overly bright.
There is an important difference between diffusion of light and
diffusion of illumination. 2 Light from a single source, no matter
to what extent that light may be diffused by the enclosing media
would, from the standpoint of illumination on a desk, not result in
diffusion. Diffusion of illumination is the important factor and is
the result you secure when the light received on the surface viewed
is from a number of directions. This may be secured through close
spacing of units or through using the ceiling as the light distributor
either with indirect or semi-indirect fixtures. Good diffusion of
illumination is essential in offices, drafting rooms and similar places
where glazed paper or desk tops with polished surfaces are in use.
For stores, a high degree of diffusion is not so necessary and is gen-
erally present to a sufficient degree with any system of lighting
because of the large number of outlets.
Maintenance of lighting equipment should not be overlooked and
it is very important that arrangements be made for a proper cleaning
366 ILLUMINATING ENGINEERING PRACTICE
of the equipment at periods v.arying from two weeks to a month or
more, depending upon the dust conditions of the location. Deteri-
oration is less with direct lighting reflector and lamp units than with
the indirect or semi-indirect units. There are locations where it
would not be possible to keep indirect and semi-indirect units clean
without a cleaning period so frequent as to be unnecessarily expen-
sive. In these cases direct lighting equipment will permit of longer
periods between cleaning.
It is well to remember in accepting tables of desired intensity,
utilization factors or constants, and methods of calculation gener-
ally furnished by the equipment manufacturers that these values are
invariably for new clean equipment. An average deterioration
factor should be used of 10 per cent, to 25 per cent., depending upon
whether the maintenance is likely to be good or average. In consid-
ering fixture design, it is worth while to note also the ease or difficulty
as the case may be with which the equipment can be cleaned.
Fixtures should be substantial and the means of removing glass-
ware for cleaning should be simple. Ordinary labor is generally used
for maintenance and holders with springs or similar complications
are not easily taken care of by the average cleaner.
OFFICE LIGHTING
Office employees as a class are subjected to more severe eye-strain
than almost any other class of workers.
In our large cities during many hours of the day and in many
instances all day, they work with a mixture of natural and artificial
light. The intensities of the latter, in these days of unwise economy
of energy for lighting, are rarely adequate: There would seem to be
little doubt that with a mixture of daylight and artificial light, a
higher intensity of artificial light is required than where artificial
light alone is used. Whether this is due to a higher eye adaptation
demand or to color differences, has not been decided.
The frequent use of an instrument for measuring illumination
intensities cannot be too strongly recommended. In a recent
installation complaint was made that the clerks were having diffi-
culty with their eyes, although apparently the lighting installation
had been given every possible design and maintenance attention.
On inspection it was shown that the spacing and kind of fixtures were
satisfactory, the contrast between the brightest and darkest object
in the room was well within the proper range and the installation had
MACBETH: LIGHTING OF OFFICES 367
every appearance of being right. Illumination measurements,
however, brought out the point that the average intensity was about
1.5 foot-candles which was certainly not high enough for the charac-
ter of clerical work performed. A simple increase in the size of
lamps used corrected the difficulty.
In a recent investigation 3 in a block of office buildings in New
York City, over 85 per cent, of the workers were under artificial
light or a mixture of natural and artificial light all day, and over 90
per cent, of them worked more than eight hours per day. Some of the
clerks and stenographers had only 0.5 to 1.5 foot-candles on their
work. Others again by the use of portables, mostly placed improp-
erly, worked under 30 to 40 foot-candles, likewise suffering from
headaches and eye discomfort. An entire floor of ledger keepers
with a system of semi-indirect units, otherwise satisfactory, had
only 0.5 to under 2 foot-candles effective at the desks. The mis-
directed economy demands of the office building superintendents or
the competition demands of the venders of lighting equipment, to
do with a less energy expenditure than necessary, was undoubtedly
accountable for these installations.
Economy is frequently referred to in considering office lighting.
The economy which is most lasting, however, is that which avoids
the waste of human energy. 4
" Such waste has no compensating return but is an irretrievable and total
loss. The man who works an entire day to accomplish that which, under
obtainable conditions, he could accomplish in half a day, has wasted a
portion of that which is above all price life Poor and insuf-
ficient light is indefensible from every standpoint The most
vital of all economies is the saving of human energy Let us
not overlook the fact that we work by sight, that we see by light."
In discussing the cost of lighting, Professor Charles F. Scott stated 5
that in one instance the cost of good light for an office was but 2 per
cent, of the wages; that "the difference in cost between good light
and poor light would be i per cent, of the wages," noting that
"One per cent, of an office day was about five minutes, that if clerical
work can be done with greater ease and figures read more accurately, if
there is greater rapidity and fewer errors, if there is less eye strain, less
headache, greater comfort and satisfaction, so that more and better work
is done in eight hours than would be done in eight hours and five minutes
with a poor light, then the extra cost is justified."
It was also shown that the difference in cost of equipment between
368 ILLUMINATING ENGINEERING PRACTICE
satisfactory and unsatisfactory methods was insignificant. He
advised that in determining the real value of good illumination the
cost of light should be determined in terms of the total cost of pro-
duction, either the labor alone, or the labor plus the various other
charges which enter into the total cost of production. This was to
be expressed either in per cent, or in minutes, adding that common-
sense judgment is a better guide than detailed systems of cost which
fail to consider the indirect and really important elements that make
good illumination worth while.
In a discussion of costs for office lighting at a hearing of the Heights
of Buildings Committee of the Board of Estimate and Apportion-
ment of the City of New York, 6 comparing the costs of good artificial
lighting service to the cost for daylight, generally considered to be
free, it was stated, that, if the buildings in New York were limited to
a height of three or four stories to secure the maximum angle of sky
effective in the interior of these buildings, considering the ground
values and office rentals per square foot per annum, the values of the
remaining office space would go up enormously; that, at the present
rentals, there were very few locations, particularly in the office build-
ing district, where artificial lighting could not be supplied for ten
hours per day, 300 days per year, at an expense not exceeding i per
cent, or 2 per cent., and certainly less than 5 per cent, of the present
rentals per square foot per year. Two or three kilowatt-hours or
100 cu. ft. of gas per square foot per year would be sufficient energy
to furnish more good illumination than 75 per cent, of the offices
in New York now have. Good artificial lighting is certainly much
less expensive than daylight when considered on this basis.
The load factor in a set of typical office buildings in Chicago was
shown to vary from 1.5 to 4 hours per day. 7
Modern office lighting practice calls for general illumination of a
fairly high intensity, evenly distributed throughout the entire room,
in contrast to the old method of supplying a low intensity of general
lighting with points of high illumination caused by local or drop
lamps over each desk. Originally this system was necessary as in-
candescent lamps were not efficient enough and the rates for energy
were comparatively too high to warrant supplying the proper illumi-
nation throughout the entire 'room, but with our high efficiency
illuminants and reduced rates the present system is justified. 8
General lighting has become practically standard. No well posted
designer thinks of providing desk lamps for typical office work.
Although in the private office a different problem often arises.
MACBETH: LIGHTING OF OFFICES 369
Local lighting has in many instances proven objectionable as there
is a great liability of glaring reflections from the desk surfaces and
glazed paper. Marked contrasts often exist between the brightly
lighted desk area and the rest of the room, both of which factors
cause a reduction in the efficiency of the eye.
An office with a multiplicity of drop lamps is unsightly, the cost
of wiring is high and there is a heavy expense when wiring is changed
as the position of desks are shifted. The employees are likely to
change the location of lamps by tying the wire to some stationary
object, a practice which is objectionable from a standpoint of safety
and forbidden by the wiring codes. Time is lost moving the light-
sources about and the breakage of lamps is likely to be high.
With general lighting, overhead units are so placed that the lamps
are well out of the angle of vision and are equipped with diffusing
glassware. The arrangement, of course, must be such that dense
shadows are avoided. Larger lamps are permissible, which, in
general, are more efficient than the smaller sizes; fewer outlets are
required, reducing the cost of wiring. When one stops to consider
all the factors that enter into an effective lighting system he is soon
convinced that general illumination is really far more economical
than local lighting.
The three general types of lighting units as ordinarily recognized
are direct, semi-indirect and totally indirect.
Direct lighting with efficient reflectors is unquestionably the most
economical for with it the color of walls and ceilings has less effect
on the resultant illumination. Direct lighting, if improperly ar-
ranged, may produce glare either from the light sources themselves
or by reflection from the object lighted, or it may not distribute the
light evenly and as a result produce objectionable shadows. It is
not generally as decorative as the other methods. Nevertheless,
thousands of satisfactory installations of good direct office lighting
are to be seen, employing translucent glassware rather than opaque
reflectors thus avoiding the undesirable condition of a dark ceiling
and a gloomy appearance of the room.
Totally indirect lighting is probably the most " fool-proof " from
a standpoint of a glaring installation. The light is usually evenly
distributed and the effect comfortable. Objections have been raised
that there is a total absence of shadow, making the room appear flat.
If the system is properly designed, however, this is not true.
Semi-indirect lighting is an intermediate practice; it may be more
efficient than totally indirect and much better for the eye than the
24
37 ILLUMINATING ENGINEERING PRACTICE
average direct lighting system. Semi-indirect lighting is not glaring
if the proper unit is chosen; it can be made very decorative, the light
can be quite evenly distributed and such shadows as are produced
are soft and do not become annoying. The fact that the place where
the light originates is readily discernible has a psychological effect
on the average individual and is said to make people feel more at
ease than under totally indirect lighting.
A semi-indirect unit, first, should be of quite dense glass; in other
words, transmit but a small portion of the light if the best conditions
for the eye are to be obtained. If light density glass is used, the
bowl becomes very bright and the system loses many of its advan-
tages, dropping back to the direct lighting class where a number of
fairly bright objects are in the field of vision.
Second, the fixture or hanger used should be of such a length, and
the socket in the proper relative position to the bowl, that the light
is directed over the ceiling in such a manner as to evenly illuminate
it. Many cases can be noted where the lamp is placed too low in the
dish, concentrating the emitted light in a fairly narrow angle, re-
sulting in a ring or circle of very bright illumination while between
units it may be comparatively dark. At other times to get rid of
this effect the lamp is raised so high that from some parts of the
room the filament becomes visible, introducing glare. On the intro-
duction of the gas-filled tungsten lamp with its rather concentrated
filament, this feature became of more importance than formerly.
Third, in most localities, the glass used should be smooth inside
and, preferably, outside also, as roughed glass collects dirt very
readily and is difficult to clean. A plain but effective equipment of
this kind is shown in Fig. i.
Fourth, the means of suspension of the bowl should be such that
there is absolutely no danger of the glassware falling and it is
desirable to have some convenient means of cleaning.
Fifth, in the commercial office the decorations of the glassware, if
any, should be very simple, for any appearance of excessive ornate-
ness would be out of keeping with the character of the room. Deep
crevices in the glass, although they may be decorative, are objec-
tionable from the standpoint of dust accumulation. Fig. 2 shows a
semi-indirect installation using a somewhat typical opalescent
blown glass dish, 17 in. in diameter and 5 in. deep. The interior of
the dish is fire polished, the exterior is roughed with an etched
decoration.
There is a factor which does not enter into the choice of the unit,
MACBETH: LIGHTING OF OFFICES 371
but which has an important bearing on the system as actually
installed, namely, is the color of walls and ceilings. With indirect
systems it is very essential that the ceiling be light in color, white or
slightly cream, to secure a maximum efficiency of reflection. Even
with direct lighting, as part of the light goes upward, light ceilings
are desirable. The upper part of the walls, also, should be light, as
considerable light often reaches this part of the room. The lower
half of the walls are not so useful from this standpoint, and it is often
desirable to decorate these in some darker neutral tint for this is in
the natural field of view and a dark surbase provides space on which
the eye can rest in comfort. Matt or dull finishes are always prefer-
able to glossy surfaces as they avoid the possibility of annoying
reflections.
The single office spaces usually have desks and cases placed next
to walls. Many have tables in the middle. 9 The center outlet
system of lighting, either direct or indirect, is not considered entirely
successful where desks are to be placed next to the walls. In con-
ference rooms where the work is done around a large table in the
middle, either method is satisfactory. It has been stated that in
semi-indirect lighting the amount of transmitted light should be
about 15 per cent. This, however, is a matter that has entirely to do
with the brightness of the surroundings or the low intensity of sur-
faces within the range of normal observation. There are many cases
where there is no apparent advantage for a single semi-indirect or
totally indirect unit in the center of an office. The distributed unit
system has been in use a great many years and has proven quite
satisfactory. It is always subject to less deterioration from dust
and is less effected by changes in color of ceilings and walls.
In planning the outlet arrangement for a large office building, it
is important to anticipate a sub-division of office space as in office
buildings the partition arrangements are particularly flexible,
scarcely two tenants requiring a similar division of space. In many
instances it is necessary to provide at least one outlet for approxi-
mately each 100 to 200 sq. ft.
Spacing of Indirect and Semi-Indirect Units. Ceiling height
largely determines the distance between outlets for these fixtures.
This distance should be approximately equal to the ceiling height or
may extend to 1.5 times the ceiling height. Where close work is to
be performed a less distance should be chosen. The distance of
units from the ceiling is more largely a matter of appearance from an
architectural point of view. If the unit is placed where it looks as
372
ILLUMINATING ENGINEERING PRACTICE
though it belonged in the room the distribution of light can be taken
care of by selecting the proper equipment for the purpose and
adjusting the filament or mantle in relation to its reflector or the
angle of cut-off of the unit itself.
SPACING AND FIXTURE LENGTHS FOR VARIOUS CEILING HEIGHTS,
INDIRECT AND SEMI-INDIRECT FIXTURES
Height of ceiling,
ft.
Fixture length,
ft.
Maximum distance between
outlets, ft.
8
I-S
7-5
10
2 .0
Q.O
12
3-o
12 .O
14
3-5
15-0
16
4.0
18.0
18
4-5
22 .O
20
S-o
25.0
Two-thirds of the above spacing distances may be used under structural
conditions that warrant other spacing than given above.
BANK LIGHTING
Artistic consideration in the lighting of banks 10 is very important.
The architectural harmony should be given as much consideration
as the utility. Present practice supplies a relatively low intensity of
well diffused general illumination produced by a decorative or semi-
decorative system, and a higher illumination by localized lighting at
the points which logically demand this. These may be divided as
follows :
Patrons' Desks in the Banking Space Proper. In the center of the
room, a floor outlet should supply service. to a standard fitted with
two brackets and diffusing reflectors, or one special trough type
reflector and clear lamps. At the sides, bracket type fixtures and
similar equipments meet the requirements.
Banking Cages. Special cornice type, mirrored trough reflectors,
or short brackets and opaque reflectors, should be located well out
of the way and strong localized light provided.
Bookkeepers' Desks. Local lamps with reflectors so designed and
located that there is no direct reflection into the eye, should provide
the desirable intensity of evenly distributed light.
For local desk lighting it is practically impossible to secure satis-
factory results by placing a lamp symmetrically on a desk as shown
Fig. i. Office 30 ft. by 32 ft., ceiling height 10 ft. 6 in. Ceiling is matt white in color,
walls medium cream. Height to bottom of units 8 ft. Seven semi-indirect lighting fixtures
are used with dense opal glass reflectors. One 300-watt gas-filled lamp per outlet.
Fig. 2. Medium-size private office. Ceiling height 10.5 ft. Ceiling finish white, walls
dark cream. Six outlets are used with three 6o-watt clear tungsten lamps in semi-indirect
dishes at each outlet. Length of fixture 2.5 ft.
(Facing page 372.)
Fig. 3. Individual desk lamp placed in the center of the desk furnishing illumination
for two workers, one on each side of the desk, a frequent but most unsatisfactory location
owing to the difficulty due to direct reflection.
Fig. 4. Sales floor of large wholesale dry-goods house using a form of semi-indirect fix-
ture with translucent bowl and wide band, the interor of which is lined with ripple mirrored
glass. 250-watt vacuum tungsten lamps were used, one at each outlet, with a spacing of
ii ft. by 15 ft. On this floor the ceiling and upper side walls are finished in a flat white.
MACBETH: LIGHTING OF OFFICES 373
in Fig. 3, without setting up a serious condition of direct reflection
from the paper surfaces on which the work is done. This placing of
desk lamps symmetrically, bringing the work to be done into a
direct line between the eye of the operator and the lamp, is almost
universal with desk portables and is perhaps responsible for more
eye discomfort than any other single condition in office lighting.
Simply shifting the portable two or three feet to the left in the
case of a right hand writer, and to the right in the case of a left
hand writer, will successfully eliminate this direct reflection possi-
bility. It is rather surprising, the large number of eye discomfort
cases among clerical workers that can be corrected in this simple
manner. It is a general conclusion also that the lamps used in desk
portables are invariably too large. There is no practical necessity
of intensities beyond 10 foot-candles for this work and yet with the
average portable the intensities range upwards to 25 and 50 foot-
candles.
As a simple test to determine the satisfactory position on a desk at
which work may be performed or to note whether a portable lamp has
been moved out of the danger zone, the operator may, when seated in
the regular working position, place a mirror at various parts of the
working plane and take observations as to whether or not this mirror
shows the reflection of a lamp or the image of any bright surface. A
satisfactory working condition may be secured by either shifting the
working area, or the lamp in the event of it being a portable, to such
a point that all reflections seen in the mirror will be of low intensity
surfaces.
Another cause of eye discomfort in offices, particularly with the
liberal expanse of window surf aces now provided for in most buildings,
is due to the large angle of sky within the normal range of vision.
Recently, in an office in one of our large modern buildings, two steno-
graphers after working under this condition where they faced a wide
angle of sky daily for a couple of months suffered from headaches
and eye-strain. One even found it necessary to wear glasses. By
turning their desks around so that they faced a moderately bright
wall rather than the bright sky, their eye difficulties were relieved.
STORE LIGHTING
The general requirements for the best illumination of stores and
especially department stores, may be considered to be: First, the
goods displayed should be properly illuminated. Second, there
374 ILLUMINATING ENGINEERING PRACTICE
should be absence of glare. Third, the lighting units or fixtures
should be attractive in appearance. Fourth, the light generated by
the lamps should be utilized efficiently.
While all of these general requirements are of great importance,
the order in which they are given above may be considered the order
of their importance, in the average case.
The primary requirement is to have the merchandise well illumi-
nated. In the first place, the intensity of illumination should be
sufficient. There is a tendency toward using higher intensities of
illumination for artificial lighting year after year. Care should be
taken, therefore, to use an intensity sufficiently high. There is no
harm or discomfort to the eyes in doing this, if the eyes are properly
protected from glare. Second, the distribution of illumination should
be reasonably uniform. In order words, one part of the store should
not be appreciably brighter than other parts.
The term uniformity is used generally to express the evenness of
illumination over a working plane and is understood to refer to the
values that would be shown by illumination measurements if at every
point on the plane the illumination intensities were similar. Abso-
lute uniformity of illumination, however, is never necessary in prac-
tice. The eye is not adapted to detect variations even as great as
35 per cent, in a room, provided the minimum intensity is above
one foot-candle.
Third, the light should have the proper color value. The color of
the light given by the standard gas and electric lamps, which are now
universally used for the larger stores, is an excellent color for illumi-
nating a large proportion of the merchandise generally displayed.
This color is not true white, however, and where the goods should be
shown in their true colors, the same as in daylight, the excess of red
rays can be filtered out by the proper kind of glass.
There has been a great deal of discussion among lighting men on
color of light for department stores. It is rather difficult to separate
the demand for color in light, or rather lack of color, for a light
tending toward white, from that due to the lamp and glassware
manufacturers ' sales enthusiasm. The fact remains, however, that
ten years ago a great many department stores were lighted with
direct-current enclosed arc lamps, from which a whiter light was
received than that given by the incandescent electric lamps that
have almost universally replaced the arcs. In the electric field the
tungsten incandescent lamp is in general use to-day with isolated
instances of an endeavor, either with colored glassware or colored
MACBETH: LIGHTING OF OFFICES 375
bulbs, to filter out some of the excess red in this lamp and so im-
prove its color value. Very few of these attempts, however, from
the standpoint of the final installation, are nearer a white light, and
in many instances are less close than was the direct-current enclosed
arc lamp which they replaced and which produced light of a better
white light approximation more efficiently than many of these later
methods. There has in the past, furthermore, been very little
demand in large department stores for incandescent mantle gas
lamps because of the color of the light, although from the beginning
these lamps have produced a light resulting in less distortion of
colors. There is an opinion that in the endeavor to see fabric, absence
of predominating color in light is most important. This is not true
unless greater attention is at the same time given to intensity of
light. Recent tests have shown that intensities from 50 to 100
foot-candles help this situation to a greater extent than does light
having the proper absence of excess color if used as an intensity of
approximately 3 foot-candles, or less. Direction of light, that is,
a minimum of diffused light, is especially necessary for the examina-
tion of fabrics.
Much of the talked-of demand for whiteness of light has been
for "color matching." In many instances the light from our
ordinary sources is better for color matching, particularly if the
fabrics selected are ever going to be seen under the ordinary lighting
of our homes or places of business. There are many colors that
will match under white light that are far enough off under ordinary
artificial light to be unsatisfactory. Accurate color matching has
only been secured where the match is effective with all the light
sources under which the materials will later be seen in combination.
Fabrics matched under a source having for instance a 30 per cent,
white- light sensation value will not necessarily match under either the
ordinary artificial light of the home or of the daylight of the street.
The near white light or so-called approximate white light is use-
ful to a more limited extent in color identification to prevent con-
fusion in selecting a blue or a green for black, a pale yellow or orange
or pink for white, etc.
Glare is produced when the light units are so bright that they
decrease the ability of the eye to see clearly or cause discomfort
to the eyes. The ordinary observer does not know what is the
cause of his discomfort, and may not attribute it to the lighting.
Customers, however, will not stay long in store where the light
is trying to the eyes, even though they do not realize why they do
376 ILLUMINATING ENGINEERING PRACTICE
not feel like staying. For example, the proprietor of a large billiard
room, after rearranging his lighting system along the lines of elimi-
nating glare, became convinced that his customers were playing
billiards from one to two hours longer at a session than formerly. He
had not realized that his old lighting system was too glaring for
comfort until he saw the difference actually working out in increased
revenue from his tables.
It is possible so to light a store that the customers will feel com-
fortable and not suffer through abuse of their eyes, and still the goods
displayed will be brilliantly illuminated. The most important thing
is to have all light sources low in brilliancy at the angle at which
they are apt to be viewed. It is further desirable not to have extreme
contrasts of light and shadow. The latter usually takes care of itself
on account of the light-colored finishes, now universally employed
for ceilings, in modern stores. It is sometimes thought a desira-
ble thing to have the light units appear brilliant so that the store
will look attractive from the outside. The comfort of the customer
when he comes inside, however, should not be sacrified to obtain a
brilliant and glittering appearance from the outside. The public
is becoming educated along these lines and is not attracted by
glare as much as formerly. Furthermore, if the goods themselves
are well illuminated, the store will look attractive. This appear-
ance of attractively good illumination may be enhanced by taking
care to display goods of light color near the entrances so that the
illumination will appear at its best from the outside or on first
entering.
It is not necessary to go into detail regarding attractive appear-
ance of the lighting units. Obviously, these should be pleasing to
the eyes and in harmony with the surroundings. The store owner
is more apt to overemphasize this point than to underestimate it
in comparison with the other requirements.
Efficiency in the utilization of light is important. This point is
often apt to be overlooked by the store owner. In comparing any
two systems of illumination, the choice should not be made alone upon
the question of appearance of the unit, or even the quality of illumi-
nation obtained. In a large store the amount of electrical energy
used is considerable and it is important for the owner to get the
best returns for the money.
Economy as applied to the lighting of a store must not be mis-
takenly understood to refer only to the cost of lamps and fixtures
or the expense of operating them. This is the debit side of the
MACBETH: LIGHTING OF OFFICES 377
account. On the credit side are the sales that result directly from
the inviting effective manner in which the goods are shown.
Lighting Systems. Stores may be properly lighted by direct, semi-
indirect or indirect lighting equipments.
In large stores direct lighting is usually the most preferred on
account of its high efficiency in the utilization of light. -Direct
lighting systems vary considerably in efficiency, however, and the
least efficient of these is on about the same plane as indirect and
semi-indirect lighting.
Semi-indirect lighting produces to a greater extent a more thorough
diffuse illumination, which is the quality of illumination obtained
by having light come from a great many directions. This is accom-
plished by having a large part of the light reflected from the ceiling
so that illumination at any one point is produced by light from many
directions. Diffusion of illumination as thus produced is character-
ized by the absence of sharp and dense shadows and by the minimiz-
ing of high lights or glint reflections. This quality of diffusion is
desirable in many classes of store lighting but not in all classes. It
is undesirable, for example, in the display of jewelry, silverware,
glassware, etc., where direct lighting should always be used, as
high lights or glint reflections are a necessary part of the display. A
combination equipment may be used to meet these conditions where,
as shown in Fig. 5, indirect lighting fixtures were installed for general
illumination with direct lighting units over the counters.
We secure a shadow effect with predominating direct lighting
which assists the eye considerably in determining the structure of
fabrics and other goods, while under illumination that is largely
diffused these details disappear.
In using semi-indirect lighting it is always desirable to use a dense
glass so that the maximum amount of light is reflected from the ceil-
ing, and the bowl is low in brilliancy preferably not much brighter
than the ceiling itself. This not only gives the maximum degree of
diffuse illumination, but it also reduces the liability of obtaining
glare as pointed out above.
Indirect lighting has the same general characteristics of illumi-
nation as semi-indirect lighting. A high degree of diffusion of illumi-
nation is obtained by having the light come from the ceiling. It is
usually considered, however, that semi-indirect lighting is more
attractive in appearance on account of the illuminated bowl. With
indirect lighting there is also a contrast between the dark bowl and
the brilliant ceiling. Semi-indirect lighting is usually preferable on
378 ILLUMINATING ENGINEERING PRACTICE
this account, although indirect lighting is often as efficient and it
gives the same desirable diffuse illumination as the semi-indirect.
Types of Direct Lighting Equipment. Direct lighting fixtures may
be fitted with open reflectors, enclosing globes, or semi-enclosing
globes.
Open reflectors are the most efficient in the utilization of light.
Prismatic reflectors, clear or velvet finish, and heavy density opal
glass, are highly desirable on account of their efficiency. These may
be used in single units or in clusters.
Enclosing globes are often preferred, however, in order to obtain a
more distinctive appearance. Any direct lighting unit which utilizes
the light efficiently, results in a great deal of the light being directed
downward and at angles not far from the downward direction. This
means that in standing under the unit and looking up, the brilliancy
will be too great. It is not possible, however, to protect the eyes
against such conditions with direct lighting. In the ordinary uses to
which a store space is put, the occupants should never have occasion
to look at the light units from directly underneath. Opal enclosing
globes, however, which do not redirect the light in useful directions
but simply diffuse it over the surface of the globe, have nothing to
recommend them for store lighting except their appearance. While
the brilliancy is not as great as that of a bare lamp, it is still higher
than is desirable. Furthermore, the light is not distributed effi-
ciently and the wattage required is consequently greater than would
be necessary for an installation of efficient units. On the other hand,
opal globes are made up in such a large variety of attractive designs
that it is not possible to condemn them entirely for store lighting.
When the globe is large so that the light is diffused over a large sur-
face and when the store owner feels willing to pay the additional cost
in order to obtain the appearance desired, it may be justified.
Semi-enclosing globes are usually made in the form of a flat or
shallow reflector with a bowl suspended directly underneath. These*
are more efficient in the utilization of light than enclosing opal
globes. The brilliancy of the bowl is usually even higher than the
opal enclosing globes and on that account the units are not as desir-
able. Semi-enclosing globes of this type should not be confused with
semi-indirect lighting fixtures. In semi-indirect lighting a large part
of the light comes from the ceiling so that the light received on the
plane where illumination is desired comes from many different direc-
tions and the illumination is diffused. In the case of the semi-
enclosing globes with flat or shallow reflecting surfaces, all the light
Fig. 5. General view of jewelry store lighted with indirect lighting fixtures for general
illumination and direct lighting units bracketed out from the shelving over the counters.
Store 22 ft. by 60 ft., ceiling height 16 ft. Length of fixtures 3-5 ft., using in the five outlets
three soo-watt lamps and two 750-watt lamps, the latter on outlets No. 2 and 4.
Fig. 6. View of floor of large department store using enamelled steel indirect lighting
fixtures with short suspension. A better distribution of light could be secured in this kind of
location if the fixtures were lower so that the ceiling would be more uniformly lighted.
(Facing page 378.)
Fig. 7- Department store floor using single-chain fixtures with one-piece opalescent glass-
ball globes. This is a typical department store floor.
Fig. 8. Semi-indirect gas fixtures as standardized for use in a chain of grocery stores.
MACBETH: LIGHTING OF OFFICES 379
comes from the bowl and the reflector, and this is, of course, a much
smaller area than the area of the ceiling. Semi-enclosing globes
should, therefore, be classified as direct lighting units and not as
semi-indirect units.
The high-grade shop is usually small in size, lavishly furnished,
located in some fashionable section and handling only the best grade
of goods. The proprietor is accustomed to spending large sums for
rent, equipment and general upkeep and more money can be spent
for individuality of layout. A distinctive lighting system is, there-
fore, appropriate. Artistic appearance is the predominating factor,
and efficiency a secondary consideration. The lighting system
should harmonize with the architecture and preferably be designed
to be strictly in accord with some predetermined plan.
SMALL STORE LIGHTING
Small stores may be divided roughly into five groups, 14 the first being
those requiring equal illumination on the side walls, shelves, and on
the counters, as bakeries, drug stores, grocery and china stores.
Stores of medium width, may be lighted satisfactorily with two rows
of lamps. This will result in a high intensity of direct light on the
counters and sufficient diffused light for the walls. In narrow stores,
one row of lamps down the center of the store will give satisfactory
results.
The second class of stores demand good illumination on the coun-
ters and a small amount of light on the side walls, such as haber-
dashery, jewelry, and stationery stores, in which locations, inspec-
tion of the goods is on the counter which must be lighted with a fairly
high intensity with a requirement for lesser intensities on the side
walls or shelving. In jewelry stores particularly, this treatment is
necessary and local lighting over the counters is desired with clear
lamps which result in a better appearance of engraved objects and
jewels.
In the third group are stores that demand the highest intensity on
the wall surfaces and a low general illumination. Art and music
stores, paint and hardware stores, are in this group. In the art
stores pictures are displayed on the walls, and in the music stores it
is necessary to have a sufficiently high intensity on the shelving
for the reading of labels on the boxes.
In the fourth group of stores are the clothing, confectionery, milli-
nery, and shoe stores. In these locations general illumination is re-
quired with local lighting at convenient places in the clothing and
380 ILLUMINATING ENGINEERING PRACTICE
millinery stores that a proper direction of light at high intensities may
enable the customer actually to see the fabrics in a manner not
possible under diffused illumination with the ordinary low intensity
values.
In the fifth group are the small barber and manicure shops which
require localized lighting.
Gas Lighting. Gas lighting is in far better condition to-day than
ever before. There is a better understanding of the kind of lamps
and fixtures required to meet the general demand and more attention
has been given to the successful production of these fixtures. Gas
companies have shown awakened interest in good gas lighting and,
in many places, the consumer can count upon a grade of service not
possible a few years ago. Maintenance with gas lamps, as with all
of our artificial light sources, is of particular importance and in most
cities to-day it is possible to secure a high grade of service from the
local supply companies at a nominal charge and, in some instances,
without charge. There is a strong recognition by the gas companies
that their service to the consumer means lighting service rendered,
rather than merely gas by the cubic foot.
Good lighting is worth its cost and the merchant to-day is more
willing than at any time in the past to meet that cost.
Semi-indirect lighting with gas has proven to be entirely satisfac-
tory. The maintenance of these fixtures is simple and the renewal
cost low. A gas lighting installation similar to Fig. 8 is all that could
be desired and is certainly a better proposition than has ever before
been offered in gas lighting to a similar class of store.
The general rules given for the use of other illuminants may be
utilized in the planning of gas lighting installations with such modi-
fications only as may be imposed by the differences in the size of
units available. For mechanical and other reasons, semi-indirect
lighting is particularly favorable to the use of gas. A large number
of mantles may be placed within a single bowl and lighted by means
of a single pilot flame. It is furthermore unnecessary to locate the
units or to select glassware with particular reference to the illumi-
nation of individual areas, and with this system it is possible to
secure much higher intensities without the resulting glare that is so
usual with the older types of gas lamps used for direct lighting. This
kind of fixture has been recommended for all classes of store lighting
with the exception of jewelry stores where direct lighting, as stated
above, will give better results from the standpoint of reflection of
light from silverware, cut glass, jewels, etc.
MACBETH: LIGHTING OF* OFFICES 381
The method of store lighting in considerable use before semi-
indirect gas fixtures were available is shown in Fig. 9. This illus-
tration shows a section of a department store illuminated by means
of direct lighting cluster units.
An installation where "gas arc" lamps are used is shown in Fig.
10. This was at one time the most common method of gas store
lighting, largely because of the lower cost of installation and main-
tenance of this lamp as compared to a cluster of small lamps. It is
only fair to the latter, however, to state that in many situations where
costs have been analyzed, the advantage has been found on the other
side, the cluster of small units being less expensive to maintain where
the conditions of gas supply were favorable and less frequent atten-
tion was demanded. 16 This large direct lighting unit is still favored
in those places where low first cost is the important consideration.
Even this field is, however, being taken care of to an increasing
extent by the large single inverted mantle lamp shown in Fig. n.
SHOW-CASE LIGHTING
The show case is a miniature show window 10 ' 17 . It should stand
out in contrast to the surroundings and should have at least twice
the lighting intensity of the store proper effective on its goods.
Good illumination renders sales work easier, for close selection can
be made without removing the goods from the case. This decreased
handling is an advantage in largely eliminating shop wear.
Show cases should be lighted by lamps placed within the case and
hidden from view. The unit used must be quite small, as it must
be placed at the upper edge in the corner of the case. It should
harmonize in appearance with the general finish of the fittings.
Lastly, the lamps used must be of low wattage to avoid heat.
Tubular lamps with suitable trough or individual reflectors meet the
requirements excellently, and with the line source form, as shown in
Fig. 12, a given wattage is spread over quite an area.
With the counter show case the goods are viewed from above, and
the general direction of light must be downward. With the high
show cases, in which lay figures are on display, the light must come
from an angle.
If the illumination in the store proper is not too great, 150 lumens
per running foot of show case is quite satisfactory, and in cases where
a high intensity exists outside of the show cases, this allowance may
be increased.
382 ILLUMINATING ENGINEERING PRACTICE
There are considerable differences in the equipment offered for
this work; many of the trough reflectors that may be secured for
use in the front corner of the case do not screen the standard bulb
lamps and even in some cases, the smaller tubular lamps that have
been used, from the eyes of the clerk behind the counter. This is
largely a matter of cheap equipment and lack of attention to this
important point, as good show case lighting equipment can be read-
ily secured. Fig. 13 is an illustration of an individual reflector
installation with which small lamps are used.
Lamps of low wattage are necessary to distribute properly the light
where the cases are relatively small and also to reduce the heating
effect to a minimum. This is of particular importance in confection-
ery stores where heat from the larger lamps is sufficient in the
summer time to melt much of the stock on display. These cases are
generally of the closed type and the heat dissipation is through the
circulation of air in the case and radiation from the glass surfaces.
Where this heat radiation has given considerable difficulty, the
problem has been met by using small candle-power, low voltage,
miniature lamps in series. While this method is not generally rec-
ommended, it has met the demand where apparently nothing else
would do. The total number of lamps required for a series should be
kept within one case.
It is advisable when determining the intensity to use to treat
all show cases in one store alike whether for light or dark goods as
the character of display may be changed.
SHOW-WINDOW LIGHTING
Undoubtedly there has been considerable guess-work and ill-
directed experiment in show-window illumination. 18
Architects and builders have apparently given very little consid-
eration to this important question. The space in the window at
the disposal of the window-lighting specialist is frequently a matter
of a few inches.
We have only to observe the show windows in our home city to
realize that this very important problem of illumination has been
given very little consideration. Windows high or low, shallow or
deep, are frequently given the same treatment. Windows containing
dark goods adjoining displays of light goods are given the same
quantity of light. Little attention has been paid to the amount
of reflection from materials or fabrics, or to the quality or quantity
of either goods or light. Windows finished in light wood or decora-
Fig. 9. Small department store using three-lamp fixtures with single inverted mantle gas
lamps with prismatic reflectors. Fixture length 2 ft.
Fig. 10. A plant and seed store using inverted mantle "gas arc" lamps. This type
of installation is favored in those places where low first cost is the most important considera-
tion.
(Facing page 382.)
Fig. ii. Bakery store using large single inverted mantle gas lamps, store width 25 ft.
spacing between outlets 10 ft. Height of ceiling 12 ft., height of lamps 8 ft.
Fig. 12. Show cases with small continuous trough reflectors using 11 in. tubular lamps
with single straight filament and contacts on each end of the tube. This equipment for
case lighting occupies a minimum of space in the corner of the case and is reasonably in-
conspicuous.
Fig. 13. Show case lighted with small individual reflectors using is-watt round bulb
candelabra base tungsten lamps. These reflectors are usually installed on lo-in. to 24-in.
centers. The reflector equipment is also adapted to small lamps and medium screw-base
receptacles.
Fig. 14. Show window using concentrated single-piece mirrored glass reflectors with
loo-watt vacuum tungsten lamps on 15- in. centers. Curtain used at top of window to
screen view of the lamps. Depth, 7 ft. height, floor to ceiling, n ft.
(Facing Figs, n and 12.)
Fig. 15. Appearance at night of show window, a vertical section of which is shown in dia-
gram, Fig. 18.
Fig. 1 6. Show window, open in the back. Lamps in trough reflector screened from the
view of those in the interior.
MACBETH: LIGHTING OF OFFICES 383
tions may have been properly and sufficiently illuminated, but when
the style of the decoration changes to mahogany or dark oak, the
illumination falls off so much that the window lighting comes in
for severe condemnation on the grounds of deterioration in the
accessories, or gross carelessness in permitting the pressure of the
supply to drop off.
Seldom has the fact been made plain that because of the darker
finishes a corresponding increase in intensities or change in dis-
tribution of light flux is necessary.
In perhaps no other location can the merchant secure a greater
return on his investment than with the comparatively small outlay
for effective show-window lighting. Window illumination is strictly
an advertising proposition and, as such, costs about 10 per cent,
of that of any other equally effective medium.
The purposes of a well-lighted show window and its tasteful
display of goods is to attract the passerby. A glaring light source
formerly in more general use is, however, about as good as none,
for the goods cannot be seen, owing to the blinding effect upon the
observer. There should be no exposed lamps within the field of
vision. In general, the light must come from in front of the goods
to avoid bad shadows, and the background of the window should
be chosen to obviate specular reflection from the lighting units.
The proper lighting of show windows has become one of great
importance with merchants and is one that demands consideration
as a commercial proposition. The merchant is coming more to a
realization that his display windows are the medium through which
a great deal of his business comes and the precentage of business that
he obtains through such display either directly or indirectly depends
upon the manner in which he has his windows dressed and the
manner in which they are lighted in comparison with that of his
neighbor merchants; assuming that the windows are equally well
dressed.
Proper Placing of Lamps for Show Windows. Due consideration
must be given in show-window lighting of the lighting conditions
on the street or on the sidewalk in front of the window, as it is
usually desired to have the windows appear bright, it is necessary
to take these outside conditions into consideration. Any rules
given for the amount of light used in show windows are therefore
subject to modification and more light would have to be used under
various street-lighting conditions to secure the desired effect of
brightness.
384
ILLUMINATING ENGINEERING PRACTICE
It is not at all satisfactory to light show windows from lamps in
front, outside of the window. Considerable of the light from these
lamps will be reflected from the outer surface of the plate glass and
tests have shown that these outside lamps have a utilization effi-
ciency in the interior of the window of only 20 per cent, of that which
may be secured from lamps placed within .the windows. 18
The illumination of store windows can, with very few exceptions,
be most effectively taken care of with lamps arranged along the
front of the window, Fig. 17. The lamps should
be placed high and out of the direct line of
vision. In some cases it is necessary to use
a painted band with a sign transparency to hide
the lamps; in others, an ordinary curtain or
shade will accomplish the purpose, or, where
a more simple, dignified treatment is required,
a wooden or metal moulding of sufficient depth
may be fitted across the window between the
lamps and the plate glass near the top. The
lamps should be equipped with reflectors, which
will direct the light downward and back into
the window; this will insure the proper direction
of light and natural shadows. Shadows are
necessary, but should not be sharply defined.
We should have no difficulty in distinguishing
detail in the shadows. This unsatisfactory con-
dition is quite noticeable in a window lighted
_ with a single high-powered unit hung in the
Pig. 17. Section of center of the window.
S. !n wh a e r e" A window lighted from the rear and below,
provision was made for with the shadows upward and forward, would
flectoTs^ Hghting Te ~ be little more unsatisfactory than the so-called
shadowless window. All sense of size, propor-
tion, distance and texture are lost or are so badly distorted as to repel
observers rather than to attract them. Windows have been lighted
successfully from the front and below where a few large objects are dis-
played on the floor of the window and where the height of the win-
dow or structural conditions were such as to render it difficult to light
the window from above. Satisfactory installations have also been
made where, in addition to the lighting from above, "foot-light"
sections have been placed along the front bottom of the window.
The purpose of these sections, which should contribute intensities
MACBETH: LIGHTING OF OFFICES 385
not more than one-third of that effective from the top of the window,
is to illuminate the shadows to a lower intensity than the high lights
resulting from the lamps in the upper part of the window. This sys-
tem is useful in windows where the objects displayed have wide pro-
jections under which there would be heavy shadows. These "foot-
light" sections are also effectively equipped with colored lamps or
color filters and are used to direct colored light into the shadows for
the purpose of rendering the objects displayed more attractive.
Light Distribution Calculation. In high, shallow windows, con-
centrating reflectors should be used; while in deep windows, these
reflectors would not be satisfactory. A very simple method for
determining the distribution characteristics of the reflector to use is
to make a scale drawing of a sectional elevation of the window,
showing the height and depth, marking in the satisfactory position
for the lamp from a structural point of view, and the assumed plane
of illumination. Radial lines should then be drawn from the lamp
center to this plane. The length of these lines can be measured with
any scale, preferably in centimeters or tenths of an inch. These
numbers squared will then be a measure of the proportionate inten-
sities required for uniform normal illumination over the section of
plane assumed. It may be desirable to increase the values toward
the front of the window and reduce those in the rear as objects having
fine detail, if placed in the front of the window, are sufficiently close
to the observer, and the high intensity would be useful, whereas, in
the rear of the window, it is more a matter of discerning form and
outline. These values are then plotted to a convenient scale which
will bring them up as shown by curve B, Fig. 18, and by considering
this specification curve with the candle-power dsitribution curves of
units that are available, it is a simple matter to select the one which
will give the best approximation. In this instance, the solid line,
curve C, was selected and illumination measurements afterward
made in the window show the close approximation of E, the measured
value, to D which was calculated, and in the vertical planes G and F,
respectively. A considerable building up of the values in the front of
the window can be counted upon through the reflection of light from
the inside surfaces of the plate glass.
The completed window is shown in Fig. 15. Sixty- watt lamps
with focusing prismatic reflectors were installed on 13. 5-inch centers.
The lamps and reflectors were directed backward into the window
at an angle of 20 degrees from the vertical. It will be noted that
there is clear glass in the upper half of the rear of this window. The
25
386
ILLUMINATING ENGINEERING PRACTICE
purpose of this glass is to admit daylight to the store. At night this
glass is objectionable because of the reflection of the lamps and re-
flectors used in front of the window. This reflection could be elimi-
nated if window shades were installed in the back of this window,
on the window side. In many instances, shades have been in-
Fig. 1 8. Diagram illustrating the method of calculation for the predetermination of the
distribution of light in a show window; the distribution curve of the unit selected to meet
the specification and also the calculated and resultant illumination values are given. A,
is the assumed line of trim; B, calculated photometric curve to produce uniform normal
illumination on A; C, polar diagram distribution curve of the unit installed; D, calculated
illumination values from a unit having the distribution of curve B; E, test values of final
resultant illumination on the horizontal plane; F, calculated vertical illumination values
from unit corresponding to curve B; G, test values of final resultant illumination on the ver-
tical plane.
stalled in locations similar to this but they are invariably improperly
placed on the store side of the glass.
It is not assumed that it is correct to base all such calculations
upon an average window trim for all windows. As a matter of fact
the line of window trim undoubtedly differs in the majority of
windows. If a window is merely flooded with light it is not neces-
MACBETH: LIGHTING OF OFFICES 387
sarily productive of the best results but may be an extravagant waste
of energy and money for the merchant. With properly designed
reflectors the goods in a window may be made to stand out more
prominently with a lower power consumption than with reflectors
with a distribution not conforming to that required to direct the
light at such incident angles upon the goods as to cause the redirected
rays to be most effective upon the eye of the observer. This is true
regardless of a possible difference in efficiencies of the reflectors.
In many stores and showrooms the show windows are merely an
extension of the sales floor. The windows are not backed up. It is
very important in lighting a window of this kind that the lamps be
screened from the range of vision of those in the store. This can be
done as shown in Fig 16, where a trough reflector was used, designed
in such a manner that the rear section of the trough cut off all view
of the lamps from the interior, the cut-off being effective right up to
the back of the window at any position above 3 feet above the floor.
Determination of Number of Outlets. After the candle-power dis-
tribution characteristics of the unit have been settled upon, it has
been found sufficient to multiply the floor area, in square feet, by the
illumination desired in foot-candles, then multiply this result by a
value which will range from two to five or more, depending upon the
efficiency of the light distribution. This result will be the total
lumens required, which amount divided by the total lumens per
lamp, will give the number of lamps. It is important to provide in
the placing of these lamps for ample illumination at the ends or sides
of the windows. The center will be well taken care of from the con-
tributions from practically all of the lamps in the row unless exceed-
ingly concentrating reflectors are used. In fact, it is desirable to use
a wider spacing of units in the center of the window than at the ends.
Many illumination measurements made in existing window installa-
tions show that with a uniform spacing of lamps, the illumination
intensities are very much higher in the center of the window than
at the ends. This is not a result of design, but of ignorance or
thoughtlessness. If the intensity at the ends is satisfactory, then
there is too much in the center, whereas, if the center is satisfactory,
either a closer spacing of units or larger lamps should be used in the
ends of the window.
Because of the amount of light absorbed by dark goods, windows
where this class of goods is to be displayed should have higher in-
tensities than where light goods only are on view. In some instances
in the past, this was provided for by switching arrangements so that
388 ILLUMINATING ENGINEERING PRACTICE
when dark goods were displayed all the lamps would be in use and a
proportionately less number could be turned on for light goods.
From the standpoint of the merchant, this did not prove a practical
consideration as in all cases investigated all of the lamps were used
together. The conclusion is reached, therefore, that if dark goods
are at any time likely to be displayed, the window lighting shall be
designed for dark surfaces.
It is important with gas-filled incandescent lamps that they be
installed in such a manner as to make it difficult, if not impossible, for
the window trimmer to place goods close to these lamps or attach
anything to them. 22 There is less difficulty with gas lamps because
window trimmers have a very clear idea that with these lamps there
is heat; there is, however, a sufficiently great fire risk with gas-filled
electric lamps to warrant this caution, and much less of the heat
association idea.
Figs. 19 and 20 illustrate the value of checking up the requirements
for light distribution in a window. This was done for the window in
Fig. 19 and Fig. 20 was a direct duplication of the installation in Fig.
19, made with the consent of the owners of the first mentioned window.
The dimensions are identical and the same kind, size and number of
lamps were used. Instead of the lamps being hung pendent with an
angle reflector, however, as in Fig. 19, Fig. 20 was fitted with a very
similar prismatic reflector which, instead of being pendent, was
tipped up at an angle which resulted in directing most of the light
into the upper rear part of the window.
The methods for installing gas lamps to illuminate show-window
displays vary somewhat with the construction of the windows. 23
In Fig. 23, are shown three methods, two for the enclosed box type
window, and one for the open type.
The enclosed type of window can be best lighted by units installed
in the front of the window through openings in the ceiling or deck.
The lamps should be provided with reflectors to direct the light
downward and back into the window. A valance or screen should
be placed at the top to result in a finished appearance and to hide the
reflectors. There should be openings in the floor of the window to
admit fresh air. These openings are to be connected to air ducts
over which cheesecloth has been stretched to prevent dust being
carried into the windows and to check the direct flow of air which
might cause sudden draughts. The cheesecloth can be stretched on
a wooden frame of such construction as to permit of its easy renewal.
The glass in a show window so ventilated will not "sweat" and, as
Fig. 19. Show window in which the arrangement of lamps and reflectors was carefully
calculated. The average intensity in this window was 30 foot-candles.
Fig. 20. Show window constructed in practically every particular similar to Fig. 19.
These two photographs were made on the same kind of plate and were given the same time
of exposure, development and printing.
(Facing page 388.)
Fig. 21. Show window of the boxed-in type illuminated with five single inverted mantle
gas lamps installed over glass panels in the ceiling of the window close to the plate glass.
Fig. 22. A deep enclosed window with glass panels in the ceiling above which gas lamps
with concentrating reflectors are used.
MACBETH: LIGHTING OF OFFICES
389
a consequence, will be free from frost during cold weather. The
arrangement of the air ducts, ventilators, and lamps causes a current
of air to pass constantly across the back of the glass. In any window
where sweating is noted, it is merely necessary to arrange the lamps,
fixtures, and ventilators in such a manner as to produce a circulation
of air over the entire surface of the glass and the trouble will
disappear.
In windows fitted as shown in Diagram A, Fig. 23, the openings in
the deck should be large enough to permit the lamps to be adjusted
from the interior of the window. Transoms above the windows
--y^lIeUl Oiling f
Light: 12 on Center!;
ITt^
Window Plmtfor^ ^ VJndow Platform
ABC
Fig. 23. Illustrating three methods of installation for window lighting with gas lamps.
A, enclosed type of window, lamps equipped with angle reflectors projecting through open-
ings in the front of the deck of the window. B, the totally enclosed type of window, with
glass panels in the front part of the deck and the gas lamps with opaque concentrating re-
flectors installed above this glass at a sufficient height to enable the reflectors, glassware,
and mantles to be removed without difficulty. C, an approved method for the installation
of gas lamps in the open type of window. The lamps are concealed in a box built in at the
top and parallel with the plate glass, the bottom of this box being fitted with glass panels.
should extend to within 3 inches of the ceiling and should be
hinged at the bottom so that when they are open they protect the
lamps in the window from draught and prevent undue heat pocketing
at the ceiling.
For corner show windows, windows that are shallow, or windows
where the display is such that access to the window can be had only
at irregular intervals, the type of construction illustrated in Diagram
B, Fig. 23, is recommended. The glass panels in the front of this
deck are of ripple or cathedral glass which serves to break up the image
of the lamp above the glass and to distribute the light effectively
390 ILLUMINATING ENGINEERING PRACTICE
throughout the window without the absorption losses of the ground
or sandblasted glass that has been used in the past. In this type of
window the lamps should be installed above the glass at a sufficient
height to permit the mantles and glassware to be removed. The
reflectors should be opaque and of the concentrating type.
Fig. 21 shows a window of this kind in which the lamps are
equipped with prismatic glass reflectors. The opaque band with
translucent letters at the top of the window serves to cut off the view
of the glass panels from the observer on the street.
In deep narrow windows the lighting units may be distributed
over the entire deck. A window lighted in this manner is shown in
Fig. 22.
For the open type of show window, the construction shown in
Diagram C, Fig. 23, is recommended. The two important factors in
a window of this type are : first, that the location of the lamps must
be such as properly to illuminate the face of the display presented to
the observer on the outside and, second, those on the inside of the
store should be protected from the glare which would result with a row
of open lamps used in this position. The lamps can be shielded
from the interior of the store by this method as shown on the diagram.
The rear section of this box may be of panelled wood or metal. The
bottom of the box should be filled in with panels of ripple glass.
Opaque concentrating reflectors should be used. As cathedral glass
can be secured in several colors, it is advisable that the framework be
arranged so that the glass can be easily removed and, in this manner,
the color effects of the lighting can be varied by the use of different
colored glass.
There are two practically standard methods used for igniting the
lamps in these show windows. First, the jump spark system, the
necessary energy for which is supplied by a battery of dry cells and,
second, ignition by pilot flames. The gas supply to the lamps can be
controlled by a magnet valve operated from the dry cells.
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MACBETH: LIGHTING OF OFFICES 391
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1912, page 132.
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"Drafting Room Indirect Illumination." Electrical World, 1912, page 832.
392 ILLUMINATING ENGINEERING PRACTICE
W. S. KILMER. "Office Building Lighting." Electrical World, 1912,
page 264.
W. S. KILMER. "The Lighting of an Office Building." 111. Eng., 1912,
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10 "Handbook on Incandescent Lamp Illumination." Edison Lamp Works of
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W. N. GOLDSCHMIDT. "Indirect Lighting in an Insurance Company's
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S. G. HIBBEN. "When Architect and Engineer Cooperate." L. J., 1913,
page 35.
D. WOODHEAD. "The Lighting of a Bank and a Large General Office."
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F. J. McGuiRE and F. R. NUGENT. "The Lighting of New York's Great
Municipal Building." L. J., 1914, page 125.
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W. R. MOULTON." Modern Lighting of a Bank by Reconstruction of Old
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C. L. LAW. "Illumination Test on Semi-indirect and Cove Lighting in a
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"Cove Lighting of a Store." Electrical World, 1916, page 378.
11 W. S. KILMER. " Semi-indirect Lighting Applied to Large Areas." L. J.,
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L. J., 1915, page 105.
E. J. DAILEY. "Clothing Store Lighting with Type C Mazda Lamps."
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13 A. L. POWELL. "Large Dry Goods and Department Store Lighting." L.
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C. L. LAW and A. J. MARSHALL. "The Lighting of a Large Store." Trans.
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"Filene Store, Boston." Electrical World, 1913, page 579.
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1913, page 17.
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1914, pages 1134, 1145, 1397.
MACBETH: LIGHTING OF OFFICES 393
"The Lighting of a Large Department Store." L. J., 1915, page 245.
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Cen. Sta., Dec., 1915, page 150.
"Lighting Features of Department Store, Boston." Electrical Review and
Western Electrician, 1915, page 312.
"An Innovation in Store Lighting." 111. Eng., 1911, page 354.
M. H. FLEXNER and A. O. DICKER. "Illumination of a Furniture Store."
L. J., 1913, page 141.
E. F. OLIVER. "Modernizing Furniture Store Lighting." L. J., 1915,
page 153.
14 A. L. POWELL. "The Lighting of Ordinary Small Stores." L. J., 1913,
page 122.
C. L. LAW and A. L. POWELL. " Present Practice with Tungsten Filament
Lamps Small Store Lighting." Electrical Review and Western Electrician,
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15 "Lighting of All-Package Grocery Stores." Gas Age, 1916, page 523.
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J. N. COOK. "Commerical Lighting." Prog. Age, 1911, page 418.
B. K. CARLING. "Store Lighting." Prog. Age., 1911, page 435.
E. H. MARTIN. "Lighting a Rug Display." Prog. Age, 1911, page 487.
R. M. THOMSON. "Holding Lighting Business." Prog. Age, 1911, page
610.
E. M. OSBOURNE. "Store Lighting with Gas Arcs." Prog. Age, 1911,
page 987.
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16 C. I. HODGSON. "The Use of Detailed Maintenance Records." L. J.,
1915, page 274.
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Western Electrician, 1913, page 162.
394 ILLUMINATING ENGINEERING PRACTICE
"Indirect Lighting in a Large Retail Clothing Store." Electrical Review
and Western Electrician, 1913, page 670.
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1915, page 251.
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page 173.
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1913, page 407.
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19 H. B. WHEELER. "The Illumination of the New Hub Store, Chicago."
L. J., 1913, page 116.
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1912, page 178.
20 A. L. ABBOTT and C. M. CONVERSE. "Show Window Installation." L. J.,
1913, page 39.
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"Show Window and Display Lighting." Electrical Review and Western
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21 E. R. TREVERTON. "Combination Gas and Electric Office Lighting."
L. J., 1914, page 264.
22 1. CLYDE. "Danger in Show Windows." Electrical World, 1911, page 335.
23 "Report of Committee on Window Display." Proc. N. C. G. A., 1914,
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page 196.
24 P. EVES. "Store Window Lighting." Prog. Age, 1912, page 572.
THE LIGHTING OF THE HOME
BY H. W. JORDAN
Many discussions coming under the title of illuminating engineer-
ing are so embellished and surrounded with technical terms and ex-
pressions that the mind of the average practical man becomes much
confused in listening to or reading about them and he is often as
much in the dark at the end as at the beginning.
The average central station operator or salesman's knowledge of
illuminating terms is more likely to be limited to a general practical
understanding of fixtures, lamps, candle-powers and wattages, rather
than lumens, lamberts or ultra-violet radiation.
I have no intent to speak lightly of the subject as an exact science,
or of the real value of technical knowledge applied to illuminating
engineering; but my experience has proven to me that there is a real
need of more simple information on this subject that could be ap-
plied alike by the central station man and the lighting service sales-
man, and it is for such men that this lecture has been prepared.
The possession by the central station man, the salesman or the
electrician, of a thorough knowledge of the principles of illumination
would be of the greatest advantage, but even though he appreciated
that it would mean much to him, the average man will not apply
himself to a technical study of these principles.
There is encountered frequently at the present time the problem
of the old installations with fixtures often of barbarous design. If
these were short-lived, one problem of poor illumination would be
solved; but while the residence owner may quickly see the disad-
vantage of antiquated plumbing and remedy it, he and his' anti-
quated lighting fixtures "grow old together." However, if the solic-
itor, directly in contact with the owner and the builder of the small
residence, had a knowledge of even the elementary rules of illumina-
tion, there would, I believe, be a vast improvement.
The effect whether conscious or unconscious of the harshly or
insufficiently lighted home, is as subtle and as uncomfortable in the
case of a small residence as in that of the large. It is true that in
the small residence the question of expense must be more carefully
395
396 ILLUMINATING ENGINEERING PRACTICE
considered than in the large, yet when the owner realizes that in-
correct illumination often means larger bills than proper illumination,
he is, as a rule, anxious to have the trouble corrected. Poor lighting
is by no means always attributable to a desire on the part of the
owner to economize, but is more often due to a misunderstanding
as to what constitutes correct lighting.
In the lighting-service salesman's mind, selling and service should
be side by side. The average residence owner must be credited
with common sense, and it is reasonable to suppose that if the solic-
itor explained intelligently the essentials of correct illumination, the
owner would not knowingly select the incorrect. When proper
lighting is better understood, it, rather than economy, will be the
primary factor.
I believe that the illuminating companies are alert to the bene-
ficial results of an understanding on the part of their solicitors as to
what constitutes a home correctly lighted, and desire to cooperate
with architects, fixture designers and decorators in this respect.
These companies do realize the great need to the public, and there-
fore, to themselves, of proper installations and illuminants in the
home. This can be judged in a way from their advocacy of the
most efficient lamps, their adherence to the policy of free advice
to present and prospective customers, and to their support of the
departments of illuminating engineering.
No fixed rule can be given for home lighting. There always arises
the question of the use to which the rooms in the house are to be
put by the individual owners, their personal tastes, whether artistic
effect is desired, or whether it is entirely a question of economy. I
am sure no matter what the residence owner's taste may be, there
always exists the desire to have the lighting artistic and efficient and
the cost reasonably low.
PSYCHOLOGICAL ASPECTS
The principal object in home lighting is without question the
psychological. It is our earnest desire to produce a plan of illumi-
nation that will be pleasing and agreeable to those who linger in
its presence. It is a recognized fact that our visual perceptions and
sensations are agreeable or disagreeable, pleasant or unpleasant.
The illuminating engineer must give this fact great consideration.
A too brilliant light source takes away that atmosphere of restful-
ness nearly always desired and under it one is prompted to sit up
JORDAN: LIGHTING OF THE HOME 397
straight on the edge of a chair rather than to sit peacefully at ease,
as one would feel like doing in the presence of a reasonable amount
of light of soft agreeable colors.
After all, proper and correct illumination is that which obtains
pleasing and agreeable results and effects. Surely, the emotional
factor, is very important in the lighting of the home on account of
its direct influence upon the emotions of both the conscious and the
subconscious mind.
People have stated that the true indirect system in small interiors
has a most peculiar effect on the mind; some complain that it gives
them the blues, others say that it makes them feel depressed, and I
personally do not favor it for residence lighting.
Psychology enters into the consideration of aesthetic effects and
also the physical. It is a familiar fact that artificial lighting has
been done heretofore practically with illuminants giving much yel-
low, the colors blue and green being deficient until very recent times.
I believe that the most agreeable effects are obtained by illuminants
that give a proper proportion of yellow and red. For instance,
light that has a sufficient percentage of yellow and red produces a
very agreeable effect upon the complexion, whereas one that does
not have a sufficient proportion of yellow and red will "show up"
wrinkles and freckles and produce a disagreeable, harsh appearance.
This fact points directly to the advisability of using shades to tone
the color.
Time will not permit going far into the psychological aspects of
illuminating engineering, my intention being to mention it briefly
in so far as it has a direct bearing upon the lighting of the home.
PHYSICAL ASPECTS
It is conceded that most of the eye troubles of to-day are traceable
to the fact that we are using our eyes much more than heretofore,
and that much of our reading is now done in the evening. By the
infinite possibilities of lighting equipment, the problems as pre-
sented to the layman are at present, and have been for some time
past, rendered comparatively easy, the limitations placed upon him
being comparatively few. If he decides that one system is bad, he
tries another, or increases the intensity of light, and the whole time
he may be getting deeper and deeper in trouble. Here is where
such unlimited freedom may and often does form a dangerous gift.
The allurement to excess in the quantity of light, is always present.
398 ILLUMINATING ENGINEERING PRACTICE
In the days of our forefathers, lighting problems were very simple;
the tallow candle or the whale oil lamp furnished all the light con-
sidered necessary, and in many cases the newspaper or books of
those days were read by the light from the fireplace. That the per-
centage of eye troubles was less than at present is probably due to
the fact that one could not read for any great length of time by those
methods, reading matter was not as common then as now, and people
usually retired shortly after dark. At the present time there is no
limit to the kinds of magazines and papers possible to obtain at any
newsstand, and the polish is such that most of their pages could
almost be used as a reflector in a projector lantern.
There is no question that the eye has become accustomed to light
received obliquely from above. This, I believe, is one of the rea-
sons the eye is affronted by light, harsh or strong, coming too
brightly from any other direction. The need of giving serious
thought to the lighting of the home from a hygienic standpoint is
at once apparent, because the faculty of sight is of supreme im-
portance. The aim should be not only to have the necessary light
to hold the eye to its regular work, but also give the eye its normal
amount of vision. Eyesight declines with passing years, and illu-
mination in the home must be of such a character as not to increase
this disadvantage.
The old rule that light for reading should come obliquely over
the left shoulder, well hints that direct rays should be kept out of
the eye. In lighting a room for reading or for work that is pro-
longed, it is always desirable to avoid too strong shadows, and glare
either direct or reflected, while not doing away with shadows
altogether.
ARTISTIC ASPECTS
The present-day lighting service requirements in the small
residences, in the homes of the middle class, is low cost and utility.
In the larger residences, the homes of the rich, the selection of fix-
tures may be governed almost entirely by artistic considerations.
Here the words science and art may be synonymous, and there
arises the opportunity for the illuminating engineer to combine
the two in the production of devices for agreeable and pleasing
effects.
It is desirable that all illumination when possible, shall be aes-
thetically correct. When one considers that the quantity or
quality of light, or type of fixture adds or detracts from the
JORDAN: LIGHTING OF THE HOME 399
arrangements and the decorative appeal of a room, one recognizes
the necessity of giving these much thought. The purpose for which
the room is to be used and its character must receive consideration.
In the large residence, in many instances, only an artist can do
justice so far as fixtures are concerned.
One of the blessings of to-day is that lighting auxiliaries are more
artistically designed than heretofore, and there is not left, therefore,
much excuse for inartistic lighting equipment.
In these days of period furnishings great care should be taken in
the selection of fixtures. There are many cases when efficiency
must be sacrificed in order to permit the use of a fixture absolutely
in harmony with the surroundings and the period. For example,
in a large parlor of the Louis IV period, with its gold furniture with
light coverings, delicate hangings at the windows, and other decora-
tions in keeping with the times, imagine how a shower, a semi-
indirect, a true indirect, or a Colonial fixture would look! A room
of this type demands a chandelier of the time with its cut-glass, and
if the client really has the courage of his convictions, he may equip
it with real candles; but if he has not quite reached this point, the
candles can be replaced by the candle lamps which probably will
be cleaner and cause less trouble.
Occasionally a dining room is furnished in early English style with
the carved table the old court cupboard and the Jacobean sideboard
and chairs, the setting being provided by a spacious room with
patterned ceiling and oak-paneled walls. In a room of that char-
acter the ordinary stock fixture would be out of the question, and
one would make use of a more ornate chandelier, multiple wall
brackets and candelabra lamps.
One of the first things to understand even from the briefest study
of period furnishings is that all furniture and all kinds of decoration
that have come down to us weighted with historic tradition, were
evolved as a natural result of certain conditions of life; hence the
various types that were commonly used together, will always look
well when brought together.
It is of very great importance to study the lighting problem from
every angle, and if necessary to arrive at the conclusions in the selec-
tion of fixtures by the process of elimination. In no case should one
upon entering a room immediately decide upon a certain type of
fixture simply because he recently saw one at a fixture house that
impressed him.
Artistic aspects and beauty in a room, are a matter of harmonious
400 ILLUMINATING ENGINEERING PRACTICE
relationships, and good taste in illumination demands a correct
association of fixture and of light, with the proper background
setting.
PRACTICAL APPLICATION
The lighting service representative in search of the residence class
of business has probably the best opportunity to start the client on
the right track, because he is usually the one to come in contact
with him first, and it is needless to say that he should possess a
comprehensive knwledge of the subject of home lighting.
He should, of course, be familiar with the different sizes of lamps,
candle-powers, wattages, cost of operation, and should possess
other practical information. It is obvious that should he be possessed
of technical knowledge, his value to his company might be in-
creased; but the salesman's scientific knowledge is usually limited
and perhaps fortunately so, for a technically trained man is seldom
a clever salesman.
Not infrequently when a customer equips his house initially with
the most efficient types of lamps, replaces the burnt-out ones with
old carbon lamps, the change comes so gradually that it is scarcely
noticed until the bills show nearly double as much energy used as
previously, and the result is a complaint.
Another factor which enters surprisingly into the economical use
of lighting is proper and convenient switching. For instance, if
the entrance hall lamp is not controlled from the second floor as
well as from the first, it may be left in service much longer than is
necessary because some one on the way up stairs may have forgotten
to turn it off, and is too lazy to retrace his steps.
Another method of wasting energy is that of the careless or
neglectful person who goes down cellar to "fix the furnace"
and allows the cellar lamp to remain in service all night. To
obviate this condition a pilot lamp could be installed over the cellar
door where it would prove as useful as one for a flatiron or a range.
Another cause of high bills is misplaced outlets. I have seen out-
lets so badly located that it was necessary to produce approximately
10 foot-candles in one end of a room in order to obtain the necessary
2 foot-candles at the other. Obviously, the lamps should be so placed
as to produce light where it will be most used, thus not only adding
to the pleasing effect of the illumination, but reducing the cost of
lighting. Such matters are entirely under the control of the builder
and contractor. When we realize how limited the appropriations
JORDAN: LIGHTING OF THE HOME 401
are for wiring some of the smaller houses, we wonder how they
provide as much as they do. This remark applies principally to the
" ready built" houses, where financial gain is the only thing thought
of. It is unfortunate that this evil exists, but at present there
seems to be no remedy.
Globes or shades, the indispensable adjuncts to the lighting
fixture, are made in all shapes and sizes and of all colors of the
rainbow. Some are so thin that they are of scarcely any help in
concealing the lamp filament, and others are so dense that barely
any perceptible amount of light can be obtained through them;
both of these extremes are to be avoided. The kind of glass selected
should be given considerable thought, as glass absorbs, transmits
and reflects.
The safest way of protecting one's self against bad fixtures is to
reduce the fixture appropriation to a point that will insure simplicity.
The worst fixtures to be seen are the gaudy ones of medium price,
where an effort has been made to obtain a highly decorative effect
without the skill in design and finish in execution really necessary
for good results.
The difference in cost of installation and fixtures between good or
bad never is so wide that the builder would not select the good if he
realized the evils of the bad. Houses sell more readily when they
contain practical and artistic electrical equipment.
COOPERATION
A realization of the importance of illuminating engineering in the
vast fields which are opening to us have demonstrated the desirabil-
ity for the cooperation of the illuminating engineer with the architect
and the decorator. A new profession, without doubt, is in the process
of development. My architectural friends inform me that they are
depending on illuminating engineers more and more every day for
knowledge and aid. It is a fact that illuminants and new devices with
their intricate details are being developed so rapidly that even those
who make special studies of them can hardly keep pace. Granting
this statement to be true, I fail to see how the architect or decorator
can afford to spend as much time on illuminating problems as
necessary, without doing so at the expense of his profession. It is,
therefore, advisable for the architect and the lighting expert to work
together to obtain the results for which both are striving.
The engineer should be consulted where architectural changes are
26
4O2 ILLUMINATING ENGINEERING PRACTICE
contemplated, or, where special lighting is wanted to emphasize
architectural effects, and the architect has an equal right to be ad-
vised of anything that concerns the house he has designed. We
have fewer occasions to consult with the decorator, but the same
conditions apply.
LIVING ROOMS
In providing the lighting for the living room, consideration must
be given to the fact that of all the rooms this one is most used
by the average family; as this room is utilized for many purposes,
a somewhat elastic lighting scheme should be arranged. In addition
to being the library of the home, it is often used for social affairs,
such as card playing, and dancing, and at other times one or more
members of the family and their friends simply desire to lounge about
and converse. In most cases, more time is spent in reading than
at anything else, and it will at once be seen that good lighting is a
very necessary source of comfort and one to which the utmost con-
sideration should be given. There are a number of ways of providing
light suitable for reading.
One way would be to illuminate the room so brightly that one
could see to read in any part of it, but this method would prove
very costly and consequently out of the question in the majority
of rooms, and certainly would not be considered artistic.
Often selection is made of a portable lamp fitted with an opaque
reflector that will throw the light on the reading matter, but this
type of lamp, while admirable for reading, is of so little service in the
general lighting of a room that it cannot, or should not, be considered
seriously in the scheme of general illumination,
Some people attempt to obtain light for reading from the chande-
lier above by directing the rays downward, or by attaching a short
extension cord to the fixture and equipping the lamp with a pris-
matic or even an opaque shade. This scheme is satisfactory for
the reader, providing the pages are turned at such an angle that he
does not receive the glare from the paper, but it is a makeshift
arrangement, unsightly, and should not be encouraged.
Often in homes where electricity is employed for lighting use is
made of a kerosene lamp, commonly called a " student's lamp" for
reading, not really as a matter of economy, but to do away with the
supposed eye-tiring, uncomfortable glare from the incandescent
lamp, which bad reputation comes from the use of a too brilliant
lamp unsuitably placed. One can quite readily duplicate the
JORDAN: LIGHTING OF THE HOME 403
effect of the kerosene lamp with electric or gas lamps and with great
added convenience.
One reason that the table lamp is commonly preferred for reading
is that it does away with the glare that is likely to come from a
chandelier or a bracket. Glare could be avoided by assuming a
proper position for reading, by the proper turning of the pages of a
book to avoid it, or by the use of suitable shading. Care should be
taken to select a table lamp that gives the proper light all around the
table and upon the reading matter, rather than on the top of the
table, where one can consider that a large amount of the light is
wasted.
The style or type of lamp does not matter much, as long as the
shades are not dark and are wide enough to allow the light to cover
the page of a newspaper, held by a person sitting near.
The selection of the shade for the reading lamp is one that natur-
ally lies within the control of the purchaser. Unfortunately, he
or she only too often considers the question of ornamentation
before the practicability of the lamp and the use to which it is
to be put. Individual taste, after all, is the root of much of the
present-day evil in illumination. Naturally, in the selection of a
reading lamp for the home, there is to be considered the number of
persons likely to use the lamp at the same time.
If the living room is small, say 14 to 15 feet, and is furnished with
a table in the center, the table is the logical place for the lamp,
and there will be ample room for several persons to sit comfortably
around it.
Where economy in maintenance is the object, the single table
lamp for a small room can be recommended, as the same lamp can
be used both for reading and for the general lighting of the room,
provided it is equipped with a globe or shade that while concentrat-
ing a considerable portion of the light within the reading area, will
also allow enough light to radiate in all directions to give fairly good
illumination in other parts of the room. When use is made of three
or four proper-sized lamps, this arrangement is admirable for read-
ing, and one is not likely to be troubled by glare from a page of white
paper because the light comes principally from one side. To supply
electrical energy to the table lamp an outlet in the floor, under the
table, is to be preferred and recommended. Of course, here enters
the question of expense, too often uppermost in the small residence
owner's mind, but the slight extra expense may be explained to him
from the artistic side and the view-point of comfort. He may think
404 ILLUMINATING ENGINEERING PRACTICE
that a cord could be extended from the fixture that is likely to have
been placed above to take care of the table lamp, but a cord dangling
above the table is not a source of eye-gratification and always seems
to have a knack of hanging in the way.
Wall brackets would hardly be required in a living-room so small,
from either a practical or an artistic stand-point, but if there hap-
pened to be a mantel, and of a style which positively demanded some-
thing to satisfy the artistic taste, two tiny portable lamps equipped
with small candle light sources or brackets could be placed, one on
either side of the shelf. In decoration they should harmonize with
the surroundings, but they have little value from a lighting stand-
point.
If the living room is a large one, there will probably be two or
three tables scattered about, and use can be made of a suitable
lamp on each table. These lamps can be used for reading, or simply
for ornamentation. A room thus equipped with lamps ornamented
with colored silk or art glass shades produces a very artistic effect
when in harmony with the furnishings of the room and adds greatly
to its charm.
The next thing to consider is the general illumination of the above
two sizes of rooms. In the case of the smaller, either direct or
semi-indirect lighting would be proper. If semi-indirect is decided
upon, and the height of the ceiling is about 9 ft. the top of the bowl
should be from 30 to 36 in. from the ceiling. Assuming that the bowl
is 6 in. deep, there would be left 6 ft. 9 in. head room. Fortunately
one has a large assortment of artistic and efficient stock bowls to
select from, some in colors and some white. In selecting colors in
a bowl care should be taken to see that they do not clash with the
furnishings of the room. If there arises any doubt on this point,
the pure white will surely give satisfaction. It is an easy matter
to install colored lamps of any size when a particular color scheme
is desired.
In case direct lighting is the choice, use should be made of the
multiple style of fixture. There are so many styles of this kind of
fixture that it would be unreasonable to specify any particular one.
To see that the lamps have frosted bowls and are properly shaded
with soft colored shades, if desired, to harmonize with surroundings,
is the most important point.
This general lighting unit should always be controlled by a wall
switch conveniently located at the entrance to the room. Economic-
ally, it would be advantageous to have the units for general illumi-
JORDAN: LIGHTING OF THE HOME 405
nation wired in two or more circuits so that a small amount of illumi-
nation can be used when full intensity is not needed.
In designing the general illumination for a large oblong room, a
more difficult problem is encountered, and it is here that the coopera-
tion of the illuminating engineer with the architect brings about the
best results. If the ceiling is plain, that is to say, has no beams and
no other fancy decorations, two ceiling outlets could be provided,
one in the center of each half of the room, the type of fixtures to be
semi-indirect or direct, as desired. Ceiling units should be so
selected and installed that they do not break up the continuity of
the ceiling. If the furnishings are to be distinctive, say, for instance,
Colonial, the semi-indirect type of fixture would be entirely out of
place, and use should be made of special fixtures of a Colonial char-
acter. Fixtures of this type have been on the market for some time
and are attractive by day and efficient by night. If the furnishings
are not of any special design or period, then the semi-indirect unit
could be employed.
In case of a very low or beamed ceiling, the fixtures could be
omitted, and a number of multiple wall brackets supplied, for when
properly designed they are very ornamental whether lighted or not.
A room thus equipped with decorative wall brackets and table or
floor lamps is very attractive.
In a room having a fireplace and mantel, there should be an outlet
on either side, their location on or beside the chimney depending
upon the type of fireplace. If the brick work extends to the ceiling
and is not too elaborate, a bracket over each end of the shelf would
be suitable. If ornamental brick-laying was attempted, the brackets
' could be installed on the wall at the sides. If the mantel is of wood
and somewhat delicate in design, a more delicate type of wall bracket
should be selected.
DINING ROOM
Probably next in importance to the living room is the dining room,
which in some small residences is used as a living room. Here, after
the evening meal, gathers the family around the table, some reading
and some otherwise engaged. In a room of this character, one would
not hesitate to recommend the art glass dome provided the table is
the center of attraction during mealtime and the light in other parts
of the room can be to a degree subdued. The dome, if so suspended
as not to obstruct the view of persons looking across the table, makes
406 ILLUMINATING ENGINEERING PRACTICE
a very effective and practical dining-room unit and is also admirable
for reading.
In selecting the dome one should be careful not to obtain too
gaudy colors, or one ornamented with a fringe hanging from the
edge the effect of which is very disturbing by casting its scraggling
alternating dark and light lines upon the faces and clothing or on
books and papers.
The dome could be equipped with two, three or four sockets and
round-bulb, all-frosted lamps which should be well shaded from the
eyes of people sitting in a normal position about the table.
A house of the class here considered would not be provided with
a superabundance of baseboard receptacles, and hence one or more
of the sockets in the dome could conveniently be used for any one
of the several cooking appliances. The dome circuits should be
controlled by a wall switch, and the individual lamps by pull chain
sockets.
In the dining room of the higher priced home, the lighting expert
has a better opportunity to exercise his art. Here the semi-indirect
bowl will usually meet with satisfaction. A ly-in. bowl, containing
three or four sockets, with 25-watt or 40- watt lamps, gives consider-
able leeway, the size depending upon the color of the walls and
^the amount of illumination desired. 'Economically, it would be
desirable to install a wall switch so arranged that one or all lamps
could be turned on or off.
In the dining room, as in the living room, period furnishings and
color schemes should be given thorough consideration. Semi-indirect
bowls are made in various gently tinted colors, any one of which
would tone down the light of the tungsten lamp to the soft, warm
tones so much appreciated in the general lighting of a dining room.
Bright or startling colors should be avoided, except on rare occasions.
For instance, at Christmas time one might wish to decorate the din-
ing room temporarily in red and green; at Hallowe'en time with
black and yellow; St. Valentine's Day with pink, and so on. With
the semi-indirect bowl or ceiling fixture one can easily produce almost
any desired color scheme in lighting by the employment of various
colored silks or papers.
Another attractive style of fixture is one with three or four lamps
pointing up, and equipped with colored silk shades, cylindrical in
shape, but smaller at the top than at the bottom, the colors selected
depending on personal taste and the surroundings. This style of
fixture should be equipped with round-bulb all-frosted lamps.
JORDAN: LIGHTING OF THE HOME 407
Ordinarily the delicate tints of rose, cream, yellow or amber, will be
found to harmonize with the other decorations.
With these last two types of fixtures it would be found desirable
to install a floor receptacle at about i ft. to the right of and i ft. out
from a point beneath the center of the table, the idea being to extend
the lamp cord over the edge of the table on the right hand of the
person liable to do the serving, and to dodge the central pillar of the
table if it has one. In addition to the floor outlet it might be found
convenient to install a baseboard outlet near the serving table, in
order that utensils could be used there when desired.
If the room contains a mantel and fireplace their charm would be
enhanced by a pair of candle lamps. Thus, to equip one's home
harmoniously, is to give a new charm and a new intimacy, for the
secret of the attractive home lies in the graceful blending of lighting
principles with the accessories.
It is with some hesitation that I approach the problem of illumina-
tion of dining-rooms of the palatial type. Here it is that we again
come in contact directly with our esteemed friends the architect and
the decorator. I am pleased to say that the meetings with these
people in such places are more pleasant now than in times gone by.
In stately, dignified dining-rooms large chandeliers may be used
with most beautiful effect; but they should be supplemented with
multiple wall brackets and candelabra lamps. As most of these
large rooms have period furnishings, we more and more realize the
value of greater knowledge in period styles.
In this class of dining-room as in many others, frequently with the
exception of a small lamp near the serving table, the other fixtures
are seldom used, and tallow-candles furnish all the illumination
desired. This effect is certainly very satisfying to the esthetic
taste.
LIBRARY
Few of the smaller residences have what would be called a library,
and in houses of the next grade the library and living-room are usually
joined in one. Occasionally a room is used exclusively as a library,
and by bad tradition it is usually in rather dark finish. Additional
absorption of light is encountered when the walls are lined with
bookcases filled with books. Here sufficient general illumination
should be provided by ceiling fixtures to enable the titles of books
to be clearly read. Wall brackets could hardly be recommended in
a room of this character, since with these it is more difficult to light
408 ILLUMINATING ENGINEERING PRACTICE
properly the bookcases which lie nearly in the same plane with the
brackets; and again, in locating the outlet for the bracket almost
any place upon the wall is liable to interfere with the bookcase space.
In this room proper reading lamps are of primary importance, and
the same suggestions for reading lamps as have been made for read-
ing lamps for the living-room previously described, would apply,
not forgetting that a generous supply of floor or baseboard plugs is
especially useful.
MUSIC-ROOM
The average home does not have a room, as a rule, devoted ex-
clusively to music, but occasionally such a room does exist. Bril-
liant lighting is not necessary here except when the room is used for
other purposes. However, near the piano, which instrument
usually gives the room its name, considerable localized light should
be provided. This is best accomplished by means of a portable floor
lamp equipped similarly to the reading lamp as previously described,
supplied with energy from floor or base-board plug. As certain
occasions may require the presence of several musicians or enter-
tainers, a soft general illumination is often necessary. In all cases
the lamps should be well shaded from the eyes of the guests.
Wall brackets are objectionable from the fact that when lighted,
they are eternally shining into people's eyes, much to their distress
and discomfort.
The music-room is probably the only one in a residence where
cove lighting could be considered. Before decision is made, how-
ever, due thought must be given not only to the client's pocket-
book, but also to the possibilities of the lamps and trough being
kept clean which they generally are not.
In case use is rriade of the semi-indirect or chandelier source for
general illumination, it must be carried high enough so as not to
distract the attention, or in any way interfere with the field of view,
and the lamps must be thoroughly shaded.
DENS
From the character of "den" rooms, it would seem desirable to
provide considerable general illumination, due to the fact that the
walls are usually decorated to the ceiling with pennants, crossed
swords, trophies, Indian relics, skulls and other articles of a rather
JORDAN: LIGHTING OF THE HOME 409
gloomy nature. It would be a pity for a guest to miss seeing any
of these most interesting curios.
The finish and furnishings of these rooms being usually dark,
more energy should be provided than in ordinary rooms of the same
size. In case of a small room, a fairly deep semi-indirect bowl
equipped with a loo-watt lamp, or, if the room is large, a shallow
bowl containing three 40-watt lamps, would make a well lighted
room.
Should localized light be required for a desk or table, I doubt if
the type of lamp that would likely be selected to harmonize with
surroundings would be suitable for either writing or reading, its
only value being for decorative purposes, and it would be serviceable
only when a little light is desired. The lamp cord should be con-
nected to a baseboard receptacle and have chain pulls. A wall
switch near the door should control the ceiling fixture.
SUN PARLORS OR CONSERVATORIES
These rooms being usually filled with plants and flowers, some of
which grow to the ceiling, soft, general illumination is required, and
the semi-indirect bowl will produce a beautiful effect. In addition
to the general illumination a table lamp will prove very serviceable.
KITCHENS AND PANTRIES
These are the working portions of the house and should receive
careful consideration. In the small kitchen a 60- watt lamp equipped
with a shallow prismatic reflector, located well up in the center of
the room, will give the light required in all parts. In the small
pantry one 2$-watt or 40- watt lamp, similarly equipped over the
working table, would provide ample light.
In the large kitchens, lamps should be installed at one or more
points, as the arrangement of the working space requires, such as
the stove, the sink, and very likely a working table. In case of a
hooded range, one or two lamps should be placed under the hood.
A ceiling lamp should be provided for general illumination, under
control of a wall switch conveniently located; in some cases it may
be found convenient to control this lamp also from the second floor.
All kitchens should be equipped with receptacles for flatirons and
other appliances. Butlers' pantries, where glasses and dishes are
washed and stored, usually require two ceiling fixtures, one over the
410 ILLUMINATING ENGINEERING PRACTICE
sink and another to illuminate the shelves. Twenty-five watt
lamps, with enclosing shades, furnish the proper equipment.
HALLS
In most of the new residences the halls are almost as much a
living part of the house as are some of the other rooms. In nine out
of ten of the houses a person in the living- or dining-room, can see
into the hall. With this arrangement the lamps should be carefully
shaded so that the glare may not be offensive to persons in the other
rooms. This remark applies in the case of direct lighting by lantern
or small fixture.
A semi-indirect unit makes a pleasing hall light source. The bowl
selected should be of the deep type and not very large. If the hall
is long, and the back part is left in comparative darkness, it may be
necessary to supplement the principal fixture with wall brackets.
In case of an exceptionally low ceiling, wall brackets are to be pre-
ferred, the lighting being balanced by means of one or two on each
side wall, depending upon the length of the space. The fixture, or
one of the wall brackets, should be controlled by a switch near the
front door and also from the second floor.
The second floor halls generally do not require as much light as
do the lower halls. If the ceilings are low, say 8 or 9 feet, a hemi-
sphere or squat ball makes a good unit. If there is only one lamp,
it should be placed near the top of the stairs. If the hall is large,
another should be installed in the other part. Both of these should
be controlled by wall switches and the one most used should
be controlled also from below.
Wall brackets if used at all in halls or passageways should be
installed rather high, because there is danger of persons running
into them in the dark.
Back halls and passageways require very little light. The loca-
tion of fixture should be such that the lamps will thoroughly light
the stairways.
In the halls of the more imposing character, fixtures of special
design will be necessary. These should give a well diffused light
and fairly soft shadows. Should the upper part of the room be
decorated with special architectural features, due consideration
should be given this fact, and arrangements should be made to give
them proper significance. Not infrequently there are paintings, and
these also may require special treatment, by lamps placed to illumi-
nate them directly.
JORDAN: LIGHTING OF THE HOME 411
BEDROOMS
In the bedrooms the handiwork of the feminine sex will be every-
where in evidence. Probably in no other part of the house will she
be as much concerned as here, she has decided upon the location of
the dressing table and planned to have the various rooms decorated
in appropriate colors. It is necessary for the engineer to know what
these colors are to be in order that the shades selected may be of a
tint to match. I mention these frivolous matters first, not because
of their importance, but because in the lighting of the home similar
matters are first brought to the attention of the illuminating engineer
by the lady of the house, and if he fails to gain her favor he may as
well stop before beginning.
It is a recognized fact that the bedroom suffers more from mis-
placed fixtures than from insufficient light, for the amount of light
required is not large in rooms of medium finish.
Now, assuming that the architect has provided space enough
between the windows for a dressing table, and then a little more, one
can recommend a pair of swing-arm wall brackets, one on either
side of the mirror, as the most satisfactory lighting equipment. If
the cheval mirror or a mirrored door is among the articles of furni-
ture, it could be equipped in a similar manner. When it is definitely
known where the other articles of furniture are to be placed, one can
easily provide lighting for the remainder of the room.
Wall brackets, properly shaded, are to be preferred for bedroom
lighting. For those who follow the practice of reading in bed, an
additional small reading lamp can be installed for use beside the bed.
This lamp should be controlled by a pull chain or pendant switch,
easily reached from the bed.
There is no serious objection to the use of a ceiling lamp in a
bedroom, but it should be resorted to only in cases where extreme
economy must be observed.
At least one lamp in the room should be controlled from a switch
beside the door.
BATHROOMS
There is little to be said about bathrooms. The best method is to
install two wall brackets about on a level with a person's face, one on
each side of the mirror. These will not only give ample general
illumination in the largest of bathrooms, but are most suitable for
shaving.
412 ILLUMINATING ENGINEERING PRACTICE
In very small rooms one center ceiling lamp does very well when
placed high. A switch should be placed near the door.
In this room should be placed a receptacle for a water heater.
CELLARS AND LAUNDRYS
Cellars usually require only a very moderate amount of lighting.
Use could be made of one 25-watt tungsten lamp near the foot of the
stairs and another in front of the heater. If the cellar is sub-divided,
other lamps placed in the dark portions will be helpful. The lamp
near the foot of the stairs and the other one near the heater should
be controlled by a switch at the top of the stairs.
There is seldom much work to be performed in the laundry after
dark, but in city houses, the laundries are often so located that no
daylight enters at all. Here there should be one lamp over the
tubs, one over the ironing table, and one for general illumination.
The lamp over the table and tubs should be equipped with steel
porcelain enamel reflectors, and controlled by key sockets. The
center lamp should be controlled by a switch located near the door.
Receptacles should be installed in the laundry for washing jnachines
and flat irons.
PORCHES
The porch does not require any high intensity of illumination,
for we would refrain from making it look like a store front. The
purpose for which a porch lamp is used is welcoming the arriving
and speeding and the departing guests, illuminating the steps and
enabling the people in the house to scrutinize a caller.
If the front porch lamp has any value for the last-mentioned
reason, then why not a back porch lamp? Its advantage would at
once be appreciated by the average woman in the home, when the
back door bell rings, perhaps late in the evening, or when she is
alone in the house. A 25-watt lamp in an enclosing ball near the
ceiling of the porch and controlled by a switch inside the door, is
all that is required.
GENERAL REMARKS
I am not in favor of elaborate massive fixtures anywhere in the
ordinary house. The more simple and unobtrusive ones are much
to be preferred. In selecting fixtures for the whole house, the nearer
they match each other in design the better. The metal parts should
JORDAN: LIGHTING OF THE HOME 413
particularly be of the same design and finish, although the glassware
or shades can be different in shape and color.
In other words, in the smaller residences all the fixtures should
be as nearly the same style as possible, or the house will look like
a second-hand fixture establishment. In large houses, the type of
decoration is the controlling element.
In conclusion, I want to call attention to the fact that central
station managers are beginning to realize the importance of in-
creasing the residence load. The great variety of house wiring
campaigns now under way certainly emphasize this fact and great
effort is being made to equip old houses for electric service.
In the territory of the Edison Electric Illuminating Company of
Boston alone, there were many thousands of unwired houses before
the house wiring campaign was started, and at the end of three
years about 4000 of these houses had been wired on the "Easy
Payment Plan" with an approximate annual income of $100,000
an average of $25 per year, per customer. This means an addition
to the lighting load of about 4000 kw. which load, coming as it does
on a late peak, is very desirable, a fact that is being appreciated
more and more.
THE LIGHTING OF STREETS PART I
BY PRESTON S. MILLAR
This lecture is to be considered as complementary to that of
Mr. Lacombe which is to follow. The two are intended to em-
brace the entire subject of street lighting. The division between
the two is arbitrary and of course cannot be absolute.
HISTORICAL
. Within the limitations of time imposed it is not desirable to dwell
upon the historical aspects of the subject. The evolution of street
lighting from the torch bearing stage through the period of candle
and oil lamps, maintained individually by citizens,* up to the begin-
ning of community or municipal maintenance, has little practical
bearing.
The modern history of street lighting begins with the invention of
illuminating gas and assumes its second interesting phase with the
development of the direct-current series carbon arc lamp. Follow-
ing these two stages of progress, the historical aspects of street
lighting naturally ramify into the history of street illuminants, of
street lamp mountings, of street lamp equipments and accessories
and of the development of ideas of street lighting principles. These
several phases of the subject are treated under their respective head-
ings in either this lecture or in Mr. Lacombe's lecture.
PURPOSES
The earliest street lighting was adopted as a measure of protection
for the wayfarer against the criminally inclined. At first this pur-
pose was served partially by the carrying of torches.
* Some milestones in the historical progress of street lighting are as follows:
1558 Paris the first modern city to attempt civic street lighting. Inhabitants ordered
to hang lighted candle lanterns in front of houses.
1766 Pitch or resin bowls substituted for candle lanterns.
1809 First street lighting by illuminating gas in London.
1821 Illuminating gas used in Baltimore for street lighting.
1884 Arc lamps used in Philadelphia for street lighting.
1896 Gas mantle lamps first used in this country for street lighting.
415
41 6 ILLUMINATING ENGINEERING PRACTICE
Then the lamps were placed permanently along the way, affording
better protection and also marking the route. From this stage street
lighting was evolved into a means of safeguarding traffic against
collision and of revealing obstructions or holes in the roadway.
More recently it has been developed into an ornate system which not
only accomplishes these purposes but also promotes commerce and
embellishes the highway both by its own artistic qualities and by the
lighting effects which it produces.
Modern street lighting serves the combined purposes which have
been named, these being fundamental perhaps in the order in which
they are mentioned. The protective element, however, is sometimes
taken for granted, and most, if not all, attention is directed to the
last purposes named. All street lighting ought to serve these several
purposes and the importance of each purpose in a particular installa-
tion must depend upon the local conditions, principal factors among
which are traffic density, real estate development and criminal
hazard. In portions of cities in which evil resorts exist, the police
protection factor is of first importance. In interurban highways
the marking of the way and provision against collision are essential.
The dignified, high-class avenue must be so lighted as to reveal its
character by exhibiting to advantage the architectural features of
the buildings. For police purposes the lighting should be designed
primarily to reveal large objects on the street and sidewalk.
To serve the purposes of the automobilist the same requirement
exists, and in addition raised places or depressions in the roadway
should be revealed, and the curb or limitations of the driveway should
be perceptible. His is the most difficult requirement to meet be-
cause of the high rate of speed at which he travels. Safety requires
that he must be able to detect the presence of objects in a single glance.
The pedestrian requires somewhat the same lighting design, though
he is especially concerned in having inequalities in the sidewalk
revealed. In the important streets he should be able to distinguish
faces of passers-by.
From the point of view of aesthetics the fundamentals of design in
all visible portions of the lighting system should be observed. This
applies especially to posts, fixtures and glassware. The incongruous
should be absent; nor should the general effect of the street be
neglected. The fixtures should be of pleasing design in themselves
and suitable to their surroundings. They should be installed at such
intervals and in such locations as to enhance the attractiveness of
the street by day and by night.
MILLAR: LIGHTING OF STREETS 417
Minor requirements of street lighting present themselves. For
example, one may wish to observe the time indication of his watch,
or to read an address on a card, or to find an article which he has
dropped, or to read a number on a house front. All such require-
ments are distinctly minor and should not have an important place
in a discussion of this kind. Many of them can be met by moving
near to a street lamp; others occur so infrequently as not to demand
much consideration. In general, lighting which serves the major
purposes outlined above serves also these minor purposes as well as
they can be served with a given expenditure for lighting.
All of the foregoing may be summed up in a statement of the
purposes to be served by street illumination put forward by the
Street Lighting Committee of the National Electric Light Associa-
tion in 1914 as follows: 1
1. Discernment of large objects in the street and upon the sidewalk.
2. Discernment of surface irregularities on the street and on the sidewalk.
3. Good general appearance of the lighted street.
In studying the street lighting requirements for a particular
locality, much time and thought profitably may be devoted to defin-
ing for that locality the principal purposes of the street lighting.
With these purposes clearly in mind, best efforts are more likely to
be made to design the installation in conformity with the require-
ments. Most serious mistakes in the design of street lighting have
occurred because those responsible have failed properly to analyze
the purposes to be served by the lighting and have proceeded in
design without sufficient thought or according to preconceived ideas
of the requirements of good street lighting. It is almost as unreason-
able to design street lighting without reference to the local require-
ments as it is to design interior illumination without reference to
the character and decorations of the room.
EXTENT AND SCOPE
Street lighting is regarded as one of the indispensable functions
of municipal government. As a rule in this country it is rendered
by a private corporation under contract with the municipality.
Cities, towns, and even the smallest villages are lighted very gen-
erally. It has been stated that the lighted streets of New York
City alone, if placed end to end, would form a single lighted highway
extending from New York to Reno, Nevada.
27
418 ILLUMINATING ENGINEERING PRACTICE
It is the usual practice to adjust the intensity of street lighting
to the real estate and traffic conditions of streets, lighting the im-
portant streets more brilliantly than the secondary streets. Ap-
propriations for street lighting purposes are often somewhat inade-
quate with the result that all classes of streets are not as well lighted
as the engineers in charge would like to have them.
In recent years the great growth of automobile traffic has com-
plicated the street lighting situation in a number of ways for one
thing the use on the automobile of too brilliant lamps at night has
introduced a serious problem. Where street lighting is so inadequate
as to make their use important, automobile lamps for lighting the
roadway safeguard the automobilist but introduce a large measure
of annoyance and some danger for other travelers. Where street
lighting is reasonably adequate within municipal limits, it is en-
tirely feasible to abolish the use of head-lamps, as is demonstrated
by the experience of some years in New York City. If instead of
seeking to eliminate the difficulty solely by limiting the light from
head-lamps municipal authorities would see to it that the street
lighting is adequate, the whole problem could be dismissed by pro-
hibiting the use of head-lamps within the city limits.
The advances of the last ten years in efficiency of light production
have led to considerable improvement in street lighting. Not all
of these advances have been put into increased lighting; part have
been realized by municipalities in reduced lighting costs. Thus
the great opportunities offered by the new illuminants for better
street lighting have not been grasped in their entirety, though very
large improvement has resulted.
In the last two or three years marked impetus has been given to
the lighting of village roads and of rural highways. 2 Counties,
towns and villages are awaking to the value of such highway light-
ing. Street lighting growth js reactive. If a street is lighted be-
cause there is traffic requirement for lighting, the result is increased
traffic, which in turn brings the demand for better street lighting.
The tendency of municipalities to appropriate inadequate sums
for street lighting has offered an opportunity which the American
business man has not been slow to grasp, with the result that there
has entered into American practice the merchants ' display lighting
system, the so-called " white way" lighting, whereby a given street
through the activity of a merchants' association is much more
brilliantly lighted than adjacent streets. This is considered good
advertising in that it attracts people to the locality. Installations
MILLAR: LIGHTING OF STREETS 419
of this sort are generally of distinctive design; many of the earlier
ones were of the incandescent lamp cluster type; 3 some consisted of
arches across the street; others of festoons of lamps mounted over the
curbs. The more recent installations have consisted of a post and a
single ornate lighting unit enclosing either an arc 4 or an incandescent
lamp.
When well organized and operated under the strict supervision
of a strong merchants ' association or of the municipality, this spo-
radic street lighting has been successful in setting a new and higher
intensity for street lighting, tending to advance the standard every-
where. When not controlled, it has sometimes led to isolated and
ill-considered instances of merchants ' lighting which has resulted in
a hodge-podge, often three or four different lighting units being
installed on a single block, some lighted at night and others
unlighted.
In a few instances municipalities have installed special high
intensity street lighting systems at a greater expense than would
ordinarily have been incurred for the street selected, the city having
in mind something of the same considerations as have actuated mer-
chants ' associations, namely the advertising of the locality in order
-to bring traffic to it.
Prevailing intensities in street lighting practice in a measure are
dependent upon the extent and intensity of private lighting with
which they are brought into frequent comparison. In a city where
the private lighting is highly developed, the general level of street
lighting intensities is likely to be higher than in other cities. In
passing it may be noted that the converse is also true, and that the
introduction of higher intensities in street lighting tends to increase
the intensities of private lighting with which it is contrasted.
The general practice is to light street lamps shortly before dark
and to allow them to continue in service until after daybreak. The
4000-hour year in this latitude is standard. No self-respecting
city continues the archaic moonlight schedule whereby street lamps
are not lighted when the almanac states that the moon should be
shining. A more reasonable modification of lighting practice,
applicable to merchants ' lighting rather than to civic lighting, is the
reduction of the amount of street lighting after the midnight hour,
as for example, the extinguishing of a certain percentage of the
lamps.
Like many other municipal enterprises, street lighting serves
the entire populace and is a matter for community interest. More
420 ILLUMINATING ENGINEERING PRACTICE
than most such municipal activities, its status is apparent to the
citizen and to the visitor. A city is likely to be judged by its muni-
cipal undertakings, and the most casual observer takes cognizance
of the condition of the street lighting.
1 'A city is judged by impressions. It may have the finest climate
in the world; it" may be fortunately situated near rivers and rail-
ways; it may have every natural advantage that a business man may
desire. Yet if it be unattractive, dirty and gloomy, its development
will be slow. When it does develop, the first impetus will be given
by changing its appearance for the better; and in that change street
lighting will play an important part." 5
Something of this view is manifesting itself in many cities of the
country, and the newer installations are reflecting in their enhanced
attractiveness and effectiveness the municipal pride which underlies
design.
The cost of civic street lighting per capita generally ranges
from 60 cents to $1.20. It amounts to perhaps, from 2 to 3
per cent, of the total municipal expenditure. Its value must be
taken to include not alone the safety features which are its primary
purpose, but as well a part of the city growth and the promotion
of the industry and the welfare of its citizens. It may be claimed
also that the large expenditures for highway construction are
rendered of greater utility by street lighting, and that to a degree
the street lighting must be credited with the promotion of the enor-
mous traffic which these highways bear. To quote a recent
expression:
"It is earnestly believed that when the economic value of a system of
street lighting is compared with other public works, such as schools, bridges,
police force, fire department, etc., the price required to be paid for the
protection of the life and limb of the entire citizenry of the inhabitants;
the protection of our wives and daughters from crime and annoyance; the
protection of our property from burglary by supplementing the police
force by adequately lighted streets; the various conveniences to the
public, secured by sufficient illumination on the streets, and the adver-
tising and aesthetic values of adequate street lighting to the city at large,
and to the individuals the cost of an adequate street-lighting system
will be found to be insignificant compared to the value received, and the
expenditure well worth while." 6
ILLUMINANTS
Recent History. The earliest development of importance to this
discussion was that of the open carbon arc lamp and series direct
MILLAR: LIGHTING OF STREETS 421
current lighting systems. The early records of street lighting in
this country show a number of competing companies manufacturing
equipment for such systems. They became recognized standard
street lighting systems and continued as such until the development
of the enclosed carbon arc lamp in i894. 7 The open carbon arc
lamp in this country, generally speaking, did not attain to the
development which it received abroad. It is understood that the
carbon electrodes produced in America were inferior to those later
employed in Europe. The superiority of the electrodes available
and the lower prevailing cost of labor in Europe led to the continua-
tion of the open carbon arc lighting system for years after its decline
began in this country.
The enclosed carbon arc lamp by reason of its relatively long
electrode life and low maintenance cost received in this country a
measure of development which it could not experience abroad. It
possessed other advantages over the open carbon arc lamp which
aided its success, including a better light distribution characteristic
and a greater steadiness of light. In the course of the decade suc-
ceeding its invention, the enclosed carbon arc lamp became the
standard street lighting lamp in this country.
As subsidiary to the arc lamp, small illuminants were used in the
lighting of streets of minor importance. These were the gas mantle
lamp, the gasoline lamp and the series carbon incandescent lamp.
The period of modern street lighting illuminants was ushered in
by the development of the metallic electrode lamp in 1904.* This
lamp, known as the magnetite lamp, and the metallic flame arc lamp
and also as the luminous arc lamp, attained great eminence in street
lighting practice in this country during the decade following its
invention.
Within this same decade the flaming arc lamp was the subject
of much experimental and development work on the part both of
arc lamp manufacturers and of electrode makers. The same diffi-
culty of high maintenance costs in this country placed it beyond
practicability to utilize the short-life flaming arc lamps of European
development. As in the case of the carbon arc lamp the difficulty
was reduced by enclosing the arc and securing a much longer elec-
trode life. For a time the enclosed flame arc lamp with long life
electrodes bade fair to become a real factor in the street lighting
situation in this country. Several important extensive installations
were made with more or less satisfactory results. The development
of the gas-filled tungsten lamp known as the " Mazda C" lamp, in
422 ILLUMINATING ENGINEERING PRACTICE
IQI4, 9 however, introduced competition which the flame arc lamps
of present types have not been able to meet except in special cases.
The Mazda C lamp superseded a number of illuminants in the
street lighting field. It hastened the displacement of open and en-
closed carbon arc lamps which were still in the field. Its availability
in small sizes resulted in its substitution very generally for the gas
and gasoline mantle lamps which had very largely claimed the
secondary streets for their own, and of course it displaced the ineffi-
cient carbon series lamps wherever they were in service.
The development of street lighting practice is so largely involved
with the development of street illuminants that this brief account
of the development of the latter serves to recall the history of street
lighting as a whole. The development of other phases of street
lighting practice has perhaps lacked the definite steps of advance
which are apparent in the record of the illuminants employed, but
the progress has been none the less real on that account.
Modern Lamps. As electric and gas illuminants are the subjects
of other lectures, their qualities need not be discussed in detail in
this connection. Street lighting lamps in this country are as
follows:
Gas filled tungsten lamps (Mazda C).
Arc lamps (principally magnetite lamps).
Low pressure gas and gasoline mantle lamps.
The incandescent lamps are usually of the series type, though in
some cities, notably New York City, multiple lamps are employed.
American selection^ of illuminants differs somewhat from that in
European countries due to the higher labor costs and greater dis-
tances prevailing here. Thus it may prove economical for us to
sacrifice something of efficiency in order to secure lower maintenance
cost, while in Europe the maintenance cost is not so large a factor
in the total. This in part accounts for the fact that flaming arc
lamps and pressed gas lamps have not come into use largely in this
country as abroad. On the contrary, the magnetite lamp has been
used largely here and hardly at all abroad.
Through the courtesy of the Lamp Committee of the Association
of Edison Illuminating Companies, the accompanying table of data
on electric street lamps is available.
In this connection Fig. i will be of interest. This shows the
lumens produced, the watts, and by reference to the diagonal lines,
the efficiency of the principal electric street lamps. The lumens
per watt output of street lamps is by no means a final measure of
MILLAR: LIGHTING OF STREETS
423
TABLE I. RELATIVE LIGHT PRODUCING EFFICIENCIES OF MAZDA C, FLAMING
ARC AND MAGNETITE LAMPS
Mazda C lamps
Description
Bare lamp
Equipped for service as-
suming 25 per cent,
absorption in accessory
Total
lumens
Average
watts
Lumens
per watt
Total
lumens
Average
watts
Lumens
per watt
6 6-amp '' 6o-cp."
600
1,000
2,500
4,000
6,000
6,000
10,000
46.9
71.9
155-0
245-0
367-0
310.0
518.0
12.8
13-97
16.12
16.32
16.32
19.3
19.3
450
750
1,875
3,000
4,500
4.500
7,500
46.9
71-9
155-0
245-0
367.0
310.0
518.0
9-6
10.4
12. 5
12 .2
12.3
14-5
14 5
10.5
II. 5
12.7
13-5
6.6-amp., "roo-cp."
6 6-amp. "2SO-cp." .
6.6-amp., "40O-cp."
6.6-amp., ''6oo-cp."
2O-amp., "6oo-cp." (compensator).,
ao-amp., " xooo-cp." (compensator).
no-volt, "200-watts"
no-volt "40O-watts"
2,795
6,130
12,740
17.960
200.0
400.0
750.0
1,000.0
13.97
15 33
16.99
17.96
2,098
4,600
9,550
13.480
200.0
400.0
750.0
1,000.0
no-volt "75O-watts"
no-volt "looo-watts"
Magnetite lamps
Description
Bare lamp
Equipped for service
Am-
peres
Electrode
Globe
Total
lumens
Average
watts
Lumens
per watt
Total
lumens
Average
watts
Lumens
per watt
4
4
5
5
6.6
Standard
Clear
Clear
Clear
2.991
4.649
5.768
7,655
8.708
310
323
390
371
SI I
9-65
14.4
14.8
20.6
17.0
High efficiency. . . .
Standard
High efficiency. . . .
Standard . . .
Clear
Clear
7.5 I (Yellow)
A.C. enclosed flame arc lamps
Clear
8.557
480 17.8
the relative economy of the several types, but it is one important
factor entering into the problem.
Fig. 2. shows change in illuminating power and output or effi-
ciency with change in electrical values for the several types of
modern street lamps.
All lamps used for street lighting are variable as to candle-power.
There are inherent initial variations. Other variations are due
to operating irregularities, including those of supply and of main-
tenance. Arc and gas lamps are subject to additional variations
involved in adjustment, and irregularities in the light-giving ele-
ments. Most of these variations, however, do not detract materially
424
ILLUMINATING ENGINEERING PRACTICE
from the utility of the lamps in service. It is important to note
their existence to avoid misapprehensions affecting engineering
practice or specifications rather than to include them as factors
affecting the value of the lighting service to the public.
10000
9000
400
500
600
Watts
Fig. i. Typical initial values of electric illuminants.
ACCESSORIES
Since lighting accessories are covered thoroughly in another
lecture, it is the purpose here to touch upon them but briefly.
In street lighting the lamp accessory serves to protect the lamp,
to redirect the light, to reduce the brightness, and to improve
MILLAR: LIGHTING OF STREETS
425
the appearance of the unit. Sometimes all of these purposes are
accomplished; again, only one or two of them may be served. While
many varieties of accessories are available on the market, yet it is
fair to say that the development along this line is far from complete.
The characteristics which are regarded as desirable in all lamp
accessories intended to serve these purposes include: Simplicity,
sturdiness, cleanliness, low cost, and low light absorption. Ap-
parently the advantages of securing the best balance among these
desirable qualities is not generally appreciated, nor are accessories
always selected upon the basis of data concerning their various
qualities.
In the selection of accessories, after the generally desirable
qualities have been sought, the question of a desirable light-direct-
SSSSS|SSf|22:S SSJSSSSS3SS2SS22 g gSS|SS|S2 22
Percent Volts or Amperes Percent Volts or Amperes Percent Volts or Amperes
Fig. 2. Variation of input and output with change in voltage.
ing characteristic remains. This should not be considered to the
exclusion of other qualities; but, other things being equal, a generally
desirable form for various classes of streets may be indicated. Thus
in the lighting of residence streets and of highways, little or no
light is desired in the upper hemisphere, and a form of distribution
curve similar to that provided by the pendant magnetite lamp or
by the tungsten-filament or Mazda lamp with reflector or refractor
may be acceptable. On the other hand, in the illumination of prin-
cipal avenues and business streets of a city, some light above the
horizontal is essential to the good appearance of the street. Here
some form of diffusing globe becomes desirable.
The absorption of light in fixtures and accessories usually is an
important factor in reducing efficiency. Much may be accomplished
through skilful design in minimizing this loss. Containers which
426
ILLUMINATING ENGINEERING PRACTICE
completely enclose the light source are of course likely to occasion a
greater absorption loss than are reflectors which receive only a
portion of the total flux without subjecting the remainder of the
flux to absorption. In considering loss due to absorption it is well
to be more tolerant of absorption which results in great reduction of
brightness and which effects desirable re-direction of the light flux.
Absorption which results in neither desirable re-direction nor sufficient
reduction in brightness is difficult to excuse. Absorption involved in
either re-direction or brightness reduction without accomplishing the
other desirable purpose, may or may not be overlooked, depending
upon local conditions. The best design of accessories should ac-
complish both objects to a reasonably satisfactory degree.
Some statistics of absorption of light in accessories for street
lighting are presented in Table II. This indicates for reflectors
which intercept only a part of the light, absorptions ranging from 1 2
to 32 per cent. For housings with enclosing globes the absorptions
range from 18 to 39 per cent.
TABLE II. ABSORPTION or LIGHT IN ACCESSORIES
(Mazda Lamps)
Lamp
Accessory
Loss of light,
per cent.
Remarks
SOO-watt
Reflectors.
Steel enameled reflector shallow
curve. . .
29
250- watt
Steel enameled reflector shallow
curve ....
32
25-250 watt
Radial wave reflector
1221
Varying with size of re-
400-cp.
Enclosing Units
Concentric reflector and refractor . . .
2o-in. concentric reflector and dif-
fusing globe
23-40
36
34
flector and location of
lamp filament.
Depending upon inten-
sity of globe and size of
housing.
4OO-cp.
i8-in. radial wave reflector and dif-
fusing globe
32
250-Cp.
Radial wave reflector and diffusing
globe ,
39
Refractor units
3139
Ornamental upright fixtures dif-
fusing globes
18-27
Enclosing Accessories alone (without
fixtures).
Diffusing globes with slight blue tint.
White diffusing glass
Approx. 45
8-29
C. R. I. glass
5- 8
Clear glass
Approx. 4
MILLAR: LIGHTING OF STREETS 427
In the design of street lighting accessories in general considerable
progress has been made during the past few years. This has consisted
principally in small improvements which have entered into general
practice. The more general use of diffusing globes and of the better
class of reflectors is a matter of common knowledge and represents
the principal advance in accessories.
In recent years there have been several attempts at radical re-
vision of accessory design to secure a particular light distribution.
Among these may be mentioned an asymmetrical prismatic device
having reflecting prisms on one side and the usual combination of
diffusing ribbings and redirecting prisms on the other side. This
was designed to direct upon the street a portion of the flux which
otherwise would fall upon building fronts or fields along the street.
Another device is the two-way (or four-way) half parabola reflector.
This is designed to direct upon the street most of the light which
otherwise would be delivered above the horizontal and much of the
light which without it would fall upon buildings or fields along the
street. A third design represents an endeavor to avoid glare.
It consists of a prismatic hood with an opal envelope. Its candle-
power distribution is symmetrical and for successful application it
requires to be installed either at relatively short intervals or at
relatively great height.
The most recent development along this line is a special prismatic
refractor, 10 which differs from the usual prismatic arrangement in
that the diffusing ribbings and the directing prisms are turned
inward, the outer exposed surfaces consisting of smooth glass. A
typical candle-power distribution curve is shown elsewhere in Fig. 4.
The refractor is entering into practice more largely than have any of
the other light-directing devices just described. A recent modifica-
tion of the refractor is known as the band refractor, differing from
the ordinary refractor in that the lower part of the bowl is missing.
The downward light from the source is therefore allowed to escape
without redirection or absorption. Candle-power distribution
curve is shown elsewhere in Fig. 6.
Diffusing globes of alabaster, opal and cased glass are available,
offering a variety of sizes and shapes, as well as a wide range of
diffusion and absorption. Also the use of segmented diffusing
globes is growing. It is therefore possible to choose for any lamp
fixture and post a globe which will possess precisely the qualities
needed to produce the desired light effects, and to comply with the
artistic requirements of the installation.
428 ILLUMINATING ENGINEERING PRACTICE
CALCULATIONS OF STREET ILLUMINATION
The computation of total or zonal light flux from candle-power
distribution curves need not be considered in this connection, as
it is treated in another lecture. While in the calculation of illumina-
tion on the street the same principles are involved as in other
illumination computations, yet the applications are somewhat
different because the surface illuminated is a long, narrow strip.*
While reflecting surfaces such as buildings play an important part
in street illumination, yet the light which they return to the street
surface is usually not large and it may be ignored in these computa-
tions. This simplifies the problem materially.
In order to carry out some simple studies of light delivered upon
the street surface, four characteristics of vertical candle-power
distribution are selected as follows: These are typical of:
Magnetite lamp with band refractor.
Magnetite lamp, pendant type, clear globe and small internal reflector.
Mazda C lamp in a bowl refractor.
Mazda C lamp in a particular fixture with diffusing globe.
These four distribution curves adjusted to the same total flux
appear in Figs. 3, 4, 5 and 6. They represent the range of practical
distribution characteristics encountered in street-lighting practice
at the present time. It is desired to emphasize the shape of dis-
tribution curve. It is the purpose to deal with the characteristics
of distribution rather than to undertake a comparison of illuminants.
In order to afford a better idea of the candle-power distribu-
tion represented by the curves, there are presented solids of revolu-
tion of the candle-power distribution curves for three of the charac-
teristics selected. These solids (Fig. 7) therefore represent the mean
distribution of candle-power. It is assumed that the lamps are
mounted 22 feet over one-curb of a level and straight street, of which
the roadway is 50 feet wide and each sidewalk is 15 feet wide. The
candle-power effective upon the street between building lines is
represented by the white portion of the distribution models.
The computation of flux delivered upon the street consists in
ascertaining what portion of the total light is represented by the
white part of the solid. It is apparent that the procedure to be
followed consists in determining the zonal flux values for the
illuminant and in ascertaining the portion of the flux in each zone
* This is complicated when a lamp is mounted at street intersections.
*
o
Fig. 7. Solids respresenting distribution of candle-power corresponding respectively with
Figs. 4, 5, and 6.
0.5
0.4
o.c
0.2
0.1
LIGHT FLUX EFFECTIVE UPON STREET
CONSTANTS FOR MEAN ZONAL CANDLE-
POWER; SOURCE MOUNTED AT ONE SIDE
f
G5
55
75
55
45
35
25
15
10 20 30 40 50
Ratio of Street Width to Mounting Height
Fig. 8. Chart for calculation of light flux delivered upon street surface directly from lamps.
(Facing page 428.)
MILLAR: LIGHTING OF STREETS
429
which falls within the street boundaries. The solid of revolution of
the candle-power distribution curve of the light source may be
divided into zones assumed as equivalent to zones of spheres.
That portion of each such zone which lies between a plane per-
pendicular to the street and parallel to the building line and a
plane passing through the opposite boundary of the illuminated
surfaces, supplies a basis for the computation of zonal constants by
means of which the proportion of flux delivered upon a street in
Fig. 3. Fig. 5.
105
IS
H
43 60 15 80 45
Fig. 4. Fig. 6.
Figs. 3, 4, 5 and 6. Vertical candle-power distribution characteristics.
each zone may be determined. These constants depend upon the
ratio of street width to mounting height.
From such constants the chart in Fig. 8 has been prepared.*
To illustrate the application of this chart, let us take the assumed
standard conditions and the curve of the complete unit in Fig. 4.
The following table shows computations of the per cent, of the total
flux in each zone delivered upon the street, employing the mean
zonal candle-power. The flux upon the sidewalk on one side of the
* Studies of light flux delivered upon the street were presented before the Illumina-
ting Engineering Society in ioio. n Improved methods of calculating these values have
been developed by Mr. E. Peterson who has prepared the chart in Fig. 8, and to whom
the lecturer is indebted for permission to use it.
430
ILLUMINATING ENGINEERING PRACTICE
lamp and that upon the driveway plus the sidewalk on the other
side have to be added together to obtain the total light flux dis-
tributed upon the street.
FLUX ON STREET
Candle-power Distribution as in Fig. 4. Mount 22 ft., Street Width 50 ft.,
Sidewalks 15 ft. Source over One Curb.
Driveway + one sidewalk
One Sidewalk
Mid-zone Angle
Mean zonal
candle-
R - |f - *.P 5
R = = 0.68
power
K
Lumens
K
Lumens
5
105
0.047
4-9
0-047
4-9
15
H5
o. 141
16.2
o. 141
16.2
25
130
0.232
30.1
0.232
30.1
35
170
0.314
53-4
0.263
44-7
45
210
0.387
81.3
0.178
37-4
55
270
0.449
121. 2
0.141
38-1
65
4 80
0.496
238.0
0.105
50-4
75
930
0.328
305-0
0.063
58-6
85
62O
0.092
57-o
O.O2O
12.4
907.1
292.8
Flux effective on street 1 200 lumens.
Total flux produced by unit 2991 lumens.
Proportion delivered upon street 40 per cent.
This method of calculating light flux delivered upon the street is
employed elsewhere in this lecture in studies of effect upon delivered
flux of changes in location of street lamps.
The characteristic of distribution of horizontal and vertical illumi-
nation intensities for each of the four illuminants which have been
chosen is indicated in Figs. 9 and 10. These show intensities com-
puted along the street in line with a single lamp. The curves of
horizontal illumination show maximum values near the lamp to be
greatest in the case of the diffusing globe equipment.
DISCERNMENT UNDER STREET ILLUMINATION
Visual Characteristics. The theoretical aspects of vision under
street-lighting conditions are covered in another lecture of this course.
It may be well, however, to indicate briefly in this connection the
salient features which have a direct bearing upon our problem. The
MILLAR: LIGHTING OF STREETS
HORIZONTAL ILLUMINATION INTENSITIES
FROM ONE LAMP
MOUNTING HEIGHT 22 FEET
Mazda Diffusing Globe
Mazda- Do wl Refractor
Magnetite Arc-Clear Globe
Magnetite Arc-Band Refractor
60 80 100 120 140 160 180 200 220 240
Fig. 9. Variation of illumination with distance.
0.7
0.6
|0.5
a
8 - 4
fa
0.3
0.2
0.1
VERTICAL ILLUMINATION INTENSITY FROM
ONE LAMP-MOUNTING HEIGHT 22 FEET
07
\
Mazda-Diffusing Globe
Maz.da-Dowl Refractor
Magnetite Arc-Clear Globe
Magnetite Arc -Band Refractor
06
\
05
\
\
1
04
Sy
5
+>
g
-0.03
02
\
\
N
^^
h
\
\
\
x
N %NV
^^^^
^
.-"""*>
A 01
x*
^
-^^
\
^v
^
"^
^^.
-^.
?^x
^
\
*^"*,
"
""J"
V
^~~!
jsiss
55=
^ss*
gSJJE
! im
m^m
20 40 60 80 100 120 140 160 180 200 220 240
Feet
Fig. 10. Variation of illumination with distance.
432 ILLUMINATING ENGINEERING PRACTICE
von Kries rod and cone theory of vision now generally accepted
allots to the retinal rods the process of vision at very low intensity.
The rods are thinly interspersed among the cones in the central or
foveal portion of the retina of the eye and are more thickly clustered
about the peripheral portions. In twilight or scotopic vision the
foveal or central portion of the retina is less sensitive than the con-
tiguous surrounding portions. In such vision sensibility in general
is a function of the dark adaptation of the retina. Complete dark
adaptation is rarely realized, but various degrees of dark adaptation
characterize vision under street-lighting conditions. Dark adapta-
tion is built up slowly for some minutes after the removal of all
bright lights and more rapidly during a period of one-half hour or
more until with continued darkness, it attains the approximate level
of retinal sensibility which the conditions will permit. Dark adap-
tation is easily destroyed by the intrusion of a bright and powerful
light source within the field of vision.
Under conditions of dark adaptation, sensibility near the cen-
tral portion of the retina is increased when the area of stimulus is
increased. When the intensity of the light stimulus becomes feeble,
the eye in general becomes relatively more sensitive to light of the
shorter wave length. This is known as the Purkinje effect and is
usually associated and complicated with the degree of dark adapta-
tion of the eye.
From the foregoing statements we may conclude that in street
lighting, especially under conditions of feeble intensity, best vision
requires (i) absence from the field of view of powerful and bright
light sources, (2) illumination of as large areas as practicable and
(3) light in which the shorter wave lengths are prominent.
Street-lighting theories should be comprehensive. While it is
often desirable to separate a given variable and to study it to the
exclusion of other variables in order to ascertain its characteristics,
yet no one variable should be considered with respect to its effect
upon the problem as a whole without taking into account the effect
of all other variables. Thus, while the foregoing general require-
ments for good vision under street lighting conditions appear to be
fundamentally sound, yet it may be quite possible that other con-
siderations may in particular instances make it desirable to trans-
gress these rules in order to obtain a better final result.
Indoor vs. Outdoor Lighting. An important distinction between
street lighting and interior lighting is this poor lighting of interiors
results in ocular discomfort due to difficulty in seeing things well
MILLAR: LIGHTING or STREETS 433
enough; poor lighting of streets results in failure to see things. In
the lighting of interiors the problem is rarely that of detecting the
presence of objects, while in the lighting of streets this, to a large
degree, is the principal object. Under interior illumination we
rarely have recourse to rod vision. Cone vision prevails almost
exclusively. In the streets at night rod vision predominates and
the situation is complicated by a frequent shifting from rod to cone
vision. Under the usual interior illumination we discriminate
fine relief; we distinguish colors. Under the usual street lighting we
discover objects and consciously or unconsciously classify them with
respect to type, but generally speaking we are not able to, and per-
haps do not desire to, discriminate details or distinguish colors.
When the lighting is intensified, as in the important streets of
the city, other secondary purposes are served. Here one may be
able to identify the color of an automobile or to recognize a passer-by.
Visual conditions and revealing processes are more nearly similar to
those which obtain in indoor lighting. That is, one discriminates
detail, color, etc. This more expensive street lighting of necessity
must be restricted to the principal streets of a city. It is limited
to a relatively small area. As shown elsewhere street-lighting prob-
lems are less difficult under such conditions.
In the great majority of cases streets are lighted to a low intensity.
Reliance is placed almost entirely in large shade contrasts and in
contour rather "than upon discrimination of detail. It is impor-
tant to remember this distinction. If the details of a surface are to
be described, theoretically at least the most important consideration
would be the securing of uniform vertical illumination. Such dis-
crimination, generally speaking, is beyond the scope of the average
street-lighting system. In practice, perception consists first in detec-
tion of the presence of an object, and second in recognition of the
object. If the object is of considerable size, it is detected because it
is lighter than its background and surroundings, or darker, or be-
cause it casts a shadow which can be seen. One can detect the
presence of an automobile, but may be unable to distinguish the
make. The size and contour classify the object, but the color is
not revealed. The size and contour may make it evident that
another object is a person. The lighting is sufficient to permit per-
ception of his movements, but it does not reveal the color of his dress.
Silhouetting. In warm weather, light-colored fabrics are common
in the apparel of women and children if not of. men. With this
exception, nearly all objects which it is important to perceive when
434 ILLUMINATING ENGINEERING PRACTICE
traversing a street, whether they be objects on the street or irregu-
larities in the street surface, tend to be either the same shade or of
a darker shade than the street surface. The majority of objects
and pavement irregularities are thus not light in color, and on this
account it is the most usual consideration that the contrasts per-
ceived are those of dark objects against lighter backgrounds. 12 It
should be noted that dark objects are the most difficult to perceive
and that their perception involves the most important part of the
street-lighting problem.
When the objects are large, in the majority of cases they are seen
as silhouettes. Fig. n is a comparison of a man seen in the street at
night; first by direct light that illuminates to a degree which makes
it possible to see him even if he were not contrasted against the
background, and second, when seen as a silhouette in another por-
tion of the street where the illumination is too feeble to reveal the
details. Even in the first case it will be observed that he appears as
a silhouette, though less strongly contrasted against the background
than in the second case.
Fig. 12 is a comparison of an observation target viewed in turn
from opposite directions. The target is painted substantially the
same color as the street. Viewed from the side it is well illuminated
from a near-by lamp. It is very difficult to see because its bright-
ness is substantially the same as that of its background. Viewed
from the opposite direction it is dimly illuminated from a distant
lamp, but is readily seen as a silhouette because its background is
much brighter than its observed surface.
While this discussion refers more especially to the great majority
of street lighting which is not of a high order of intensity, yet it
may not be out of place to note at this point that no street lighting is
so intense as to eliminate silhouetting as a fundamentally important
method of discernment. Fig. 13 is a picture of a silhouette in a
brilliantly lighted street.
The difference between brilliantly lighted and dimly lighted
streets in regard to silhouetting is that on the brilliantly lighted
streets one is not dependent solely upon the silhouette effect for
discernment, whereas in many parts of a dimly lighted street he
must rely exclusively upon this method. Even in brilliant sun-
light most large objects are seen on the street as silhouettes for the
reason that at a distance one cannot discriminate fine details of
relief and color, and that upon a near view one ordinarily is not
Fig. ii. Man seen by direct illumination and by silhouetting.
Fig. 12. Observational target seen by silhouetting and by direct illumination.
(Facing page 434.)
MILLAR: LIGHTING OF STREETS 435
sufficiently interested to do so, especially when moving rapidly, as
in an automobile.
When the illumination is markedly variable, objects between
lamps are seen as silhouettes offering still greater contrast to their
background because a part at least of such background is more
brilliantly lighted than under uniform illumination. When objects
are near to and just beyond a lamp on a non-uniformly lighted street,
they are more likely to be seen as under interior illumination through
the discrimination of surface details because, other things being
equal, the illumination at such points is more brilliant than is the
case of a uniformly lighted street. Thus a pedestrian seen through-
out the length of a uniformly lighted street as a silhouette offering
mild contrast against the background would appear on a non-
uniformly lighted street first as seen under dim interior illumination
and then as a strongly contrasting silhouette.
Direction of Light. Consider an abrupt depression in the road-
way (see Fig. 14). This is discerned readily if the rim or the exposed
floor of the depression is markedly lighter or darker than the sur-
rounding roadway. If the light falling upon the depression is
derived from a distant lamp opposite the observer, the observed rim
is likely to be left in darkness and to appear much darker than the
roadway, thus revealing the presence of the depression. If the light
falls at an acute angle from a lamp behind the observer, the observed
rim is likely to appear brighter than the surrounding roadway, thus
revealing the presence of the depression. In either case the floor of
the depression may be darker than the surrounding roadway, be-
cause it lies in the shadow and if seen it also will reveal the presence
of the depression. On the other hand, if the light is received from
a near-by lamp slightly nearer the observer than is the depression,
the rim and floor of the depression may be illuminated to about the
same brightness as the surrounding roadway and there may be little
or no contrast to reveal the presence of the depression. Thus with
one-tenth the illumination a depression midway between lamps may
be more readily discerned and avoided than a depression near the
lamp illuminated to ten times the intensity. This is an illustration
of the importance of contrast as affecting street-lighting visibility
and incidentally of the fact that lighting effectiveness is by no
means dependent exclusively upon illumination intensity.
DESIGN
Street-lighting design is subject to certain fundamental conditions
as follows:
436 ILLUMINATING ENGINEERING PRACTICE
Class of city.
Municipal appropriations.
Class of street.
Buildings along the street.
Trees.
Roadway and curvature.
Lamp characteristics.
Human characteristics.
Let us consider briefly these several unalterable or nearly un-
alterable conditions.
Class of City. -Cities differ greatly in respect to their industries
and activities. In some cities, such as New York and San Fran-
cisco, night life is highly developed. In others, which the writer
does not have the temerity to illustrate by examples, the streets are
not used largely by night. This difference in characteristic may
reasonably be expected to have a bearing upon the intensity standards
of street lighting. Differences in real estate values and in traffic
density also distinguish cities, and these too have a direct bearing
upon municipal appropriations for street lighting.
Municipal Appropriations. Money expended for street lighting
is fixed usually as a compromise between the wishes of the engineers
having the lighting in charge and the desires of those who pass
upon the appropriations. Ordinarily the problem is to secure the
most effective street lighting with the money which is available.
Class of Street. In every city, streets vary in importance through-
out a wide range. It is customary to adjust intensity standards of
street illumination with reference to relative real estate valuations,
traffic density and safety requirements for each locality.
Roadway. The nature of the pavement, street gradient and
curvature are important conditions which have to be taken into
account. All affect directly the problem of lamp locations and the
distribution of roadway brightness with respect to incident light.
Trees. The presence or absence of trees likewise enters into the
design in an obvious manner. If lamps are mounted well out over
the roadway, the sidewalks are likely to be lighted poorly. If
lamps are mounted low over the curbs, the roadway suffers.
Lamp Characteristics. These are treated under "Illuminants."
Human Characteristics. These are discussed in another lecture
at length, and visual characteristics are referred to briefly elsewhere
in this lecture.
Within the limitations of the above conditions which cannot be
MILLAR: LIGHTING OF STREETS 437
changed or which can be changed only with great difficulty, design
ought to be based upon a thorough understanding of what the street
lighting is intended to accomplish. Before addressing himself to
design the engineer in charge should determine beyond all question
what he expects of the street illumination. Does he want uniform
illumination, and if so what component or quality of the illumina-
tion does he desire to have uniform? Does he want to avoid glare,
and should he design the installation with this in view as a para-
mount object, or, as appears to the writer to be the more logical and
reasonable procedure, should he seek to accomplish the purposes
set forth as the three major objects of street lighting described in
the early part of this lecture? Not until this point is settled should
he proceed to consideration of means of accomplishing the desired
objects with the lighting which is to be designed.
With the purposes to be served denned after careful consideration,
actual design of the lighting installation may be undertaken. At
this point there are certain principles of good street lighting which
as generalities find proper place in a lecture of this kind. These
embrace generally accepted rules which may be regarded as estab-
lished, and other propositions, ranging from notions to rather gen-
erally accepted tenets which are subscribed to by various engineers,
but which have not yet received general acceptance. Some of
these latter are at present moot questions and it is the lecturer's
purpose in such cases to indicate the fact in order to enable his
audience to attribute proper weight to each such proposition.
There are certain obvious important features of street lighting
which are essential to effectiveness, including good maintenance of
posts, lamp fixtures and lamps, reliability and continuity of service,
etc. For the purposes of this lecture such features characteristic of
a first-class street-lighting service may be assumed. With these
disposed of, effectiveness of street illumination may be said to depend
upon the following factors:
Intensity of light upon the street average and variability.
Brightness of street surface.
Visual angle between lamps and street surface.
Extremes of contrast between lamps and street surface.
Extent to which the visual field is illuminated.
Extent to which the visual field immediately adjacent to light source
is illuminated.
Contrasts produced on the street surface.
Contrasts produced on objects on the street.
438
ILLUMINATING ENGINEERING PRACTICE
Appearance of installation by day and by night.
Appearance of street and buildings by night.
Average Intensity of Light upon the Street. The intensity of light
upon the street is one important factor affecting the value of the
street lighting. The total flux of light or the average intensity
throughout the length of the street should however be considered in
connection with the variability of intensity and with the proportion
and utility of the light delivered upon buildings. The weight to
be attributed to different degrees of variability as increasing or
decreasing the utility of the illumination cannot be stated definitely.
Considering first average intensities the following table is offered
to show modern practice in this country.
TABLE III. TYPICAL STREET LIGHTING INTENSITIES
Class of street
Average horizontal
illumination intensity
Desirable characteristic
Important avenues and
0.5 to i.o foot-candle
Ample light on building
heavy traffic streets.
(lumen per square foot).
fronts.
Secondary business streets.
o.i to 0.2 foot-candle
Ample light on buildings.
(lumen per square foot).
City residence streets ....
0.05 to o.io foot-candle
Subdued light on building
(lumen per square foot) .
fronts.
Suburban highways
0.02 to 0.04 foot-candle
Maximum light on road-
(lumen per square foot) .
way.
Suburban residence streets.
0.005 to 0.02 foot-candle
Very subdued light on
(lumen per square foot) .
building fronts.
The above intensities should be considered in connection with the
fact that municipal appropriations very generally are inadequate.
They therefore do not necessarily represent modern ideas of best
practice; they represent rather the status attained in practice, the
occasional exception conforming to the criterion of desirability en-
tertained by street-lighting engineers. As a standard for guidance
therefore it is probable that the higher value shown may be accepted
with greatest safety.
Per Cent. Flux Delivered Directly upon the Street. With a method
available for the ready and convenient computation of light flux
delivered upon the street surface, we may carry out some simple but
instructive studies of the influence of changes in location and equip-
ment of street illuminants upon delivered flux. For the purpose
we shall adopt certain standard conditions which will obtain unless
MILLAR: LIGHTING OF STREETS
439
otherwise stated. These include a level and straight street free
from trees and other obstructions, the width from curb to curb
being 50 feet with sidewalks 15 feet wide, lamps being mounted at
a height of 22 feet.
Fig. 15 shows for each of the four candle-power distribution
characteristics which have been referred to, the change in per cent,
flux delivered directly upon our arbitrarily selected street due to
alteration in the location of the illuminant transverse of the street.
Thus when the lamps are mounted over the middle of the street,
the maximum flux is of course delivered upon the street surface.
When these lamps are moved over to the curb 25 feet away from the
EFFECT OF LAMP LOCATION UPON
FLUX DELIVERED UPON STREET
10 20
Lamp Distance from Point over
Middle of Street
Fig. 15. Variation of percentage of flux delivered on the street with position of lamps.
center of the street, the same standard mounting height of 22 feet
being preserved, the reduction in flux delivered directly upon the
street ranges from 12 to 16 per cent. This is of course based upon
the assumption that the street is free from interference due to the
presence of trees and other causes.
From Fig. 16 we may derive some interesting relations between
lamp mounting height and per cent, flux delivered directly upon the
street. In order to make the diagram useful for other purposes,
the scale of abscissa shows ratio of width to height. The four curves
are applicable to our four selected candle-power distribution charac-
teristics. The variation in per cent, light flux delivered upon the
street due to a change in the height of the lamps or in the width of
440
ILLUMINATING ENGINEERING PRACTICE
the street when the lamps are mounted over the curbs is indicated.
When the values are taken from the chart it will be seen that, for the
candle-power distribution characteristics represented, when the
lamp is raised from a height which is half the width of the street to
one which is three-quarters the width of the street, the reduction in
36
20
16
12
I
- 4.0 Amp. Standard-Magnetite Arc -Reflector
-- 4.0 Amp. Standard -Magnetite Arc-Refractor
--- Mazda C Series Refractor Unit
--- Mazda C Multiple Diffusing Unit
012 345
Ratio Street Width. One Side of Lamp to Mounting Height
Fig. 1 6. Variation in light flux delivered upon streets of various widths with change in
lamp mounting height.
flux delivered upon the street ranges from 12 to 21 per cent. In
order to consider a particular case, it may be assumed that the lamps
are mounted over one curb of our standard street. The per cent,
light flux delivered upon the street with various mounting heights
will then be as indicated in derived Fig. 17.
Light on Building Fronts. The total or average light flux delivered
MILLAR: LIGHTING OF STREETS
441
upon the street surface of course does not represent a complete state-
ment of lighting value, though it is a very important part of such
complete statement. Light delivered upon building fronts ranges
from perhaps a negligible value in residential streets, where it is
often undesirable, to a high value in important avenues where it is
indispensable. Figs. 18 and 19 are examples of building front illu-
mination representing extremes of good and bad practice.
Variability of Illumination along Street. The complete qualities
of the illumination cannot be known unless there is a statement
66
62
58
, 54
w 50
%
c &
!
88
34
30
26
FLUX DELIVERED UPON 80 FT.
STREET BY LAMP MOUNTED
22 FT. OVER MIDDLE
OF STREET
\
\>
V
\\
,\
Magnetite Arc.Clear Globe
Magnetite Arc-Dand Befractor
Mazda-Bowl Refractor
Mazda -Diffusing Globe
-v>
s \ :
S^
^<\
tS;
^^V
TS
i?
\
iL_
f
^-*
\
32
^
r "V
s;
>K
10
15
26
Mounting Height
Fig. 17.
indicating the variability of the distribution of light on the street
surface itself.
It has long been a tenet of street lighting that invariable or uni-
form illumination is the great desideratum. 13 This may be quali-
fied perhaps by saying that those who so believe are satisfied to
obtain illumination which is nearly uniform, varying from maximum
to minimum by no more than say 5 to i. Recently it has been
further qualified by the concession that when the total light produced
falls below a certain minimum which corresponds roughly with
residence street lighting, the uniformity criterion may have to be
abandoned as impracticable for the reason that it may be possible
442 ILLUMINATING ENGINEERING PRACTICE
to light such streets better with a degree of variable illumination
than with uniform illumination.
While many writers insist upon the general quality of uniformity,
most of them are a little indefinite in stating what component of
the illumination should be uniform. All desire uniformity of incident
light, but opinion seems to divide some favoring uniform horizontal
illumination and others uniform vertical illumination.
It would appear that the desire for uniformity arises from the view
that by this characteristic the best lighting effects will be obtained
throughout the length of the street. If the horizontal illumination
is desired to be uniform, it is in order that the street surface may
be seen to best advantage. If the vertical illumination is desired
to be uniform, it is to the end that faces of passers-by may be dis-
tinguished. It may appear reasonable to consider that in order to
maintain visibility conditions uniformly along the street, the
intensity of incident light must be uniform. The chain is only as
strong as its weakest link. The minimum illumination intensity is
said to be the weakest link in the chain of street illumination. There-
fore, the minimum must be increased until it attains the average. So
persuasive is this view that in England there appears to be rather
widespread belief that a correct basis of rating street illumination
must be closely associated with a minimum intensity.
It is perhaps inherent in the problem that differences of opinion
should exist as to the relative importance of uniformity in horizontal
and vertical components of the illumination. The two serve quite
different purposes; each is important. Unfortunately the require-
ments for the two are not identical. Uniform vertical illumination
imposes a requirement for a somewhat higher angle of maximum
light distribution than does uniform horizontal illumination. Ad-
vocates of uniformity in general manifest a tendency to accept either
of the two distribution characteristics which it is possible to obtain,
being satisfied if they can realize either the one or the other of these
uniformity conditions.
Uniformity of illumination is naturally approximated when a
street is lighted brilliantly by closely spaced lamps. It does not
have to be striven for and can hardly be avoided. In such streets
there is ample intensity for all purposes and the desirability or
undesirability of strict uniformity of lighting need not be discussed.
It is only in streets illuminated to lesser intensities where uniformity
can be attained only by using a larger number of small lamps or by
modifying radically the light distribution curve that the desirability
MILLAR: LIGHTING OF STREETS
443
or undesirability of uniformity of light distribution along the street
becomes a matter for discussion.
In the practical lighting of such streets uniformity of illumina-
tion can be obtained either by raising the angle of maximum dis-
50
45
40
35
S
3
J
20
15
10
VARIABILITY OF HORIZONTAL
ON STREET.
I
4
INTENSI
Mazda-Diffusing Globe
Magnetite Band Refractor
Magnetite Clear Globe
Mazda-Bowl Refractor
4 6 8 10. 12 14 16
Ratio Spacing to Mounting Height
Fig. 20. Variation of horizontal illumination with mounting height.
tribution about the source of light or by multiplying the number of
sources. Assume that it is desired to increase the uniformity of
horizontal illumination by substituting for the diffusing globe equip-
444 ILLUMINATING ENGINEERING PRACTICE
ment shown in Fig. 6, the prismatic refractor equipment shown in
Fig. 5. Fig. 20 shows that if the spacing interval is ten times the
mounting height, this change of accessories will reduce the ratio of
maximum to minimum horizontal illumination intensity from 34 to
7. This change would be attended by certain changes in the
lighting conditions as follows:
(a) The proportion of light utilized would be decreased. It is not
altogether a simple matter to compare these distribution curves to derive
the percentage of light utilized. If the illuminants are employed on a
business street, much of the light delivered above the horizontal by the
diffusing globe will have to be considered as useful. If the illuminants
are employed in outlying streets, the light delivered above the horizontal
would not be of much utility. Considering the lamps to be mounted
over the selected 8o-foot street, 23 per cent, of the light in the lower
hemisphere would be delivered upon the street surface if the refractor
is used and 32 per cent, if the diffusing globe is used. Fig. 16 shows that
if we consider the total light produced, the diffusing globe delivers a
larger proportion on the street if the street width on each side of the lamp
is no greater than 1.8 times the mounting height, while the refractor
delivers a larger proportion of the total light upon the street if the ratio
is greater than 1.8.
(6) The maximum intensity delivered upon the street surface near the
lamp is much reduced.
Such reduction is at all times likely to reduce the effectiveness of the
lighting and especially so when the lamp is located over street intersec-
tions where the bright pavement near the lamp is visible from four
directions.
(c) The effect of glare is increased.
The brightness of the light source from 65 to 75 degrees above the nadir
is increased about four fold, this being due in part to higher intensity in
this zone and in part to smaller size of the accessory.
To sum up, when the variability of horizontal illumination as
measured by the ratio of maximum to minimum is reduced from 34
to 7 by substituting a bowl refractor for a diffusing globe, the
utilized light flux is reduced, the maximum intensity delivered upon
the street near the lamp is reduced and the glare is increased. A
comparison of two particular equipments has been chosen as the
basis of this discussion. Some other comparison might modify
the discussion somewhat. The general tendency, however, would
be in the direction indicated and the case chosen is peculiarly
appropriate in that a choice between these two forms of accessories
presents itself in most present-day designs.
MILLAR: LIGHTING OF STREETS
445
In the second case (numerous small lamps) there is involved:
(d) Increased expense for both installation and operation.
(e) Likelihood of reduced effectiveness due to multi-directional light
and consequent reduction of contrasts.
This effect was brought out clearly in Dr. Bell's lecture.
It might be well to incur the foregoing disadvantages involved
in securing uniform illumination if any important purpose were to
be served by uniform illumination. But it is a fact that uniformity
of incident light is not necessary to the unvarying maintenance of
the most important visibility conditions throughout the length of
the street. This is evidenced by the following:
0.6
ILLUMINATION INTENSITY AND BRIGHTNESS
ASPHALT STREET- LAM PS 260 FT. APART
100 120 140 160 180 200 220 240
Feet
20 40 60
Fig. 24. Illumination intensity and brightness.
(/) At points between lamps where intensity is feeble, superior direction
of incident light tends to compensate.
Figs. 21 and 22 show two somewhat similar depressions in the street
surface. The first is illuminated principally by a lamp which is removed
about 25 feet and which is behind the camera. The latter is illuminated
by a lamp which is removed about 150 feet and which is beyond the de-
pression. Unquestionably in driving or walking, one would be more
likely to see the depression in the pavement in the latter case in spite of
the fact that the intensity is much less than that at the depression shown
in Fig. 21. This is because the direction of the light is such as to produce
in Fig. 22 a strong contrast which reveals the presence of the depression
in spite of the low intensity. Generally speaking, light delivered at acute
angles midway between lamps is more effective in revealing surface
irregularities than is light near the lamps.
446 ILLUMINATING ENGINEERING PRACTICE
(g) At points between lamps where intensity is feeble, silhouetting is
effective.
For the principal purpose of the street lighting, the discernment of large
objects, the serviceability of this illumination is greater midway between
lamps than it is at the point of highest intensity. (See Figs, u and 12.)
(h) At points between lamps where intensity is feeble, brightness is
likely to be maintained at a fair value.
Fig. 24 shows illumination data for Avenue A between 68th and 6gth
Streets, New York City. You are asked to observe that in the region
between adjacent lamps along a line halfway between the curb and the
center of the street the variability of illumination as measured by ratio
of maximum to minimum is 40 to i for the horizontal illumination and
8.4 to i for the vertical illumination. Now note the curve of street bright-
ness under this illumination as measured at an angle of 3 ^ degrees (known
colloquially as "chauffeurs' angle" and illustrated in Fig. 23). This
brightness is as nearly uniform as might be wished, the ratio of maximum
to minimum being 2.17 to i. A similar discrepancy between horizontal
illumination intensity and brightness is encountered when measurements
are made along a line transverse of the street. The brightness between
lamps when viewed at "chauffeurs' angle" is many times as great as the
normal brightness when viewed from directly above. Measurements of
brightness viewed from above show that this value is substantially pro-
portional to the horizontal illumination intensity.
Avenue A is paved with asphalt. It is in the poorer section of the
city; its vehicular traffic consists principally of horse-drawn vehicles.
Judging the pavement in the daytime an inexperienced observer would
conclude that it offers but little specularity; certainly it is less specular
than is the average asphalt street, yet on this street variable illumination
intensity is translated into practically uniform brightness when the street
surface is viewed as in driving.
As a matter of fact, little if any consideration has been given by uni-
formity adherents to the importance of brightness uniformity as distin-
guished from uniformity of some component of incident light. This
aspect of the matter was perhaps first emphasized by the lecturer in 1910. n
It is the lecturer's view therefore that uniformity of incident light
on the street is unnecessary because : (i) with moderately variable
illumination, because of more favorable direction of incident light,
one sees surface irregularities as well in the darker regions between
lamps as in the more brightly lighted regions; (2) one sees large
objects on the streets as silhouettes in the dimly lighted regions even
more surely than in the brightly lighted regions and (3) the ap-
pearance of the street surface approximates uniform brightness
MILLAR: LIGHTING OF STREETS
447
even with marked diversity of incident light. These views have
been fully confirmed in the investigations of street lighting effective-
ness conducted during the past two years under the auspices of
LAMPS:
PONY BROAD CARBON OPEN ARC LAMPS
o 4 AMPERE MAGNETITE LAMPS H. E. ELECTRODES
INSTALLATION:
SPACING 210 FEET. MOUNTING HT. 22 FT.
AVERAGE FOR ENTIRE WIDTH OF STREET
AVERAGE PER.
CEPTION DISTANCE
Fig. 25. Illumination intensity and target findings.
the Street Lighting Committees of the National Electric Light
Association and the Association of Edison Illuminating Companies.
An example of findings in local observational tests* will further
Courtesy of the Philadelphia Electric Company.
448 ILLUMINATING ENGINEERING PRACTICE
illustrate these points. 14 Fig. 25 shows the distribution of
horizontal and vertical illumination along the street under two
systems of lighting. Below these curves are the findings in observa-
tional tests. The targets (see Fig. 34) of the disc type were found
by pedestrians quite as generally between lamps as in the more
brightly lighted regions near lamps. The targets of the cylindrical
type were seen at quite as great distances when located between
lamps as when located near lamps, yet the horizontal illumination
intensities under these two systems varied respectively from 23
to i and from 17 to i.
It is the lecturer's view that strict uniformity of illumination on
all but the most brilliantly lighted streets can be attained only
with added expense and with some difficulty; that its attainment
is attended by disadvantages; and that visibility requirements do
not demand it. It is submitted therefore that a moderate diversity
of intensity along the street is a more reasonable criterion.
GLARE
Glare in street lighting manifests itself principally in reducing
visual ability, in causing ocular discomfort or annoyance, and in
rendering an installation less attractive. In a general way it may be
said that methods of reducing glare influence all of these manifesta-
tions, although in some cases the effect upon one manifestation may
be more pronounced than upon another. Little is known concern-
ing the numerical relations involved in the reduction of glare in
street lighting. 15
We do know, however, that glare is reduced: 16
i. If the power and brightness of light sources at observed angles
are reduced.
The customary method of reducing glare consists in surrounding the
light source with a diffusing globe of as large size as may be regarded as
desirable. It is to be remembered that assuming complete diffusion by
the accessory the brightness of a globe decreases inversely as the square
of its radius, and that therefore with the same lamp a 1 6-inch globe will
be only 40 per cent, as bright as a lo-inch globe.
A more elaborate method recently advocated and embodied in some
modern practice consists in limiting within a lower zone which is relatively
free from observation, the greater part of the light flux, and allowing little
or no light to emanate from the source at angles which will be observed
in the ordinary use of the street. This method is open to the objection,
(i) that it requires for successful application either excessively great
MILLAR: LIGHTING OF STREETS 449
mounting heights or very short spacing intervals, both involving large
cost; (2) elimination or reduction of flux at angles which are very useful
when the characteristics of specular pavement are considered; (3) as ap-
plied thus far this method usually carries with it so great a reduction
in the light delivered upon building fronts as to make the effect unsat-
isfactory on streets of commercial importance.
2. If the visible region immediately contiguous to the light source
is made bright.
This is a little apprehended effect. It is probably associated with the
ocular characteristic described elsewhere according to which, under con-
ditions of twilight vision, retinal sensitiveness is increased if the area of
the stimulus is increased. One reason why the exposed arc of the magne-
tite lamp has been found to be a less serious source of glare than might
have been anticipated is doubtless the presence of a reflector immediately
adjacent to the arc. When mounted in streets any light source presents
a less serious source of glare if it is seen against a background of a light-
colored building than it does if its background is in darkness.
3. If the surfaces viewed are made bright.
In street lighting the surface viewed is generally the street itself. This
may be increased in brightness by reason of more powerful lighting or by
reason of a higher albedo, or, under certain conditions, by reason of in-
creased specularity. The effect of glare from a given source is diminished
if the street surface is rendered brighter in any of these particulars.
4. If the visual angle between light sources and the observed sur-
face is increased.
The general application of this is to be found in a demand for increased
mounting heights as light sources become more powerful and more bright.
On curved roadways this finds application if a lamp is mounted over the
inner curb, the visual angle between it and an object in the distance be-
yond the curb is likely to be small and the glare reduces visibility markedly.
If the lamp is mounted over the outer curb, the visual angle between it
and the roadway beyond the curb is increased and the glare is less serious
(Fig. 32).
5. If a large portion of the field of view is illuminated.
Also, if the general field of view contains many lighted surfaces, the
effect of glare will be less than if large portions of the field of view are left
in darkness.
STREET PAVEMENTS
The light reflecting qualities of street pavements both as respects
reflection coefficient and specularity are of prime importance in the
street lighting problem. A street pavement which is naturally
29
45 O ILLUMINATING ENGINEERING PRACTICE
light in color and which can be kept from darkening seriously under
use obviously is capable of enhancing greatly the effectiveness of the
street lighting provided by any system. Most pavements darken in
use, especially under automobile traffic, where oil drippings bring
early discoloration. This would result in rendering ineffective the
most powerful of street lighting systems if it were not for the saving
fact that in practically all such cases the pavement takes on a con-
siderable degree of specularity, especially under automobile traffic.
Fig. 26 is a view of the wooden block pavement of Columbus Circle,
New York City. The background reveals the polish resulting
from automobile use. The foreground consists of the same pave-
ment which is not traversed by automobiles and is not specular.
The rapidly increasing use of automobiles is exerting a marked in-
fluence on this aspect of the street lighting problem and specularity
of street pavement must now be taken into account in practically
all streets that are lighted artificially.
The higher spots of the pavement take on a polish and become
small mirrors on the street surface. In driving one views the pave-
ment at an angle of perhaps from i to 5 degrees. Each street lamp
may be seen in many of these small mirrors, the result being a broken
streak of light along the street not unlike moonlight on the water.
It should be noted that it is the distant lamps and not the nearby
lamps which are seen reflected in these little mirrors. If a large
number of distant lamps are within view at a given time, especially if
they are distributed across the street, the result will be many streaks
of light side by side, all contributing to render the pavement bright.
Generally speaking, specular pavements are dark in color, and under
diffused light, as in the daytime, appear incapable of reflecting light
advantageously as compared with other pavements. At night,
however, specular pavements usually exhibit characteristics which
promote visibility. Figs. 27 and 28 for example are comparisons
between suburban roads, one roadway in each case being specular
and the other a dirt road which does not reflect light specularly.
While the lighting systems are not strictly comparable, yet both
comparisons indicate the fact of favorable reflection from the specu-
lar pavements and unfavorable reflection from the mat surface
pavements. Fig. 29 is a view of a fairly wide street lighted by lamps
which are mounted over curbs. It will be observed that there are
many such streaks of light along the sides of the street, but that the
center of the street appears dark. This installation, by the way,
was designed to produce uniform illumination.
. 26. Showing effect of automobile traffic (background) in polishing pavement.
Fig. 27. Mat and specular road surfaces.
(Facing Page 450.)
Fig. 28. Mat and specular road surfaces.
Fig. 29. Uniform intensity of incident light. Variable brightness.
Fig. 30. Driveway illuminated by three rows of lamps.
(Facing insert Figs. 28 and 29. )
Fig. 31. Two views of a drive, without and with lamps concealed.
MILLAR: LIGHTING OF STREETS 451
On the other hand, Fig. 30 is an illustration of one driveway of a
wide street illuminated by three rows of lamps. The streaks of
light are in this case distributed to better advantage across the street,
creating a general appearance of uniformity which was lacking in
Fig. 29. The characteristics of street pavements here introduce
a condition which makes the skillful location of light sources a most
important factor, affecting the value of the street illumination to
the detriment of intensity considerations.
Fig. 31 offers two views of a drive in Central Park, New York,
lighted by lamps which are mounted n feet over each curb and
spaced along each curb at intervals of about 75 feet staggered. At
the top the view shows usual lighting conditions. The view below
shows the results when the lighting is modified by covering the lamps
with white pasteboard reflectors which limit the light below an
angle of 65 degrees. These effectually conceal the lamps from view
and increase materially the light on the street within the illuminated
area. They leave the pavement relatively dark between lamps.
As the pavement reflects specularly the downward lighting is not so
effective as it would be otherwise. The reflectors eliminate glare
but at the same time make it impossible to avail of the advantageous
reflecting qualitities of the pavement by intercepting all of the light
which would be reflected specularly to the user of the drive. It is
possible that if this pavement reflected diffusely, the covered lamps
might provide superior lighting. With the specular pavement they
undoubtedly provide an inferior lighting.
LAMP LOCATIONS
In locating lamps on the principal business streets of a city, a
standard arrangement is usually desired and required and but little
deviation is warranted. The precise location of the lamps is
relatively unimportant as far as illumination is concerned.
In secondary streets where the lamps are likely to be rather
inadequate, it is often desirable to mount them well out from the
curb either on suspensions or on mast-arm posts. Usually such
streets do not have many trees, and the sidewalk lighting does
not suffer as a result of the central mounting.
In residence streets, where trees are likely to abound, the loca-
tion of the lamps becomes much more important, the more so since
fewer lamps are employed and the utmost must be made of the
materials at hand. So far as the roadway is concerned, it is usually
452 ILLUMINATING ENGINEERING PRACTICE
difficult to improve upon a location over the middle of the street low
enough to escape serious interference from trees and high enough
to avoid serious glare. In such lighting, however, the sidewalk is
likely to be neglected. If the lamps are mounted low over the curbs,
in order to keep the light well beneath the limbs of the trees, the
sidewalk is likely to be taken care of to a somewhat better degree,
but the roadway lighting is likely to be ineffective. Abroad to
some extent a combination of the two lamp locations has been
found effective, large lamps being mounted over the middle of the
streets at intersections, small supplementary lamps being mounted
low over the curbs between street intersections. Similar arrange-
ments have been tried in this country.
In outlying districts, parks, etc., lamp locations are usually some-
what optional. Here the illuminating engineer has an opportunity
to exhibit his skill as an engineer and as an artist. By studying
the topography and curvature of the roadway, by making due
allowance for glare, and by taking full advantage of pavement
specularity, the skillful engineer may so locate his lamps as to obtain
with a given expenditure much more effective street lighting than
could be had with perfunctory location of the lamps. An excellent
illustration of the importance of lamp location under such condi-
tions is afforded by the two views in Fig. 32. In the one a lamp is
mounted over the inner radius of a curve in an automobile driveway.
There is a great deal of light on the pavement at the curve, but the
roadway beyond is obscured. The glare is very serious. In the
other view the lamp has been moved to the outer curb of the curved
roadway. There is less light upon the pavement in the foreground,
but the curb can be seen readily. The distant roadway may be
seen readily as a result of specular reflection from the next lamp,
which, by the way, is located at a distance of about 900 feet.
SUMMARY
Summing up the foregoing comments on the design of street
illumination, it will be noted that the simple method of calculating
flux on the street leads to the ready establishment of relations in-
volving the location of the lamp, its height and its equipment. Not
only must the total flux delivered upon the street surface be taken
into account, but the amount of light delivered upon building fronts
is important. Moreover, the variability of the illumination along
the street requires careful thought, especially where low intensities
MILLAR: LIGHTING OF STREETS 453
prevail. The means for reducing glare are indicated, the character-
istics of street pavements are illustrated and the importance of
this factor is emphasized. The possibilities of improvement by
skilful lamp location is the last point mentioned.
^COMPARISON AND TESTS
As street illumination is generally supplied under a contract be-
tween a municipality and a public-service corporation, there is an
ever-present desire to provide some means of proving the adequacy
of the service rendered. In the past too much emphasis has perhaps
been placed in this connection upon the candle-power of the lamps
or upon the illumination intensity. It is evident that a street-light-
ing service must include such important elements as reliability and
continuity of operation, good maintenance of lamps, poles, lines,
etc., and a satisfactory attitude on the part of the contracting com-
pany as well as reasonably good maintenance of the candle-power
of the lamps. Lamps may be shown to be of adequate candle-power
and yet the service in general may be unsatisfactory. On the other
hand, the lamps at times may not be quite up to par in candle-power
and yet the street-lighting service as a whole maybe eminently satis-
factory. It is desired therefore to deprecate the tendency of the
past to over-emphasize this one phase of street-lighting service to
the exclusion of other equally important features.
Nevertheless for engineering or political reasons the demand
recurs for a measure of the iUuminating value of street-lighting
systems. It has been the writer's privilege during the past six
years to be closely identified with efforts which have been put forth
in this country to solve the problem of providing a satisfactory
measure of street-lighting values for this purpose, and the statements
on this subject which follow are largely based upon the experience
which he has had in the conduct of investigations in this field for
the Street Lighting Committees of the National Electric Light
Association and of the Association of Edison Illuminating Companies.
The problem of testing street illumination is divided naturally
into two parts. The first has to do with means of determining
whether or not a stipulated lighting service is being rendered; the
second has to do with the determination of the relative illuminating
value of two different street-lighting systems.
The first of these is by far the simpler. A contract or specifica-
tion for street lighting under which tests are to be performed ought
454 ILLUMINATING ENGINEERING PRACTICE
to include a description of the lamps and accessories to be em-
ployed and a statement of their photometric values, including the
total flux of light, the candle-power distribution curve and a range
of toleration above and below the standard within which the lamps
may be allowed to fluctuate. Test of fulfilment of this part of
the contract then consists in determining the total flux of light of
the lamps. This may be accomplished either by determining the
operating electrical values of the lamps, removing them from the
circuit and sending them to a laboratory in their operating condi-
tion, or, where practicable, in subjecting them to test in situ by
bringing an integrating sphere photometer to the street for the
purpose (see Fig. 33). These methods are not simple. Such tests
do not need to be made often, but in the event of a serious question
arising concerning the adequacy of the service, they afford means
which experience has shown to be most reliable for determining
accurately the illuminating values in terms of the contractual
provision.
The history of attempts to arrive at a satisfactory method of
testing street illumination is a record of confusion, 17 and it now
appears that much of the confusion has arisen as the result of a vain
attempt to adopt some method which would at once prove fulfil-
ment of contractual obligation and indicate the usefulness of the
illumination. Thus the 1907 "Committee to Consider Specifica-
tions for Street Lighting" of the N. E. L. A. sought a measure of
the illuminating value and finally recommended the mean normal
illumination produced by a lamp in the street at the height of the
observer's eye and at a distance of not less "than 200 and not more
than 300 feet from a point immediately below the lamp, as com-
pared with the illumination provided by a standard lamp under like
conditions. It would appear likewise that the Joint Committee on
Street Lighting Specifications, which has labored recently in Eng-
land, was actuated by a desire to combine both purposes when they
recommended the minimum horizontal illumination as a measure
of street lighting. The whole problem is immensely simplified
if the purpose of proving fulfilment of contract is divorced once and
for all from the purpose of comparing relative illuminating effective-
ness of different street-lighting systems. Considering exclusively
the latter problem, let it be noted that a difficulty which existed
until recently has been the lack of any method for definitely com-
paring the illuminating effectiveness of street-lighting systems.
There has been no way to determine whether the notion of minimum
Fig. 32a. Good lighting of curve-
Fig- 33- Integrating sphere photometer in service tests.
Fig. 34. Target painted similar to street surface.
MILLAR: LIGHTING OF STREETS 455
illumination or of average vertical illumination provides the nearer
approach to a real measure of effectiveness. There has been no
test for such proposed measures. It has been my privilege, with
the aid of associates at the Electrical Testing Laboratories, to devise
and with the assistance of the Street Lighting Committees which
have been named to put into effect certain methods calculated to
furnish a means for determining street-lighting effectiveness in
several important respects and, therefore, for testing these several
proposed measures of effectiveness. These methods are described
in detail elsewhere. 18 They consisted first in classifying the pur-
poses of street illumination through inquiry and consultation into
the following three principal purposes:
Discernment of objects on the street.
Discernment of surface irregularities.
Esthetic qualities.
Second, in devising tests to obtain relative discernment values for
large objects on the street and for small objects on the street surface
and in recording opinions of qualified observers in respect to the
aesthetic qualities. The means for measuring discernment which
were finally adopted consisted in determining the maximum dis-
tances at which automobilists could see targets painted similar
to the street surface (see Fig. 34) and in determining the percentage
of small targets similarly painted which pedestrians could find in
walking through the street. By carefully eliminating variables and
standardizing conditions, comparative discernment values were
obtained for any two systems compared upon the same street at
one and the same time by a given group of observers (see Fig. 34).
It is the writer's opinion that these observational tests, 14 when
impartially conducted, provide a closer approximation of the real
illuminating values of street-lighting systems than have heretofore
been available, and are the only reasonably adequate means thus far
developed for testing the validity for proposed measures of street
lighting effectiveness. With the results of the Street Lighting
Committees' investigations before us, it is possible to put these pro-
posed measures to the test and to ascertain whether or not they
afford reliable measures of lighting effectiveness. From the mass
of data available on this subject, I have selected a few striking in-
stances illustrating the weaknesses of the several proposed measures ;
these are presented in the following table.
456
ILLUMINATING ENGINEERING PRACTICE
Description
Proposed
measure of
lighting
Ratio A to B
Comment
Proposed
measure
a
If
Jh
System A
System B
250-c.p. lamps,
25o-c.p. lamps,
Flux in low-
0.72
1.03
With system B all light was
light diffusing
refractor, 24
er hemi-
concentrated in the lower
globes, 24 feet
feet^overjnid-
sphere.
hemisphere. Much of it
over middle of
dlejof street,
was redirected at angles well
street, 150 feet
i5O_feet^apart.
up toward the horizontal.
apart.
The greater bulk of such
light, of course, fell upon
houses, trees or lawns and
did not contribute materially
to the street lighting. As a
result substantially the same
total quantity of light was
delivered upon the street
from system A and system B.
2SO-c.p. lamps,
2SO-c.p. lamps,
Intensity
0.41
1.03
The refractor equipment re-
light diffusing
refractor, 24
10 below
ceives an especially favora-
globes, 24 feet
feet over mid-
horizontal.
ble rating in terms of in-
over middle of
dle of street,
tensity 10 below horizontal.
street, 150 feet
1 50 feet apart.
It is very evident from the
apart.
comments made above that
there is no such difference in
illuminating value as this
form of rating would indi-
cate.
125-c.p. lamps,
250-c.p. lamps,
Minimum
2.6
0.95
light diffusing
light diffusing
horizontal
globes, 1 8 feet
globes, 24 feet
illumina-
over middle of
over middle of
tion.
street, 75 feet
street, 150 feet
apart.
apart.
Each of these three proposed measures which has been prominently
urged upon attention in recent years is shown by these single in-
stances to be in valid, as judging by the observational tests which have
been made. It does not seem necessary to cite other instances or to
justify the observational tests as a basis for judging the proposed
measures. It would seem to be evident that each of the latter
fails to measure real lighting effectiveness and that each offers a
basis of rating of which advantage can easily be taken to secure a
higher rating for illuminants of no greater illuminating value. Both
the rating in terms of intensity 10 degrees below the horizontal and
the rating in terms of minimum horizontal illumination have to do
with light at a particular angle, and the rating may easily be in-
* Automobilists* and pedestrians' tests combined with equal weight.
MILLAR: LIGHTING OF STREETS 457
fluenced by altering such light without changing the illuminating
value. Safer and more reliable but still inadequate measures are
afforded by the average horizontal illumination and the average
vertical illumination on the street. All of these, however, fail to
give any credit for light directed above the horizontal, which in
modern street-lighting practice, holds a very definite value. The
total flux of light is a fairly reliable measure, comparing favorably
with the best of the others, but inadequate in that it fails to dif-
ferentiate between desirable and undesirable distribution character-
istics. As previously stated, experience in the work of this Com-
mittee results in the view that the most useful single method of
rating is to be had by combining with a statement of the total light
flux or the mean spherical candle-power a candle-power distribution
curve. The two should then be interpreted according to best judg-
ment, the distribution characteristic being considered in the light
of the facts first, that where it is desirable to illuminate building
fronts, a certain proportion of the light should preferably be directed
above the horizontal; and second, best lighting effects in general
are obtained with a moderate diversity of illuminating intensity
along the street, avoiding on the one hand, that degree of non-
uniformity which results in unlighted areas between the lamps and,
on the other hand, that degree of uniformity which tends to reduce
contrast and definition.
Measures which have been proposed cannot be relied upon to
afford an accurate and final test of street-lighting effectiveness.
They may show the amount of light produced and something of its
distribution. While these are important factors, yet they do not
give wholly complete information. No appraisal of a street-lighting
installation can be made reliable until in addition to these facts
information is available including a complete description of the
illuminants and their accessories, the candle-power distribution
characteristic, the location of the lamps, including height and
spacing, and the brightness of the light source in directions in which
it is likely to be viewed. If all of this information were available,
one could form a good idea of the merits of a street-lighting system
for general purposes, but its effectiveness when installed on a par-
ticular street could not be approximated unless in addition informa-
tion were available including a photograph and description showing
the buildings and trees along the street, and indicating in a general
way the extent and character of traffic and the criminal hazard in
that locality.
458 ILLUMINATING ENGINEERING PRACTICE
If these statements are correct, it must be evident that there has
not yet been devised any thoroughly satisfactory measure of street
lighting which can be employed to show street lighting effectiveness.
Through test data and capable observation the relative effectiveness
of two different systems of lighting may be established, but more
than this cannot be said at the present time.
For proving contractual obligations the selection of a suitable
measure is simpler and nothing seems indicated which, upon the
whole, is quite so generally satisfactory as a measurement of the
total flux of light. Another question arises in this connection,
however, which is not so simple to dispose of. This is the proper
sampling of lamps in order to secure reliable indication. It should
be taken for granted that the purpose of testing is to secure represen-
tative and reliable information regarding the service, and that the
purpose of including test provisions in specifications for street light-
ing is to hold the service up to a high standard, reducing to a mini-
mum the number of individual lamps which are of inadequate illumi-
nating power and providing an additional incentive to keep the
average illuminating power at a reasonably high value. Starting
with this assumption it is regarded as good practice to select a
particular area of district for investigation to secure representative
sample lamps from such a district and to provide for reliable tests
of such samples. The selection of samples ought to be made with-
out prejudice and without a knowledge of the condition of the lamps
which are chosen for test. Where lamps are of the incandescent
type, the testing work is simplified and the following sample sched-
ule may be expected to provide a reasonable sampling.
SAMPLING SCHEDULE
Number of lamps in district Minimum number or per cent.
investigated. lamps to be tested.
Less than 300 130 lamps
300- 499 10 per cent.
500-2499 7 . 5 per cent
2500-9999 5.0
The lamps ought to be tested in their operating condition if
practicable, either by bringing an integrating sphere to the lamps in
the street or removing the lamps after ascertaining their operating
values and having them tested in a suitable laboratory. The average
illuminating power of the samples tested ought to be regarded as
applicable only to the district investigated.
MILLAR: LIGHTING OF STREETS 459
Bibliography
1 Report of Committee on Street Lighting. Transactions National Electric
Light Association, Technical Section, 1914, page 589.
2 Report of Special Committee on Commercial Aspects of Municipal and
Highway Lighting. National Electric Light Association, May, 1916.
3 Report of Committee on Electric Advertising and Decorative Street Lighting.
Transactions National Electric Light Association, Vol. II, 1912, page 188.
4 C. A. B. HALVORSON. "Ornamental Luminous Arc Lighting at New Haven."
General Electric Review, 1912, page 221.
& WALDEMAR KAEMPFFERT. "Ornamental Street Lighting." Published by
the National Electric Light Association, Commercial Section, 1912.
6 F. A. VAUGHN. "A Practical Application of the Principles of Scientific
Street Lighting." Transactions Illuminating Engineering Society, 1910, page
282.
7 L. B. MARKS. "The Invention of the Enclosed Arc Lamp." Sibley Journal
of Engineering, Vol. XXII, Oct., 1907.
8 C. P. STEINMETZ. "The Magnetite Arc Lamp." Electrical World, Vol.
XLIII, 1904, page 974.
I. LANGMUIR and J. A. ORANGE. "Tungsten Lamps of High Efficiency."
Proceedings American Institute of Electrical Engineers, Vol. XXXII, 1913, page
1935-
10 "Refractor for Street Lighting." Electrical World, Vol. LXIV, 1914, page
439-
11 P. S. MILLAR. "Some Neglected Considerations Pertaining to Street
Lighting." Transactions Illuminating Engineering Society, 1910, page 653.
12 P. S. MILLAR. "An Unrecognized Aspect of Street Illumination." Trans-
actions Illuminating Engineering Society, Vol. V, 1910, page 546.
13 A. J. SWEET. "An Analysis of Illumination Requirements in Street
Lighting." Journal of the Franklin Institute, 1910.
14 P. S. MILLAR. "Tests of Street Illumination." Transactions Illuminating
Engineering Society, Vol. XI, 1916, page 479.
15 A. J. SWEET. "Glare as a Factor in Street Lighting." Electrical Review
and Western Electrician, Vol. XL VI, 1915, page 439.
16 P. S. MILLAR. "Effective Illumination of Streets." Transactions American
Institute of Electrical Engineers, Vol. XXXIV, 1915, page 1429.
17 J. W. LIEB, Chairman. "Report of Committee on Street Lighting."
Transactions National Electric Light Association, 1913, page 357.
18 "Report of Street Lighting Committee." Transactions National Electric
Light Association, 1914, page 589, and 1915, page 710; also P. S. MILLAR.
"Tests of Street Illumination." Transactions Illuminating Engineering Society,
1916, page 479.
THE LIGHTING OF STREETS PART II
BY C. F. LACOMBE
The subject of street lighting in this lecture course has been divided
between Mr. P. S. Millar and myself, and as the subject has been
summarized in the syllabus of the course by the Committee on
Lectures, the main subjects assigned to me will be taken up without
further introduction.
REQUIREMENTS OF CITY LIGHTING
The problem of lighting a city is to distribute the illumination in
proportion to the streets, within the funds available, from that of
the most congested thoroughfare to that of a sparsely settled suburb ;
the maximum requirement being the lighting of great squares and
street intersections under conditions of heavy congestion of traffic;
and the minimum being that necessary for policing the city and the
prevention of accidents. One should therefore study the classes
of streets existing in cities of various grades. We may arrange the
grades of cities and classes of streets about as follows:
Grades of cities, by
population
Class of street
Description of use
I 500,000 and over
II 250,000 to 500,000
III 100,000 to 250,000
IV Less than 100,000
Special or Class AA
Class A
Class B
Class C
Class D
Class E
Class F
Class G
Very important. Crossing of
great streams of traffic.
Important streets, greatly used
at night.
Well used streets.
Ordinary night use, best resi-
dence streets.
Ordinary residence.
Suburban residence.
Parkways or boulevards and
suburban roads.
Connecting country thorough-
fares, State or County roads.
In all cities certain streets pass from one class to another in their
course and some care is necessary in classifying them for lighting
461
462 ILLUMINATING ENGINEERING PRACTICE
intensities; the different grades of streets can perhaps be best
identified by some examples:
Class A A. Parts of Wabash Avenue and Dearborn Street,
Chicago; Broad Street, Chestnut and Walnut Streets, Philadelphia;
Times Square and portions of Broadway and Fifth Avenue, New
York.
Class A. Parts of these same streets contiguous to the most
congested sections; such as streets within the Loop, Chicago; Fifth
Avenue, Pittsburgh; Broadway from 6oth Street to 72d Street,
New York; parts of Walnut, Twelfth and Market, Chestnut and
Broad Streets, Philadelphia.
Class AA streets ra/rely exist outside the very largest cities of
Grade I in the country.
Class A streets represent the most important streets in the usual
city between 250,000 and 500,000 inhabitants of Grade n, and the
White Ways of smaller cities. For instance, parts of Pennsylvania
Avenue, Washington; Grand Avenue, Milwaukee; Main Street,
Rochester; Seventh and Third Avenues, New York.
Class B streets are those probably greatest in number in all
cities of any size in the country. These are well used, often have
street railways on them, with wholesale or retail stores, and usually
toward their farther ends develop into residence streets, for instance:
Broad Street, Newark, and upper Broadway, New York.
Class C streets are streets of ordinary night use, except as
they may lead to amusement centers or contain street-car lines.
Examples are: Commonwealth Avenue, Boston; Park Avenue,
New York; Michigan Avenue, Chicago.
Class D streets are usual residence streets of good quality
throughout cities generally. In the larger cities the houses are
in blocks of buildings, but in smaller cities this class of streets gener-
ally develops, even near the center, into
Class E or suburban residence streets with detached houses,
and usually full of trees.
Class F streets, or special boulevards and parkways, have been
developed of late by demands of more rapid transportation, which
has been made possible by motor cars, and in this way the country
has been brought in close contact with the city. Usually no street-
car traffic exists on these roads and they are used almost exclusively
by automobiles running at a speed of 20 miles an hour, or over.
Examples of this are: the Shore Boulevard from Boston to Lynn,
LACOMBE: STREET LIGHTING 463
the Ocean Parkway to Coney Island, the Parkway Systems of
Chicago and Boston.
For the same reason, increased ability to travel at high speeds,
these boulevard systems may be said to be further extending into
cross-country, county, state and national highways which can be
classified as Class G roads. Examples of this are certain roads in
New Jersey: for instance, between Jersey City and Paterson; certain
roads in Eastern New York; Nahant Road near Nahant, Mass.;
Lincoln Highway between Jersey City and Newark, and the same
highway in Salt Lake County, Utah. Comparatively little has yet
been done in lighting such highways, but the prospect is encouraging.
Assuming that these grades of streets cover the general scale of
street lighting, they will receive different amounts of lighting in-
tensity, somewhat in accord with the necessities of vision, but the
dominant factor is the amount of money devoted to street lighting
by various municipalities. The question of appropriations really
decides the type of lighting that can be planned for a given city,
assuming always it is properly proportioned to the use of the respec-
tive streets. It appears that at present these appropriations are
usually too small and the amount and intensity of lighting too low
for the reason that within the last fifteen years automobile traffic has
developed in its entirety, and the congestion in night centers has
increased in proportion to the population and to the increase of
transportation facilities. In this regard it is well to remember that
while the development of better illuminating appliances and the
demands for better lighting have increased the intensity of interior
illumination probably five or six fold in the last fifteen years, general
street illumination has increased but little.
PHENOMENA OF VISION
The faculty of vision ranges from full direct sensation from re^
fleeted light to what is termed adaptation to darkness, where we
can distinguish only shades and contrasts. As you know, the
retina of the eye receives light upon an arrangement of sensitive
nerve termini known as rods and cones. In this arrangement the
cones are rather at the center of the retina and the rods with scattered
cones radiate toward the periphery. The cones are supposed to
give us the sensations of color, and detail we have with direct vision
by reflection, such as we get in daylight or under high artificial
illumination. As the intensity of the light diminishes, the cones
464 ILLUMINATING ENGINEERING PRACTICE
begin to lose their sensibility, we slowly lose the sense of color, dis-
tinctness and so on, and our vision is restricted largely to the sensa-
tion given by the rods. These rods retain their sensitiveness; in
fact, this sensitiveness really increases with darkness until in full
adaptation it has become very acute. Be this as it may, however,
rod vision is poor vision. In consequence, as Dr. Bell pointed out
in the Johns Hopkins lectures of 1910, there is a physiological divid-
ing line between that illumination by which we can see well and
with ease, and one by which we can only see forms or contrasts and
shadows. It is the first class of vision which requires the most ex-
pensive lighting, and which naturally we prefer ; but, unfortunately,
on account of insufficient appropriations, and so on, we generally
have to deal with the lower grade of vision, and the difficulties in
deVeloping illumination of this class produce most of our problems.
Referring again to the grades of streets, direct vision, as in day-
light, is required on Class AA streets and areas, and to a large degree
on Class A and B streets, and particularly at intersections of streets
of this character. Powerful lighting is necessary where street-car
lines cross each other and turn in various directions, and where
intermingling streams of pedestrians are 1 persistent and continuous..
Police statistics have shown that the greatest number of street
accidents occur at places where traffic crosses or changes from
one direction to another. In consequence, where such streets cross,
each of them contributing streams of traffic, high illumination is
required for safety if nothing else. If such lighting is provided in
these places one has a degree of visual acuity approaching that of
daylight, and feels the same sense of safety in that one can note the
color, speed and detail of approaching objects and so direct one's
movements as to avoid them.
Foreign cities have developed lighting which produces direct
vision on their more important streets to a greater extent than in
this country. We have few examples of lighting of the order of
5.25 foot-candles average horizontal measurement over a con-
siderable length of street, as exists in London, nor have we any such
examples of high illumination as is shown in the Potsdamer Platz, in
Berlin, where an average of 1.75 foot-candles is developed over the
entire Platz.
While the sense of vision varies with the individual, this direct
vision by rods and cones together prevails down to about o.i foot-
candle; below that we begin to get into rod vision, not entirely by
any means at 0.05 foot-candle, but completely so at o.oi foot-
LACOMBE: STREET LIGHTING
465
candle. When low orders of intensity prevail in certain sections of
a city and reliance must be placed on adaptation of the eye to a
certain degree of darkness, it is well to avoid sudden changes to
bright lighting within the section, as the eye adapts itself to such
changes quite slowly. With this in view, we can approximate a
scale of illumination for the various requirements of the grades of
streets.-
REQUIREMENTS OF CITY LIGHTING FOR SEVERAL CLASSES OF STREETS
Grade of city
I
500,000
& over
II
250,000
& over
HI
100,000
& over
IV
under
100,000
Degree of intensity
Grade of street
Hor. foot candles
Average
Minimum
High. Excess at street crossings
Giving clear vision
AA
A
B
C
D
E
F
AA
A
B
C
D
E
F
G
A
White
B
C
D
E
F
A
ways
B
C
D
E
0.5
0.35
O.2
0.075
0.04
0.02
From that
D&Ewit
o.oi for
roads.
From direc
ing merely
interurbar
0.25
O.I
0.05
0.02
O.OI
0.003
of Classes
bin a city to
interurban
tional light-
T to that of
i roads.
Lower but vision still distinct . . .
Above clear moonlight
About clear moonlight, average
higher
Vision by silhouette or con-
trast. Dark adaptation
Vision by silhouette or con-
trast. Dark adaptation
Vision by silhouette or con-
trast. Dark adaptation
CLASS AA. FOREIGN STREETS AND PLACES 1913
Horizontal foot-candles
Max.
Aver.
Min.
Cheapside, London.
2.00
9OO
1.23
0.72
0.2
0.5
0.24
0.12
Regent Street, London .
Piccadilly, Manchester.
1.4
0.64
i-75
Freidrichsstrasse, Berlin..
1.0
7.6
Potsdamer Platz, Berlin
The general question now arises as to how we are to produce the
illumination required on the streets. Only a short time ago one of
3'J
466 ILLUMINATING ENGINEERING PRACTICE
our greatest problems was the fact that for practical use we had only
two classes of lighting units; one, the larger unit, typified by an
arc lamp; the other, the small unit, such as an incandescent or mantle
gas lamp. It is obvious that it was impractical to properly grade
the lighting just described with such limited means.
ILLUMINANTS AND LAMPS
To-day there are two improved types of arc lamps, namely, the
flaming arc lamps of two or three intensities; and the luminous arc
lamps of three. In incandescent lamps we have now the complete
system of gas-filled tungsten or "Mazda, Type C" lamps of prac-
tically any output desired for street lighting from 60 to 1500
candle-power, suitable for both multiple and series circuits. In gas
lamps, the inverted mantle lamps with a variable number of mantles,
and the vertical single mantle lamps are valuable and eminently
practical.
In the brief time given to this lecture, it is impossible to cover
older types of lamps, which are now superseded. As a matter of
fact, the enclosed arc lamp and the vertical mantle gas lamp are and
will be in general use for a long period undoubtedly, although they
may be said to be superseded and obsolete.
Lamps as usually equipped for street purposes in standard forms
may be described as follows, so far as concerns the light distribution.
The enclosed carbon arc lamp gives its maximum ray at about
40 from the vertical, the flaming arc lamp not enclosed gives its
largest flux nearer the vertical, the enclosed type with standard equip-
ments gives its maximum flux along and to about 30 from the
horizontal, the luminous arc and the "Mazda Type C" along and
usually below the horizontal at from 10 to 30 or 40, depending
on the equipment; inverted mantle gas lamps give their largest
flux downward and around the vertical, and the vertical mantle
lamp around the horizontal and downward.
STREET LIGHTING WITH LARGE AND SMALL UNITS
In a lighting system with large units it is characteristic that when
they are closely spaced the direction of the rays relatively is not as
important as when spaced far apart, when such direction for pro-
motion of perception has a decided effect in producing the necessary
contrast. Where closely spaced the equipment would be such as to
LACOMBE: STREET LIGHTING 467
give a distribution below the lamp approaching the hemispherical.
An important characteristic in the case of arc lamps is that the
light is usually white, either the violet white of the carbon arc, the
clear white of the luminous arc, or the creamy or pinkish white of
the usual flame arc. These light units scintillate, giving the effect
of brilliance, vary in intensity and appear alive. The color effect
produced is in contrast to the yellower light given from store win-
dows and seems to belong characteristically to the street. This
result is quite desirable.
Large incandescent lamp units equipped and spaced like arc lamps
producing the same general effect as the large arc units of similar
distribution characteristics, but giving a light tending toward yellow,
are still and continuous in their performance and in consequence do
not give the brilliant lively effect of the arc lamps. There is little
differentiation in color with the light from store windows and,
therefore, the street seems to take on the effect of one tone somewhat
duller and more monotonous than with the arc system.
Lighting with small units spaced at distances proportionate to
those for large units has the same characteristic of a lighted spot and
a darker area. This was very familiar under the usual treatment of
vertical single mantle gas lamps with their average candle-power and
distribution. The substitution of electric lamps of much higher
intensity broadened the scope of the small unit to a large extent.
With lamps of from 80 to 100 candle-power at greater heights from
the street, the illumination was much increased and brightened.
The effect of a number of smaller lamps on a block formerly lighted
by two arcs, one at each end, was to break up the large dark area,
decrease the extreme contrast between maximum and minimum
intensity, and generally resulted in much greater visibility and safety.
Where not too greatly diffused by enclosing glassware, care being
taken to reduce glare, and arranged parallel or on one side of the
street only, the contrast or unidirectional effect is' maintained.
Where strongly diffused, arranged opposite each other or staggered,
and with relatively short spacing, the lighting loses contrast effect,
and perception is affected detrimentally.
SOME FEATURES OF ILLUMINATION SYSTEMS OF ARC AND
INCANDESCENT LAMPS
The lighting system now made possible by the use of various
intensities in arc lamps and with "Type C " incandescent lamps, the
468 ILLUMINATING ENGINEERING PRACTICE
latter ranging in intensity from very high to low candle-powers,
enables one to grade the lighting of streets as to their use, much more
accurately than was possible in the past. The gas-filled incandescent
lamps particularly are available practically on all systems of dis-
tribution, except that there are maintenance conditions which must
be considered when they are used on the same circuits with arc
lamps.
Lamps using the same current but of various intensities are
available on any series system, so that we no longer have to provide
special arc lamp circuits for the supply of current to large units.
With this lamp we now have a series unit graded in consumption
and intensity to meet all conditions necessary to fit the lighting to
the street. They are economical and efficient, practically inter-
changeable, having no mechanical moving parts; are susceptible of
artistic treatment and are in every way flexible and adaptable to
street lighting conditions.
There is little question that the system of lighting with "Type
C" lamps of all required sizes, will replace the direct and alter-
nating current enclosed carbon arc lamp almost entirely, as soon as
the equipment can be economically changed. It is also true that
larger units will probably take the place of flaming arc lamps, in
spite of the improvements that have been made in these lamps and
their higher initial efficiency. It is unnecessary, therefore, to devote
any particular time to the discussion of these types of lighting units.
The formidable rival of the " Type C " lamp is the luminous arc which
is somewhat more efficient. It is available in three current ratings,
4, 5, and 6 amp., with standard fixtures of several forms. It gives
a very brilliant white and scintillating light of more accurate
color value. It is also susceptible of ornamental treatment and by
the use of refractors can meet conditions of almost any required
spacing. Its high initial candle-power enables it to be widely dif-
fused and yet develop strong lighting on the street. It is par-
ticularly practicable for business streets where not only the street
but the building fronts should be well illuminated.
An advantageous feature of the magnetite arc lamp for street
lighting is the brilliant white light it gives. A unit of this character
is distinctive of the street itself, the light produced being in great
contrast to that given by store lighting. Compared with incandes-
cent lamps, the luminous arc lamp differs in that it operates mechani-
cally, is limited to large units and cannot be used directly on alter-
nating current circuits but requires the use of current rectifiers.
LACOMBE: STREET LIGHTING 469
For the last year there has been a close rivalry between the two
systems; both have definite characteristics which in specific prob-
lems will lead to a choice of one or the other. For the general re-
quirements of illumination in the average city, however, it appears
that the lower initial investment, when combined with the extreme
adaptability, the ease of operation and satisfactory service perform-
ance of the "Type C" incandescent lamps, make them the more
general choice.
In view of the availability of graded units of illumination, in both
arc and incandescent lamps, the development of the lighting on the
streets requires more careful adjustment than in the past, when we
had only two units of illumination, one large and one small. One
must also carefully study the development of such a graded system
with reference to first cost as well as of operation.
One of the greatest obstacles to the improvement and increase
of street lighting is the cost of the equipment and its installation on
the streets; the most careful attention should be paid to this, and
where it can be kept down without loss in appearknce or in the effi-
ciency of the illumination, it should be done. The utilization of
present equipment so far as is possible to produce good results, will
secure the greatest economy in first cost.
DEVELOPMENT OF ILLUMINATION ON THE STREET
Assuming the use of graded units, if we should use magnetite
lamps for the large units, the circuits and locations of these lamps
would naturally be in the central section of the city and on important
radial streets leading therefrom, the smaller units of the gas-filled
type would cover those portions directly enveloping the central
section and the various residence and suburban districts with sepa-
rate circuits therefor. In the simplest form, assuming that "Type
C" unit alone is used, the general design of lighting the city would
be about as follows : The various streets of the night center or centers,
usually easily determined, would receive the most brilliant lighting
from large units spaced regularly and closely along the streets.
Special intersections, where streams of traffic meet and diverge,
should be very adequately lighted with double lighting at the inter-
sections and close spacing along streets. Where such intersections
become open squares or plazas, excellent effects can be obtained from
tall standards with lamps 40 to 45 feet from the ground. In such
cases, with proper reflectors for throwing all the light below the
horizontal, a high intensity can be obtained over the whole area.
470 ILLUMINATING ENGINEERING PRACTICE
The light need be diffused only slightly, if at all, and full efficiency
can be obtained without danger of glare. It is in these sections of a
city that one should see clearly by direct reflection. From this
central section, avenues and streets will lead to the residence and
other sections, and should receive the next lower grade of lighting,
"B," amply sufficient for fast moving street-car or automobile
traffic. This lighting may properly be so arranged as to be brightest
nearest the centers and decrease to Class C as the distance increases
and the traffic diminishes. This can be done most easily and cheaply
by increasing the spacing between lamps, retaining at least one per
street intersection. Where these streets intersect other streets of
the same character, the lighting should be reinforced at the inter-
section. This grading of street lighting is further aided by the
ability we now have to decrease the candle-power, and by the light-
directing devices now available. Generally outside of the main
night center of the city other local centers will be found much used
at night, which must receive augmented lighting for a few blocks.
In such cases the lighting should usually be sufficient to produce
direct vision by reflection. As a general rule in business districts,
the equipment should be of a type which will light the building
fronts as well as the streets. In residence districts this should be
avoided above the first story in sections where houses are in blocks,
and further minimized in suburban districts. In the latter sections
the average resident objects to almost any form of visible light source.
In every city will also be found the wholesale business and finan-
cial section which is usually deserted at night but requires ample
lighting for police purposes, about Class C. As such sections are
usually treeless they may best be lighted by large units at street
intersections and alley entrances. Where blocks are short and mid-
block alleys exist, mid-block lamps may be of a smaller size and with
different equipment.
The residence sections of a city take varied treatment. In almost
all cases, silhouette lighting must be depended on in these sections.
The density of population, the character of the houses, and the
trees each has a strong effect in determining the lighting of residence
streets. In the residence sections of the city where the houses are
practically continuous, the illumination intensity should be main-
tained at about moonlight value Class "D." As one leaves these
sections, however, and reaches the suburban residence district, with
detached houses becoming further and further apart, the lighting in-
tensity would naturally diminish to Class "E." In this case where
LACOMBE: STREET LIGHTING 471
trees are few in number, large lamps at street intersections placed
fairly high, with directing refractors give good results and may be
spaced at considerable distances without intermediate lamps. Where
in suburbs the spacing can be moderately short, even more agreeable
results may be obtained by the use of reflectors and diffusing globes.
Where heavy foliage exists, small lamps at comparatively short
distances apart on standards low enough to allow the light to spread
under the trees gives the most satisfactory results to the residents.
In the suburban sections where overhead construction is usual,
very satisfactory lighting can be obtained by the smaller incandescent
units placed on each line pole or on every other line pole, due regard
being had for the street intersections. If available, the use of the
poles of other lines than those of the lighting company for street
lamps in the residence district, would be valuable in such cases,
particularly on those streets having trolley lines and acting as
arteries or feeders leading from the outskirts to the center of the
city.
In general, a low order of intensity may be used throughout
suburban residence sections sufficient for police purposes, and yet
fairly below the intensity of moonlight. Unidirectional lighting
in such sections is particularly valuable as full use of the silhouette
effect is necessary.
Boulevards may be treated like residence streets with either the
large or the small unit. Where there is little interference with light-
ing and the boulevard is so wide that trees do not meet, and center
suspension cannot be used for aesthetic reasons, large lamps on higher
standards with long arms bring the light source well over the road-
way and give excellent results. Where the reverse conditions pre-
vail smaller lamps should be used. Where the boulevard is parked,
smaller lamps attractively mounted and properly equipped give
satisfactory results. If center plots are available, the boulevard
being quite wide, the cheapest and most efficient lighting will be
obtained by large lamps properly arranged in these plots. If the
traffic is sufficient, and in any case at important intersections, the
center plot lamps would properly be supplemented by lamps ar-
ranged along the outer lines of the roadways. When boulevards are
extended and become interconnecting roads, passing temporarily
from city conditions, the lighting must be graded in accordance with
the amount of traffic. Originally lighted to the degree of "D"
or "E" streets, they may change to Class "F" conditions, and the
lighting be diminished accordingly with less expensive equipment.
472 ILLUMINATING ENGINEERING PRACTICE
Interconnecting country roads, county roads or highways, except
within town limits, are rarely lighted at this time. A movement
in this direction is beginning, however, and should be encouraged.
To make this type of lighting popular it must be very inexpensive
in so far as equipment is concerned and low in operating cost. In
consequence it usually consists of lamps mounted on line poles.
These lamps are placed at distances varying from 300 to 900 feet
and more apart. It is obvious that the lighting at such distances
apart is largely directional in character. Four-ampere luminous arc
lamps, from 600 to 900 feet apart, properly equipped with refractors
and reflectors, give about the best lighting of this type in use at this
time, and objects are quite visible in silhouette. This condition
exists even up to spacing 1 200 feet or 1 500 feet apart. If the smaller
incandescent lamps are used with refractors they should not be
spaced at distances over 500 feet apart for the loo-c.p. size.
ELECTRICAL DISTRIBUTION SYSTEMS
In the lighting of streets, the electrical distribution system is
important, from its bearing on the question of first cost and also of
operation. Distribution systems can be generally stated to be
either series or constant current systems, and multiple or constant
potential systems, while combinations of these in one system are
sometimes used. The second or multiple system, usually low ten-
sion, is generally limited to the central or business area of a city
where the density of consumers is at the maximum. Energy for the
lighting is taken directly from the general supply net work. Except
in the largest cities this system is not generally in use for street
lighting, and elsewhere the series system is practically universal.
Originally it was used for open direct-current series arc lamps operat-
ing from arc-lighting generators. Speaking broadly, with the ad-
vent of the enclosed arc lamps came the alternating-current series
circuits with series transformers, improved later by the constant
current regulator, and then with the development of the luminous
or magnetite lamp, came the mercury are rectifier. To-day the
alternating constant-current series system in one of its forms is used
in most localities, as it is available for carbon or flaming arc-lamp
circuits or "Type C" incandescent lamps and, with rectifiers, can
be used for luminous arc lamps.
With this system the energy is distributed from central stations or
sub-stations feeding the various circuits. Two methods seem to be
LACOMBE: STREET LIGHTNG 473
in use. In the first, constant-current regulating transformers change
the constant-voltage alternating current to constant-current series
alternating, being so regulated that the sizes of the lamps may be
varied independently of each other. In the other, series transformers
are used to supply the energy to the series circuit with special taps
on the primary and secondary of these transformers for regulating
the current within the limitations of the transformer, in accordance
with the number of lamps on the circuit. In some systems a reac-
tance is added in series with the lamp so that in case a lamp goes
out, constant current will be maintained throughout the circuit.
Each system has its advantages, particularly under special circum-
stances. The automatic regulation of the constant-current regulat-
ing transformer is more accurate and less awkward in adjustment
than the series transformer system with taps. On account of the
importance of the street lighting to the night life of a city, it is wise
to have the lighting circuits originate at one point, as with the
constant current regulator system, where it is under the observation
of an attendant, as this would tend toward better operation and
quicker repair in case of interruption to service.
So far as illumination is concerned, the question of the location
of the lamps is relatively of great importance in the amount of
expense that may attach to the first cost of the system, on account
of the expense of lamp posts, their erection, and the connection of
the lamps to the system. For instance, in overhead systems line
poles can be used as lamp poles, particularly if at every street inter-
section there was at least one pole in a suitable position. This
should be provided for in the construction of new lines. In such
case the first cost of lighting units is at a minimum. The objec-
tions to overhead lines are the well-known ones of obstructions on
sidewalks, ugliness and interference with trees. These objections
may be greatly minimized if proper precautions are taken, careful
preparations made, permits obtained in advance, the poles painted
so as to be unobtrusive, and the structure maintained in first-class
condition. The trimming of trees may also be accomplished if
carefully negotiated, but it is best to conduct such operations with
the authorities having jurisdiction, usually the Park Department of
a city. Attention is drawn to these points for the reason that a
part of the demand for underground construction and its conse-
quent expense is caused by the use at times of somewhat arbitrary
methods and the neglect of neat and workmanlike construction.
Where underground construction is warranted, or one is compelled
474 ILLUMINATING ENGINEERING PRACTICE
to use it, the question of equipment cost becomes even more im-
portant, the relative expense is greater and every effort should be
made to utilize any available equipment and to locate the lamps so
that the minimum expense be incurred. The cables, conduits,
manholes, hand-hole boxes, street openings and settings and founda-
tions of heavy iron poles involve a heavy first cost. Where iron
trolley poles exist, as they usually do in downtown districts, very
advantageous results can be obtained by utilizing them, with either
the parallel or the staggered arrangement of lamps. By this sensible
and economical method a number of quite successful installations
have been made throughout the country, by which the street lighting
has been greatly improved at a minimum cost.
Where underground construction must be extended into the outer
sections of the city to avoid pole lines, armored cable may be used
at much less expense than standard underground construction.
Where iron lamp posts are available, such as in the substitution of
electric lighting for gas lighting, they may be used to advantage,
with modern diffusing appliances, in economy of first cost.
LOCATIONS, SPACING AND HEIGHT
This brings us naturally to the question of location, spacing and
height of lighting units. A very careful study and survey of the
streets to be lighted should be made before lamps are located, keep-
ing in mind the kind of illumination to be used, the various grades
of streets and the results to be obtained. Full field notes should be
made covering the characteristics of each street, its use, character
and direction of general traffic; the type and position of buildings,
particularly of special buildings; the color, type and reflection char-
acteristics of the pavement and buildings; curves in streets, alleys;
parking, if any; special open areas along streets, special street inter-
sections, and streets containing street-car lines, and particularly
intersections of such streets. The existing lamp posts, distribution
system, trolley poles and construction of pavements are also of
great importance. These data should be transferred to a large
street map for record.
So many different conditions may appear and the results desired
are so varied, that it is impossible to lay down any general rule of
procedure other than in the general description already given, except
to emphasize the importance of studying the streets and designing
the lighting in accordance with field conditions, even to the extent
LACOMBE: STREET LIGHTING 475
of making trial installations in the most important places. A care-
ful study of accurate field notes in connection with the general
description of the laying out of lighting already given, will generally
cause the situation to clear up and definite lines of procedure will
develop.
If one had complete control of height, spacing and location of
lamps, theoretically almost any desired lighting could be obtained,
uniform or non-uniform, with minimum glare. Mr. Millar's tests
for the National Electric Light Association and the Association of
Edison Illuminating Companies, have shown that for visibility,
uniform lighting on streets, particularly of a low order, is not the
best, thereby exploding an old theory. In view of the results he
has obtained, one should work toward lighting giving reasonable
contrasts, that is, lighting which, predominating in one direction,
creates contrasts. Such contrasts alternating between each light
source thus create a moderate diversity of intensity between the
extremes of uniformity and non-uniformity. We rarely have
complete control of height and location, so that heights and spacing
in actual practice must be considered. In general the height at
which lamps are placed is limited by the expense of the equipment
and the difficulties in getting at lamps at considerable heights for
cleaning and renewal. From 20 to 25 ft. is the general limit for
large units, and 10 to 16 ft. for the smaller ones. Within such
limits, the higher the better, as wider light distribution is obtained,
the high lighting under the lamp being decreased while the minimum
normal illumination within the usual radius is only slightly affected.
At these heights the larger lamps are generally out of the direct line
of vision which, of course, is an advantage. It is rare to find large
units at less than 18 ft. above the surface of the street except in
ornamental and well diffused lighting systems.
Larger lamps can be used with the higher posts and in conse-
quence there would be fewer posts. It will be noted that the
generally available heights are limited within small variations, and
hence the spacing is the dominant factor in determining the size of
the unit with which the desired illumination is to be obtained. The
limit of practical spacing where large lamps are used, has been some-
what increased lately by reason of the introduction of refractors
which distribute the lighting flux at a further distance from the post.
In certain cases, therefore, the distances between posts may be in-
creased from that of older practice and the use of some intermediate
lamps, with their extra installation cost, be avoided.
476 ILLUMINATING ENGINEERING PRACTICE
Concerning the relative effect of height and spacing on the hori-
zontal illumination of the street within the usual working limits,
it can be stated that with a spherical or uniform candle-power distri-
bution curve below the horizontal, a change in height from 19.5 to
26 ft. will decrease the mean horizontal illumination by 14 per cent.,
and would increase the minimum illumination by 20 per cent.;
whereas, assuming a fixed height, the mean horizontal illumination
is practically inversely proportional to the distance from the lamp.
The maximum, of course, varies very little as the spacing is increased,
but the minimum decreases very rapidly, and in consequence the
uniformity of the lighting becomes less and less.
Many arrangements of locating lamps on streets are in use, vary-
ing with their importance. Where large units are utilized on streets
of ordinary use, such as C, D, and E, they are usually suspended at
the center of street intersections, or on brackets from poles at such
points. On more important streets, however, where center sus-
pensions have many structural objections, lamps are usually placed
on posts arranged in parallel along the curb, and staggered, or else
placed parallel along the curb and opposite each other. This prob-
lem frequently comes up in connection with "White Way" lighting,
and at times it is quite difficult to tell which is most desirable. As a
matter of fact, parallel and opposite lamps do not give as uniform
lighting along the center of the street, although similar in each space,
as is given by the parallel and staggered arrangement. The ar-
rangement of lamps opposite each other is usually admired for its
symmetry of location, but it is more expensive than the usual ar-
rangement. In general, except where artistic requirements dominate,
the staggered location, except where closely spaced, is preferable.
At street intersections under the parallel and opposite arrangement
along any one street, lamps are not usually placed at street corners,
but some little distance back from the house line, so located as to
apply properly certain zones of light on the intersection from each
of the units.
The parallel and opposite arrangement further requires that the
intersecting street must be independently lighted and hence this
system becomes more expensive in first cost and operation, so much
so, in fact that it is rarely used on city streets except for display
lighting or where trolley poles are available at small expense. The
parallel and staggered system usually involves the use of two stand-
ards at each street intersection at diagonal corners, not exactly at
the corners but approximately on the house lines. The intermediate
LACOMBE: STREET LIGHTING 477
lamps are then placed alternately on opposite sides of the street at
curb lines, the spacing depending on the intensity of lighting desired,
length of block, location of alleys, and so on. With this arrangement
the intersecting streets are taken care of, the intensity required ob-
tained, and contrast and unidirectional lighting are maintained
without the use of as many lamps and standards, and consequently
at less expense in first and operating cost.
The position of the lamps, whether placed along the curb or sus-
pended in the center of the street, has an effect on the general ap-
pearance of the street. Under the first type of lighting, the street
appears narrower than under the second. Under metropolitan
conditions, however, it would be very difficult to suspend lamps over
the center of the street, and hence they are usually placed on the
curb. A row of lamps opposite each other along the curb, or stag-
gered along the curb, arranged regularly and carefully coordinated
with the curb line as to height, distance from curb, and so on, both
give the street more or less the same effect of parallel lines of light
along the curb, with a long open field of vision, for at a little distance
staggered lamps at night have about the same appearance as lamps
placed opposite.
The theoretically desirable position for large units in the metro-
politan districts would be suspended above the center of the street
at a good height, using powerful lamps and varying the illumination
intensity by closer or wider spacing. While this arrangement is real-
ized abroad by suspension from the buildings or from posts on isles
of safety, it is rarely possible in this country.
EFFECT OF PAVEMENTS AND BUILDINGS
The effect of lighting a city street of one of the higher classes
depends very largely on the characteristics of the street, its pave-
ments and its buildings, for in such streets it is usually desirable to
light the building fronts. A straight street with light-colored
smooth pavement, broad sidewalks, and buildings of a light color
is most favorable to lighting, allowing long symmetrical lines of
lamps, the light from which can be diffused, and which will be
reflected specularly by the pavement and the buildings. A note-
worthy instance of this, brought about by the cooperation of mer-
chants and authorities, is Regent Street in London, which is lighted
by powerful lamps giving white light erected on isles of safety in the
center of the street. The street is paved with asphalt, has broad
sidewalks, and the stores, which vary from 3 to 6 or 7 stories in
478 ILLUMINATING ENGINEERING PRACTICE
height, were by agreement painted a light color, the result being a
brilliantly lighted and effective street, although there is little auxil-
iary window lighting. Pavements, as a matter of fact, have a
greater influence in the appearance of a lighted street, than have
buildings. Specular reflection from the pavement is very useful,
particularly with lower grades of illumination where it is important
to develop the silhouette effect. In most instances we see objects
against the background of the pavement rather than against the
buildings, particularly the more distant objects.
The lighter-colored the pavement the more the reflection obtained,
the lighter the street appears, and the better the general effect.
A given lighting system in a street with a dark dirty pavement,
houses back from the street, would look dull and dim; whereas,
under favorable conditions it would be entirely adequate. If
municipal affairs could be so coordinated that the road surface
would be always light in color the appearance of the streets
at night would be improved and less intensity would be required
to produce the desired effect.
GLARE
The blinding effect of glare is one of the most perplexing prob-
lems in illumination work. Practically it cannot be entirely elimi-
nated; it occurs in daylight and even in moonlight. Its effects are
almost independent of distance and it would seem that nature in-
tended that the eye should be subjected to a certain amount of it.
With artificial lighting it can be relieved by removing the source
from the line of vision by a considerable angle, say 25. This result
is accomplished by raising the lamp above the observer. Or, it can
be relieved by enlarging the source of light by diffusing globes
and large diffusing reflectors. Refractors are also used to deflect
the rays from those angles near the line of vision. Little contrast
between the source of illumination and its background also aids in
reducing glare. Recent instances of these methods of minimizing
glare for large units may be quoted :
One instance, three magnetite lamps with diffusing globes of some
absorption are to be placed on the top of 3o-ft. trolley poles.
A magnetite lamp is placed in a large sectional diffusing globe at
1 6 feet from the sidewalk.
In one case looo-c.p. "Type C" lamps were placed in refractors for
redirecting the rays, and suspended at a height of 30 feet above the
street.
LACOMBE: STREET LIGHTING 479
Another instance, looo-c.p. or i5oo-c.p., "Type C" lamps were
equipped with band refractors and placed 15 feet above the street
in large lanterns with diffusing glass.
With small units at low height and short spacings it is even more
difficult to prevent glare; obviously the source of light is nearer the
line of vision, yet the flux is small and if diffused too much becomes
too weak to produce the lighting effect desired. With the "Type
C" lamp, however, the small source of light is of such intense bright-
ness that the light must be diffused, redirected, or the source raised
as far as possible from the line of vision. In many cities, diffusing
globes are placed around these lamps where they are used in connec-
tion with former gas lamp posts. In other cities, lamps of loo-c.p.
are placed at intervals of from 24 to 1 5 feet in domed reflectors with
the filament so focussed that one can only get the lessened glare
effect of the large white disc of the reflector. This arrangement was
found not to produce excessive glare. In another arrangement a
25o-c.p. lamp was placed at a height of 16.5 feet with diffusing globe
of low absorption value. In another instance, ico-c.p. lamps were
placed in bowl refractors at a height of 15 ft. in attempting to pre-
vent glare, but the effect was to make the lighting look dull. It is
believed that an arrangement of a 100 c.p. lamp in reflectors with
band refractors, the source of illumination being above the lower
edge of the reflector and the bowl of the lamp being frosted, would
give the brilliant effect of the bright light source without injurious
glare.
ACCESSORIES
In one form or another, reflectors, refractors or diffusing globes
are in general use on all lamps, for the reason that it is desired to
redirect the rays of the lamp towards the surface to be illuminated.
Diffusing globes are used often with reflectors where it is desired to
throw most of the diffused light downward. Globes alone are used,
sometimes of special design, where a part of the light is to be used in
lighting the upper parts of the buildings. Reflectors should be
carefully designed in connection with the lamp and the position of
the light source, so that this source is at the proper focussing point
in connection with the reflector for the light distribution desired.
Where this is done, and with the addition of a diffusing globe in case
of a light source of high intrinsic brilliancy, the effect of a large light
source is obtained with fairly well distributed illumination. Where
the sources of light are spaced far apart, refractors add much to
480 ILLUMINATING ENGINEERING PRACTICE
their effectiveness, the action of the refractor being to redirect e
rays of light emitted by the light source into prescribed direct ^s
for which the refractor is designed, usually at from 10 to 15 de< a
below the horizontal. This will increase the illumination HIK ay
between lamps, diminishing it near the lamps, thereby reducing the
spot effect. Refractors, which have been in use abroad for a com-
paratively long time, came into use in this country with improvem :nts
of the luminous arc and the "Type C" lamp, in which the position
of the light source does not vary.
POSTS AND MOUNTINGS
Among the many other details necessary to successful street
lighting, one must consider posts and mountings, particularly under
city conditions, where the more simple standard apparatus, such as
are used for overhead circuits on wooden poles, is not desirable
except in so far as the lamp itself is concerned. It is necessary
that the posts and mountings of lighting units should be as attract-
ive as possible in appearance in the daytime; in any case they should
be neat, well painted and workmanlike in effect, and kept so.
Posts for metropolitan use are usually built of steel and iron, or iron
and concrete, treated more or less ornamentally, and developed in
many forms.
Lamps are supported on top of the- posts, hung in lyre tops, or
are placed in diffusing glass globes and in specially designed lanterns,
for use either on pole tops or on brackets, and one or more brackets
with a lamp on each bracket are often used. In all these forms, one
of the principal considerations should be the ease with which the
lamp can be reached for operation and maintenance. When lamps
are set at from 22 to 25 ft. in height where pavements are smooth
and traffic is dense, they are frequently attended to by men on tower
wagons. As this plan is expensive, the lamps should be so arranged
that they can be lowered to the street when possible to do so. Auto-
matic hangers are frequently used; they have the advantage of
detaching the lamp from the circuit while it is lowered to the street
for attention.
The expense of the post and lantern itself varies over wide limits,
depending on the elaboration of artistic design and finish. On
metropolitan streets of the AA, A and B class, use is made *f iron
and steel posts, costing from $35 to $125 and above depenuing on
the strength required, height and ornamentation. A full standard
LACOMBE: STREET LIGHTING 481
ar... ornamental equipment for luminous arc lamps has been de-
\r' ed and used with good effect in many cities. The reinforced
r-te post has proved to be serviceable and cheap. It is par-
ticuv.oiy adapted to artistic treatment at a minimum expense, and
has . sen used with great success in parks and parkways particularly
with the smaller lighting units. A very attractive post can be ob-
tained for this service for about $9. A successful attempt at a tall
conclete post has recently been made in a Western city, where re-
irforted posts 30 ft. in height, with reinforced concrete brackets,
' ave been constructed. Each of these posts with lamps cost about
$115 installed. With the exception of this one case, the estimates
given above do not include the cost of setting, or of lamps. The
cost of setting depends on variable conditions; with the larger and
taller posts concrete foundations are used, and the necessary excava-
tion under congested city conditions, is extremely costly in many
cases. It is desirable, so far as is reasonable, to use standard designs
of posts and lanterns, thereby avoiding excessive first cost. Where
special posts and lanterns are designed for artistic effect, the first
cost is largely increased and the operating costs also, for renewal
parts have to be specially made and are expensive.
In general, where large units are employed, it is desirable to use
brackets and bring them well out over the roadway of the street,
in order to put the light where it is needed for the greatest traffic.
With the ordinary foliage encountered on most avenues and streets in
the central part of a city, long brackets will be found very desirable,
with lamps at 20 ft. or higher, in order to bring the light out beyond
the foliage. This arrangement also serves to place the lamps at a
point where they not only light the roadway but throw a considerable
portion of the light under the trees and on the sidewalk.
About the usual limit in length for big units under metropolitan
conditions is from 8 to 10 ft. and in the suburbs where overhead
construction can be used from 10 to 12 ft. In small units at heights
of from 14 to 15 ft., 4-ft. brackets are quite sufficient to improve
materially the lighting in the roadway and yet throw the light under
the trees.
The housing of the light source itself; in other words, the lantern,
must be weatherproof, designed for its type of lamp, to allow ease
in repair, cleaning, and the renewal of glassware. It should be de-
signed 9~>r the best light distribution and be attractive in appearance.
The standard apparatus to-day generally meets these conditions.
Special lanterns, like special posts, are expensive in first cost and
31
482 ILLUMINATING ENGINEERING PRACTICE
maintenance, and they are warranted only where artistic design is
required to harmonize with that of the post. They are rarely
justified except for points where great artistic merit is desired.
GRADED ILLUMINATION AND RESULTS ON CERTAIN STREETS
In establishing a certain grade of illumination on a street, a de-
termination of the average and minimum illumination having been
made, the size and number of lamps can be established quite easily
by the flux method. Manufacturers now supply candle-power dis-
tribution curves of the complete unit, including globes, reflector or
refractor. They show the spherical or hemispherical candle-power
and the total or the downward useful lumens. From these data
can be determined the number and size of lamps required per block
or unit area to obtain the average foot-candles (lumens per square
foot) desired. The location of the lamps must then be made after
a study covering the many local factors that determine this. The
locations should be finally checked by inspection, particularly on
important streets where many factors may effect the desired result.
With these points established, the minimum and maximum illu-
mination can be determined usually by the point-to-point method.
When very low average illumination is to be established, the de-
termination of the minimum illumination may be all that is neces-
sary. Some typical installations illustrating the grades of street
lighting given earlier in this lecture may be described as follows:
The so-called 'Times Square" or "Longacre Square," New York,
may be cited as a good example of an area where Class AA lighting
is desirable. The " Square" is formed by the two triangles meeting
at their apexes and extends from 43d to 46th Street. Two double-
track street railways cross each other at an acute angle and great
intersecting streams of traffic of street cars, motors, carriages and
pedestrians occur until very late at night. A certain illumination is
obtained until quite late from electric signs, etc., and from street
lamps on the edge of the Square; we assume this to average o.i foot-
candles, and that it was desired to bring up the average of the Square
to 0.5 foot-candles. It is noted that the sidewalks around the Square
are well lighted and the area over which the lighting is to be increased
is in, the middle beyond the sidewalks. Three ornamental poles, one
placed at the apex near 46th Street, one near 44th Street, on an isle
of safety, and one on the sidewalk at center near 43d Street, equipped
with four brackets suspending the lamps at 45 ft., would be the best
LACOMBE: STREET LIGHTING 483
height and location. Four lamps per post of about 1500 mean hemi-
spherical candle-power each, so equipped with reflectors as to give
about 9000 lumens within 75 from the vertical would produce the
desired result. Auxiliary but smaller lamps would be placed on
posts at 45th Street. This is the apex of each triangle and the large
posts would be, respectively, 300 ft. to the north and 240 ft. and 560 ft.
to the south of this point. A standard type of "Type C" lamp and
pendant fixture with reflector will fill the requirements. At the
height stated the glare is negligible.
A type of street of the A Class, which would be a "White Way"
street in a smaller city, having an average illumination of about 0.35
foot-candles and a minimum of 0.15 would be equipped about as
follows: On a street 50 ft. wide 4-amp. luminous arc lamps, one per
post staggered and 65 ft. apart, 18 ft. high, with diffusing globe and
high efficiency electrodes, would give 2500 lumens per lamp, which
would produce the required average. Standard globes, mountings
and posts can be obtained for this equipment.
Class "B" lighting would be produced on a street 95 ft. wide
with 75o-watt "Type C" lamps at 18.5 ft. from the ground, with
standard lantern and diffusing globe, arranged staggered 100 ft.
apart between streets and 60 ft. apart at street intersections, this
arrangement giving by measurement an average of 0.25 foot-candle
with a minimum of 0.044.
Class "C." This illumination can be obtained from 100 c-p.
"Type C" lamps, 120 ft. apart on a street 45 ft. wide, lamps placed
on poles along one side, 15 ft. in height, equipped with slightly coned,
radial wave reflectors with filament of lamp carefully focussed in
reflector, the resulting average illumination being 0.075, with a
minimum of 0.015 foot-candle.
Class "D" lighting is about that needed for a boulevard street of
high class with many trees. A very effective installation giving
about this illumination would be one with 25o-c.p. lamps in orna-
mental fixtures with reflector and diffusing globes, mounted about
15 ft. in height, placed parallel and opposite along a street 100 ft.
wide and 1 10 ft. apart. Concrete posts like those in the Parkways of
Chicago would be attractive in this case. Another method would
be placing i5o-c.p. lamps with light diffusing globes at a height of
14 ft., no ft. apart along the curb, staggered on a street 60 ft. wide.
. Class "E" lighting is about that produced by the familiar vertical
mantle gas lamp 9 ft. 10 in. high when in first-class condition, spaced
90 ft. apart, staggered on a street 60 ft. wide.
484 ILLUMINATING ENGINEERING PRACTICE
Class "F" intensities depend largely on the amount of money
that can be devoted to them. The minimum is given for interurban
conditions; it is very low and when within a city the illumination
should be raised to Class "E" or "D. " Such lighting as designated
by "F" can be obtained with 400 c.p. "Type C" lamps with 80
refractors 22 ft. high, center suspension, 500 ft. apart.
Class " G" streets or country roads so far have been lighted mainly
for directional effect only and have already been described.
It is well to note in connection with the general fashion of " White
Way" lighting with its increased intensities over those of a few
years ago, that such lighting has benefited only small portions of a
city's streets and that the balance suffers from the appropriation of
funds for this purpose only. Usually little effort has been made
to improve properly the remaining lighting in the municipality
since improved appliances became available.
It is necessary to explain that the intent of this lecture has been
to cover briefly the utilitarian side of street lighting. The most
attractive side, that of ornamental and artistic street lighting and
fixtures, could not be touched on in the time allotted. However,
it is proper to emphasize the desirability of such illumination, and
while it is usually expensive and limited to certain streets in the
larger cities, every effort should be made to attain it, even in more
commonplace installations. A pleasing effect does not depend on
expense alone, and good taste may be exercised with simple standard
forms as well as with more expensive ones.
CONTRACTUAL RELATIONS
The increase and development of good street lighting depends to a
very important extent on the contractual relations between the
public utility and the municipality, and these relations should be
cooperative, harmonious and progressive to develop this work to its
full growth and usefulness which have not yet been attained. The
provisions of a contract between the two parties should cover fully
the complete illumination service, the equipment and investment
necessary thereto, and the remuneration therefor. It should pro-
vide for the continuous production of a stated quantity of light at
the points desired for certain hours in a given period of time, and also
the service by which the quality of the illumination is maintained.
It should also be flexible and provide for increases or decreases in
number of units and changes in type of units and appurtenances with
LACOMBE: STREET LIGHTING 485
the normal development of the art. The full details of such a con-
tract cannot be given here. The legal form is usually provided by
legal advisors of a city, and will embody the statutory or local legal
requirements. The specifications covering the work of the lighting
contractor should include the following requirements and conditions:
CONTRACT REQUIREMENTS
I. General. (a) Equipment to be first-class, efficient and safe.
(b) Contractor to be responsible for injury or damage and to
indemnify the city against patent infringements.
(c) Contractor to exercise skill and foresight in carrying out the
provisions of the contract.
//. Work to be Performed. (a) Requires the furnishing of all
lamps, supports, connections, appurtenances, electric energy, repairs
and all service for operation and maintenance.
(b) States the respective numbers of each size and kind of lamp
to be furnished at the beginning of the contract, including technical
description of the same with its equipment and operating supplies,
if any. The rated energy and illumination performance when prop-
erly operated to be described and submitted.
(c) States the respective lamps to be supplied with energy from
underground and from overhead circuits, with the type and method
of support. A list of locations should be furnished, preferably with
a map indicating the kind of service connection and support.
(d) As the locations and kind of supports are usually prescribed
or approved by the city, this clause would provide for submitting
the necessary samples, photographs or maps to the city within a
specified time and getting its approval of these items.
///. Operation. Recites the conditions of the operation and
maintenance of the lamps and all their appurtenances, practically
covering the lighting service, including the uniformity of lighting,
elements of the same, regulation, trimming or replacement, cleaning,
painting and prompt repairs. This clause should require complete
operation in accordance with the best modern practice.
IV. Testing. Should cover the tests to be made by city of the
fulfilment of the contract requirements as to the light given by
the unit. This is usually required either in the terms of energy or
illumination or both; tests to be made either on the streets or in the
laboratories or both. This would usually be accompanied by clauses
covering the quality of service and inspection thereof.
486 ILLUMINATING ENGINEERING PRACTICE
V. Increase or Decrease in Number of Lamps. -(a) In case of
additional lamps, would require compliance with previous specifi-
cations as to type of supports, connections, lamp units and so on,
and locations as specified by the city.
(b) Should cover the installation of additional lamps and damage
and allowance for delay.
(c) Would cover limitations as to distance of extensions from the
present circuits on either the underground or the overhead circuits,
without cost to the city. Would also cover the payment by the
city if the lamps are placed at distances greater than the limits
agreed upon.
(d) Defines the right of the city to discontinue lamps entirely or
change from overhead to underground circuits, depending on the
rates for the lamps on each circuit or with provisions for refunding
to the company the unamortized cost of the equipment.
(e) Would state the conditions in long-term contracts as to the
lamps and equipment ordered late in the life of the contract and the
unamortized cost of the same toward the end of the contract term,
if it was not renewed.
(/) Requirements to be stated covering deductions as liquidated
damages for outages.
(g) Provision covering arrangements by which another type of
improved lamp may be tried and adopted if desirable to both parties.
This clause would also provide method of estimating the cost of
such change relative to the cost of the original equipment.
(h) Provision should be made for covering arbitration of disputes
between the parties to the contract, or of amendments to the
contract.
(i) Conditions to be stated covering payments by the city under
the contract in accordance with the local statutes governing this
process.
It will be understood that this list of contract provisions is general
and will require considerable amplification and change in many
instances. It is understood, of course, that every contract stands
on its own bottom and no general rules can be laid down to cover
local policies and conditions.
PRESENT PRACTICE IN CONTRACTS
When one considers the many varied circumstances that have
existed in the development of the electric lighting industry since its
LACOMBE: STREET LIGHTING 487
inception and the various political forms of government under which
contracts have been made, it is not strange that these contracts
vary very much in certain provisions, and that no standard form
seems to exist, even at this late date. It is interesting therefore to
observe the data obtained by an inquiry on this subject and others,
instituted by The Milwaukee Electric Railway and Light Company
in 1915. Through the courtesy of the company a partial summary
of it can be submitted to you, the limited time requiring that only
the most important and usually disputed points be touched on.
It might be deduced from the data collected that contracts were
divided into two great classes depending on the theory on which
the contract is based. One class obviates specific clauses covering
removals, changes of equipment, types of units, etc., by providing
a margin or factor of safety in its rates to generally cover these
points. The other class takes up special charges in detail and pro-
vides for them specifically outside of the rates for the illumination
service itself. Data were obtained from about 128 different com-
panies in cities varying from 20,000 to over 200,000 in population,
but omit Chicago, Philadelphia, and portions of Greater New
York.
From the data it appears that the length of term of contract
In 21 cases was under 5 years
In 44 " " 5 "
In 7 " " over 5 " and under 10
In 48 " 10 "
In 4 " " over 10 "
In 4 " there was no contract.
As to the question of the right of a city to increase the number of
lamps, there was no restriction in 96 cases. As to the right of remov-
ing lamps from one location to another, there was no restriction,
except that in a little more than half the cases the companies bore
all the costs, while in the balance the city generally paid the costs.
As to ordering lamps changed from overhead to underground
circuits, in over half the cases the city had no right to do so. In
39 cases the city had the right, but it was rare that the company
was remunerated for such change.
No provision was made for tests of illumination or energy in 58
cases, nearly half the total number. In 34 cases the city may test ; in
the majority of these cases the tests were for energy consumption
only.
Referring to the distance of the location of a new lamp from the
488 ILLUMINATING ENGINEERING PRACTICE
nearest circuit, it was found in 72 cases that there were no limits.
In 49 cases it was limited by various distances. Where the distance
to the location of the new lamp exceeds the limit, in 36 cases no pro-
vision was made to cover excess cost, but in 20 cases the city paid
the cost for the excess distance.
The discontinuance of lamps is allowed to a minimum number in
45 cities; in 35 cities the city may discontinue as much as it pleases,
in 22 cases no provision is made, and in n cases discontinuance of
lamps is not allowed at all.
Provisions for substitution of improved lamps are found in the
contracts of only 29 cities. In 27 of these, the conditions on which
such changes may be made vary from no additional charge to the
full cost in excess of the original contract requirements.
Outage regulations and penalties are enforced in only 40 cities.
Contracts are subject to revision as to price by agreement in
14 cities.
Contract rates are subject to regulation and amendment by
national, state and municipal authorities as follows:
In 1 8 cases by the municipality.
In 55 cases by public service commissions.
In 5 cases by the state authorities.
In 4 cases by the state and municipal authorities.
In i case by Congress.
The queries just mentioned are those which might cause more or
less serious disputes between the parties to a street-lighting contract,
and this extremely wide variation in practice is difficult to explain.
It certainly shows a state of looseness in contracts which under usual
business conditions would produce needless disputes, but this gener-
ally does not seem to have been the case.
MEASURE OF ILLUMINATION SERVICE
Among the most frequently discussed questions in contract
requirements is that of a satisfactory measure of the illumination
service rendered to the city. It is a complicated matter and from
the municipal standpoint in actual practice should be handled on
administrative engineering lines based on correct technical data.
Good illumination service implies two things. One is a satis-
factory light-giving source, and the other is the attention given to
it or the service which keeps the source of illumination at its maxi-
mum efficiency, and provides for its continuous and regular opera-
LACOMBE: STREET LIGHTING 489
tion. These two should be taken together in measuring the
result.
The older types of lighting units such as the carbon arc lamp
varied and were irregular in intensity, so that much difficulty was
encountered in establishing a satisfactory measure of illumination.
It was very difficult, if at all practical, to transport the units to the
laboratories and duplicate street conditions. It was also difficult
to handle smaller and fragile units, such as mantle gas or vacuum
tungsten lamps. The more modern illuminants are comparatively
rugged, vary little in actual operation and may be tested with greater
ease and more definite results. A satisfactory specification can now
be drawn covering such lighting units as the luminous arc or the
"Type C" incandescent lamp, either of which, with specific equip-
ment, will develop a predetermined distribution of intensity from
the light source when a specific amount of energy is delivered at the
lamp and the variations of this may be fixed within close limits
with modern regulating appliances. A diagram showing the candle-
power distribution with the mean spherical candle-power and the
flux data for a given lamp and equipment should be required and this
from time to time may be used as a check on the fulfillment of the
contract.
On the usual series circuits a simple system of recording ammeters
operated and owned by the city would check the energy used
daily and a reasonable inspection force can determine whether other
service conditions were fulfilled. With limiting requirements as
to the economical life of incandescent lamps, in view of loss in
candle-power, inexpensive street or laboratory tests by the nearest
university or laboratory, on units taken at random, would afford
a practical check on the illumination service rendered and be
within the means of almost any city spending a reasonable sum for
its lighting. In large cities, involving various types of units and
service, an elaboration of this system with the aid of a testing
laboratory would accomplish satisfactory results. In such cases the
inspection service becomes even more important, as usually in such
cities there are a number of light sources which vary largely in accord-
ance with the attention given them, such as mantle gas lamps.
These vary from so many causes that the inspection of the results of
the attention given to the unit and service given by it is most im-
portant, for the illumination shown by tests of these lamps on the
street varies within a considerable percentage even when they are
well maintained.
490 ILLUMINATING ENGINEERING PRACTICE
UTILIZATION OF IMPROVEMENTS
Another important provision of a lighting contract of any length
is a clause providing a means of utilizing improved lighting appliances.
General dissatisfaction will occur when the people of one city
see in other cities considerable improvements in lighting which they
cannot have on account of unsatisfactory contract conditions. It is
wise public policy, therefore, for many reasons to include in a long-
term contract a clause which will allow for the trial and possible
substitution of improvements in lighting units and adjustments of
costs. Such a clause should provide for a thorough service trial
and conservative methods of change; for if lighting systems had been
changed as rapidly as the successive leading improvements have
taken place in the past few years, a heavy financial loss would have
ensued.
You will note from the statistics already given of the length of term
of contracts, that 103 out of 128 were for 5 years or over in length.
This is due undoubtedly to the general business advantages of long-
term contracts. Formerly there was little if any possible revision
during the term of the contract, but conditions have changed. A
large number of these contracts are now subject to possible revi-
sion as to rates or improvements during the life of the contract, a
few by provisions of the contract itself, the larger number by public
service commissions which on their own initiative, or on request
by the city, may investigate and revise certain contract conditions.
STATE CONTROL OF CONTRACTS AND EFFECT
State control of public utilities has a decided effect on contracts
for street lighting. With well-administered companies it removes
possible competition. Such a company normally, therefore, will
continue to perform the street-lighting service and its equipment
therefore will be available for its whole economic or useful life. To
this extent, therefore, the former value of a long-term contract is
lessened.
Rates for street lighting have also been affected. Where this
question has been investigated by public service commissions the
trend of their decisions generally follows the theory of cost of
service with a reasonable rate of return. In many cases they have
obtained the approved contract rate through an inventory and cost
apportionment precisely similar in method to that used in estab-
lishing rates for commercial business.
LACOMBE: STREET LIGHTING 491
Generally this has been done without allowing any marked dis-
crimination in favor of a municipality. Free service particularly
is condemned and is apportioned as part of the cost of street light-
ing. The commissions rarely interfere, however, in a street-light-
ing contract so long as its terms are reasonable and encourage a
practical and liberal policy of adjustment during the life of the
contract. They usually allow rates for energy lower than published
tariff rates for various reasons, among them being the facts that the
city is a single customer, generally requires no meters, contracts for
long periods, and its uses of energy develop a favorable load-factor.
Apparently, therefore, the question of rates for street lighting in
the future under public service commissions, will tend toward cost
of service plus a reasonable return, both of which depend to a certain
degree on the size and length of the contract, the service required,
and the general business of the company in the community in which
it operates. The most advanced form of a contract based on cost
and return is that which has been recently adopted in the state of
Wisconsin and known as the " Indeterminate Contract." This
has been fully described in the technical literature of the day. In
brief, it is a contract where the company becomes the agent of the
city, carrying out its wishes at cost and receiving for its remunera-
tion, in addition to ah 1 costs including amortization of equipment,
only a reasonable return on the business involved. The initial
investment basis and rate of return is established by agreement or
by a decision of the Public Service Commission and the items of all
accounts are rendered to the city yearly. This form of contract
should encourage the use of higher intensities of illumination, for
under it a larger illuminant may be used in place of a smaller one
with a relatively small increase of the cost of the service.
REDUCTION OF COST AND INCREASE IN USE OF LIGHT
Summing up the situation, it may be said that the general trend
of the various factors bearing on rates for street lighting, such as
improved and more efficient units and state control, is toward low
rates. This in itself encourages a more liberal use of street lamps
and better lighting. This greater volume of business tends to offset
the lower return per unit.
There is a large field of needed improvement in street lighting
in this country, not only in the normal increase in numbers of lamps
but in the increased intensity really required by modern condi-
4Q2 ILLUMINATING ENGINEERING PRACTICE
tions. Its development may be greatly accelerated when costs are
decreasing, and a wise business policy will urge the advantages of
improved lighting where favorable prices occur.
GOOD PUBLIC POLICY
A careful far-seeing public policy, after providing for good ser-
vice with all that that requires, will proceed to the education of the
public in the possibilities of the illumination of the city and the
ability of the contractor to furnish such illumination at reasonable
rates.
This function naturally devolves on the public utility. With
this in view, it is not enough to restrict one's efforts to the require-
ments of the day. The relations between the city and company
should be such as to make it possible for the company to show the
city the improvements in the art as they occur, and to explain
how the city can best use its available funds in the increase and
improvement of the street lighting so as to attract people and busi-
ness to it. This effort should be continuous and of the same general
persistence used for commercial customers, and the utility should
remember that it has a dual responsibility in the improvement and
welfare of the city, for what helps the city helps the company.
Even if this were done without profit or with a relatively low rate
of return, the effort would be justified from the increased business
derived from the general improvement. It is difficult to imagine
a better method for the advancement of street illumination or for
retaining the good will of a community.
RAILWAY CAR LIGHTING
BY GEORGE H. HULSE
The proper lighting of railway cars has always offered special
problems, both in regard to the methods employed in producing
the energy for lighting, and in the application of the light sources
to obtain proper illumination.
As methods of lighting have been improved, the new methods have
been applied to the lighting of cars with such modifications as the
special conditions make necessary. Oil lighting superseded can-
dles, gas displaced oil lighting, and for a time completely dominated
the field, but at the present time, as in other places, the field is
divided between gas and electricity.
GAS LIGHTING
Practically all cars using gas light employ oil gas as the illuminant.
As the storage space available is limited, it is necessary to carry the
gas under pressure in order to have a sufficient supply on the car,
and also to have a gas of comparatively high illuminating value.
Coal gas, of low candle-power primarily, loses at least 50 per cent,
of its illuminating value when compressed to a point high enough
to give sufficient storage. Oil gas has not only a much higher candle-
power uncompressed, but when compressed to ten atmospheres,
loses only 10 per cent, of its illuminating power.
Oil gas is made by the distillation, or "cracking" of petroleum
oil in cast iron or clay retorts, or in steel generators filled with fire
brick checker work. A fixed gas is formed which has for its prin-
cipal ingredients methane and heavy illuminants with a very small
amount of hydrogen. It has a heating value when compressed of
1250 heat units per cubic foot. After passing through proper
washing and purifying apparatus, the gas is compressed to 1 2 atmos-
pheres in store holders, from which it is carried to the railroad yards
by suitable pipe lines. The car holders are filled from these pipe
lines. The car equipment (see Fig. i) consists of one or more welded
steel holders to contain the gas supply, two filling valves, a pres-
sure gauge, a regulator for reducing the holder pressure to that at
493
494
ILLUMINATING ENGINEERING PRACTICE
which the lamps operate, the pipe line for carrying the gas from
the holders to the lamps, and the lamps or burners. All fittings
for both the low-pressure and high-pressure piping are especially
designed for the work. The pressure regulator is placed under the
car near the holders so that the amount of high-pressure piping is
small and none of it is inside the car.
The pressure regulator reduces to the proper pressure and main-
tains this pressure constant with the varying amounts of gas used.
At the beginning oil gas was burned in regenerative lamps with
a cluster of from two to four burners of the union jet type. All
lamps used at the present time are fitted with incandescent mantles.
One of the early, and probably the earliest completely successful
application of the inverted mantle was to car lighting. This was
Fig. I. Diagram of Pintsch gas equipment using mantle lamps.
due to the fact that while the development of the inverted mantle
for general use had to contend with low and varying pressure, in
car lighting a sufficient and uniform pressure was at all times
available.
In this country two sizes of mantle are used, one which gives
28 candle-power, with a gas consumption of 0.8 cubic feet per hour,
the gas pressure being i pound per square inch. The use of this
size mantle is limited to bracket lamps, and a few installations in
which four of these mantles are employed in a cluster for center
lighting.
The other size of mantle, which is used for center lamps gives
90 candle-power, with a gas consumption of 2 cubic feet per hour
at 2 pounds pressure per square inch.
The mantles used are of a special form and composition to with-
HULSE: RAILWAY CAR LIGHTING 495
stand the rigors of railway service, and give three months average
life in service.
There are upward of 85 gas plants for the manufacture of oil
gas in the United States and Canada. Gas is delivered to the car
holders and charged for at a uniform rate, the amount of gas sup-
plied being measured by the increase of gauge pressure.
The holders are made exact size and the contents of a holder can
always be determined by multiplying its capacity by the gauge pres-
sure in atmospheres. This feature, besides furnishing a means of
measuring the amount of gas supplied, is important for determining
the hours of lighting which a holder contains and also for the purpose
of car interchange.
Cars using oil gas are dependent upon stationary plants, but this
has not been found to be a disadvantage, principally because the
time required to charge a car is so short. It can be done, if neces-
sary, at a division station stop.
ELECTRIC LIGHTING
Three methods of electric lighting for railway cars are in use:
1. The head-end system.
2. Straight storage.
3. Axle-driven generators.
The Head-end System. In the head-end system use is made of a
generator driven by a steam engine at the head of the train, either
in the baggage car or on the locomotive. Electrical energy is car-
ried back from the generator to the cars to be lighted by means of a
train line on the car roof and connectors between the cars.
In this country the generator of a head-end system is usually
installed in the baggage car, and is driven by a steam turbine, steam
for its operation being brought from the locomotive through suit-
able hose connections. A very few equipments are in service
with the generating set mounted on the locomotive, but this entails
heavy installation cost, since several locomotives may be used in
hauling one train over its trip.
As the steam supply is shut off when the locomotive is detached
from the train it is necessary to have a storage battery on one of
more of the cars to supply light during such time as the locomotive
is detached at terminals or division points.
The head-end system gives efficient and economical results, but
496 ILLUMINATING ENGINEERING PRACTICE
its great disadvantage is that light can only be used when a car is
in a train with a generator equipment. If the cars are equipped with
batteries to supply light during such times as the locomotive is dis-
connected, the proper arrangements for charging the batteries entail
a sacrifice of simplicity and economy.
Straight Storage. -In the straight storage system each car is
equipped with a set of storage batteries of sufficient capacity to
supply energy to the lamps for the desired trip. As ordinarily ap-
plied, the equipment is simple, consisting of lamps, storage batteries
and charging receptacles, with necessary wiring. At terminal yards
the batteries are charged with energy obtained from a stationary
power plant. The lamps operate directly from the batteries, no
voltage regulator being used.
This system of lighting would be ideal if it were not for the fact
that the charging of the batteries consumes too much time. Gen-
erally cars are not available in one location long enough to receive
proper charge.
The cost of equipping a railroad yard with the proper charging
lines is considerable.
Car lighting systems dependent upon stationary plants are fea-
sible as shown by the oil gas system, but the time required for charg-
ing must not interfere with car service.
Axle Driven Generators. In this system the car axle is used to
drive a generator which supplies energy for the lamps in the car,
and for charging a storage battery which supplies energy to the
lamps when the car is running below a certain speed.
The equipment consists of the following:
A generator mounted either on the car body or truck with some
form of driving system between the car axle and the generator, a
storage battery to maintain the light when the speed of the generator
falls below that at which it gives the proper voltage, regulating
apparatus to govern the output of the generator at varying speeds,
to give the proper charge to the storage battery, and to maintain
constant voltage at the lamps, and some means of keeping the polar-
ity of the battery charging current constant when the direction of
the movement of the car is reversed.
Various systems have been devised to meet these conditions and
a large number are in successful operation on railway cars. The
best practice is exemplified by an equipment in which the generator
is mounted on the car underframe, the generator controlled for out-
put at varying speeds by a carbon pile rheostat in its field circuit,
HULSE: RAILWAY CAR LIGHTING 497
which give a constant current output until the battery approaches
full charge, when the control is automatically changed to constant
voltage, thereby preventing overcharge of the battery. The voltage
at the lamps is held constant by an automatic carbon pile rheostat
placed between the battery and the lamps.
Of the three different systems of electric car lighting, the axle-
driven generator system is and, no doubt, will continue to be the
one most used. This system renders the car absolutely independent
of a stationary plant and, in spite of its seeming complexity, is the
only one of the three systems capable of general application to cars.
A properly designed axle system is superior to head-end or straight
storage system as it renders the car available for use in any territory
and does not necessitate lay-overs at charging plants.
CAR ILLUMINATION
Adequate and proper illumination of passenger cars of various
types presents difficulties not met with in other lines of illuminating
engineering. Car construction limits to a considerable extent the
location of the lighting fixtures, which, in combination with the
seating arrangements makes ideal illumination conditions hard to
realize. It is practically impossible to have the lighting fixtures
out of the range of vision, and very adequate screening of the light
source is necessary. In addition to this the constant motion of the
car makes it necessary to have a greater amount of illumination
than is required in places where this condition does not exist.
There has been in the past a tendency to sacrifice proper illumina-
tion results in favor of the appearance of lighting fixtures, and also
to look for an appearance of light in the car, rather than for proper
illumination; but these mistakes are rapidly being corrected, and I
believe that the practice of proper illumination has advanced as far
in car lighting as in any other fields.
Passenger Coaches. The passenger coach is the type of car that
is used in greatest numbers, and its proper lighting is most important.
It is the dividend paying car, and carries the great bulk of the travel-
ing public.
Aside from the general illumination necessary, the principal use
of artificial illumination in a coach is for reading, and the lighting
system should be so designed as to give proper illumination on the
reading plane, which is 45 to the horizontal and at right angles to
the center line of the car.
32
498
ILLUMINATING ENGINEERING PRACTICE
Owing to car design and construction, two methods of lighting are
available, as regards the placing of the lamps. They may be hung
from the roof in a single row down the center line of the car, or a
row may be placed on each side deck, directly over the seats. An
-DATA ON I L
TEST No..jS.a> .......
EFFICIENCY AND UNIFORMITY
ILLUMINATION 33 IN. ABOVE FLOOR OF CAR
45 reading plan
Horizontal plane
>0"U.'
Per cent, of total light delivered on -horizontal plane
Mean variation from average, 45 reading plane
Lowest four values (excluding station 20), 45 reading plane
a.sof.c z.<oo\ c. a.aof.c.-s.aa f.c.
LIGHT OUTPUT OF ONE UNIT
Total light, lamp alone := 7QO lumens
Lamp with reflector:
Distribution 100 to 180= G4.E lumens
SO" to 100 =236.5 lumens
O e to 50=E94-.Slumen
Total
&&$._ LIGHTING
CEMIEBL DUCK S...SKAT SPACING
Type of Car _7O ft COACH
Description of Unit "Pr.l5inat.IC
Co.
Screening Angle of Reflecto
Coefficient ol Reflector of
lining = 6.3 %
ILLUMINATION CUKVE.4.5....RADlN(i.PLANE
Aisle scat illumination shown by full line Window sent illuminttion shown bj dotted line
For Cm Plan and Teil Station Locations see flan fro. i.
.scc. of Ry. ECec. Engr
6 7 6 9 10 II IZ 13 14 15 16
STATIONS
Fig. 2. Illumination results in coach with gas mantle center lamps.
elaborate series of tests made a short time ago demonstrated that
equally good illumination results can be obtained with either type of
installation. Practical considerations, however, make for center
lighting on account of the fewer number of fixtures used and the
lesser number of lamps and reflectors to maintain. Another con-
HULSE: RAILWAY CAR LIGHTING
499
sideration is that with side lighting shadows are likely to be cast by
a passenger's head which will interfere with the proper illumination
of his own or another person's paper. This occurs with center light-
ing only when people are standing in the aisles.
TEST No 31
DATA ON ILLUMINATION EFFICIENCY AND UNIFORMITY
ILLUMINATION 33 nt. ABOVE FLOOR OP CAR
FlffTglC
w^.
^TH
1.73
a.oe
2.<Z>I
2.ee
2^>
LIGHTING
SFACIXG
Tvpe of Car fOft
Per cent of total light delivered on hon
Mean variation from average. 45 reading plane
Lowest four values (excluding station 20), 45
l.5O(.c
f.c. 1.59
LIGHT ODTTOT OF Out Uurr
Total light, lamp alone := !
I^amp with reflector:
Distributkn 100 to 180 = S^.llumeus
50* to 100= Vl-Olnmens
Description of Unit Prismatic
*Rgf \gctor "B owl Unit _ '
Hnlophang Wk^.No.ia31Q,
-_JaCLwatt.Gi3aTnngsten Lamps
Base Contact of Electric Lamp_l3(fc
above top of_BEELECTOR
Screening Angle of Reflector " _".;;_
Coefficient of Reflector of car head-
lining = 26.3 /. _
ILLUMINATION CURVE 451 JVC AB.LNJ&. PLANE
Aid* mt illnmintion ibowu by fall lint Window *e>t i
Far C*r Pl^n **4 Ttst Station Loca!:mt srt f.'e* A-. i-
Cenyrlght 1t<4 by u Anoc. of Ry. E:c. Era.
i jhown by dotted line
Fig. 3. Illumination results in coach with electric center lamps, enclosing bowl type.
The following are some of the results obtained in the test of which
I spoke above:
Fig. 2 shows results obtained with center lamps using Pintsch
gas, with mantle. The reflector unit is a prismatic reflector having
a prismatic bowl under it to give proper light distribution. The
5oo
ILLUMINATING ENGINEERING PRACTICE
prismatic reflector is covered on the outside by an opal-glass en-
velope, adding to the appearance and serving to keep the prismatic
glass clean. This arrangement gives very uniform lighting; the
illumination of the aisle and window sittings being practically equal.
TEST No. 7
D-ATA ON ILLUMINATION EFFICIENCY AND UNIFORMITY
ILLUMINATION 33 IN. ABOVE FLOOR OF CAR
-^^T
.!i
M,..,.
Ai.l
H..i,.
45 reading pUne
I.-33
2.>1
a/30
Horuont.
plane
3.0 a
4.1 fo
3.30
Per cent of total light delivered on horizontal plane 4-6-7%
Mean variation from average, 45 reading plane 16.0%
Lowest four values (excluding station 20), 45 reading plane
1.72 f,c -Z-Otri cZJO-bif 2.O5f c
LIGHT OUTPUT OF ON
Total light, lamp alone :
Lamp with reflector :-
400
..100 6 to 180= 56.4-lumens
50" to 100= 72-4-lumens
to 50 =219. 7 lumens
Total =3483 lumens
-.-ELECTRIC LI GHTI NO
CENTE.RDECK Z. .SEAT SPACING
Description of Unit Cl.e.3 r
P.ci.s.m aii c...Re/f i g CT o r
-HQlo.ph.anc NO. I.B22.6,
_.5O watt.G-3.OTungsten Lamps
Base Contact of Electric Lamp i'/ft
above top of REFLECTOR
Screening Angle of Reflector 3"2>
Coefficient of Reflector of car head-
Fig. 4. Illumination results in coach using electric center lamps, with open-mouth pris-
matic reflectors.
Fig. 5 shows the construction of the lamp used in the foregoing
test.
Fig. 3 shows test with a similar type of fixture using the 5o-watt
train lighting electric lamps. The illumination is considerably
Fig. 5 Lamp used in test, Fig. 2. Fig. 6. Lamp used in test, Fig. 4.
Fig. 7. Lamp used in test, Fig. 8.
(Facing page 500.)
Fig. 8. Interior of dining car.
F'g- 9- Interior of dining car with indirect center lighting, and direct side lighting.
HTJLSE: RAILWAY CAR LIGHTING
lower than in the preceding test and less uniform, there being more
difference between the aisle and the window sittings.
Fig. 5 shows the result with center lighting, using prismatic open-
mouth reflectors, and Fig. 6 shows the type of unit used in this test.
TEST No.
DATA ON ILLUMINATION EFFICIENCY AND UNIFORMITY
ILLUMINATION 33 w. ABOVE FLOOR OF CAR
4Sr<Uc1>U~
Z^5
^OA
LIGHTING
AT SPACIKC
Type of Car ..7Q. f*
Per cent, of toul light delivered on horizontal pUne
linn variation from -average, 45 read ng plane
Lowest four value* (excluding station 20). 45 reading plane
-4<V3%
I 8.8*
Description of Unit Medium Den-
sity Opal- M*fh
LIGHT Otrmrr or Ottm Uxrr
Total light, lamp alone :
Lamp with reflector :
Distribution
40Olnr
100 U>180 l> = < 78ul
50* to 100 =| I e.felumens
0* to 50*=|57felnmens
Total
_5O i watt;fi3OTungsten Lamps
Bse Contact of Electric LampjSfa'L
above top of gF Fl FCTOQ
Screening Angle of Reflector .
Coefficient of Reflector of car head-
ILLCMINATION COBVB 45 KEAD1N& PLANE
showa bj foil lioe Window ml illmnin
Far Cfr Plan tmJ Tat Stotia* Locttient tee plan Aa.__JL
CopyHght
i shown by dotted hue
Fig. 10. Illumination results in coach using electric center lamps with medium density
opal open-mouth reflectors.
Fig. 10 shows the results obtained with medium density opal
reflectors.
Fig. ii shows the results obtained with heavy density opal
reflectors.
502
ILLUMINATING ENGINEERING PRACTICE
Fig. 7 shows the type of unit used in the preceding test.
Fig. 12 is interesting as it shows side lighting with clear pris-
matic reflectors. The wattage is the same, as with the center lamps,
and the results compare very closely.
TEST No. H
DATA ON ILLUMINATION EFFICIENCY AND UNIFORMITY
ILLUMINATION 33 IN. ABOVE FLOOR OF CAR
Window
Mem oa
Only
Car
45 reading plane
2.01
2-74
2.2>2>
Horizontal plane
J5. 26
4-.s i s
2>.51
Per cent of total light delivered oh horizontal plane 4,
Mean variation from average, 45 reading plane J
Lowest four values (excluding station 20), 45 reading plane
9.7*
_ EL,E.CTR1C_ LIGHTING
.CENTER DECK ..?....SBAT SPACING
Type of Car 7O ft. CoacVt
Description of Unit . He.ftV >/..Dn-
si4j/ Opal- Open-mouhRef.
LIGHT OUTPUT OF ONB UNIT
Total light, lamp alone :=
Lamp with reflector:
Distribution
. . . 100 to 180= 27 2> lumens
50 to 100= 72. lumens
0" to 50== 216.4lumens
T<ftal = 5 15-7 lumens
..50. .. watt.fi^3O.Tungsten Lamps j(
Base Contact of Electri
above top of
Screening Angle of Reflector..
Coefficient of Reflector of car head-
linin g = g-3 %
ILLUMINATION CURVE jr5._R.EAP!NS PLANE
Aisle teat illumination shown by fall line Window seat illumination shown by dotted line
For Car Plan and Test Station Locations ste plan fro. _ J,
Copyright 1914 by the Aiioc. of Ry. Elec. Engn.
A k k A A i i Jr '-*''-"i|
JJJJJJJJJJJJJJJJMM
Pig. n. Illumination results in coach using electric center lamps with heavy density opal
open-mouth reflectors.
Fig. 13 shows results with side lighting with medium density
opal reflectors, and this test compares very closely with center lamp
test, using the same class of reflector.
These tests show that by using units best adapted for car-lighting
HULSE: RAILWAY CAR LIGHTING
503
service, illumination of about 2.5 foot-candles can be obtained on
the reading plane by spacing the center units 6 feet apart and using
lamps with an output of approximately 390 lumens per lamp. The
same results can be obtained in side lighting by placing the units
TEST No. _.-**.
DATA ON ILLUMINATION EFFICIENCY AND UNIFORMITY
ILLUM 1NATION 33 IN. ABOVE FLOOR OP CAR
HoruoaUl plant
1.85
__HALF DECK .2..SKAT SPACING
Type of Car 7CLf COQCb
ofUnit Pri
Per cent, of total light delivered on hor
Mean variation from average, 45 reading plane
Lowest four values (excluding station 20), 4, ~
Afel
. ... 5 reading plane
toz f.c i.oa i c i.o4f cj.oz
LIGHT OUTPUT OF ONE Unr
Total light. lamp alone : = - .
Lamp with reflector :
Distribution
\ZQ lumens
100 to 180= V5-Z lumens
50 to 100 = ZO- Alumens
to S0"= 64.7l
Total =e&.3. lumens
VA Q \QpV.ang No. \S221
__\S wartjajb^Tungsten Lamps
Base Contact of Electric LampJ/fc. .....
above top of gfFVECIQg
Screening Angle of Reflector ____ 3.<&.*
CotiScient of Reflector of car head-
JJJJJJJJJJJJJJJJJJJU
Fig. 12. Illumination results in coach using electric side lamps with open-mouth prismatic
reflectors.
6 feet apart, and using lamps with an output of 220 lumens per lamp.
This allows a depreciation of 40 per cent, in the efficiency of the light-
ing system before the illumination drops to 1.5 foot-candles.
Direct lighting with a reflector of medium or heavy density is most
504
ILLUMINATING ENGINEERING PRACTICE
satisfactory for a coach, as it is much more efficient than either an
indirect system or a direct system using a light density reflector
since the walls and ceilings cannot be kept in proper condition to
reflect any appreciable amount of the light which falls on them. The
DATA ON ILLUMINATION EFFICIENCY
TEST No. _.-!*. ?
AND UNIFORMITY
ILLUMINATION 33 IN. ABOVE FLOOR OF CAR
i=n
Oily
E"
45 reading plane
Horizontal plane
\.OA
1.20
\.\-z
l.^V5
\.V9
T3T~
Per cent, of total light delivered on horizontal plane
Mean variation from average, 45 reading plane
Lowest four values (exc.uding station 20^ reading
LIGHT OUTPUT OF ONE UNIT
Total light, lamp alone :=
Lamp with reflector :
Distribution
;.... I2O lumens
.100" to 180= 25.4lumens
50 s to 100"= e.e lumens
to 50 =Adfe lumens
Total =98.3 lumens
.... S...SKAT SPACING
Type of Car VQjCO<Xcb L
Description of Unit Medium _
JQejii J.ty-O.paL.'B.eilecijor
.SiifiiyLCcu. NO. .9.0 U ...
15 watt.<3.rl.8.Tungsten Lamps
Base Contact of Electric Lamp....Q *
above top of._REF-LECrK>R.
Screening Angle of Reflector S7 C
Coefficient of Reflector of car head-
lining =.. .fc.3.J/o
ILLUMINATION CURVE 45 REACllHGL-
i by full line
For Ctr Plan and Tat Station Locations
, M*t illumination ihcwn by dotted line
A Jc Jc Jc Jc Jc Jc
f
Pig. I3 . Illumination results in coach using electric side lamps with medium density opal
open-mouth reflectors.
direct system with a reflector which properly screens the light source
also affords, with the darker ceilings and walls, spaces of low illumina-
tion for the eye to rest.
Dining Cars. Dining-car lighting is in a class apart from that of
HULSE: RAILWAY CAR LIGHTING
505
other classes of cars, and it is in the dining car that most of the
novelties in lighting are used. Obviously the table is the most
important item in the car, and a high illumination must be con-
centrated on each table, although a fairly high general illumination
has been found necessary so that the car will present a cheerful
appearance to the person entering it. Installations with high
intensity on the tables and low general illumination have not been
found satisfactory. Good general illumination can be obtained
from center fixtures mounted on the center deck, either direct or
indirect lighting being used. For table illumination, fixtures should
be mounted over each table, and no more satisfactory type of unit
has been developed than that which uses a concentrating reflector
and redirecting plate under it to give the proper distribution with
maximum light on the table top.
Fig. 14. Dining-car lamp.
Fig. 8 shows a dining car equipped with this type of fixture.
Fig. 14 shows the construction of the table lamp and Figs. 9
and 15 types of indirect center lamps used. The most satisfactory
dining-car illumination is obtained by lighting the tables with this
type of fixture, and using a semi-indirect unit for the center lighting.
Sleeping Cars. Sleeping cars require lighting for general illu-
mination, for reading or working at the tables in the sections, and
for illumination of the berths after they are made up.
General illumination is obtained by center lamps placed close to
the ceiling to prevent interference of the fixture with the upper berth.
Small units placed in the corner of 'each section provide additional
local illumination for reading and to light the made-up berth.
Figs. 1 6 and 17 show sleepers equipped with such fixtures.
Fig. 19 shows the results of an illumination test made on the car
shown in Fig. 17.
506
ILLUMINATING ENGINEERING PRACTICE
The staterooms of a sleeping car and of a compartment car are
lighted in the same way.
Smoking rooms have a center lamp for general illumination and
bracket lamps back of the fixed seats to afford proper lighting for
reading.
The passageways are lighted by ceiling fixtures, using either an
open-mouth reflector, or a fixture with reflector and directing plate
set flush with the ceiling.
CAR J5LEEPER
LAMP FIXTURES
+ USHT ENCLOSED CENTER LAMPS
I LIGHT BCRTH LAMPS
N OF LAMPS 6CENTER.ZBeRTW
GLASSWARE
CENTER LAMPS -OPAL REFLECTOR.FK05TED BOWL
BERTH LAMPS - PRISMATIC GLOBE
BULBS 8C.P CARSON
VOLTAGE . *S
TOTAL WATTS 1148
AVERA8E FOOT-CANOUM V9
MAXIMUM- - *i
MINIMUM " l-Si
EXTREME VARIATION .__ -_' .TS
% VARIATION ABOVE MEAN Vt
((.VARIATION BELOW MEAN Z*
SOUARE FEET ILLUMINATED SAO
FOOT CAN bLESX FLOOR AREA .S*
tit
Fig. 19. Illumination results in sleeping car, Fig. 17.
The proper lighting of the berth section of the car after all the
berths are made up is best accomplished by a bracket lamp placed
on the bulk-head facing the aisle; this allows all the center lamps to
be extinguished, and affords sufficient light for passing through the
car.
Parlor Smoking Cars. This type of car presents a rather difficult
problem, as the seats are arranged so that the occupant faces the
center of the car. Best results are obtained by the use of a few center
Fig. 15. Indirect lamps used for dining-car lighting.
Fig. 16. Interior of sleeping car.
(Facing page 506.)
Fig. 17. Interior of sleeping car.
Fig. 1 8. Interior of parlor smoking car.
Fig. 20. Interior of parlor car with side lighting.
Fig. 21. Bag-rack portion of postal car.
(Facing page 506.)
Fig. 22. Letter-case portion of postal
Fig. 23. Observation room of private car.
HULSE: RAILWAY CAR LIGHTING 507
lamps for general illumination, and bracket lamps placed on the side
of the car back of the chairs for reading light. Cars having a flat
side deck can be fitted with reflecting units directly over the chairs
instead of bracket lamps.
Such an installation is shown in Fig. 18.
Parlor Cars. The conditions in this class of car are very similar
to that in the passenger coach, and the same type of installation is
used, although indirect lighting can be better used in parlor cars
owing to the' fact that the walls and ceilings can be maintained in
better reflecting condition.
A parlor car equipped with side lighting is shown in Fig. 20.
Postal Cars. Postal cars require greater illumination than those
of any other class, as the work done demands constant and arduous
use of the mail clerk's eyes. A very thorough investigation was
conducted some years ago by one of the large railroads assisted by
various manufacturers and participated in by the Standard Car
Committee of the Post Office Department. Various types of in-
stallation were tested, and determinations were made of the amount
of light necessary for the postal clerks to work. From the results
obtained by these tests the committee issued specifications for light-
ing which must be met in every postal car.
Postal cars are generally divided into three sections: the letter
distributing cases, the bag distributing racks, and the storage space.
The distributing cases require high illumination on the vertical plane
of the box labels, and on a horizontal plane for reading the addresses
on the mail. The bag distributing racks require high illumination
on the horizontal plane, for the labels on the bag racks. The storage
end requires a fair general illumination.
The following are the specifications for illumination of postal
cars issued by the Post Office Department:
These initial values are set to give proper illumination with a 40
per cent, depreciation in the efficiency of the.installation.
Fig. 21 shows the bag-rack portion, and
Fig. 22 shows letter case of a postal car equipped to meet this
specification.
Private Cars. A private car is a combination of several types of
cars and the lighting is accomplished in the different sections some-
what as it is in the class of car to which that section corresponds.
The observation room resembles somewhat the parlor smoker,
and in this part of the car general illumination is obtained by center
lighting, with bracket lamps behind the chairs, or half deck units
508
ILLUMINATING ENGINEERING PRACTICE
INITIAL VALUES OF ILLUMINATION REQUIRED
Foot-candles
Minimum
Maximum
Bag Rack Portion:
Center of Car Horizontal..
3-75
2.OO
3-75
1.66
2.OO
I2.OO
12.00
IQ.OO
19.00
12 .OO
Mouth of Bags, measured 18 inches from side of car
Horizontal.
Letter Cases:
Over table Horizontal
Face of Case Vertical.
Storage Portion
for local lighting. In private cars, however, more latitude is allowed
in fixture design, and frequently the center lighting is obtained from
one large fixture placed in the ceiling of the observation room.
Fig. 23 shows the observation room of a private car so equipped.
Local lighting is provided for the gauges and speed indicating
devices with which most business and private cars are equipped,
so that these instruments may be read when the other lamps in the
car are not in use to allow track inspection from this part of the car
at night.
The dining room of a private car is best lighted by a single unit
placed directly over and throwing a high illumination on the table,
with local lighting for the buffet.
The staterooms are lighted with center lamps, with local lighting
for the mirrors. Where berths are used, berth lamps are provided ;
and at the beds special reading lamps are attached to the bed posts.
All cars having vestibules have lamps over each step, directing
light on the steps. A flush type metal reflector is generally used.
Street and Interurban Cars. The proper lighting of this type of
car has, until recently, been neglected, both as to the amount of
light furnished, and the proper application of the light sources.
The carbon lamp was kept in use for a considerable time after the
metallic filament lamp had displaced it in almost every other field.
No means was used to direct the light to that portion of the car
where it was to be used or to shield the filament from the eye. At the
present time the metal filament lamp with proper reflectors are
being used exclusively.
Two general arrangements of the units are in use. Cars having
cross seats are lighted by a single row of units placed on the ceiling
HULSE: RAILWAY CAR LIGHTING
509
along the center line of the car, the spacing varying from 4.5 to 10 ft.
according to the size of lamp to be used. This spacing is also depend-
ent on the length of the car, as the lamps are operated in series and
must be in multiples of five.
When the seats are longitudinal, along the sides of the car, the
units are arranged in two rows, about 22 in. from the side of the car.
The same spacing is used as with the center lamps.
The lighting system is designed to give a minimum illumination
on the plane of utilization of 1.5 foot-candles under conditions of
80 per cent, normal voltage, which means that at normal voltage the
illumination will be about 3.75 foot-candles. This variation is a
condition which will have to be corrected before street-car lighting
can be called satisfactory. Up to the present time no device has
been produced which satisfies the operating officials of this class
of car as to cost and simplicity.
REFLECTORS AND GLASSWARE
A number of types of reflectors and enclosing units have been de-
veloped for car lighting uses.
For coach lighting, and other classes of cars where efficiency is
the prime object, and appearance a secondary consideration, the open-
mouth reflector is in almost universal use. Best results are obtained
with a reflector giving the maximum candle-power at 45.
The following are the principal types of this class of reflector,
together with the illumination obtained and the efficiency using a
6-ft. spacing, giving 66% generated lumens per running foot of
the car:
Average illumination on 45 reading
planes, foot-candles
Illuminating
efficiency on
45 plane
Aisle seats
Window
seats
Average
Prismatic Clear
Heavy Density Opal .
2.66
2.41
2.00
I.Q4
1.79
2.17
I.8 7
1.65
1-50
1-52
2.42
2.14
1-83
1.72
1.66
34-2
30-3
25-9
24-3
23-5
Medium Density Opal. . .
Prismatic Satin Finish.
Light Density Opal.
Where appearance is the primary consideration enclosing units are
used, and the energy efficiency somewhat sacrificed.
ILLUMINATING ENGINEERING PRACTICE
The following results are obtained with this class of unit under
conditions similar to those stated above:
Average illumination on 45 reading
planes, foot-candles
Illumination
efficiency
on 45
plane
Enclosing units
Aisle
seats
Window
seats
Average
Light Density Opal
1.09
i-39
1.44
1-56
1.36
1.17
0.97
.09
.24
.24
.11
13
1.03
1.24
i-34
1.40
1.23
1. 15
14.6
17-5
19.0
19.8
17-4
l6. 3
Shallow Prismatic Reflector
with Light Density Bowl
Reflecting and Diffusing Globes
Semi-indirect .
Total Indirect
Bare Lamp
All of the foregoing are for electric light. For gas lighting the
following results were obtained, the generated lumens being 130 per
running foot of car:
Average illumination on 45 reading
planes, foot-candles
Illumination
efficiency
on 45
plane
Enclosing units
Aisle
seats
Window
seats
Average
Deep Prismatic Reflector &
Bowl
3-65
2.74
2.08
1.92
3-72
2-34
1-52
1.6 7
3-69
2-54
i. 80
i. 80
26.8
18.4
I3-I
I3-I
Reflecting & Diffusing Globes.
Medium Density Opal Globes.
C.R.I Diffusing Globes
Aluminized metal reflectors are in very general use in postal and
baggage cars due to their high efficiency and durability.
FIXTURES
Lighting fixtures for use in railroad cars require special design and
construction, and embody some features not found in fixtures built
for other purposes.
1. They must be substantial to withstand the constant vibra-
tion to which they are subjected.
2. They must be easily removable for refinishing when the car
goes through its regular shopping.
3. The arrangements for holding the glassware must be such that
HULSE: RAILWAY CAR LIGHTING 511
it can be easily applied, or removed for cleaning, but at the same time
must be securely held so that there is no danger of its jarring loose.
4. They must be of suitable color and design to harmonize with
the interior treatment of the car.
5. The mechanical design must be simple and all working parts
must be easily accessible.
The first condition is met by careful mechanical design, sug-
gested by experience in this class of work, as fixtures built for other
uses are wholly unfit for use in railway cars.
The second feature is generally covered by a type of construc-
tion in which a plate or spider is firmly fastened to the car ceiling,
this plate forming the support for the socket. The ornamental part
of the fixture is secured to this plate, but may be removed without
disturbing the electric connections or the attachment to the ceiling.
The arrangements for holding the glassware in enclosing units
must be worked out for each type of glass employed. With a large
proportion of the fixtures for electric lighting use is made of open
mouth reflectors and for these, holders have been developed which
fulfill the condition admirably. The ordinary type of holder
equipped with set screws was quickly abandoned as being unsafe.
One of the best holders developed consists of a spring clamp com-
prising a number of metal fingers which spring over and grip the
neck of the reflector. In order to make the action of this spring
clamp positive, a cap nut is screwed down against the spring clamp,
locking the fingers against the neck of the reflector in such a manner
that the spring of the clamp takes care of expansion in the glass and
cushions it against vibration.
The question of suitable design is one which is governed to a
certain extent by the wishes of the purchaser, but I believe that the
results obtained in car lighting work compare very favorably with
that in other lines.
Electric lamps are built so that the bulbs may be easily renewed,
and the sockets and wiring easily accessible. Gas lamps are made
so that they can be lighted without opening the bowl, mantles ap-
plied without the mantle being removed from the container until
it is properly attached to the lamp, and no adjustments to the air
or gas supply are necessary; in fact the lamps are made without any
means of adjustment.
Much remains to be done before the lighting of railway cars will
be all that can be desired, but I know of no other field where more
effort is being expended to obtain proper and adequate illumination.
THE LIGHTING OF YARDS, DOCKS AND OTHER
OUTSIDE WORKS
BY J. L. MINICK
It is a notable fact that illuminating engineers generally have given
only casual attention to the field of lighting in railway service and it
is largely with the hope of stimulating interest in this field that this
lecture has been prepared. Probably no other single industry offers
such a wide variety of interesting problems for solution in which
practically every known form of illuminant may be used to advan-
tage. This field is open to the illuminating engineer if he will avail
himself of the opportunities offered. Many railroads of importance
have large engineering organizations but only a few employ men
sufficiently well trained in this important branch of science to solve
properly the many problems that constantly arise. Many of these
problems are common to other industries which have come within
the range of the illuminating engineer and their solutions are there-
fore well known. Many others are peculiar alone to railway service
and it is from among these that the material for this lecture has
been selected.
American railroads derive approximately three-quarters of their
gross income from the handling of freight and about one-fifth from
passenger service. All problems whose solution will in any way,
improve, facilitate or stimulate the movement or handling of either
freight or passengers, are of prime importance to the railroads in their
endeavor to furnish adequate service to the public. The lighting of
yards, docks and other outside works has been selected for review in
this lecture as these problems are intimately connected with the
handling of transportation and their importance is not generally
well appreciated.
In presenting these problems it must be understood that the
solutions suggested are not to be considered as final or conclusive.
They represent the labors and investigations of only a limited number
of engineers during the past five or six years. Improvements in
illuminants and in- the control of light are constantly removing many
of the difficulties commonly encountered and the investigations of
33 513
514 ILLUMINATING ENGINEERING PRACTICE
railway committees and individuals are resulting in many changes
in methods of operation which tend to simplify the lighting problems.
Few tests have been conducted to show what intensities of illumina-
tion are required for different classes of service and it is for this
reason that data of this character are not presented. The material
presented deals largely with the practical aspect and gives some con-
ception of the difficulties encountered in railway service.
FREIGHT YARDS
Description. The economical operation of important railroads
requires that the roadway shall be divided into sections or divisions,
each of sufficient length to give the train crews a full day's run under
normal operating conditions. Shops, roundhouses and other facili-
Direction of Traffic *-
^Grade-Sfto-e*
--Not less than Length of Train--
Receiving Yard
Direction of Traffic >
Grade
Not less than Length of Train >i
Classification Yard
Fig. i. Diagrammatic sketch of freight yard.
ties are provided at division points for the inspection, maintenance
and repair of the rolling equipment, also weigh scales and yards for
weighing and classifying freight.
Freight originates in small package quantities, at stations provided
for that purpose, and is here weighed and loaded for shipment, usually
in box cars. Full car loads are received from warehouses, factories,
mines, etc., which have siding connections with the main line. All
of this miscellaneous freight, both small package and full cars, is
collected by "way collection trains" and transported to the end of
the division for weighing and classification.
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 515
Two generaltypesof freight yards are in common use to-day; one in
which the tracks are approximately level, requiring the constant use
of locomotives for moving the cars, the other in which the tracks are
arranged and graded to permit the movement of the cars by the
force of gravity. This latter type is known as a "gravity" or
"hump" yard, and since it is rapidly superseding the first-mentioned
type, this type of yard has been selected for discussion.
Pull-in Yard. When a freight train arrives at the end of a division
it is delivered to the " pull-in" yard where the road crew turns it
over to the yard crew. The pull-in yard consists of several tracks or
sidings each long enough to receive a full train, which, in the case of
full car loads, consists of from forty to sixty loaded cars or possibly
as many as one hundred and fifty empty cars. Convenient track
connections permit of easy access to the roundhouse and storage
track for the locomotives and cabin cars. The yard crew moves the
train into the "receiving" yard, as soon as possible to make room for
the arrival of other trains.
Receiving Yard. The receiving yard contains usually two or three
times as many tracks as the pull-in yard and is of about equal length
though the character of the freight handled, local conditions, etc.,
play very important parts in the arrangement and length of tracks
in both of these yards. The tracks in the receiving yard are graded
slightly in the direction of traffic to reduce to a minimum the steam
power required for their movement.
The work in both of these yards is of such a nature as to require the
train crews to be on their trains the larger part of the time. No work
of any account is performed on the ground. The lighting require-
ments are not severe and usually whatever suffices for the neck of the
classification yard or ladder tracks can be used in these yards to
advantage.
Weigh Scales. The success of rapid and accurate weighing is
dependent upon the constant movement of cars across the scales at
uniform speed. Weighing is usually continued throughout the entire
twenty-four hours, and hence the artificial illumination provided
must be such as to enable the weigh-master and yard crews to per-
form their duties equally well during all hours of the day and night.
In the lower end of the receiving yard an abrupt change from
negative to positive grade takes place, the rise in track continuing
until the rails are somewhat higher than the scale platform. From
this point, commonly known as the "hump," they drop rapidly to
the end of the scale platform. The scale platform also has a slight
516 ILLUMINATING ENGINEERING PRACTICE
negative grade. In operation a locomotive is attached to the rear
end of the train in the receiving yard to regulate its speed. The
grade is such that the weight of the train moving down the grade
toward the scales is sufficient to force the first two or three cars up
the grade over the hump, thus requiring the locomotive to regulate
the speed only by applying additional power as the train is decreased
in length.
As the cars pass over the hump a point is reached where the re-
verse in grade causes the last car to pull away from the train. At
this point the car is cut loose and allowed to "float" across the scale
platform. The grades and distances are so arranged and propor-
tioned that all cars, regardless of size or weight, move across the
scales at approximately the same velocity. Usually there is an in-
terval of about one-half car length between cars. At the point of
cutting the cars the grades are such that the cars move a distance of
not to exceed 6 ft. during which the couplers may be opened. Dur-
ing the interval of cutting a visual inspection is made of the running
gear, brake rigging, etc.
These operations require illumination of fairly high intensities
localized within a comparatively small ground area. The couplers
must be sufficiently well illuminated to enable the car cutter to
note quickly when they begin to part. All parts underneath the
car must be well illuminated to enable the detection of loose, bent
or broken parts. Usually a single large lamp will serve for this
purpose. Experience shows that it should be placed thirty or more
feet above the rail and at least 10 ft. from the center of the track
to give proper clearance between the pole and the sides of passing
cars. The pole should be set opposite the point where the cars are
cut. The distance from here to the end of the scale platform will
vary with the type of hump used. With earth-fill humps it may be as
great as 60 ft., while with the more modern mechanical humps, with
which changes in elevation are secured at will by mechanical means,
it will vary from 25 to 36 ft. In several instances two lamps have
been used, one on each side of the track.
Several types of lamps have been tried in this service but none of
them have proven as satisfactory as the incandescent lamp. The
5 oo- watt vacuum type lamp has been generally used, though a
lamp of slightly larger size would probably have been used had it been
available at the time of making the initial installation. The recent
introduction of the gas-filled lamp, with its increased candle-power
for the same wattage, provides a satisfactory means for increasing
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 517
the illumination without great expense. The reflector should be
of the distributing type and designed to give the maximum inten-
sity at about 45 degrees. Its skirt should be extended to shield
the lamp from the range of vision of the weigh-master and others
employed in the vicinity of the scale house. All illumination under-
neath the car must come from reflection from the ground. In
many instances clean river gravel, crushed oyster shells, white-
wash and other light colored substances have been spread over the
ground to increase the reflection. Arc lamps have proved unsatis-
factory principally on account of the unsteadiness of the arc.
As previously stated the grades, dimensions, etc., of the hump
are so proportioned as to give each car, as nearly as possible, the
same momentum as it passes over the scale platform. The average
rate of movement is from ten to fifteen seconds for each car, from
the instant the front wheels mount the platform until the rear
wheels leave. There is an interval of about one-half car length be-
tween cars which gives an interval of from twenty to twenty-five
seconds in which all of the operations incidental to weighing must
be performed. During this interval the weigh-master must read
the initials, number and light weight of the car and verify the
records on the manifest in his possession and in addition he must
weigh the car and make record on the manifest of the total, light
and net weights.
The weight must be taken after the rear wheels have mounted and
before the front wheels leave the platform. The car is free to move
from 10 to 15 ft. on the scale platform under this condition.
This represents an interval of time of from one and one-quarter to
two seconds during which the weight may be taken. Under these
conditions all cars are weighed to within less than 200 pounds though
many of them exceed 175,000 pounds in total weight. The average
rate of weighing is slightly less than three cars per minute while
under favorable conditions it may run as high as four per minute.
This rate of movement makes it necessary for each operation to be
conducted with certainty as a single failure will interrupt the opera-
tion of the entire yard until the car involved can be returned to the
receiving yard for re weighing.
The illumination at night must be the equivalent of that under
daylight conditions. There are three opportunities for the weigh-
master to read the initials of the car, etc. ; on the front end of the car
as it approaches the scales, on the side of the car as it passes over the
scales, and again on the rear end of the car as it leaves the scales.
ILLUMINATING ENGINEERING PRACTICE
40 30 20 10 10 20 30
Fig. 2. Candle-power curve of scale-lighting unit with special reflector; 200- watt lamp;
horizontal plane.
180 170 160 150 140
10 20 30 40
50
Fig. 3. Candle-power curve of scale-lighting unit with special reflector; 200-watt lamp;
vertical plane.
MINICK: LIGHTING or YARDS, DOCKS, ETC. 519
A special reflector has been designed by one of the eastern rail-
roads for this service, as it was not possible, at the time the original
installation was made, to purchase on the open market a reflector
giving the desired distribution. This reflector is of the angle type
with the lamp pendant and the axis of the reflector in the horizontal
plane or at right angles to the axis of the lamp. It is approximately
13 in. in diameter by 5% in. deep and was designed to accommodate
a 2 50- watt vacuum- type lamp. The 2oo-watt gas-filled lamp has
lately been substituted with satisfactory results and with the latter
lamp the reflector gives maximum ca'ndle-power at about 45 degrees
in the horizontal and at 90 degrees in the vertical plane. The design
is such, however, that the larger part of the light flux in the vertical
plane lays above the horizontal or between the angles of say, 70
and 140 degrees. A few of the recently developed types of porcelain
enameled reflectors give very close to this distribution and may now
be purchased on the open market.
Six of these fixtures are mounted along the front of the scale
house facing the track at a height of 7.5 ft. from the rail to the bot-
tom of the socket. The spacing varies slightly with local conditions
and the type of scales used. The fixture nearest the hump is ad-
justed to illuminate the front end of the car as it approaches the
scales, particularly the number panel which invariably appears in
the upper right-hand corner of the end of the car, the observer fac-
ing the car. This lettering covers an area approximately 16 in.
high by 30 in. long, with the bottom from 9 to 10 ft. above the rail.
The next four fixtures are adjusted to illuminate the lettering on the
side of the car, which covers an area about 40 in. high by 10 or 12 ft.
long, though in many instances the initials extend the entire length
of the car. The bottom of this lettering is about 4 ft. above the
rails. The last fixture illuminates the rear end of the car as it passes
off the scales.
Oil-fuel head-lamps, mounted along the front of the scale house,
were originally used in this service. They were very unsatisfactory,
as the chimneys quickly became smoked and, even under the most
favorable conditions, the intensity on the side of the car was low.
Flame arc lamps were next used and, although the intensity of the
illumination was greatly increased, these also were unsatisfactory
principally on account of the unsteadiness of the arc and the welding
of the electrodes.
Since the body of the car overhangs the trucks at each end by
several feet, the trailing wheels are not illuminated and it is difficult
52O ILLUMINATING ENGINEERING PRACTICE
for the weigh-master to determine quickly when the car has left the
platform. A spot-lamp is placed on the side of the scale house,
several feet above the rail and near the end of the platform, with the
beam trained upon the ends of the rails. The reflector used is of
the concentrating type to insure high intensity of illumination.
The scale beam is mounted within a bay window opposite the
center of the scale platform with a clearance of about 3 ft.
between the car and the edge of the window. It is illuminated by
means of three pendant fixtures, one opposite the center and one
opposite each end of the beam. These are mounted 7 ft. from
the floor to the bottom of the reflector and in a line 12 in.
back from and parallel to the scale beam. Special reflectors and
25-watt lamps are used to illuminate only the scale beam and
counterpoise. These reflectors are special in that they have
unusually long skirts to prevent direct light striking the glass of the
window and being reflected in such manner as to obscure the weigh-
master's vision. Great care must be exercised to see that none of
the polished metal parts give objectionable reflection.
As each car moves off the scales and down the grade into the classi-
fication yard a car rider mounts it and regulates its speed by the
hand brakes so that it will strike the cars standing in the yard with
only sufficient force to close and lock the coupler. A second large
lamp mounted on a pole, usually a duplicate of the equipment at
the hump, furnishes sufficient illumination for the rider to mount the
car safely.
All of the lamps in the immediate vicinity of the scales are oper-
ated from multiple circuits with control switches inside the scale
house within easy reach of the weigh-master. This enables him to
make use of artificial illumination during the hours of dusk and
dawn and during dark periods^ of the day when the natural illumina-
tion is ample for all other yard operations. All reflectors outside
the scale house, particularly those in the immediate vicinity of the
scales, must have skirts not only to shield the lamps from the range
of vision of the weigh-master but to prevent reflection on the glass
of the windows, since much of the weighing must be done while the
windows are closed.
The scale mechanism requires frequent inspection, particularly
the knife edges, knuckles, etc. Artificial lighting is necessary for
this purpose as all of the mechanism lies within the vault under-
neath the platform. Many structural shapes are used and their rein-
forcements and connections offer so many obstructions to general
MINICK: LIGHTING OF YARDS, DOCKS, ETC.
521
illumination that this service is necessarily restricted to the spot
lighting of important parts, supplemented by a limited number of
bare lamps for general illumination. Spot lighting is secured by the
use of porcelain enamel metal angle reflectors of the concentrating
type fitted with 25-watt or 40- watt lamps. Portable lamps with
extension cord connections are required for the examination of the
parts lying within the regions of shadow.
Classification Yard. As the car leaves the scales it passes down
an incline of fairly heavy grade, of from 250 to 500 ft. in length,
leading to the " ladder track" connecting with the yard tracks.
40
40
30 20 10 10 20 30
Fig. 4. Candle-power curve ladder-track unit, maximum and minimum.
The yard may vary in width from eight or ten to as many as twenty-
five or more tracks so that the ladder track will contain a large
number of switches set closely together. These switches are oper-
ated electrically or electro-pneumatically from a tower provided
for that purpose. Since it is necessary that the car riders see all of
the switches clearly, and as they ride on the rear ends of the cars,
excellent illumination of the switches must be provided.
At the opposite end of the classification yard there is a ladder track
which connects with the pull-out yard. The type of lighting units
serving the ladder track may usually be utilized in the pull-in, re-
522
ILLUMINATING ENGINEERING PRACTICE
ceiving and pull-out yards to advantage. Two principal con-
ditions must be satisfied. With a mounting height of 35 ft. the
candle-power values in the 3o-45-deg. zone must be relatively
high to illuminate properly the ladder track switches, and again in
the 6o-7o-deg. zone high values are required for the illumination
of the tops of the cars, particularly in the receiving yard which may
be six or eight tracks wide. At 30 degrees the candle-power value
should be not less than about 600 nor more than 1300. At from
60 to 65 degrees it is fixed very closely at 1000. Fig. 4 shows the
maximum and minimum values of candle-power for 3 5 -ft. mounting
height.
Traffic rules require that there shall be safe clearance between the
poles and the sides of cars and the allowances are such that the poles
Height in Feet_pf Unitsabove Track*
/
>/
x
x
s
x
x
x
X
^
x
X
) 20 40 60 80 100 120 140 160 180 200 220 24
Distance in Feet between Units
Fig. 5. Mounting heights and spacings, ladder-track units, pull-in-receiving and
pull-out yards.
must be set at least 10 ft. from the center of the track. Pole
steps must be set parallel to the track rather than at right angles to
it. On straight track, where switches and other local conditions
permit, pole spacings should not exceed 100 ft., while on curved
track and in the vicinity of switches, they should be much
less for this type of fixture. Good practice requires that each
switch shall have at least one lamp not further than 30 to 40
ft. from it. Fig. 5 gives maximum spacing for mounting heights
up to 80 ft. Under normal conditions spacings should preferably
ably be about 75 per cent, of the values given.
The common conception of the problem of classification yard
lighting is that of providing a fairly even distribution of light over a
large expanse of railway tracks. This, however, is an erroneous
impression as the absence of cars means that no work is being per-
MINICK: LIGHTING OF YARDS, DOCKS, ETC.
5 2 3
formed and light is then unnecessary. This problem has on several
occasions, been likened to that of attempting to light a large city,
of relatively tall buildings and narrow streets and alleys, entirely
from the outskirts of the city. Owing to the large space required
for clearance purposes between cars and poles or other obstructions,
it is seldom possible to secure sufficient space for pole lines through
the center of the yard. The relatively high cost and the difficulty
of securing straight poles 40 ft. or more in length usually makes it
necessary to confine the pole line construction to 35-ft. poles. When
set in 5-ft. holes and fitted with appropriate pole tops for supporting
40 30 20 10 10 20 30 40"
Fig. 6. Candle-power curve, classification yard unit; maximum and minimum.
the lamps, this length of pole gives a clear height of the lamp above
the rail of from 32 to 35 ft.
The tops of the, cars must be well illuminated. The riding of
moving cars is a more or less hazardous occupation and as the riders
perform their principal duties on the tops of cars it is essential that
they be given sufficient illumination to make their positions secure.
The illumination must be sufficient to enable them to see clearly other
cars on the same track and thus judge distances and so regulate the
speed of the cars as to make the coupling without injury to the load
or equipment. Finally the illumination between the cars must be
524
ILLUMINATING ENGINEERING PRACTICE
sufficient to permit each rider to climb off a car and cross the yard
without danger of accident.
It is obvious that the units most satisfactory for lighting the body
of the yard, under the conditions described, are those which have
very flat candle-power distribution curves. With an effective
mounting height of only about 20 ft. above the tops of the cars it
10
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance in. Feet between Units
Fig. 7- Mounting heights and spacings classification yard fixtures.
is necessary that candle-power values shall be very high immediately
below the horizontal, or at say the angle of 80 to 85 degrees, if yards
of great width are to be properly illuminated. Fig. 6 shows the
maximum and minimum candle-power values required for a mount-
ing height of 35 ft.
Multiplying Factor
o S S g
/
/
/
/
/
^^
^
10 20 30 40 50 60 70
Height in Feet of Units above Tracks
Fig. 8. Multiplying factors yard lighting fixtures.
While it is desirable to mount the lamps as high as possible, local
conditions will largely govern this feature. As previously stated,
poles more than 35 ft. in length are usually difficult to secure. On
the other hand, the possibility of having to shift the pole line to a
new location at some future time, by reason of changes in track
arrangement, etc., makes it particularly desirable that short, stout
poles be used. Lamp spacings will vary with the mounting heights
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 525
and should not exceed 200 ft. for 35 ft. in height. It is good practice
to stagger the poles along one side of the yard with reference to those
on the opposite side. Fig. 7 shows the maximum spacings for mount-
ing heights of 70 ft. and less. It is common practice to limit the
spacings used to approximately three-quarters of these values.
The size of the lamp will vary with the spacing and mounting
height. The candle-power curves shown in Fig. 4 and Fig. 6 are
based upon a mounting height of 35 ft. If for any reason it is found
necessary or desirable to use a different mounting height, in any of
the yards to which reference is made, the size of the lamp must be
increased or decreased sufficiently to give candle-power values
which may be determined by multiplying the values shown in Fig.
4 and Fig. 6 by the factors shown in Fig. 8 for the several mounting
heights.
Pull-out Yard. The pull-out yard lies immediately beyond the
classification yard. Here full trains are made up from cars removed
from the classification yard and prepared for movement over the
main line. Like the pull-in yard, this yard consists of only a limited
number of tracks, each long enough to accommodate a full train.
Very little of the work requires men to pass between or climb over
cars, consequently the lighting requirements are not severe. The
type of lamp used in the pull-in and receiving yards and along the
ladder track is satisfactory for this service and the same mounting
heights and pole spacings will apply.
Service and Types of Illuminants. Since the lamps must be
placed overhead, poles for supporting them are necessary. Changes
and improvements in operating conditions and methods frequently
require that the tracks, and consequently the lamps, be changed in
location. This precludes the possibility of employing anything
approaching permanent construction, such as underground conduit
systems, steel poles or towers, catenary construction for suspending
a large number of small lamps over the body of the yard, etc. The
enormous length of tansmission line required, because of the neces-
sity for locating the poles along the outer edge of the yard and on
other unoccupied ground areas, prohibits the use of multiple
circuits. Several of the larger yards in use to-day are from 4 to
6 miles in length and require from 10 to 15 miles of pole line
construction. Series circuits are therefore commonly used.
Practically all forms of arc lamps have been used in this service
with varying degrees of success. Arc lamps are not entirely satis-
factory. They require frequent trimming and trimming in an active
526 ILLUMINATING ENGINEERING PRACTICE
freight yard is a more or less dangerous occupation. Runways for
repair and supply trucks cannot be provided, and hence the trimmer
must make his rounds on foot and all supplies and all lamps removed
for repairs must be transported by hand. These items must be given
serious consideration and that system of lighting should be selected
which reduces to a minimum the difficulties of operation and mainte-
nance, even though the illumination of the yard be interfered with
to a slight extent.
The old open arc lamp was probably the first type of electric
lamp used in this service. It was undoubtedly a big improvement
over the trainman's hand lantern then in common use. This lamp
soon gave way to the direct-current enclosed arc which in turn was
superseded by the alternating-current enclosed arc lamp. All
of these lamps were expensive in operation as compared with later
types. They were inefficient and did not give proper light distri-
bution for the service required.
The luminous or magnetite arc lamp approaches most closely to
the ideal from the standpoint of light distribution. It gives its
maximum candle-power at from 80 to 85 degrees, thus making it
possible to light the center of the yard fairly well even though it be
as much as 400 ft. wide. The "flat distribution" also permits
the use of comparatively short poles which is a big advantage from a
construction standpoint. Flame arc lamps have not given satis-
faction for two principal reasons: first, while the candle-power in-
tensities at 15 degrees below the horizontal are sufficient to illuminate
the center of the yard, the intensities at lower angles are great enough
to give very bright spots in the immediate vicinity of the lamp poles,
which is objectionable to the car riders; and second, the excessive
flicker of the arc and the frequent welding of electrodes causes annoy-
ance and inconvenience.
Large size incandescent lamps, fitted with refracting glassware,
are the most attractive units at the present time. By their use
the hazards of cleaning and trimming are reduced to a minimum.
The shape of the refractor is such that a slight shifting of the lamp
with reference to the refractor, will give almost any shape
of candle-power distribution curve desired. Finally the energy
consumption is nearly as low, for equal yard illumination, as for
any of the series arc systems now employed. At the present time,
however, the sizes of street series incandescent lamps regularly
manufactured in ampere ranges up to 7.5 are probably not large
enough to be competitive with the luminous arc lamps for wide
MINICK: LIGHTING^ OF YARDS, DOCKS, ETC. 527
yards, unless pole space can be reserved through the yard for
the incandescent lamps. If pole space can be reserved for this pur-
pose, the distances shown in Fig. 5 may be used as the spacing
between pole lines through the yard. The 6oo-c.p. gas-filled lamp
with refractor unit is competitive, both in power consumption and
candle-power distribution, with the 4.o-amp. luminous arc lamp.
The latter lamp, however, should not be used with spacings of more
than 200 ft. The 6.6 amp. luminous arc, which may be used with
spacings of 300 ft. or more, is a much more powerful unit than
any of the incandescent lamp units now regularly manufactured,
unless it be the IQOO candle-power 2o.o-amp. series lamp, which is
not a desirable lamp on account of the necessity for using a
" compensator."
Projector lamps have been used to a limited extent in this service.
The lamp is mounted near the hump and the beam is directed
against the rear end of each car as it passes into the classification yard.
The principal objections to this method of lighting are: the extreme
high cost of operation, the necessity for the yard crew to avoid fac-
ing the light source thus increasing the difficulties of performing their
duties, and finally, the danger of accidentally playing the beam on
passenger trains operating on the adjacent main line, thus obscuring
signals and possibly interfering with the vision of the engineman and
fireman. Mercury vapor lamps of high candle-powers and flood-
lighting units have also been used to a limited extent. In each of
these installations the use, of steel poles or towers, 75 ft. or more
in height, is necessary though the number of lighting units is mate-
rially reduced. The use of towers or similar structures require per-
manent locations which are not always to be had at reasonable
expense.
Transfer Station. Small package freight must be classified or
sorted for destination exactly as are full cars, and for the same pur-
pose. There is a difference in the method of operation, however.
The character of the freight handled requires that each package shall
be removed from the way collection car and placed in another car
consigned to, or to some point near, the destination of that package,
where another classification will be made. Since much of this
freight is heavy, trucks, sometimes two-wheel hand operated and
other times four-wheel hand or motor operated, are used in handling
the individual packages. This necessitates the construction of heavy
platforms between adjacent tracks at approximately the elevation
of the car floor, for the operation of the trucks. Much of this freight
528 ILLUMINATING ENGINEERING PRACTICE
must be protected from the weather so that roofs over the platforms
are required and frequently sheds are provided for storing freight
which has been unloaded but which cannot be loaded until the next
or a succeeding day.
Usually two or more tracks, sometimes as many as ten or fifteen,
are assigned to transfer service. The platforms will vary in length
to accommodate from two or three to as many as 20 to 25 cars. They
must be about 15 ft. wide to permit the operation of two lines of
trucks on each side of the platform if necessary. The roof is sup-
ported by posts, the more modern types of construction employing
the "umbrella" type of roof, supported by a single row of steel
columns along the center of the platform. The tracks are so ar-
ranged that the edges of the cars are close to the edges of the plat-
form. Where two or more platforms are used two or more tracks
are placed between the platforms.
A single row of incandescent lamps, with distributing type porcelain
enamel reflectors, will furnish illumination sufficient for the opera-
tion of trucks on the platform. The reflectors should shield the
filament of the lamp from the natural range of vision. With a
mounting height of 10 ft. and a spacing of not greatly to exceed
20 ft., loo-watt lamps give satisfactory service.
The difficult part of this problem, however, comes within the car.
Here the address and lading of each package must be read and com-
pared with the manifest. No satisfactory method of illuminating
the 'interior of the car has yet been developed. The fixtures used
must necessarily be of the portable type and of inexpensive construc-
tion, as many of them are lost through being left in the cars. They
must be supported from overhead as the trucks must have access
to all parts of the car floor. At the present time portable hand
lamps without reflectors are used. Plug connections for this ser-
vice are usually supported by metal conduits from overhead or
mounted under the edges of the platform.
Docks and Terminal Yards. Export freight is delivered to sea-
coast points. Package and perishable freight is delivered to large
covered piers where it is stored until boats can be secured for load-
ing. Car load freight which can safely be exposed to the weather, is
delivered to a yard in which it is unloaded and stored on the ground
to await water shipment. Upon arrival of the boat it is reloaded
on cars and shifted to the pier or dock where it is transferred to
the boat.
The lighting of the covered pier is a comparatively simple problem.
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 529
* Light is required only for the purposes of trucking and piling or
stacking and for the verification of the manifest. Usually several
rows of incandescent lamps fitted with distributing type porcelain
enamel reflectors are used in this service. The candle-power of the
lamp will of course vary with the mounting height and spacing. The
spacing, however, should be so arranged as to give good light
distribution without shadows on the truck-ways and the lamps
should be as large as possible consistent with these and other local
conditions. An illumination intensity of from 0.75 to 1.25 foot-
candles along the truck- ways is sufficient. "All night circuits"
are desirable along the truck-ways for the use of the night
watchman.
The lighting of the storage yard represents a much different prob-
lem. Here freight is unloaded by locomotive type cranes and stored
on the" ground until a boat arrives, when it is reloaded and moved
to the pier for loading on board ship. The tracks are arranged in
parrs, one on which the cars are placed to be unloaded and the other
for the use of the crane. The crane has an extreme reach of from
30 to 40 ft. so that the pairs of tracks are spaced on about 6o-ft.
centers to permit the storage of materials between them. This
space, which has a clear width of possible 48 to 50 ft., is sometimes
filled completely to a height of from 20 to 25 ft. The crane may be
turned 360 degrees if necessary and when lifting loads close to the
crane the lifting arm stands nearly vertical, the extreme height
being probably 50 ft.
The handling of freight must be carried on during night hours, and
hence artificial illumination is necessary. The extreme range of
operation of the lifting arm of the crane prevents the use of pole
lines with lamps suspended from the poles. Some attempt has been
made to furnish lighting service by the use of portable standards
carrying one or more large lamps and connected by flexible cables
and plugs to a system of underground cables installed between the
unloading tracks. This arrangement has not been satisfactory or
successful for several reasons. The cost of installation for the ser-
vice performed is very high and usually not warranted. The light-
ing standards must be short enough to permit free range of move-
ment of the crane above them which places the lamps too low to
be of value in unloading gondola cars or in stacking on top of large
piles. The cable connections offer obstruction to walking in the
vicinity of the crane and they are frequently broken or cut by
materials falling upon them. The standards must be light enough
34
530 ILLUMINATING ENGINEERING PRACTICE
for convenient handling and hence the weight is such that they may
be overturned readily and the lamps broken.
An attempt has also been made to mount the lighting units on
the crane structure itself and by means of angle reflectors to dis-
tribute the- light in useful directions. While this arrangement has
given most excellent results from the standpoint of illumination a
number of operating difficulties have been encountered which have
not been entirely overcome. The use of a flexible cable for supply-
ing current to the crane from an underground distributing system
is undesirable for reasons already mentioned. The use of a small
steam-driven generator, mounted on the crane and supplied with
steam from the crane boiler, will probably not give satisfaction as
the crane boiler is designed for intermittent duty in which the peak
demand for steam may be four or five times the normal rating of
the boiler. A gasoline or oil engine-driven generator, mounted on a
small truck attached to the crane, has been used in this service, and
where lifting magnets are used for the handling of iron and steel
products, this is probably the most satisfactory arrangement for
securing electrical energy in sufficiently large quantities to meet the
demand. The vibration of the crane is excessive, especially when
releasing loads suspended in the air by the lifting magnet, and lamp
breakage is excessive unless shock absorbers are used in mounting
the fixtures. Several devices of this kind are being developed to
relieve the situation.
In this service four fixtures are mounted on top of the cab, two
along each side. An additional fixture is mounted to the rear and
one in front over each door. The lifting arm carries two fixtures, one
about one-third the distance up from the bottom, the other about
one-third the distance down from the top. The seven fixtures
around the cab are adjusted to illuminate the complete circle sur-
rounding the crane to a radius of about 80 ft. This plan permits the
preparation of a new load while the crane is engaged in stacking.
The two fixtures mounted on the lifting arm are arranged to illumi-
nate the load while suspended in the air, regardless of the position of
the lifting arm, and also while it is being stacked. Five-hundred-
watt gas-filled lamps with porcelain enamel angle reflectors of the
distributing type are used and the light is directed away from the
crane so as not to interfere with the vision of the crane operator.
Heavy materials are usually handled by gantry, or bridge type,
cranes from pier to boat or boat to pier. All important work takes
place in the immediate vicinity of the crane and the lighting require-
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 531
ments in general are similar to those of the locomotive crane. Fix-
tures are mounted on the crane structure and appropriate designs of
reflectors are employed to turn the light in useful directions, always
keeping in mind that the light must be directed away from the crane
operator. The lamps used vary in consumption from about 100 to
500 watts and even 1000 watts, depending upon the character of
the work to be performed, the locations of the fixtures, mounting
heights, etc.
PASSENGER PLATFORMS
Description. Stations are provided at frequent intervals along
the main or running tracks for the accommodation of passengers and
the handling of mail, baggage and express matter. The stations at
the ends of divisions and at other important points where crews and
locomotives are changed, are known as "Terminal Stations." All
others are classed as " Way Stations." Terminal stations are usually
large and serve many tracks and trains. Some idea of the density of
traffic at a large terminal may be gained from the following approxi-
mate figures :
Station
Trains per day
No. tracks
Broad Street Station Phila . .
t;oo
16
Pennsylvania Station, Pgh
480
19
South. Station Boston
700
28
Pennsylvania Station, New York
525
21
Both foot and truck traffic is heavy and continuous, requiring the
use of long, wide platforms between tracks for loading and unloading.
Way station traffic is generally light and intermittent.
Terminal Stations. The tracks at terminal stations are usually
arranged in pairs with platforms between, so that one platform will
serve two trains. These platforms are usually about 15 ft. wide,
though the tendency of late has been to increase this dimension
slightly. With two trains at the same platform, it may be necessary
to accommodate as many as 1000 passengers and 20 four-wheel
trucks. The trucks are loaded with baggage, mail and express
matter, each piece of which bears an address or number tag. The
baggage checks are made of either cream, red, blue or green-colored
cardboard printed in black ink. Addresses on mail bags and express
matter are usually written with indelible pencil on white or yellow
532 ILLUMINATING ENGINEERING PRACTICE
paper. These must be identified and compared with the way-bills
which may have corresponding colors. Colored berth and seat
checks in Pullman service must also be identified. In the older
stations the platforms are protected from the weather by a single
arched roof spanning all of the tracks and platforms. In the more
modern stations the platforms have individual roofs each supported
by a single row of columns along the center of the platform.
Fairly even, though not necessarily high, illumination is required
for foot and truck traffic. Fairly high illumination is required for
the verification of the way-bills. Finally, light is required on the
tops of the cars to permit icing, watering, etc., and all lamps should
be shielded to enable the enginemen to judge distances and stop
his train at the proper point. Metal reflectors, having an angle of
cut-off of not to exceed 75 degrees, are commonly used in train sheds.
The mounting height should be not less than 22 ft. nor more than
25 ft. above the rail. The spacing, and consequently the size of the
lamp will vary with local conditions, particularly the location of the
roof -supporting members to which the fixture must be attached.
The average illumination intensity should be from i.o to 1.25 foot-
candles. Glass reflectors should be used where the platforms have
individual roofs.
Way Stations. Each way station is provided with one or more
platforms for the loading and unloading of passengers, mail, etc.
They are paved with wood, brick or concrete and are illuminated at
night by small lamps mounted on poles along the edge of the plat-
form farthest from the track in the case of a side platform, or along
the center of the platform in case it is between the tracks. The
poles are spaced at intervals of approximately one-half car length to
enable the enginemen, by counting them, to judge distances and
stop his train at the proper point. At points 200 ft. at each side of
the center line of the station "approach signs" attached to lighting
poles, display the name of the station. The bottom of this sign is
approximately 8 ft. above the platform as this has been selected as
the proper height for convenient reading, either from within the car
or from the station platform.
Distributing type porcelain enamel metal reflectors, with 25-watt
lamps are used in this service. At the more important stations
40- watt lamps are sometimes used. The lamps are mounted on
metal poles about 9.5 ft. above the platform to clear loaded baggage
and mail trucks. At this height the ordinary types of reflectors
shield the lamp completely from the view of the enginemen. A
MINICK: LIGHTING OF YARDS, DOCKS, ETC. 533
special reflector has been designed to expose a small portion of the
incandescent filament so that the engineman may count the poles
by the flashes of bright light as he passes, without looking in that
direction. Where shelters are used transparent numerals are some-
times set in the longitudinal roof trusses opposite the lamps to indi-
cate the stopping points for trains of various lengths. The average
illumination of way station platforms need not exceed i.o foot-candle.
CONCLUSIONS
It should not be inferred from the foregoing that only a limited
number of sizes and types of lighting units can be used to advantage
in railway service. The scope of railway lighting is so broad that
there should be, and there probably is, a distinct field for each of the
common illuminants of the present day. Few railroads have es-
tablished definite standards for their lighting service. There are,
however, a limited number of general conditions which are beginning
to be accepted as good practice in railway service as follows:
1. Incandescent lamps should be used in preference to arc lamps if local con-
ditions will permit.
2. The candle-power of units and spacing intervals should be as large as
possible consistent with good illumination.
3. Glass reflectors should be used inside office buildings, stations, etc., alu-
minized metal reflectors in shops and semi-exposed places and porcelain enamel
reflectors in outside service.
4. All reflectors should shield from view the incandescent filament under
normal operating conditions. This means, generally, an angle of cut-off of from
60 to 75 degrees.
5. All fixtures and auxiliaries should be as strong and rugged as possible
consistent with the cost of installation.
Bibliography
H. KIRSCHBERG and A. C. COTTON. "Railway Illuminating Engineering
Track Scale and Yard Lighting. " Pittsburgh Section I. E. S., February 13, 1914.
"Report of Committee on Outside Construction and Yard Lighting." Pro-
ceedings A. R. E. E., 1914.
L. C. DOANE. "Lighting Railroad Yards with Large Incandescent Lamps."
Railway Electrical Engineer, December, 1914.
"Lighting Classification Yard, New York Central R. R., Air Line Junction,
Ohio." Railway Electrical Engineer, December, 1915.
"Report of Committee on Illumination." Proceedings A. R. E. E., 1916.
Unpublished reports of the Pennsylvania Railroad.
SIGN LIGHTING
BY LEONARD G. SHEPARD
I have made no attempt to find a record of the first sign. It is
well known that signs have been in common use for centuries and
many of these were undoubtedly more or less illuminated.
The sign of this age, with which we have to do is the sign made
possible by the modern electric lamps. It was probably about 1880
that signs of this type began to be used.
To appreciate the present-day development, it will be well to
know something of the earlier electric signs and their construction.
In 1883 a temporary sign reading "Welcome" was made by plac-
ing the old style wooden base sockets on a wooden background.
The wiring between sockets was done on the back of the sign. The
letters were 2 ft. in height and were formed of 16 candle-power
lamps, spaced about 6 in. apart.
In 1884, an electric sign reading " Boston Oyster House" was de-
signed for more permanent use. To make the sockets weatherproof
they were filled with putty and the wire being the old style known
as underwriters wire was wrapped with tape. The sign body and
frame were made entirely of wood and over all a glass case was built
like an ordinary show case. The lamps of that time had large
plaster-of-Paris bases which were protected in this sign by covering
them with soft rubber bands which covered also the outer end of
the sockets.
A double-faced sign made about this time reading "Dime Museum"
is said to have been about 12 ft. long, 3 ft. high and 2 or 3
ft. thick with i8-in. letters made up of electric lamps. Its clumsy
bulk made it look like a dog house hung up over the sidewalk.
It is interesting to note that the electric flag sign, patriotically
displayed throughout this country in the last year was anticipated
twenty-eight years ago at the convention in Chicago where a sign
made up with miniature or candelabra lamps was flashed on during
the singing of the "Star Spangled Banner."
One of the first flashing signs was made for the World's Fair in
1893. The letters were 4 ft. high and of skeleton construction
535
536 ILLUMINATING ENGINEERING PRACTICE
attached to a wire mesh backing. The mechanism which flashed
on one letter at a time was a crude affair made entirely of wood
with brass strips and bronze contact brushes. It was operated by
a 34-hp. motor. It is said that almost as much light came from the
arcing of the flasher contacts as from the sign. The sign was con-
sidered so dangerous that a man was kept constantly in attendance.
Possibly the largest electric sign ever made was constructed in
1899 during the reception to Admiral Dewey on his return from
Manila. Letters 50 ft. high reading " Welcome Dewey " were placed
on the Brooklyn Bridge and could be read from Staten Island five
miles away. There was no background, the lamps being arranged
on streamers to form the letters. No sockets were used, the leading-
in wires from the lamps which were made up without bases being so
connected that five lamps were put in series, the total electromotive
force used being 550 volt. The lamps, of which over 8000 were re-
quired were spaced 12 in. apart throughout. This sign represented
a remarkable example of series wiring. It must be remembered
that the failure of a single lamp would have made a dark section
5 ft. long in the outline of the letter.
MODERN SIGN TYPES
The illuminated sign of to-day is made in so many different varie-
ties and is used in such varying surroundings that it will be neces-
sary to classify and explain briefly the construction of the several
types to give an adequate idea of the state to which the industry
has developed.
Roof signs (see Figs, i and 2) may be taken as including all the
large, more or less, skeleton types installed above the roof level.
In these signs the steel supporting structure or framework is usually
the most important item. To place the electrical display at a
proper height and in the best location from an advertising standpoint
often requires a considerable structure and frequently too, it is
necessary to reinforce the building and to carry the anchorage mem-
bers way below the roof. This is mentioned, because at night the
framework is not seen and no idea of the investment involved can
be obtained without a full appreciation of this item.
The electrical work required to connect the sign parts to the near-
est service including the installation of the flasher and fuse blocks
depends more upon the flashing effects than upon the size of the sign.
The sign proper or sign face is usually quite simple in construction
Fig. i. Roof sign.
Fig. 2. Roof sign.
(Facing page~S36.)
536 ILLUMINATING ENGINEERING PRACTICE
attached to a wire mesh backing. The mechanism which flashed
on one letter at a time was a crude affair made entirely of wood
with brass strips and bronze contact brushes. It was operated by
a M-hp- motor. It is said that almost as much light came from the
arcing of the flasher contacts as from the sign. The sign was con-
sidered so dangerous that a man was kept constantly in attendance.
Possibly the largest electric sign ever made was constructed in
1899 during the reception to Admiral Dewey on his return from
Manila. Letters 50 ft. high reading " Welcome Dewey" were placed
on the Brooklyn Bridge and could be read from Staten Island five
miles away. There was no background, the lamps being arranged
on streamers to form the letters. No sockets were used, the leading-
in wires from the lamps which were made up without bases being so
connected that five lamps were put in series, the total electromotive
force used being 550 volt. The lamps, of which over 8000 were re-
quired were spaced 12 in. apart throughout. This sign represented
a remarkable example of series wiring. It must be remembered
that the failure of a single lamp would have made a dark section
5 ft. long in the outline of the letter.
MODERN SIGN TYPES
The illuminated sign of to-day is made in so many different varie-
ties and is used in such varying surroundings that it will be neces-
sary to classify and explain briefly the construction of the several
types to give an adequate idea of the state to which the industry
has developed.
Roof signs (see Figs, i and 2) may be taken as including all the
large, more or less, skeleton types installed above the roof level.
In these signs the steel supporting structure or framework is usually
the most important item. To place the electrical display at a
proper height and in the best location from an advertising standpoint
often requires a considerable structure and frequently too, it is
necessary to reinforce the building and to carry the anchorage mem-
bers way below the roof. This is mentioned, because at night the
framework is not seen and no idea of the investment involved can
be obtained without a full appreciation of this item.
The electrical work required to connect the sign parts to the near-
est service including the installation of the flasher and fuse blocks
depends more upon the flashing effects than upon the size of the sign.
The sign proper or sign face is usually quite simple in construction
Fig. i. Roof sign.
Fig. 2. Roof sign.
(Facing Page~536.)
Fig. 3. Street sign.
Fig. 4. Street sign.
Fig- 5- Porcelain enameled steel embossed letter sign.
SHEPARD: SIGN LIGHTING 537
consisting of light galvanized sheet steel cut out to form letters,
figures or designs and backed up to give stiffness and to enclose the
wiring. No internal frame is necessary as it is customary to place
a lattice work of light channels on the face of the framework to
support the display. The galvanized sheet steel serves as a support
for the sockets and as a background for the painted designs.
The face of the letters and display parts is most commonly flat
or flush as it is called. Sometimes where the parts are close together
a flange from 1.5 in. to 4 in. high is carried around the edge of the
face to give contrast and contribute to a clean-cut design. The
flange is a bad collector of dust, and is not necessary where there is
plenty of spacing.
Letters with a flange around the edge of the face or stroke are
called trough letters. There is a special bevel trough letter made
with a broad flange parallel to the face, on the outer edge of the bevel
trough. This flange is painted black to improve the daylight effect
as it makes a strong contrast against a light sky. An example of
this construction may be seen in the i5~ft. letter roof sign on the
Walkerville factory of the Ford Company.
Contrast between light and dark colors or between light and
shadow is very necessary to a sharp clear sign. Frequently a flange
or trough is used to emphasize an important line or to separate por-
tions of a surface which flash or light up at different times.
While the painted surface of a new sign may be relied upon, at
least in clear weather, to reflect enough light to bring out the features
of a design under ordinary conditions, the lamp filaments must be
depended upon to give the chief outline. The sockets therefore
must be carefully located to obtain good results. The distance be-
tween sockets depends upon whether the sign it intended principally
for distant reading or for a maximum effect at close range. In the
first case large candle-power lamps can be spaced well apart but in
the second the sockets must be close together to avoid a crude re-
sult. About 4 in. may be taken as a good standard spacing for
close work.
While many excellent signs are equipped with low candle-power
lamps the modern tendency, especially in the large cities/ is toward
greater brilliancies. Fifteen, twenty and twenty-five watt tungsten
lamps are frequently used although the popular lamp is the ten-
watt unit.
The beautiful color effects are obtained by the use of colored
lamps either natural or dipped or with color caps or hoods of colored
538
ILLUMINATING ENGINEERING PRACTICE
glass fitted over the lamps The dipped lamps probably allow the
greatest variation in tint, but as it is practically impossible to
obtain a lacquer that will adhere permanently to the bulb in all
kinds of weather, the color caps or hoods are more satisfactory. A
good illustration of the difference in the effect of the cap and the
hood may be seen in the many flag signs installed during the last
year or two (Fig. 6). In the popular 4 ft. flag the lamps in the
union are placed in the white stars. A blue cap gives the proper
blue effect for that portion of the flag while the direct rays from
the filament behind the cap light up the white stars. On the red
Fig. 3. Electric flag sign.
and white stripes the hoods give a solid red or white effect as is
desired.
Certainly any discussion of modern roof signs would be incomplete
were the flashing effects omitted. On the other hand a book
could easily be written in explanation of the many variations of this
phase of the sign industry.
All flashing effects are produced by switching on certain lamps or
groups of lamps in a certain sequence and with due regard to time
periods. The mechanism, usually motor driven, which makes and
breaks the many different electric circuits, some as fast as 200 or
300 times a minute, some with currents as large as 100 or 200 am-
peres, is a very important part of the installation. As might
naturally be expected, without attention, the rapidly vibrating
SHEPARD: SIGN LIGHTING 539
parts will often work out of adjustment thereby spoiling the effect
of the entire sign.
In general the skyrocket, crawling snake, or script writing effects
are the most expensive in both flasher and wiring while the con-
tinuously moving border or the simple on and off effects are the
easiest to obtain.
Too much attention cannot be given to the care or maintenance
of an electric sign. A large investment may be robbed of the greater
part of its earning capacity by neglect. A sign that is not clean
and bright and in which the lamps are not all active or one in which
a word is spelled out unevenly or with one letter omitted is a liability
rather than an asset. Proper provision should always be made for
maintenance with the same care as for installation.
The discussion of the roof sign has included many items which
pertain to all kinds of signs. There are some features of construc-
tion, however, which are very different in signs used for other
purposes.
The sidewalks or street signs as they may be called are so much
nearer to the observer that the matter of detail must be given close
attention.
This class (see Figs. 3 and 4) usually has an internal frame of
steel, sufficiently rigid to prevent the buckling of the face under fairly
heavy stress. The faces or sides are secured to the frame near the
edges and frequently across the middle especially when a large
number of sockets weakens the face.
The varieties of ornamentation are innumerable. The most
common include the raised mouldings and the use of gold leaf.
On these flat sign bodies, the illuminated letters (see Fig. 7) are
flush, raised, skeleton letter, trough or sunken, but in each case the
sockets are inserted in the letter stroke to make the reading matter
stand out. The use of color caps and hoods is common as with
roof signs.
Porcelain enameled steel has been found very satisfactory as a
sign material. The surface is an excellent reflector and can be very
easily cleaned. The letter stroke is often made of enamel even
when the body of the sign or letter is painted steel.
In one special form of construction (Fig. 5) each letter is made of
an embossed porcelain enameled steel plate. The form of the plate
is such as to give strength to the sign and allow the use of a neat
narrow steel frame not over 2 in. in width.
In another familiar form of street sign (Fig. 8) a border of lamps
540
ILLUMINATING ENGINEERING PRACTICE
is provided around the reading matter or design. Where the lamps
are raised perceptibly above the panel surface by the use of a
special frame or otherwise the illumination of the panel is usually
quite satisfactory, but where the socket is inserted flush with the
sign face very poor results are obtained. All the slight imperfec-
tions in the surface are brought out. Again the angle of incidence
is so large that very little light is reflected toward the observer.
Type A
Q
O
Q
GTc
TypeB
TypeC
Type D Type E
Fig. 7. Type A, flush. Type B, raised. Type C, skeleton. Type D, trough. Type E,
sunken grooved.
Many forms of transparencies are used for street signs (Fig. 9).
As a class they are of little value from the standpoint of street
illumination but they are often very pleasing in appearance and
have their field. In this class are the lens sign (Fig. 13) the per-
forated or cutout letter, the canteen and the ornamental glass types.
As a class almost by themselves are the changeable letter signs
(Fig. 10). Many of these, used principally for theatres, are so con-
structed that the individual letters may be easily removed and re-
placed for a change of reading. Even in the transparencies there
are changeable letter types. For general advertising, there are
FURNISHINGS
-
Fig. 8. Panel sign.
Fig. 9. Transparency.
Fig. 10. Changeable letter sign.
(Facing page 540.)
Fig. ii. Motograph.
Fig. 12. Miniature lamp letter.
SHEPARD: SIGN LIGHTING
54i
others which change automatically from one reading to another
until quite a story may be told.
A late form of changeable letter sign which combines the change-
able feature with a fascinating moving effect is the motograph
(Fig. n). In this sign, the letters appear on the right and move
evenly and rapidly across the face as though attached to a belt. To
produce this effect a large number of lamps arranged in horizontal
and vertical rows in a lamp bank on the sign face are connected
individually with contact brushes arranged in the same order but
very close together in a brush board in the control machine. A
perforated ribbon something like the roll in a piano player is drawn
continuously across the brush board. Any figure such as a letter
perforated in the ribbon will appear on the lamp bank. The brushes
make contact through the perforations with a fixed metal plate
Type A
Fig. 13. Type A, lens sign.
TypeB
Type B, cut out glass letter sign.
closing the electrical circuit and lighting up the proper lamps to
form the letters.
A careful observer of this sign will note that the vertical strokes of
the letters do not appear as bright as the horizontal strokes. The
contacts controlling the lamps are made and broken so quickly that
the lamp filaments do not reach their full brilliancy. If the observer
is quite near the sign he will note a slight blur or streak, an effect like
the tail of a comet following each letter across the sign, because
filaments do not lose brilliancy fast enough. If there could be found
a lamp in which the filament came to its full brilliancy and cooled off
more rapidly than in the present standard tungsten lamp the speed
of the sign could be materially increased.
Indoor Signs including the more common types of window signs
are usually transparencies but one or two very attractive special
types have been developed such as the miniature lamp letter
sign, Fig. 12. Each letter is in itself a lamp. It is formed by
bending a glass tube into the form of the letter and then inserting
542 ILLUMINATING ENGINEERING PRACTICE
small filaments at frequent intervals. These filaments are con-
nected in series thereby enabling each lamp letter to be connected
across the ordinary service wires.
Flood lighting of signs is a new development of the art. Under
certain conditions the effects obtained are very satisfactory.
The flood of light illuminates everything in its field, the iron
framework as well as the letters. For this reason flood lighting
would seem to be better adapted to the illumination of large solid
areas such as wall signs, buildings or bulletin boards than skeleton
types of signs.
The flood lighting reflectors in sign work, therefore, supplant
the bulletin board reflectors rather than the lamp letter signs.
ENGINEERING FEATURES
Except in the case of transparencies and possibly bulletin board
lighting the development of the electric sign has depended upon the
development of the sign lamp.
When the standard lamp available was the 16 c.p. carbon filament
unit consuming as much energy as the 5o-watt lamps of to-day and
the cost of energy was many times as high as now, the electric sign
of large size was out of the question. Again, the large size of the
available lamp bulbs made a neat well-proportioned sign impossible
in the smaller sizes. The development of the 8-c.p. and then the
4-c.p. carbon lamps made in the small bulb for sign use gave some
encouragement but even then the panel or border lamp signs with
their comparatively small number of lamps were the popular types
on account of the expense.
All of the early lamps were used at the regular service voltage and
consumed considerable energy. Reducing the diameter of the
filament to reduce the current consumption made the lamp too frail.
It was then found that by cutting the 4-c.p.,no-volt filament in half,
two lamps of 2 c.p. each could be made to be used in series. The
appearance of this new lamp about 1903 made it possible to form
properly letters by using enough lamps and without too much expense
although the signs were not equal to the brilliant present-day
standard.
In experimenting with the 55-volt lamps in signs previously made
for no volts it was found necessary to rewire the sockets two in
series an expensive process. The adoption of a new plan in sign
wiring called the series-multiple system solved this difficulty. Two
SHEPARD: SIGN LIGHTING 543
equal banks of lamps as in the two sides of a double-faced sign were
connected in series.
The arrangement called series-multiple wiring meaning a series of
multiples is apt to prove perplexing to the wireman especially when
there are several circuits on the same sign but with all its disad-
vantages the system has been thoroughly worth while for those
who understood it as its use means a large saving in energy cost.
The 2-c.p. 55-volt carbon lamps consumed from 10 to 12 watts
each. When the tungsten lamps with a large reduction in con-
sumption were produced the sign industry naturally was eager to
take advantage of the saving.
To put out a lamp with a filament sufficiently fine to hold the con-
sumption below 10 watts on 115 volts was apparently impracticable
especially with the delicate tungsten filaments; the 5-watt lo-volt
sign lamp which was announced in Jan., 1909, was the result. The
wiring for this lamp was arranged on the multiple system with a
transformer when alternating current was used, although due to the
half ampere current consumption per lamp the heavy currents at low
voltage meant considerable loss in voltage drop and, therefore, in lamp
brilliancy. With direct-current special wiring was imperative and
naturally the series-multiple system employed with the 57-volt
carbon lamps was arranged to include 10 or n groups in series.
Where the sign was small each bank had only a few lamps, thus
causing too great a stress on the remaining lamps when one or more
burned out. Hence for small signs the series wiring was used with
10 or ii lamps in each series. While the series system is the most
economical from the standpoint of lamp renewal cost, it is very un-
satisfactory in large signs because such a large portion of the sign is
placed in darkness whenever any one lamp fails. In large signs
where it is to be expected that one or more lamps will burn out fre-
quently the sign would be constantly spoiled in appearance if the
series wiring system were used, and hence the series-multiple system
is employed where the number of lamps per circuit runs over 100.
With the series-multiple system using 5-watt lo-volt lamps it is
possible without putting over 15 amperes on a circuit to include
330 lamps on a single circuit and thereby reduce the amount of
wiring.
The appearance of the lo-watt no- volt, and the 5-watt 55-volt
sign lamps in June, 1912, and of the 7. 5-watt no-volt lamp in Sept.,
1915, made it possible to eliminate the more complex series-multiple
wiring without increasing the current consumption. At this date,
544 ILLUMINATING ENGINEERING PRACTICE
however, the 5-watt lo-volt lamp is still very widely used on account
of its ruggedness.
An important problem related to sign design is that of legibility.
The size, shape, spacing, and brilliancy affect the readability to a
marked degree. Many signs that are otherwise well made are ab-
solutely unreadible even from a moderate distance. This feature
must receive attention.
THE ELECTRIC SIGN AS A LIGHT SOURCE
The illumination produced in its vicinity by the average, fair-sized
electric sign should not be overlooked as it is a point in its favor. In
exterior illumination the electric sign as a light source gives ideal
results. The lighting of a street with large numbers of small candle-
power lamps would be out of the question because of the high cost
of such a method. However, when merchants along any good
business street make use of a quantity of properly designed signs,
that street needs no other illumination, and is the best and the most
satisfactorily lighted street in the city. The signs need not neces-
sarily project even over the sidewalk. The sidewalks can be clear
of lamp posts, while the illumination is pleasing and adequate.
It frequently happens in a poorly lighted district that the electric
sign on the corner drug store stands out as the only bright spot.'
Its cheering influence is unconsciously appreciated by all.
THE ELECTRIC SIGN INDUSTRY
It is estimated that there are approximately 15,000 electric signs
in New York City and 10,000 in Chicago. In many small cities
there are more signs per capita than in either of these.
If we consider only the cities and towns in the United States with
a population of over 5000 and assume that the number of signs per
capita is 80 per cent, of the New York average there would be a total
of 240,000 signs in this country.
In an ordinary installation, the principal items of cost are the
sign proper or the sign body; the hanging and wiring; the lamps,
color caps and flasher and the permits.
The average total cost might be taken as $65. The capital in-
vested in electric signs would then be about $15,600,000.
It is probable that this is considerably underestimated as many
roof signs cost between $5000 and $10,000 each. The lamp invest-
ment alone is about $4,000,000.
The average power consumption per sign in Chicago is 760 watts.
$357>o in electrical supplies.
SHEPARD: SIGN LIGHTING 545
The rated sign load for the country on this basis would be 182,000
kw. and at 5 cents per kw.-hour with a four-hour service period per
day the energy used would bring a return of $i i ,000,000 per year to
the central stations.
It is estimated that the sign business has not been developed to
20 per cent, of what it should be. If properly developed the esti-
mates given would be multiplied by 5.
To realize what the electric sign business means to other industries
consider one class of signs, the roof signs, and note the material and
labor involved.
One thousand roof signs averaging 40 ft. high by 50 ft. long would
require :
7500 tons of steel @ $200 = $1,500,000 installed.
1,000,000 sign lamps. . .$160,000
3,000,000 ft. of wire 40,000
1,000,000 sockets 50,000
10,000 Ibs. of tape 2,000
500,000 ft. of conduit 35 5 ooo
10,000 gal. of paint 70,000
besides 30,000 flashers, 20,000 time switches, 5000 meters, 2000
transformers, many cutouts, fuse plugs, color caps and probably
about 600,000 board ft. of platform lumber.
The labor cost for designers; sheet metal workers, painters,
electricians, structural iron workers, teamsters and roofers costs
about $675,000. The railroads get about 2,200,000 ton-miles of
freight. One sign manufacturer alone pays $36,000 a year for
freight.
The figures above apply to only a portion of one class of signs.
Another class, the porcelain enamel steel signs, would keep an
immense enameling factory busy, while the many varieties of glass
signs mean a large glass business. One sign company alone employs
upward of fifty traveling men continually throughout the year.
Much more data could be presented to show the extent to which
the application of artificial light to the illuminated sign has been
carried. Sufficient has probably been given to indicate in a general
way the extent to which the art has been developed.
' ORDINANCES
Having in mind the size of the electric sign business one realizes
that those who advocate abolishing signs altogether are hardly apt
to succeed, at least without a long fight and a hard one.
35
546 ILLUMINATING ENGINEERING PRACTICE
For the good of the industry itself, however, as well as to meet
the radicals part way, certain consideration should be given to
the size, type and location of signs. Proper ordinances should be
enacted and enforced to prevent the installation of a huge ungainly
and possibly dangerous street sign. Municipalities are considering
this subject with more and more care. Probably the majority of
towns and cities of over 5000 population now have sign ordinances.
Unfortunately many of the ordinances seem to have been introduced
with the object of discouraging the industry rather than of controlling
it. It is important to use a proper hanging rig with a street sign
including sufficiently heavy chains and cables and adequate wall
attachments. A wise manufacturer will make these parts unques-
tionably strong, and a wide-awake city inspector will see that they
are properly installed.
THE EFFECT OF THE ILLUMINATED SIGN ON A COMMUNITY
From what has been said in reference to street illumination
alone, a strong argument can be made for the illuminated sign.
This point is relatively unimportant, however, as the big value of
illuminated signs to any community lies in their wonderful bright-
ening, boosting and cheering influence. Compare any two towns
with and without these signs. The one is alive while the other is
dead. The merchant, the real estate man, the politician, everyone
realizes what it means to a town to have the reputation of being alive.
Certainly it means enough to warrant a place in any list of industries
of the country.
1
o
I
tj
. o
<gw cy
I'l
s. ^ ^
The celebr o, ^ O
- additional light f c 3
Prior ^ s O
procedur JJ*
a.
8*8
BUILDING EXTERIOR, EXPOSITION AND PAGEANT
LIGHTING
BY W. D'A. RYAN
Since the days of the cave man, fire or light has been a factor in all
festivities. The victories of the barbarians were usually celebrated
at night around the blazing camp fires. The war dance of the
American Indian would not have been complete without fire.
Decorative lighting for commercial purposes originated with the
Chinese. They employed paper lanterns in festooning, and gas
jets in accentuating the architectural lines of their shops and build-
ings. The celebrations of all ages, when carried into the night, have
called for additional lighting, both for utilitarian and spectacular
purposes. Prior to the advent of electricity such lighting was ac-
complished mainly by gas jets and fireworks.
Systems of illumination falling within the classification forming the
title of this lecture differ from utilitarian lighting systems in that ef-
ficiency of light production assumes a secondary role. The import-
ant point to keep in mind in planning lighting schemes of this char-
acter is the necessity for conforming to the architectural ideas or at
least to accomplish the lighting results without conflicting with the
architecture. To do this successfully requires cooperation between
the architect or decorator and the illuminating engineer to a much
greater degree than is common practice to-day. The lighting
features should receive study from the time of the conception of the
architectural details and be developed along with the structural plans.
Of course many creditable lighting installations have been made in
buildings which were completed before the lighting was considered,
but such procedure is a mistake when it is possible to develop the
illumination plans in parallel with the architectural plans.
All architecture is created primarily for observation in daylight;
therefore, if we are to retain the architectural details of a structure
when viewed by artificial light we must approximate daylight
conditions. Daylight consists of a strong directional light from the
sun supplemented by diffused light from the clouds; the former creates
sharp shadows, and the latter relieves them to a greater or less extent.
547
548 ILLUMINATING ENGINEERING PRACTICE
A total absence of sunlight (unidirectional light) would create a
shapeless monotonous mass because there would be no shadows,
while a total absence of reflected light (diffused light) would create
extreme high lights and harsh shadows. A combination of these
two kinds of light is, therefore, necessary to give proper perspective
to architectural forms, and if we are to be successful in displaying
the work of artists and architects by artificial light we must approxi-
mate the distribution found in nature, that is, a strong directional
light coming from above if possible and supplemented by relief
lighting.
For a good many years the only method employed in lighting
a building for emphasis was outline lighting, that is, locating rows
of incandescent lamps around the cornices, windows, etc. This
method is still used for amusement parks, motion picture theatres,
etc., but is too closely akin to the bizarre for our modern monumen-
tal and commercial buildings. In such a scheme of lighting the
building merely serves as a background upon which to display lamps
and when the lamps are lighted only a skeleton of light appears and
the building is obscured by the glare. Its other disadvantages are
the diminution of artistic effectiveness at close range, similarity of
effects from different view points, the suppression or complete ob-
literation of architectural features, the economic necessity of exten-
sive untreated surfaces and severe eye and nerve strain resulting
from the glare.
Very little can be stated in the way of concrete rules covering
spectacular lighting. Such lighting depends for its success on an
appeal to the senses in which respect it is similar to paintings and
music. Light, color and motion are essential features in any spec-
tacular display and by arranging them in interesting combinations
many varied and beautiful effects may be obtained. Steam, vapor
and water afford excellent reflecting media upon which to play light.
Color always adds interest and is readily obtained by passing
white light through light-absorbing gelatine sheets or colored glass.
The illuminating engineer must use his own ingenuity in devising
spectacular lighting features.
EXPOSITION LIGHTING
In our study of the lighting treatment of expositions, it is well
to review briefly the methods and results of former expositions.
From such a study we learn that after the introduction of electricity
would create a
be no shadows,
it) would create
.nation of these
:ul in disp ;
VG must aj
:rong direc
icnted bv
w
;net.hod employed in lighting
locating rows
.tc. This
e theatres,
; ern monumen-
dvantag
ange, simila
jtrain re
a ts success c> ;
O
cu
;..
tich to pi;
ined by
ngenuity
of expositions, it
ler expo
titroduction of ele
RYAN: BUILDING EXTERIOR 549
for illumination purposes at the Louisville, Ky., Exposition in
1883, outline lighting has characterized all expositions prior to the
Panama-Pacific International Exposition in 1915. The exposition
at Louisville was notable for two reasons, first because a method of
bringing the lighting up from zero to full candle-power was in-
troduced, and secondly because it was the last exposition in this
country where gas was utilized in the plans for the main portion of
the lighting.
In the Court of Honor at the Columbian Exposition, Chicago,
1893, outline lighting with incandescent lamps was used extensively
while arc lamps and ornamental posts were employed for the lighting
of the grounds. An electric fountain and the Edison Tower forming
part of the exhibit of the General Electric Company and made up of
10,000 i6-c.p. carbon-filament lamps were the spectacular features.
The first complete method of decorative and grounds lighting
by incandescent lamps was at the Mississippi and International
Exposition, held at Omaha, in 1898. The illumination received the
highest award at this exposition.
At the Fourth Universal Exposition held at Paris in 1900 was
found a mixture of all types of illuminants; incandescent lamps
predominating with a total of 76,720 used for decorative purposes.
The Electricity Building and the Eiffel Tower were the principal
features. The crest of the Electricity Building was covered by the
" Great Star" placed behind the figure of Marqueste, the genius of
electricity. The star had 68 points made up of gilded tubing and
was lighted from the rear by six 6o-amp. projectors. Electric foun-
tains illuminated by arc and incandescent lamps equipped with
color screens were used. Other lighting features were the Palace of
Optics with 14,000 i6-c.p. and 32-c.p. lamps, the Palace of Light with
12,000 lamps, Monumental Gate with 1385 incandescent lamps in
colored globes and the Trocadero Palace festooned with thousands
of gas jets.
At the Pan American Exposition, held at Buffalo in 1901, in-
candescent outline lighting probably reached its maximum effect-
iveness. Incandescent lamps of low candle-power were used ex-
clusively and the number of lamps was gradually increased so as to
form a climax at the Tower.
At the Louisiana Purchase Exposition of St. Louis, 1904, because
of its size, the ideas carried out at Buffalo could not be adopted,
so it was necessary to spread out the lighting to a certain extent
and accentuate certain prominent features. All of the main build-
550 ILLUMINATING ENGINEERING PRACTICE
ings were lavishly festooned with incandescent lamps. Each lighting
unit along the colonnade was provided with three lamps, white,
emerald and amethyst, each color on a separate circuit. Water
rheostats were used for dimming each color and to bring the general
illumination gradually up to full candle-power. Arc lamps were
employed throughout the grounds and Nernst lamps were used in
the Exposition buildings. The monumental feature of the Exposition
was the Cascades, illuminated by incandescent lamps.
At the Lewis and Clark Exposition at Portland, Oregon, in 1905
incandescent lamps were used exclusively. One of the features of the
Exposition was the illumination of the lake. Around the lake
5o-c.p. lamps were placed in marine receptacles on the bottom of the
lake at intervals of three feet.
At Jamestown in 1907 the decorative lighting of the buildings
was done with 8-c.p. lamps placed on approximately i2-in. centers.
A water rheostat of 25oo-kw. capacity was used to " build up"
the lighting. Four minutes were allowed for the lamps to reach
their normal candle-power. One of the illuminating features was the
Administration Building with the projector beams forming a fan
behind the dome. Another pleasing feature was Flirtation Walk
where flashing lamps were placed in trees and shrubbery to give a
firefly effect.
One of the best examples of outline lighting as it should be em-
ployed for exposition buildings was shown at the Alaska- Yukon
Pacific Exposition in Seattle in 1909. A total of 250,000 lamps was
used there for all purposes. For decorative purposes 8-c.p. all
frosted carbon lamps were employed. The grounds were lighted
by 200 special electroliers, each containing 39 2o-c.p. lamps.
Gas was found admirably suited to outline the buildings at the
coronation in London, 1910. The effect produced, because of the
continual flicker of the light, was interesting and lively.
The illumination of the Panama-Pacific international Exposition
represented the latest developments in exposition lighting and
marked an epoch in the science of lighting and the art of illumination.
Previous expositions had depended upon outline lighting for night
effects and this method had probably reached the limit of its possi-
bilities at the Pan-American Exposition at Buffalo. The outline
method, therefore, was set aside and a system of screened or masked
flood and relief lighting was employed.
During the period elapsing between the Louisiana Exposition
and the Panama-Pacific International Exposition wonderful advances
O
g*
W <u
<3 rt ^
8s*
O o
O
3 '
I
ps. Each lighting
; h three lamps, white.
a a separate circuit, \\
rheost >r and to bring the general
all candle-power. Arc lamps were
emplo\ ind Nernst lamps were used in
theExi lonumental feature of the Exposition
'incandescent lamps.
Portland, Oregon, in 1905
^ . One of the features of the
2- e lake. Around the lake
n the bottom of the
I*
o
.o cv -ting of the buildings
* j in. centers.
jjf & g build up"
uT^ J ie lamps to reach
2 . U imi nating features was the
2. cj ,. beams forming a fan
. -s} . cu : : . . e - Flirtation Walk
'* urubbery to give a
o O
: ^Sighting as it should be effi-
gy snown at the Alaska-Yukon
a total of 250,000 lamps was
>es. Forj-^lecorative purposes 8-c.p. all
SC >unds were ]i
*f /> 2o-c.p. lamps.
N" hidings ;
^ :-d, because
and lively,
international E .
exposition lighting and
and the art of illumin
iline lighting for night
Cached the limit of its
!i at Buffalo. ..The outline
'em of screened or me
Dur the Louisiana Exposition
and tl ' on wonderful advances
t
sumr
RYAN: BUILDING EXTERIOR 551
had been made in the efficiencies of all types of lighting units.
Thus it was possible to illuminate, in the main groups of buildings
and grounds, approximately 8,000,000 sq. ft. of horizontal and ver-
tical surfaces with intensities ranging from o.i to 0.25 foot-candle
in the incidental gardens and roadways, from 0.25 to 3 foot-candles
on the building facades and adjacent lawns and gardens, and from
5 to 15 foot-candles on the towers, flags and sculptural groups. The
lighting load on the main group of buildings, including the window
lighting and the scintillator, was approximately 5000 kw. The
total connected load for all purposes, including the "Zone" foreign
and state sections and exhibitors, for lighting, incidental heating,
motor, and other service was 13,954 kw. with a maximum peak of
8200 kw. and an average peak of 7880 kw.
While the lighting of the Exposition was primarily electric, all
modern light sources of intrinsic merit were utilized, and a number
of excellent gas features were introduced. About four miles of
streets in the foreign and state sections were illuminated with high
pressure "gas arc lamps" equipped with 2o-in. opal globes mounted
with their centers about 16 ft. above the roadways on ornamental
poles spaced approximately 100 ft. apart, staggered. The same
type of lamp was used for emergency lighting on the kiosks through-
out the grounds. Five-mantle "enclosed gas arc lamps," in the
"Zone" section and the same type of lamp, in smaller sizes, fur-
nished emergency lighting at the gates and important exits from the
main group of buildings. Gas flambeaux were introduced in the
effects in the "Court of Abundance" and the "North Approach."
The total gas flow for the purposes mentioned was approximately
15,000 cu. ft. per hour.
Furnishing wonderful contrast to the soft illumination of the pal-
aces, was the "Zone" or amusement section, with all the glare of the
bizarre, giving the visitor an opportunity to contrast the illumination
of the future with the light of the past. As we passed from the
"Zone" with its blaze of light, we entered a pleasing field of entice-
ment. We were first impressed with the beautiful colors of the her-
aldic shields, on which were written the early history of the Pacific
Ocean and California. Behind these banners were luminous arc
lamps in clusters of two, three, five, seven and nine, ranging in height
from 25 to 55 ft. We looked from the semi-shadow upon beautiful
vistas and the Guerin colors, which fascinating in daytime, were even
more entrancing by night. The lawns and shrubbery surrounding
the buildings and trees with their wonderful shadows appeared in
552 ILLUMINATING ENGINEERING PRACTICE
magnificent relief against the soft background of the palaces. The
"Tower of Jewels" with its 102,000 "Novagems," which suggested
the official title "Jewel City" standing mysteriously against the
starry blue-black of the night, might be said to have surpassed the
dreams of Aladdin.
The Courts of Flowers and Palms each received treatment in keep-
ing with its oriental architecture. The towers were flood lighted by
arc standards and projectors and the shadows thus created were
illuminated with colored light at the various levels.
In the Court of Flowers the incandescent standard and lantern
served to give a subdued illumination throughout the court. The
balustrade standard in the Court of Palms was equipped with a 20-in.
glass sphere and the Colonnade was lighted by large staff hanging
basket fixtures.
As we passed through the approach to the "Court of Abundance"
from the east, with its masked shell standards strongly illuminating
the cornice lines and gradually fading to twilight in the foreground,
and entered the Court, we were impressed with the feeling of mys-
tery analogous to the prime conception of the architect's wonderful
creation. Soft radiant energy was everywhere; lights and shadows
abounded, fire hissed from the mouths of serpents into the flaming
gas caldrons and sent its flickering rays over the composite Spanish-
Gothic-Oriental grandeur. Mysterious vapors rose from steam-
electric caldrons and also from the beautiful fountain group sym-
bolizing the earth in formation. The cloister lanterns and snow-
crystal standards gave a warm amber glow to the whole Court, and
the organ tower was illuminated in the same tone by colored projec-
tor rays.
Passing through the "Venetian Court" we entered the "Court of
the Universe," where the illumination reached a climax in dignity
thoroughly in keeping with the grandeur of the Court. Here an area
of nearly half a million square feet of horizontal and vertical surface
was illuminated by two fountains rising 95 ft. above the level of the
sunken gardens, one symbolizing the rising sun and the other the
setting sun.
The shaft and ball surrounding each fountain was glazed in heavy
opal glass, which was coated on the outside in imitation of travertine
marble, so that by day they did not in any sense suggest the idea of
light sources. High efficiency incandescent electric lamps installed
in these two columns gave a combined initial (bare lamp) candle-
power of approximately 500,000 and yet the intrinsic brilliancy was
COURT OF ABUNDANCE
Showing the Organ Tower and the fiery serpent flambeaux. The orange colored cloister
lanterns, the flaring gas and ruby steam caldrons, and the torches on the tower
combined to produce a feeling of mystery in this Court at night.
55
iground of the palaces. The
" which suggested
against the
-.rpassed the
di c
ved treatment ink
The towers were flood lighted'
e shadows thus created Y.
i the va
<ard and Ian 4
i.it the court.
ppedwitha 2o-in.
;e staff hanging
Court of Abundance' 7
illuminating
the foreground,
3DWACWUaA 1O mJOO ,e feeling
loteiofo boioloo agjtBio ariT .xuBddmBft Jnsqiaa ^i9& oili bim iswoT afigiO 9tti
no aerioio* 9iit bnB ,anotbiB3 niB^Ja y;cfin bnB 8Bg gnhfift
.irigin JB JiuoD aifft ni ^6*a^m lo snifeai B aouboTq o^ bariicfraoD
I
.-e composite Spanish-
5 rose from steam-
iful fountain group s
h in formation. The cloister lanterns air
- gave a warm amber glow to the whole Court,
the organ t<
Court of
i iimax in dignity
:i the grandeur of t. Here an area
and vertical su
ft. above the lev;
n and the other
n was glazed in he
in imitation of tr,'
mn a of
" scent electric lamps :
ic brillia
RYAN: BUILDING EXTERIOR 553
so low that the fountains were free from disagreeable glare and the
great colonnades were bathed in a soft radiance. For relief lighting
three incandescent lamps were placed in specially designed cup
reflectors located in the central flute to the rear of each column.
This brought out the Pompeiian red walls and the cerulean blue ceil-
ings with their golden stars, and at the same time the sources were so
thoroughly concealed that their location could not be detected from
any point in the court.
The perimeter of the sunken gardens was marked by balustrade
standards of unique design consisting alternately of Atlante and
Caryatides supporting an urn in which were placed incandescent
lamps of relatively low candle-power. The function of these lamps
was purely decorative.
The great arches were carried by concealed lamps, red on one side
and pale yellow on the other, thereby preserving the curvature and
the relief of the surface decorations. The balustrade of this court,
70 ft. above the "Sunken Gardens/' was surmounted by 90 sera-
phic figures with jeweled heads. These were cross lighted by 180
incandescent projector lamps, the demarcation of the beams being
blended out by the light of the fountains of the "Rising Sun" and
"Setting Sun."
Passing through the "Venetian Court" to the west, we entered
the "Court of Four Seasons" classically grand. We were then in a
field of illumination in perfect harmony with the surroundings, sug-
gesting peace and quiet. The high-current luminous arc lamps
mounted in pairs on 25-ft. standards masked by Greek banners were
wonderfully pleasing in this setting. The white light on the columns
caused them to stand out in semi-silhouette against the warmly il-
luminated niches with their cascades of falling water, and the placid
central pool reflected in marvelous beauty, scenes of enchantment.
Having reviewed in order the illuminations mysterious, grand and
peaceful, we emerged from the west court upon lighting classical and
sublime, the magnificient "Palace of Fine Arts" bathed in what
might be called "Triple moonlight," casting reflections in the lagoon
impossible to describe. The effect was produced by projectors
located on the roofs of the "Palace of Food Products" and "Palace
of Education" supplemented by concealed lamps in the rear cornice
soffits of the colonnade.
Having passed through the central, east and west axes of the
Exposition, there were many more marvels to be seen. If one had
wished to study the art of illumination he could have visited the
554 ILLUMINATING ENGINEERING PRACTICE
Exposition every evening throughout the year and still have found
detail studies of interest. For instance, he could have seen artificial
illumination in competition with daylight. On certain occasions the
projectors flood lighted the towers before the sun went down. If
some were fortunate enough to have been present in the northwest
section of the "Court of the Universe" and watched the marvelous
effect of the "Tower of Jewels" as the daylight vanished and the
artificial illumination rose above the deepening shadows of the night,
they saw the prismatic colors of the jewels intensify and the " Tower "
itself become a vision of beauty never to be forgotten.
At night the "South Garden" could very properly have been
called the "Fairyland of the Exposition." When light was first
turned on, the five great towers were bathed in ruby tones and they
appeared with the iridescence of red hot metal. This gradually
faded to delicate rose as the floodlight from the arc projectors con-
verted the exterior of the towers into soft Italian marble. The
combination of the light from the projector arc lamps (white) and
that from the concealed incandescent lamps (ruby) produced shad-
ows of a wonderful quality. Each flag along the parapet walls had
its individual projector which converted it into a veritable sheet of
flame. As a primary line of color the heraldic shields and cartouche
lamp standards produced a wonderful effect against the travertine
walls bathed in soft radiance from the luminous arc lamps, which also
brought out the color of the flowers and lawns and created pleasing
shadows in the palms and other tropical foliage. This was supported
by a secondary effect in the decorative incandescent electric stand-
ards along the "Avenue of Palms" and throughout the gardens.
A finishing touch was added by the effect of "Life within " created by
the warm orange light emanating from all the Exposition windows
supported by* rose red light in the towers, minarets, and pylon
lanterns.
To the west the enormous glass dome of the "Palace of Horti-
culture" was converted into an astronomical sphere with its revolv-
ing spots, rings and comets appearing and disappearing at the horizon
and changing colors as they swung through their orbits. The action
was not mechanical, but astronomical.
To the east the "Festival Hall" was flood lighted by luminous
arc lamps and accentuated by orange and rose light from the corner
pavilions, windows, and lanterns surmounting the dome. All of
this view was reflected in the adjacent lagoon and possessed a dis-
tinctive charm which will long remain in the memory.
SOUTH PORTAL, PALACE OF INDUSTRIES
Showing 35-foot Cartouche Standards. This illustrates
excellent depth of detail with normal shadows.
554 RING PRACTICE
liout the year and still have fo
stance, he could have seen artificial
iaylight. On certain occasions the
towers befon n went down. If
to have bee in the nortK
ed the marvelous
/els" as. the daylight vanished and the
the deepening shadows of the night,
he jewels intensify and the "IV
t y never to be forgotten.
h Garden" could very properly have "been
-lion." When light was first
!.n ruby tones and they
This gradually
ire projectors con-
marble. The
ite) and
ed shad-
wi 'io aoAJAq ,jAToq Hiuoeet .wall* had
.aW*bnBl8 eriDuolfifiD ioo^StghiWckfifcle sheet of
^wobjjria Lranoti rftiw liuteb to rffqsf) Jabl^xa ; n & cartouche
uced a wonderful effect against the travertine
is bathed in soft radiance f ous arc lamps, which
the color of the / and created pleasing
alms and other tropical foliage. This was suppor
by a secondary effect in the decorative incandescent electric sta.
ards along the "Avenue of Palms" and throughout the g
rushing touch was added by the
.
Ion
.lie " Palace of Horti-
phere with its revolv-
.appearing at the hor i
ugh their orbits. The action
* as flood lighted by lumir-
arc lamps ar, , n d rose light from the corner
.lions, windc mounting the dome. All
view was reflected in the adjacent lagoon and possessed a
tinctive charm which will long remain in the memory.
RYAN: BUILDING EXTERIOR 555
Purely spectacular effects were confined to the scintillator at the
entrance of the yacht harbor. This consisted of forty-eight 36-in.
projectors having a combined projected candle-power of over
2,600,000,000. This battery was manned by a detachment of
U. S. Marines.
A modern express locomotive with 8i-in. drivers was used to
furnish steam for the various fireless fireworks effects known as " fairy
feathers," "sunburst," "chromatic wheels," "plumes of paradise,"
" Devil's fan," etc. The locomotive was so arranged that the wheels
could be driven at a speed of fifty or sixty miles per hour under
brake, thereby giving forth great volumes of steam and smoke
which, when illuminated with various colors, produced a wonderful
spectacle.
The aurora borealis created by the projector beams reached from
the Golden Gate to Sausalito and extended for miles in every direc-
tion. The production of " Scotch plaids" in the sky and the "birth
of color," the weird "ghost dance," "fighting serpents," the "spook's
parade," and many other effects were fascinating.
Additional features consisted of ground mines, salvos of shells
producing flags of all nations, grotesque figures and artificial clouds
for the purpose of creating midnight sunsets.
Over 300 scintillator effects had been worked out and this feature
of the illumination was subject to wide variation. Atmospheric
conditions had a great influence upon the general lighting effects;
for instance, on still nights the reflections in the lagoons reached a
climax, particularly the "Palace of Fine Arts" as viewed from
"Administration Avenue;" the facades of the "Palace of Educa-
tion" and "Palace of Food Products" as seen in the waters through
the colonnade of the "Palace of Fine Arts;" the "Palace of Horti-
culture" and "Festival Hall" from their respective lagoons in the
"South Garden;" the colonnades and the Novagems on the heads
of the seraphic figures and the "Tower of Jewels" as reflected in the
water mirror located in the north arm of the " Court of the Universe."
On windy nights the flags and jewels were seen at their best. On
foggy nights there were produced over the Exposition wonderful
beam effects impossible at other times.
When the wind was blowing from the land the scintillator display
was different from nights when the wind was blowing from the bay.
A further variety was introduced in the action of the smoke and
steam on calm nights.
On the evening of St. Patrick's day all the projectors were screened
556 ILLUMINATING ENGINEERING PRACTICE
with green, and not only the towers but every flag in the Exposition
took on a new aspect.
Orange in various shades was the prevailing color for the evening
of "Orange Day," and on the ninth anniversary of the burning of
San Francisco, the Exposition was bathed in red, with a strikingly
realistic demonstration of the burning of the "Tower of Jewels."
Never before was there such flexibility in lighting on so large a
scale, making it possible at very small expense and on short notice
to introduce modifications in the illuminating effects. This was
made feasible by use of the great number of projectors, which on
ordinary occasions projected white light, but by the introduction of
screens the coloring could be completely changed.
Briefly, the lighting equipment consisted, primarily of direct,
masked, concealed and projector lamps, representing an harmonious
blending of luminous arc, projector, incandescent electric and gas
lamps.
The high current luminous arc lamp was selected for general
flood lighting of the facades, lawns, and shrubbery on account of its
high efficiency, and relatively low maintenance cost where great
quantities of white light was required.
Projectors were used for illuminating the towers and minarets,
flags and other features where concentration was necessary.
High efficiency electric incandescent lamps m all ratings from 10
to 1500 watts were employed generally throughout the Exposition,
especially where space was limited, warm tones were required and
flexibility was of fundamental importance.
High-pressure gas lamps played an important part in street light-
ing in the Foreign and State sections; as did also low-pressure gas
lamps for emergency purposes and gas flambeaux for special effects.
o
C/3
ery flag in th
hepreva;. for the
; ' - irnirig of
g as bathed in red,
^ burning of tru
( iRh flexibility ir rg e a
| y small expense and on short ;
^.o the illuminating effects. Thi:
g V number. of projectors, which on
|- 5; g ! ight, but by the introduc!
| i g changed.
to *? 3 !, primarilv
^ /* OQ
S - "j n harm'
-
f B: g -ted for gen<
* 8 >
o -fg ghout the Ex]
5' ^ rm tones were requi
High-pressure gas lamps Si
n and Sta|jrg
GENERAL DESCRIPTION OF EXHIBITION
The exhibition was collected in rooms adjacent to the Lecture Hall of the
Engineering Building of the University. It covered about 8000 square feet of
floor area.
Exhibits were supplied by manufacturers in the lighting field, electric and gas
lighting companies, research and development laboratories, the Navy Depart-
ment and others. It was the intention in designing and collecting these exhibits,
to minimize the commercial element and to emphasize the educational value.
It was the purpose not only to have apparatus, equipments and illustrations
available, but to afford those taking the lecture course every opportunity to
familiarize themselves with the material exhibited and with its use and signifi-
cance. The scope will be indicated by the following partial list of exhibits:
ILLUMINANTS
Set of mazda lamps
Set of miniature mazda lamps
Historical collection of incandescent
electric lamps
Historical collection of gas "arc"
lamps
Comparison of gas lamps of 1906
with those of 1916
Collection of metallic flame and
flaming arc lamps
ILLUMINANT ACCESSORIES
Window lighting reflectors
Reflectors for interior illumination
Reflectors for exterior illumination
Collection showing the development
of diffusing glassware
Decorative lighting glassware
Fixtures electric and gas.
LIGHTING UNITS FOR PARTICULAR
PURPOSES
Artificial daylight equipments
Mine lamps
Street lighting units
Car lighting equipments
Flood lights and searchlights.
PHOTOMETRIC APPARATUS
Bar photometers
Integrating spheres
Portable photometers
Light niters
Photometer heads
Electrical instruments
Physical photometers
Spectrophotometers
Flicker photometers
Photoelectric cell
Color vision correction screens
Acuity apparatus.
OPTICAL APPARATUS
Spectrum projector
Comparison spectroscope.
OPHTHALMOLOGICAL APPARATUS
Visual acuity devices
Ophthalmoscopes
Visual sensitometer
Pupillary diameter wedges
Apparatus showing retinal inertia
Threshold photometer
Illuminated test cards
Color test apparatus.
COLOR APPARATUS
Spectrophotometer
Colorimeter
Color mixture wheel
Color booths
Color triangle
Variable neutral tint scheme.
MANUFACTURING PROCESSES
Cabinet illustrating the manufacture
of mazda lamps
Display illustrating characteristics
of gas lamps.
LIGHTING PRACTICE
Statistics showing importance of illu-
mination in mining operations
Model of dark room lighting
Model of street lighting
Display showing present methods of
interior illumination
Model showing art gallery lighting
Relation of illumination to safety
and output.
SPECIALTIES
Exhibit of Bureau of Standards illus-
trating the work of the Bureau
Exhibit of Navy Department, illus-
trating signaling by lighting
Exhibit of fluorescence due to ultra-
violet light
Diagrams of luminous efficiencies.
557
Fig. i. View of exhibit room No. I, looking west.
Fig. 2. View of exhibit room No. i, looking east.
558
Fig. 3. Exhibit of Benjamin Electric Company.
Fig. 4. Exhibit of Bosch & Lomb.
559
Fig. 5. Exhibit of Bureau of Standards.
Fig. 6. Exhibit of Central Electric Company.
Fig. 7. Research laboratory exhibit of Eastman Kodak Company.
Fig. 8. Electrical Testing Laboratories, interior lighting model.
Fig. 9. Electrical Testing Laboratories, interior lighting model.
Fig. 10. Electrical Testing Laboratories, interior lighting model.
562
Fig. ii. Electrical Testing Laboratories, interior lighting model.
Fig. 12. Electrical Testing Laboratories, street lighting model.
563
Fig. 13. Electrical Testing Laboratories, exhibit of street lighting units.
Fig. 14. Electrical Testing Laboratories, exhibit of lighting accessories.
564
Fig. 15. Exhibit of Nela Engineering Department, General Electric Company.
Fig. 16. Exhibit of Xela Engineering Department, General Electric Company.
565
Fig. 17. Exhibit of Edison Lamp Works, General Electric Company.
Fig. 1 8. Exhibit of Consulting Engineering Laboratory, General Electric Company.
S66
Fig. 19. General Electric flood lighting projectors.
Fig. 20. Exhibit of General Gas Light Company.
567
Fig. 21. Exhibit of Leeds & Northrup Company.
Fig. 22. Exhibit of Macbeth-Evans Company.
568
Fig. 23. Exhibit of National X-ray Reflector Company.
Fig. 24. Exhibit of the Philadelphia Electric Company.
569
Fig. 25. Exhibit of Simon Ventilighter Company.
Fig. 26. Portable lamp exhibit of Frank H. Stewart Electric Company.
570
Fig. 27. Light signals exhibited by. U. S. Navy.
Fig. 28. Optical instruments exhibited by Wall & Ochs.
S7i
Fig. 29. Gas lamps exhibit, Welsbach Company.
Fig. 30. Electric lamp exhibit of Westinghouse Company.
572
INDEX
Abbreviations, photometric units, 34 Candle-power curve explanation, 4
Absorption-of-light method of calcu-
lation, 15
Accessories, bibliography, 210
car lighting, 509
glass structural characteristics,
1 86
applications, 199
light absorption table, 426
lighting, 183
mirrored, 203
prismatic, 200
street illumination, 424, 479
Acetylene flame, brightness, 47
Arc, carbon, brightness, 47
intrinsic brilliancy, 216
lights, brightness, 47
mercury, brightness, 47
Art museum lighting, 264
Auditorium illumination, 326
Automobile headlights, 220
bibliography, 250
regulations various states, 222
Banks, illumination, 372
Bar photometer, 109
Bibliography (various subjects, see
name of subject).
Brightness, artificial sources, 47
conversion table, 39
definition, 31
measurements, 123
units, 25
conversion table, 39
Brilliancy projection sources, 216
Burners, gas, characteristics, 168
Calculations, absorption - of - light
method, 15
illumination, i
Candle, brightness, 47
definition, 30
definition, 30
diagram, flux summation, 12
distribution, cylindrical source, 8
space representation, 10
spherical source, 8
per sq. in. method to determine, 27
Space distribution, i
various definitions, 33
Carbon filaments (see filaments).
lamps (see lamps, also filaments).
Cars, electric lighting, 495
axle driven system, 496
headend system, 495
straight storage system, 496
gas lighting, 493
illumination design, 497
driving, 504
fixtures, 510
glass ware, 509
intensities, 497
interurban, 508
parlor, 507
passenger coaches, 497
postal, 507
private, 507
reflectors, 509
sleeping, 505
smoking, 506
street, 508
Churches, illumination, 296, 326
ritualistic illumination, 301
Color, brightness definition, 270
in lighting, 267
lighting media, 287
measurements, 271
mixture applications, 291
photometry, 271
saturation, 269
science practice, 268
terminology, 269
Colorimetry, 272
573
574
INDEX
Contracts, street illumination, 484
Cost data, various illumination sys-
tems (see name of system).
Curve characteristic, definition, 33
performance, definition, 33
Daylight, artificial, 290
production, 282
illumination, 19
measurements, 124
Decoration by illumination, 253
Dining room lighting, 263
Dirt cause of depreciation, 48
table, 49
Distribution circuits failing lighting,
355
street lighting, 472
curves, definitions, 33
Docks, illumination, 528
bibliography, 533
Drafting room illumination, 319
Exposure, definition, 30
Factories, illumination, bibliography,
361
cost data, 358
distribution circuits, 355
intensities, table, 350
lamps, available, 343
maintenance, 357
requirements, 344
typical plans, 351
values for manufacturing
spaces, 342
production, relations to lighting,
337
Filaments, carbon, brightness, 47
intrinsic brilliancy, 216
tungsten, brightness, 47
Filters, light, 102
Fixtures, car lighting, 510
design, 207
gas lamps, residence lighting, 180
Flame, acetylene, brightness, 47
candle, brightness, 47
intrinsic brilliancy, 216
kerosene brightness, 47
Flicker photometer, 100
Flood lighting, 239
architectural results, 261
bibliography, 251
calculations, 92
Flux, luminous, definition, 29
summation, candle-power dia-.
gram, 12
graphical construction, 14
Freight yards illumination, 514
Gas burners, characteristics, 168
lamps (see lamps),
fixtures (see fixtures).
lighting, cars (see car lighting).
mantles (see mantles).
Gasolene lamps (see lamps).
Glare avoidance, 57
characteristics headlights, 224
definition, 55
elimination, 65
street illumination, 90, 448, 478
Glass accessories (see also accessories),
structural characteristics, 186
colored, transmission coefficients,
200
light losses, 195
transmission coefficients, 193
Glassware, bibliography, 210
car lighting, 509
light losses analysis, 195
uses, 199
Globes (see also glassware).
light absorption, table, 426
Gymnasium illumination, 322
Harcourt lamp, characteristics, 105
Headlighting, bibliography, 250
Headlights, automobile, 220
glare characteristics, 224
railway equipment, 229
Hue, definition, 269
Ignition, gas lamps, 173
Illumination, aesthetic effects, 61
application of color, 289
architectural aspects, 253
artistic, 253
aspects, 398
INDEX
575
Illumination, auditorium, 326
banks (see banks),
calculations, i
absorption-of -light method, 15
daylight, 19
flood lighting, 92
point-by-point, 50
zone-flux methods, 51
cars (see car lighting),
churches, 297, 326
colored light, bibliography, 294
surfaces, 280
daylight (see daylight),
decorative, 253
definition, 30
depreciation due to dirt, 48
table, 49
design effect of accessories, 42
examples, 73
location of units, 71
process, 64
selection of source, 64
docks (see docks),
effect of ceiling bright, 24
on production, 337
expositions, 547
history, 549
exterior, building fronts, 440
calculation, 84
choice of lamps, 95
classification, 81
color effects, 96
principles, 77
public monuments, 91
strict classification, 86, 89
factory (see factory).
classification, 347
costs, 340
legislation, 341
requirements, 344
typical plans, 351
fixtures (see fixtures),
flood lighting, 239
fundamental characteristics, 309
gas developments, 165
fixtures, 1 80
hygiene, 53
indoor vs. outdoor, 432
industrial establishments, 337
Illumination, intensities industrial serv-
ices table, 350
interior design, calculations, 37
principles, 37
large rooms, 329
library, 323
measurements, 116, 122
compensated test plates, 112
nomenclature, 29
office (see office).
outside works, 513
pageants, 547
physical aspects, 397
psychological aspects, 396
public monuments, 91
quantity for eye efficiency, 59
railway cars (see cars).
residences (see residences).
schools, 311
static tester, 121
store (see store).
street (see streets).
sunlight, 46
theaters, 330
units, i, 29
values, manufacturing spaces, 342
street lighting, 89
various classes of service, 61
window (see window).
yards (see yards).
Illuminometer, Macbeth, 116
Intensities illumination, various ser-
vices (see name of service).
Intensity, luminous, definition, 30
Kerosene, flame, brilliancy, 47
Lambert, definition, 25, 31
Lamp, definition, 32
Lamps, accessories, definition, 34
acetylene, efficiency, 41
alcohol, efficiency, 41
arc development, 146
enclosed carbon engineering
date, 154
flame engineering data, 154
illumination characteristics, 149
luminous, 154
bibliography, 162
576
INDEX
Lamps, carbon (see filaments),
care, 160]
comparison, definition, 32
electric classification, 132
developments, 131
factory lighting, 343
filament, development, 133
gas, filled, 134
incandescent developments, 133
operation, 136
Mazda, engineering data, 138
street lighting data, 139
train lighting data, 138
miniature developments, 143
specific output, units, 34
factory illumination, 343
freight yard illumination, 525
gas, distant control, 176
efficiency, 168
electro-magnetic values, 178
fixtures, 1 80
ignition, 173
photography, 182
pilot consumption, 176
special application, 181
gasoline efficiency, 41
Harcourt characteristics, 105
kerosene, efficiency, 41
Moore carbon dioxide develop-
ment, 145
old-fashioned simulation, 285
selection, 161
signal, illumination characteris-
tics, 245
standard electric, characteristics,
106
tests, basis, 34
definition, 32
tube, carbon dioxide, 283
development, 145
mercury vapor, development,
157
quartz tube, 158
X-ray development, 146
tungsten (see filaments).
lumens output table, 40
Libraries, illumination, 307, 323
Light, absorption by accessories, 195
table, 426
Light, colored, bibliography, 294
distribution calculation, 385
filters, 102
losses, glassware, 195
projection applications, 213
sources, brightness, 47
brilliancy, 216
transmission coefficient, colored
glass, 200
Lighthouses, bibliography, 252
projector applications, 242
Lighting accessories, 183
(see illumination).
Lumen, definition, 30
Lux, definition, 30
Mantles, intrinsic brilliancy, 216
gas, brightness, 47
physical character, 166
Mazda lamps (see lamps, electric).
Mirror, accessories, 203
Moore lamp (see lamps, tube).
Motion picture projectors, 246
Museum, art, lighting, 264
Office illumination, 363, 366
bibliography, 390
cost, 368
design, 371
types, 369
Photography, gas lamps, 182
Photometer, bar, 109
flicker, 100
converted from Zummer-Brod-
hun, 100
physical, 99
Sharp-Millar, 117
tests, definition, 33
Photometry, abbreviations, 34
gas-filled lamps, 125
integrating sphere, no
bulky lamps, 112
lamps, quick handling, 113
standard, 103
shades and reflectors, 126
liquid filters, 102
practice, 99
projection apparatus, 129
standard lamps, 103
INDEX
577
Pintsch gas car lighting, 494
Point source, study, 1 1
Posts for street lamps, 480
Projection, general principles, biblio-
graphy, 250
transparencies, bibliography, 252
Projector applications, transparencies,
246
Projectors, photometry, 129
Quartz tube lamps (see lamps, tube).
Radiation, luminous, definition, 31
Railway headlights, 229
bibliography, 250
Rating, illuminants, 34
Reduction factor, definition, 33
Reflection, buildings, 477
coefficient, definition, 32, 191
table, 193
measurements, 127
pavements, 449, 477
walls, ceilings, floors, 17
Reflectors (see also accessories).
aluminium, properties, 197
bibliography, 210
car lighting, 509
design, 196
effects on illumination design, 42
enameled properties, 198
gas, 184
glass manufacture, 189
light absorption table, 426
losses, analysis, 195
projection, 214
metal, 184
manufacture, 188
optical properties, 191
parabolic characteristics, 217
photometric properties, 197
reflection coefficients, 193
uses, 185
utilization factors, table, 52-56
Residences, basement illumination, 412
bath room illumination, 411
bedroom illumination, 411
den illumination, 409
dining room illumination, 405
hall illumination, 410
Residences, illumination, 395
artistic aspects, 398
gas lamp fixtures, 180
physical aspects, 396
psychological aspects, 396
practical applications, 400
kitchen illumination, 409
library illumination, 407
living room illumination, 402
music room illumination, 408
porches illumination, 412
sunparlor illumination, 409
Schoolrooms, illumination values, 315
Schools, illumination, 307, 311
Searchlight, equipments, 235
Searchlighting, bibliography, 251
Shade, definition, 271
Shadows, effects on eye, 58
Sharp-Millar photometer, 117
Shop room, illumination, 319
Signal lamps (see lamps).
lights, bibliography, 252
Signaling, projector applications, 245
Signs, electric, effect on community,
546
engineering features, 542
industry, 544
modern types, 536
ordinances, 545
lighting, 535
Sky, brightness, 46
Sky-light, artificial, 289
glass, light losses, analysis, 195
illumination calculations, 21
Spectrophotometry, 272
Standard, luminous, primary defini-
tion, 32
luminous, representative defini-
tion, 32
reference, definition, 32
working definition, 32
Stations, freight illumination, 527
passenger illumination, 531
Steradian, definition, 3
Stores, direct lighting equipment, 378
gas lighting, 380
illumination, 363, 373
systems, 377
578
INDEX
Stores, show case lighting, 381
window lighting, 382
Street illumination accessories, 424, 479
arc lamps, 467
bibliography, 459
calculations, 428
city requirements, 461
contracts practice, 486
requirements, 485
contractual relations, 484
cost reduction effect, 491
design, 435, 456, 469, 474
effect of pavements, 477
electric circuits, 472
flux on street, 430
glare reduction, 90, 448, 478
graded results, 482
history, 415
illuminants, characteristics, 423,
465
recent history, 420
incandescent lamps, 468
lamp locations, 451
large units, 466
measure of service, 488
pavement reflection, 449
posts and mountings, 480
public policy, 492
purposes, 415
scope, 417
small units, 467
state control, 490
street classification, 465
tests, 453
typical intensities, 438
values of illumination, 89
variability along street, 441
Street illumination, visual characteris-
tics, 430
Streets, classification for illumination,
465
Sunlight, artificial, 289
Tint, definition, 271
Theaters, illumination, 330
Transmission coefficients (see name of
material) .
light through glass, 192
measurements, 127
Tungsten lamps (see filaments, also
lamps).
Units illumination, i
photometric, table, 34
Utilization factor, definition, 18
factors, table, 52-56
Valves, electro magnetic, 178
Vehicle head lights, 220
Visibility, definition, 30
Vision phenomena, 463
Window illumination, 363, 382
bibliography, 390
light distribution, calculation, 385
source, calculation, 21
X-ray lamps (see lamps).
Yard illumination, bibliography, 533
freight classification illumination,
521
illuminants, 525
illumination, 514
SEVENTH
OVERDUE.
-At)^i^4989
oo / u /
GENERAL LIBRARY U.C. BERKELEY
B000330B1M
THE UNIVERSITY OF CALIFORNIA LIBRARY