Data on artificial lighting; supplementary to section II of "Scientific offices management"

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Ftb 15 ISit

DATA ON ARTIFICIAL LIGHTING

Supplementary to Section II of ff Scientific
Office Management" by W. H. Leffingwell

A. W. SHAW COMPANY

CHICAGO NEW YORK
LONDON

COPYRIGHT, 1917, BY
A. W. SHAW COMPANY

PBINTED IN U. B. A.

DATA ON ARTIFICIAL LIGHTING

Where gas light is used incandescent inverted mantles wil
usually give the best results. In electric lighting rapid prog-
ress within recent years has made possible a wide range of
choice in the kind of lighting, and there is naturally more or less
dispute as to which light is really the most efficient.

In considering the efficacy of lighting methods it is important
to know in just what respect one method is more efficient than
another. It is possible that while one method may consume less
current it will give light of much poorer quality or with a
harmful effect upon the eyes of the workers. It is therefore
necessary for us to know which is more important, economy in
the consumption of current or economy in labor.

There are perhaps three methods of lighting in common use.
In the direct lighting method the source of light is visible and
the light rays shine directly upon the work. In the semidirect
or semiindirect lighting systems the source of light is visible,
but only a small portion of the light rays shines directly upon
the work, and these rays are still further modified by translucent
globes. In the indirect lighting systems the source of light is
invisible, all the rays being thrown against the ceiling or some
other light colored surface and reflected down upon the work.

Direct lighting is the oldest and most common method, and
so far as mere consumption of current is concerned, much the
cheapest of the three. Nowadays, however, the actual economy
of this method is being sharply questioned. One of the most
frequent objections to direct lighting is the glare which is caused
by the exposed light. One authority states :

What constitutes scientific and practical illumination ?
There are so many varieties of industries and the conditions
of production are so diverse that it is at once apparent that

a)

no one specification can fit them all. Illumination entirely
satis i'&(fto"ry under on^. set of conditions might be wholly
inadequate, not to say impossible, in another. There are,
however, some general principles involved which apply to
practically all situations.

There is a physiology as well as a psychology of indus-
trial lighting. The psychological states — the mental alert-
ness and the attitude toward the work — are mostly reflexes
of the physiological. It is worth while to consider how
these come about, and how those that tend towards ineffi-
ciency may be eliminated.

The eye is a most delicate mechanism, just as delicate
in the roughest laborer as in the trained artist. It is a
living photographic camera. It has every essential feature
of the camera and some besides. The iris, for example,
which is a diaphragm regulating the amount of light ad-
mitted to the eye, is wholly automatic, adjusts itself to
meet varying conditions of illumination; the lens which
focuses the light upon the retina (the sensitized plate in
the camera) is also automatic, and adjusts itself for dis-
tance. The retina, under stimulus of light rays, becomes
fatigued ; the lens and iris are controlled by minute muscles
which also tire under constant use. Abuse of these deli-
cate mechanisms leads to impotency, just as overuse of any
other organ does — a set of muscles, say, which, when tired,
cannot be made to work so strongly, as continuously, or
as accurately as when normal.

Everybody is aware of the effect of gazing, even for
a short time, at any bright light. The eye becomes dazzled,
irritated ; if continued there is even pain. The retina
becomes fatigued by overstimulation. Also the muscles
of the iris, and lens, in their effort to adjust themselves,
are tired. The effect of this does not wear off immedi-
ately. The blinding effect persists to some degree for a
long time, often a half hour or more, according to the
length of exposure and the intrinsic brightness of the light.

The tense feeling, the strain, and often the tingling
and itching of the eye following continued close applica-
tion, as in reading fine print, or closely observing fine
work, is also well known. It is in part, at least, the direct
and indirect result of a fatigue of the ciliary muscles
controlling the shape of the crystalline lens. When look-
ing at objects near at hand the muscles tighten and make
the lens more convex. For distances of fifteen feet and

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beyond, in normal eyes the image focuses on the retina
when the ciliaries are relaxed. If the eye is allowed to
relax occasionally by a look at relatively distant objects,
fatigue is to a considerable extent avoided. Likewise the
iris, the disklike curtain which controls the size of tile
pupil and therefore the amount of light admitted, if kept
tense by an exposure to high light, or if subjected to fluc-
tuating intensities, also tires and the sense of discomfort
is increased. Moreover it is a peculiarity of the sense of
light that movement or change of any sort taking place
outside the limited field of direct vision, but still within the
range of perception, is percieved more quickly even than
when in direct range; and the effort of the eye to adjust
itself to these environmental conditions when the light is
unsteady or the surroundings dark in comparison with the
field upon which attention is concentrated, is not only
annoying and fatiguing, but becomes intensely distracting
to anyone compelled to face it.

Here then we have a combination of paraial visual
paralysis, optic fatigue, general discomfort and distraction
from the work in hand, with all the physical and mental ills
and the inefficiencies following in their train. Evidently
the physiological effects of lighting are very practical fac-
tors in illumination, quite as practical as the immediate
dollars-and-cents considerations which are more easily
sensed.

It must be apparent that scientific and therefore eco-
nomic lighting not only provides a sufficiency of light flux,
steady and so distributed as to avoid obscurity and deep
shadows anywhere within the ordinary range of the
workers' eyes, but takes care that the source of illumination,
if considerably concentrated as in the case in most lighting
units now in use, shall be concealed, enclosed in a diffusing
envelope, or placed outside the range of vision .unless at
such a distance that the brilliancy is greatly reduced before
the rays reach the eye. Otherwise the main purpose of
lighting is defeated, and the workers themselves are physi-
cally harmed and are rendered less efficient for their work.

Common practice is to have individual desk lamps. These
throw a 'very strong light upon the work and nowhere else.
In spite of the fact that most clerks working with a lamp
of this sort wear dark green eye shades, the strain upon the eyes
is very great. In still other places there are individual lamps

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as well as overhead illumination. This arrangement gives bet-
ter light than the individual lamps but it is not economical.
If direct lighting be used it is much better to have uniform
illumination at least eight or ten feet above the floor, using as
many lamps as will give each desk enough light for comfortable
work. The light coming from so many directions reduces the
glare more or less and heavy shadows are not so noticeable. It
is helpful if the light units be placed so that the upright posts
do not shut off she light. The diagram on this page shows
Mr. Becker's plan for the proper arrangement of direct lights.

Extent of Effective Reach of

HOW TO SPACE LAMPS

The diagram shown here gives a good idea of one of the ways that light may be
evenly distributed. Many offices have an uneven distribution of the light and
in such cases desk lamps are an expensive necessity. With the spacing indi-
cated in the diagram, satisfactory results are usually obtained

If you use the 60 or 100 watt tungsten lights you will probably
get the best results simply because the light source is not brilliant
enough to cause excessive glare. Ordinarily the unprotected
nitrogen or gas-filled lamp of high intensity, although the eco-
nomy in current consumption is more apparent, should never
be used for the direct lighting of offices, since the glare from
this light is often injurious to the eyes.

The term semidirect, or semiindirect lighting, is applied to
the method of casting light on the walls or ceiling by means of
translucent reflectors, which is the only difference between

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indirect and semiindirect system of lighting. The current
consumption for an equal amount of semiindirect light is said
to be from forty to fifty per cent more than for a direct light-
ing system properly designed, while the current consumption
for a purely indirect system is stated to be from fifty to seventy-
five per cent more than the direct. This difference probably
explains the frequent compromises resulting in the semiindirect
systems. Undoubtedly the semiindirect system of lighting is
less tiresome to the eyes than the direct system, but some office
managers question whether there is any real saving over the
indirect systems.

With indirect lighting the eyes are protected from the bril-
liancy of the lamps, since the light reflected from the ceiling
is largely diffused before reaching the working plane. Indirect
lighting is perhaps the only practical means of hiding the lamp
without cutting down its light giving qualities. Where this
system is used the ceiling or reflecting surface must of course
be white. When offices are located in factories or buildings of
mill construction this is often impossible, but usually even there
it is possible to get a fairly good reflecting surface by painting
the ceiling properly.

From the standpoint of eye efficiency, indirect lighting is
about the nearest approach to sunlight that can be devised. It
is true that the consumption of current by this method is from
forty to seventy per cent greater, but some managers who are
using this system believe that the increased efficiency of the
workers p&ys for the difference in cost many times over.

Dr. C. E. Ferree has arrived at some very interesting con-
clusions. Some of them have been disputed by electrical illum-
inating engineers, but his experiments were very thorough and
the results should cause a widespread investigation.

Dr. Ferree started out to find whether or not the eye shows
a loss of efficiency after three or four hours' work under
a given lighting system. He prepared a series of test cards
from which the factor of memory was eliminated, containing
a number of conventional letters and lines at which the subject
was required to look intently and record on a rotating drum the
time during which he saw them clearly and the time he saw them
blurred. This test was continued during periods of three and

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four hours and a ratio established showing the proportion of
clear vision time to blurred vision time. Those who wish to
go further into the detail of Dr. Ferree's experiments will find
his paper unusually clear and interesting.* Dr. Ferree states:

The intensity recommended by the Illuminating Engi-
neering Society in its primer issued in 1912 ranges from
two or three to seven or ten foot candles, depending upon
the kind of work. Five foot candles are taken as a medium
value. This medium value is approximately three times
the amount we have found to give the least loss of efficiency
for the type and installation of semi direct lighting we
have used. The intensity we have found to give the least
loss of efficiency for this type of lighting does not give
maximum acuity of vision as determined by the momentary
judgment. At an intensity that does give maximum
acuity of vision as determined by the momentary judg-
ment the eye runs down rapidly in efficiency. That is, in this
type of lighting one or the other of these features must be
sacrificed. High acuity and little loss of efficiency cannot be
had at the same intensity. This can be had only under
daylight or under the indirect systems of artificial lighting.
However, the amount of light we find to give the least loss
of efficiency seems to be sufficient for much of the work
ordinarily done in the office or home.

The semiindirect systems of lighting are intended to
represent a compromise between the direct and indirect
systems. A part of the light is transmitted directly to the
eye through the translucent reflectors placed beneath, and
a part is reflected to the ceiling. By giving better distri-
bution, this system is also supposed to be a concession to
the welfare of the eye, but our tests show that the conces-
sion is not so great as it was supposed to be. In fact, in-
stalled at the intensity ordinarily used, or at an intensity
great enough for all kinds of work, it is little better for
the eye than the direct system. At these intensities the
bright sources of light, which seem to be the fundamental
cause of damage, have not been eliminated, nor even re-
duced, so as to give much relief to the suffering eye. Until
this is done in home, office and public lighting we cannot
hope to get rid of eye strain with its complex train of
physical and mental disturbances.

*"The Efficiency of the Eye under Different Systems of Illumination," by C. E.
Ferree, a paper read at the seventh annual convention of the Illuminating Engineering
Society, Pittsburgh, Pa., Sept. 22-26, 1913.

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THE 1ABT
^

OVERDUE.

APR 3 1933

VC '12914

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