Glare.-1. 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 portion of the field of view, resulting in reduced vision, fatigue or discomfort 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 defective 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 magnitude 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 illuminations than 6 foot candles this limit of contrast should be less than 100 to 1 while for lower limits it may be more than 100 to 1. Brightness for bowls and globes for locations where they are continuously 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 safely conform to the 100 to 1 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 systems of lighting and are likely to be especially pronounced with artificial 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 positions 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 illumination 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 annoyance 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 except 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 changed and the illumination must be sufficiently good at any point in any position to permit of efficient work.

The ratio of illumination in the shadow to illumination just outside of the shadow with large sources or indirect light may be as high as one to two without causing annoyance provided the illumination in the shadow is sufficient for the purpose in hand. Because of the nature of these shadows with indirect lighting the ordinary 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 magnitude 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 system as with a direct, provided the spacing of the outlets is the same in both cases. The direct lighting shadows have sharp edges, however, 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 purposes in terms of the illumination incident upon the work. This incident 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. Nutting from work by König and himself shows the sensibility of the eye

TABLE VIII.-EYE SENSIBILITY AT DIFFERENT MAGNITUDES OF SURROUNDING

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.

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 footcandles on a certain plane where the work is being done. The question is fundamentally one of producing a sufficient contrast of brightness 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 considered 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 purposes. 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 illumination is required although it cannot be said that with veiling glare present it is ever possible to produce as satisfactory and comfortable illumination, no matter what the intensity, as can be obtained with veiling glare practically absent.

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 b.

1.25-2.5 Note c.

3.5 -10 Note c.

0.25-1

3-7

5-10 Note d.

1-3

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.

(b) 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 æsthetic 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.

ÆSTHETIC EFFECTS

It is not the function of this portion of the lecture to give a dissertation on art but rather to call attention to methods by which certain desirable effects can be produced and undesirable ones avoided.