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 confined 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 conditions. 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.

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 killing 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.

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 definite knowledge. Points which may be considered psychological by some are taken up later under the head of æsthetic effects.

The eye is concerned chiefly with two things (a) sufficient brightness 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:

1. Sufficient quantity of steady diffusely reflected light from the object viewed.

2. Minimum flux of light emitted in the direction of the eye by specular or spread reflection from the objects viewed.

3. Absence of violent brightness contrasts within the field of vision.

4. Freedom from sharp shadows.

Glare Defined.-The 1915 Committee on Glare of the Illuminating Engineering Society in its report on Interior Illumination, page 36, I. E. S. Transactions, 1916, tentatively offered the following defi

nitions which express more definitely than heretofore attempted what constitutes glare. Three alternative definitions were offered as follows: