IN THE course of ordinary industrial tasks, the eyes are required to distinguish detail in fields where the intensity of illumination frequently varies greatly from place to place. Even in the simplest sort of visual work the eyes are constantly in motion, focussing and adapting themselves to new conditions.. If the requirements of the work are such that the eyes must shift from bright to less bright areas, or ~4ce versa, it is obvious that such conditions impose a handicap on quick and clear seeing.
The effectiveness of lighting is not necessarily determined by the amount of light flux falling upon or reflected by an object, but rather by the ease with which the object is seen. It has been well established, assuming a proper distribution of light for the particular work to be done, that the amount of visual work that can be accomplished per unit time varies with the intensity of illumination. These ideal conditions are often lacking in practical installations, and therefore experimental work has been undertaken to determine, quantitatively, the effect of variable intensities of illumination within the field of vision upon the rate with which visual work can be accomplished. The rate of visual work was chosen as a criterion of the effectiveness of the illumination since: it is a resultant of all the factors influencing vision, assuming that factors other than visual are eliminated or reduced to the second order of importance.
Vision on alternating light and dark fields is subject to all the influences affecting vision in general; and of these, the factors of intensity of illumination and the adaptability of the eye to changing intensities must be considered as of primary importance. These essential factors may be regarded as working in opposition to each other since intensity can be made a factor in aiding vision * Communicated by the Director of the I.aboratory. 73I