FUNDAMENTAL PRINCIPLES OF PROPER LIGHTING Wherever light is applied to seeing it is necessary to consider the chief factors which enhance vision. The neglect of a single one may destroy the effectiveness of an otherwise good installa tion.
Glare is light out of place and is most frequently the result of a brilliant light source in the field of view. Usually the sky with a brightness on the order of 2 candle-power per visible square inch is not uncomfortable, while the recommended limit of brightness for glass enclosing globes of lighting fixtures is on the order of 3 and not in excess of 5 candles per square inch. The filament of a bare tungsten lamp often reaches i000 c.p. per square inch. Thus all artificial light sources should be well shaded and bare lamps are only permissible when of the decora tive, diffusing or tinted types that are not regularly in the field of view. An excessive amount of light (called flux) if directed toward the eye is glaring. Light from an unaccustomed direction such as when reflected from snow or water, or originating be neath eye level, produces discomfort and often destructive glare, as of ten found through reflections from glass and polished table tops. The extreme contrast of a brilliant object against a dark background is glaring and lowers the visual acuity. Such contrasts in the field of view induce muscular strain since for darker objects the pupil tries to enlarge in order to permit more light to strike the retina, but when viewing a brilliant object the reverse takes place. The eye in protecting itself against excessive light from a bright object would automatically shut out the needed light from surrounding darker objects and hence cause these latter to be invisible. An avoidance of glare can best be achieved by shield ing all exposed sources, by avoiding wide contrasts and by placing lighting units so that the illumination will fall on the work from a customary direction and be not specularly reflected therefrom into the eyeball.
The retina possesses a property of retaining the image such that, as in the case of the motion picture, if the successive images occur faster than about 20 per second there will be no sense of discontinuity or flicker. However, any
sudden fluctuation of lighting intensity caused either by temper ature variations of the source or by combinations of moving sources and objects, should be avoided. The lower the foot-candle values, the more becomes the apparent blurring of the moving object and the faster it seems to move. In the filament lamp there is sufficient retention of heat in the wire between current reversals, in the case of alternating current supply, to smooth over the flicker effect; however on frequencies less than 20 cycles the flicker becomes noticeable. In the case of gaseous vapour lamps having no inherent thermal capacity, the light fluctuates instan taneously with the current alternations, giving rise to a succession of images of moving objects or the so-called stroboscopic effect. In general the best work-light is one which remains constant in amount and in colour quality.
Light.—In determining the correct amount of light for best vision, one must consider whether the requirements consist of merely "seeing," or whether they include seeing with least strain for protracted periods, or whether they provide for constant seeing with the least expenditure of bodily energy. To meet the first criterion or for the non-critical and easy tasks, i o foot-candles have been considered satisfactory. This amount barely suffices for occasional reading of the type on this page or of the average telephone directory. For clerical work, ordinary reading and average sewing, the accepted range is 20 to so foot-candles. For severe and prolonged tasks and in order to relieve specific strain it is considered necessary to provide 5o to ioo foot-candles,--more in the case of children and aged people. From zoo to possibly 50o foot-candles are considered pref erable in the best conservation of physical energy required for seeing and for any persons of sub-normal vision. Several manda tory state lighting codes specify absolute minimums in industry ranging from about o.5 foot-candles for storage and non-working areas, to 5 foot-candles for usual seeing jobs. Extensive tables of recommended foot-candle intensities are available and have been accepted by the foremost engineering associations and eye sight specialists.