THE COMPARATIVE VISUAL ACUITY AND EASE OF READING IN WHITE AND COLOURED LIGHT

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Abstract
A grating object, with a half-tone screen (55 lines/in.) printed on it, was used under exptl. conditions to test the visual acuity in white light, light transmitted by Chance''s Ruby No. 1 glass, and light transmitted by Board of Trade Red glass. Three illumination levels were adopted, 0.06, 0.6 and 6.0 foot-candles. The visual resolving limit was regarded as the angular subtense in the observer''s visual field of the apparent size of one grating element. There appears to be some improvement in acuity and in speed of reading at the low levels of illumination in red as compared with white light, but this advantage disappears or even becomes reversed with increasing intensity of illumination. Other expts. were conducted in daylight and the visual acuity of the naked eye was compared with that of the eye covered with various color filters. Each filter caused a deterioration in acuity and this appeared to be directly attributable to the loss of light. In general, it seems that visual acuity is dependent on local brightness discrimination which is a function of the intensity level. The small relative advantage in acuity and ease of reading associated with red light may follow the improved definition and contrast of the monochromatic image.

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