The problem of light tolerance has been studied from numerous points of view but most of the studies have not been fruitful so far as producing usable constants are concerned. Van Lear (1940) has touched on some vital related points in his study of the optical properties of reflector signs. It is shown that a very small object, such as a reflector button % inch in diameter, will be visible for a distance of from 1000-2000 feet. At such distance the wave front from a headlight cannot exceed .05 f. c. with the best luminant now in use on automobiles and is more likely to be of the magnitude of .01 f. c. or less. The return beam has been calculated by Lear (1940) and found not to exceed .0001 foot candle at 100 feet, which would be equivalent of 1/1,000,000th c. p. at a distance of 1,000 feet. Although the glare effect is small at this distance it indicates in a degree, the enormous sensitivity of the retina. A % inch button cannot be perceived as such, at this distance but it stands out strikingly as a reflector of such small amounts of light. According to the Snellen standard of a 1-minute angle, it would need be at least 6 inches in diameter or 10 times larger to be discriminated in daylight. Lebensohn (1937) has given interesting data on the amount of light needed for vision while Roper and Scott (1939) have done extensive work on seeing under headlight illumination.
Proceedings of the Iowa Academy of Science
©1945 Iowa Academy of Science, Inc.
Lauer, A. R. and Silver, Edwin H.
"Certain Factors Influencing the Tolerance of Lights and Visual Acuity,"
Proceedings of the Iowa Academy of Science, 52(1), 265-270.
Available at: https://scholarworks.uni.edu/pias/vol52/iss1/36