The effect of glare on the brightness difference threshold

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Abstract
A method is described for determining the brightness difference threshold in the presence of a point source of glare. The % probable error for a single observation of the threshold is approximately 7%. The threshold is subject to fluctuation during the course of an extended series of readings, particularly in the case of inexperienced subjects. The value of Fechner''s Fraction in the absence of glare, for 2 subjects, has been found for field brightnesses ranging from .001 to 2.2 candles/ sq. ft. The threshold in the presence of the glare source is best expressed in terms of the equivalent background brightness [beta]. The following formula is established for [beta] and represents the results tolerably well over the range indicated: [image] 0<Bs <0.22 candles/sq. ft. 0<EG <l ft. candle 1[degree]<0<10[degree]. For background brightnesses greater than 0.22 the formula breaks down. A systematic deviation from the formula given (and from any formula which makes [beta] an increasing function of Bs under any glare condition) occurs for EG=0.40 and [theta] = 3[degree]. The threshold with these values of Eg and 6 is about 10% greater for BS=0 than for BS = .0037 candles/sq. ft. The magnitude of this effect is not sufficient to interfere seriously with the use of the formula as a good overall representation of the measurements.

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