The effect of a glaring light source on extrafoveal vision

Abstract

In foveal vision, the effect of a glare source on the brightness difference threshold can be reproduced by superposing a uniform brightness [gamma] = kE/[theta]n on the field brightness B. [beta], the equivalent uniform field brightness, is given by the formula [beta] = B + kE/[theta]n, where E is the illumination on a plane containing the eye pupil, [theta] the angle of the glare source with the test object, k and n constants, for which the best values are 10 and 2 respectively. For extrafoveal vision, the formula above does not hold, since the illumination at the eye, E x, with the larger glare angles used is not identical with E. Exps., made on 2 subjects with a modification of the authors'' "threshold apparatus 1," were designed to test the validity of the formula [beta] = B + kE N/[theta]n for extrafoveal vision. In the dark-adapted eye, 1. b. i., the liminal brightness increment (the minimum difference in brightness between the test object and the field for them to be distinguishable) was at a maximum for the fovea. With the same E n, varying the angular diameter of the glare source from 1.92 to 0.34[degree], and positions of the test object from [PHI]T 5-50[degree] and [PSI]T 0-270[degree], produced no significant differences in 1. b. i. At constant glare angle, with variations in the orientation of the glare source to the test spot, no systematic variations in 1. b. i. occurred, even when the glare fell on the blind spot or the fovea centralis. But for certain positions of the test spot, varying in different eyes, the effect at a given glare angle may depend on the orientation of the glare source in respect to the test spot. When [theta] was altered by moving a glare source of constant intensity in a fixed plane, log.10 [theta] bore a linear relation to log.10 ([beta]/E N), giving a value for the constant n of 1.97-2.04. To test the proportionality between [beta] and E n, 1. b. i. was measured in the absence of glare for a series of uniform field brightnesses, and for different intensities [phi]E n of a glare source in fixed position. The resultant curves (log.10 l. b. i. against log.10 [beta] or log.10 E n) were parallel; k was found to have a representative value of 16. The fact that the effect of an imaged glare source on the sensitivity of other parts of the retina can be represented by the relatively simple formula, [beta]=B + kE n/[theta]n. for the equivalent uniform brightness suggests a simplicity in the mechanism of the glare effect. Qualitatively the main results can be explained by considering that the equivalent field brightness corresponds to illumination of the retina through a scattering of rays from the glare source in some part of their path in the eye. The equation must be modified in considering glare sources approaching the limit of the field of vision, where the effect is determined not by E x, but by E n R [phi]g, where R is the ratio of the apparent pupil area viewed from the glare source to the normal pupil area.