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Abstract
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A study was conducted to develop a device which can be used to measure directly the veiling brightness produced in the eye of an observer. To design such a device, it is necessary to know what effect a glare source at a given angle from the primary line of sight will have and how the effects of two or more glare sources add.
METHODS are reviewed of measuring the effect of a glare source at a given angle from the line of sight. The facts relating to headlights in several different ways: (1) diffuse transmission through the iris and sclera, (2) flare, produced by multiple reflections at the different refracting surfaces, (3) specular reflection from the front surface of the retina, (4) halation produced by reflection at the pigment epithelium, choroid and sclera, (5) light reflected through the vitreous from one part of the retina to another, (6) fluorescence of the crystalline lens, (7) bioluminescence, and (8) scatter by the media of the eye. An equation is developed to calculate the veiling brightness produced by a glare source in a given situation. Evidence is presented to show that the effect of a peripheral glare source on foveal vision is mediated by stray light. It is difficult to measure the veiling brightness for small angular displacements of the flare source from the line of sight, but the empirical data can be extrapolated to zero angles of glare by the use of stray light theory. It is concluded that it is possible to design a device which will measure directly the veiling brightness produced by the headlights of an approaching automobile. |