Sources of Entoptic Scatter in the Human Eye*

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Abstract
The percentage of intraocular scattered light (in the direction of the fovea), which is attributable to the cornea, has been estimated for two human subjects to be about 25%. The method is purely psychophysical: a glare beam is employed which is very small, yet sufficiently intense to produce measurable glare effects, evaluated in terms of the rise in threshold of a foveally presented test flash. Scattered light from the glare stimulus is evaluated by the equivalent veiling technique, by determining the luminance of a veiling field which produces the same rise in threshold as that caused by the glare stimulus. For conditions in which the pupil is small and the glare stimulus enters the eye near the edge of the cornea, the scattered light from the cornea can illuminate only the peripheral retina and produces no glare effect, while the image-forming rays of the glare beam pass unobstructed through the pupil, allowing the glare effect attributable to scatter in the lens and vitreous to occur. A converse condition is possible where the scattered light from the cornea illuminates the fovea, producing a glare effect, but where the image-forming rays are absorbed by the iris. Other conditions allow both components of scatter to be effective at the same time. The disagreement of the result with that of previous physical measurements in the steer eye is discussed.
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