Pupil size and retinal straylight in the normal eye

PURPOSE. Glare problems originating from bright lights are generally experienced more strongly at night. The typical disability glare is known to result from retinal straylight. In this study, the effects of pupil diameter and, especially in the case of small pupils, of eye wall translucency on the amount of retinal straylight were investigated. METHODS. Straylight was measured as a function of pupil diameter ranging from 1.3 to > 8 mm, in five normal subjects by using a white-light, CRT-based system for scattering angles of 3.5 degrees, 7 degrees, and 14 degrees. In the study of red-free light, a yellow-LED based system was used with the same five subjects for scattering angles of 3.5 degrees, 10 degrees, and 28 degrees. Data were analyzed to assess effects of (1) inhomogencity of light-scattering over the pupil plane, (2) translucency of the eye wall, and (3) effects of the periphery of the lens. To estimate the order of magnitude of pupil contraction in the typical glare situation, pupil reflexes resulting from the sudden appearance of headlight-equivalent bright lights were recorded in three subjects in a laboratory environment. RESULTS. For natural pupils (between 2 and 7 mm diameter), straylight weakly depends on pupil diameter (within 0.2 log units). For large scatter angles and small pupil diameters, eye wall translucency contributes significantly to straylight in a wavelength- and pigmentation-dependent manner. Pupil diameters decreased to photopic values under typical night-driving glare conditions. CONCLUSIONS. In normal eyes, straylight values measured with photopic pupils are by approximation also valid for mesopic and scotopic pupils, such as in night driving. Measurement of straylight under large angle and small pupil conditions can be used for quantitative assessment of eye wall translucency.