Abstract
Abstract: :
Purpose: The eye’s optical point spread function can be assessed using wavefront aberration, interferometric contrast sensitivity or double pass imaging, but the useful range of these techniques is limited to a few minutes of arc. Scatter over large visual angles can be assessed using increment thresholds (Holladay and Stiles). Light spread over intermediate ranges of distance is important (for instance as a source of complaints after surgery), but good techniques for assessing it are lacking. Here we investigate the utility of glare threshold for measuring light spread in the range from a few minutes of arc to 1 degree. Methods: Using the BIG MAX high-intensity data projector we describe elsewhere, a tiny test flash was presented in the center of a black hole in a synchronously flashed surround. Surround intensity and hole radius were varied randomly to find the intensities needed for a criterion elevation of the test flash threshold. Under conditions where threshold is set by light at the test flash location, the needed surround intensity varies inversely with the fraction of surround light scattered onto the test. Results: A subject with normal acuity, but post-operative complaints of glare, had a ten-fold increase in light spread compared to normal subjects. For normal eyes, the inferred point spread function fell more steeply than inverse square, and more steeply than double-pass physical estimates of light spread suggest. This is plausible if double-pass measures are contaminated by light spread behind the photoreceptors. Conclusions: Glare threshold measurements may be the best indicator of light spread in the range from a few minutes of arc to 1 degree. They may be useful for quantifying previously difficult-to-evaluate complaints of glare.
Keywords: physiological optics • optical properties • visual acuity