Relationship between halo size and forward light scatter.

PURPOSE To determine the relationship between the size of a halo induced by a glare source and forward scatter or visual acuity (VA) in healthy eyes. METHOD Measurements were made in the right eyes of 51 healthy individuals of mean age 29.3 ± 7.5 years. Halo radius was measured using the Vision Monitor and low luminance (1 cd/m(2)) optotypes presented at a distance of 2.5 m. The visual angle subtended by the radius of the halo was calculated in minutes of arc (arc min). Forward scatter or, straylight, was measured using the compensation comparison technique. Best-corrected distance VA was measured using high contrast (HC) (96%) and low contrast (LC) (10%) Bailey-Lovie logMAR letter charts under photopic (85 cd/m(2)) and mesopic (0.15 cd/m(2)) luminance conditions. RESULTS Mean halo radius was 202 ± 43 arc min (3.4 ± 0.7°) and mean retinal straylight was 0.95 ± 0.12 log units. Mean photopic distance HC-VA and LC-VA were -0.02 ± 0.06 and 0.12 ± 0.09 logMAR, respectively. Mean mesopic distance HC-VA and LC-VA were 0.35 ± 0.11 and 0.74 ± 0.11 logMAR, respectively. Forward stepwise regression analysis revealed that halo radius was significantly correlated with straylight (r=0.45) and mesopic LC-VA (r=0.48), but not with photopic HC-VA and/or LC-VA and mesopic HC-VA. CONCLUSIONS In healthy eyes, the larger the halo size induced by a given glare source, the greater the forward-scatter (straylight) and worse the mesopic LC-VA. Halo size seems to be independent of photopic HC-VA or LC-VA and mesopic HC-VA.