A comparison of the effect of reduced illumination and tinted lenses on stereospsis at near

Abstract

Relative depth may be appreciated with the use of one eye using linear perspective, shadows, parallax and texture as monocular cues to depth. Stereopsis, on the other hand, is the directappreciation of relative depth that requires the use of both eyes to construct a three-dimension-al percept from disparate two-dimensional retinal images. The advantage of stereopsis is with respect to complex visual tasks especially that requiring accurate hand-eye coordination. Tinted lenses are prescribed for a variety of reasons, including but not limited to photophobia, asthenopia, improving colour perception in colour deficient individuals, enhancing cosmesis and protection against glare or harmful radiation and enhancing visual performance as in sports. The aim of this study was to investigate the comparative effects of six specific CR39 tinted spectacle lenses (grade B), and a white CR39 lens, against a no lens condition, on stereoacuity over a range of illumination levels. Illumination was varied with the use of neutral density (ND) filters, while the Titmus Fly Stereotest (TFS) wasused to measure stereoacuity. Participants (n =60) between the ages of 17 - 29 years (mean =23.58; sd = 3.14) were purposively sampled from a clinical practice to participate in this research study.  Using repeated measures ANOVA and appropriate post-hoc multivariate analysis, it was evident that there was a significant decline in stereopsis as the level ofillumination decreased,regardless of tint condition; also there was no statistically significant difference in stereopsis between the no lens and white lens conditions at each level of illumination; and stereopsis wassignificantly superior with the no lens conditioncompared to all six other tint conditions (gradeB), at each level of illumination. These results indicate that stereoacuity, as measured by the TFS, is adversely affected by adecline in retinal illuminance and by the use of tinted lenses. This information could be utilised to advise patients on the performance implications of the six tinted lenses tested with respectto their effects on stereoacuity under different illumination levels.