Validity and reliability of viscual acuity measurements

New design principles have been applied to the construction of an illiterate E chart. A chart can thus be produced in which the only variable in the visual acuity task for letters of different size is the letter size itself. A simple method for using this chart in field studies is described.

The effect of pupil size on the relation between Snellen visual acuity and corrected and uncorrected myopia was examined for 22 young subjects with degrees of myopia ranging from 0.75 D to 7.5 D. Effective pupil size was varied by inducing mydriasis and then placing artificial pupils of between 1.0 and 8.0 mm diameter before the eye. Both a constant chart luminance of 120 cd/m2 and a constant retinal illuminance of 2150 trolands were used. There was little difference in results for the two lighting conditions. For the corrected myopes considered as a group, maximum visual acuity occurred for 2--3 mm diameter pupils, but larger pupils reduced acuity only marginally. For the uncorrected myopes, variation in pupil size produced a large variation in visual acuity, and for refractive errors greater than about 1.5 D, the optimum pupil diameter was less than 1 mm. For uncorrected myopes of 3.0 D or less, visual acuity was nearly as good with a 1-mm pupil as for corrected myopes. The presented data are a useful guide to the clinician.