Retinal Illuminance and the Dissociation of Letter and Grating Acuity in Age‐Related Macular Degeneration

Purpose. To report on the differential effect of retinal illuminance on letter and grating acuity. Methods. For 13 subjects with age‐related macular degeneration (ARMD) and four subjects with normal vision, standard distance and near letter acuity, as well as grating acuity data were obtained at mesopic to high‐photopic light levels. Results. In general, both acuity forms improved with increasing light level, but not in proportion with one another. The ratio of letter/grating acuity separated the ARMD subjects into two subgroups, one with relatively low letter acuity scores for which the dissociation of the two acuity forms increased with retinal illuminance, and another with higher letter acuity values for which the ratio converged toward unity. Conclusions. These findings demonstrate that both letter and grating acuity increase with retinal illuminance and that equating grating acuity with optotype acuity is untenable in subjects with ARMD, irrespective of light levels. The latter conclusion is of importance whenever acuity is used as a criterion to classify the visually impaired with regard to their legal and social status.