Abstract

Visual acuity measurement is a critical tool in the clinical assessment of visual function and in the identification of changes affecting the visual system. Traditionally, visual acuity is measured with letter charts with black letters presented in high contrast against a white background. While authoritative bodies^l.2 have made various recommendations about test chart design and calibration of optotypes, there is not a universally accepted standard. It has become common in clinical research to use charts which follow the design principles of Bailey and Lovie³ which ensure that the task is essentially the same at all size levels so that size remains the only significant variable. This requires that the chart has rows of optotypes descending in size in constant ratio steps, there should be the same number of optotypes at each size level, the spacing between adjacent optotypes and successive rows should be proportional to optotype size, and optotypes should have approximately equal legibility and they should be combined so that each row forms a set with equal difficulty. Spacing ratios can have a significant effect on the visual acuity scores⁴ of normally sighted subjects and in patients with amblyopia or with macular disorders, visual acuity performance can become substantially reduced when the task becomes more crowded and congested.⁵

© 1993 Optical Society of America