[Contour interaction in the examination of visual acuity]

The author discusses the problem of the interoptotype distance in visual acuity charts. A small interoptotype distance (5,5' minutes) kept constant throughout the chart enhances the sensitivity of the visual acuity measurement for the detection of a functional deficit, without decreasing its specificity. A specification of the distance between the last recognized optotypes should be given with the visual acuity in order to permit an estimate of the measurement's validity. A simple formula for the measurement in minutes of the angular interoptotype distance is presented.