Correlation of optotypes with the Landolt ring--a fresh look at the comparability of optotypes

The legibility of three selected sets of optotypes: (1) letters; (2) Snellen E; and (3) KOLT test have been compared to the Landolt ring in a study based on the German Standard DIN 58220, Part 2 and the International Standard ISO 8597. The letters were printed in the standard typeface used on German street signs (sans serif Linear-Antiqua, Typeface B). The main results are: 1. A set of letters can be selected in such a way that the differences in legibility displayed between different letters in the set are smaller than the ISO/DIN allowance of 0.05 logarithmic units of the visual angle, although the letters are not constructed according to the traditional 5 by 5 or 5 by 4 construction principles. 2. The eight letters C, D, E, K, N, P, U, Z are proposed for standardized visual acuity tests because of their almost equal legibility. 3. The height of the typeface used for this selected set of letters should be 5% less than the diameter of the Landolt ring in order to achieve the same legibility as the latter. 4. Both shape optotypes (Snellen E and KOLT test) must be approximately 15% smaller than the diameter of the Landolt ring in order to obtain comparable visual acuity scores.