CORRELATION OF OPTOTYPES WITH THE LANDOLT RING - A FRESH LOOK AT THE CO MPARABILITY OF OPTOTYPES

The legibility of three selected sets of optotypes: (1) letters; (2) S nellen 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 Interna tional Standard ISO 8597. The letters were printed in the standard typ eface 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 loga rithmic units of the visual angle, although the letters are not constr ucted according to the traditional 5 by 5 or 5 by 4 construction princ iples. 2. The eight letters C, D, E, K, N, P, U, Z are proposed for st andardized visual acuity tests because of their almost equal legibilit y. 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 a chieve the same legibility as the latter. 4. Both shape optotypes (Sne llen E and KOLT test) must be approximately 15% smaller than the diame ter of the Landolt ring in order to obtain comparable visual acuity sc ores.