[Correlation of letter optotypes with Landholt ring for different degrees of visual acuity]

According to the German visual acuity standard DIN 58220 part 2 we compared in 1990 a set of 10 capital letters (C, D, E, F, K, N, P, U, V, Z; type DIN 1451; Serifless Linear-Antiqua) with the Landolt Ring regarding their legibility. The measurements at that time had been accomplished with young adults whose visual acuity was around 2.0. The results showed that the letters at the same height as the Landolt-Ring are easier to see. The difference of legibility was 4.7%. So the set of letters in comparison to the Landolt Ring was equivalent within the required tolerance of +/- 0.05 log units of visual acuity as defined in the standard. For visual acuity measurements which need only eight instead of ten different letters, the set C, D, E, K, N, P, U, Z has been suggested.

With this set the experiments were repeated on 48 subjects. Nine of these had a reduced visual acuity from 0.32 to 0.6. Twenty subjects had amblyopia with a visual acuity of 0.02 to 0.2. The last nineteen subjects had a normal visual acuity, but 10 persons hereof had been artificially made ametropic by a plus-overcorrection and 9 by holding a frosted glass disk in front of their eyes. These subjects then had a visual acuity of about 0.1.

The results of the measurements showed that in this visual acuity range the letters are easier to see than a Landolt Ring of the same size. The difference in legibility of the letters in comparison to the Landolt Ring was up to 0.3 log units of the viewing angle.

To achieve an equivalence with the Landolt Ring even in this visual acuity range, the height of the letters have to be reduced at a higher percentage.