Letter legibility and the construction of a new visual acuity chart.

The ability to identify letters of different size remains, in relation to the allowed effort, the most informative measure of visual capacity. Numerous letter charts have been produced with varying design, size, and size progression of the letters. Attempts to 'standardize' letter testing have been fruitless. In Sweden, the most commonly used letter charts show deficiencies in letter selection, size, and progression. In the present study, letter legibility was studied. 10 letters of average legibility were selected and a new chart constructed using these letters in sizes corresponding to acuities from 0.1 to 2.0. A geometric size progression was found optimal (factor 1.26).