Optonet Vision Unit Documentation

Please note that this User Guide is being currently updated; new sections will be added shortly.

3.5.1.Sloan letters #

This is the recommended optotype for VA measurement and clinical refraction. It consists of 10 letters designed in 1959 by Louise Sloan: (C, D, H, K, N, O, R, S, V and Z).

The Sloan letters have the same height to width ratio, similar to the structure of the Snellen letters, drawn on 5×5 matrix.

The Sloan letters are used in the ETDRS charts, which also employ the logMAR design proposed by Bailey-Lovie.1,6 The ETDRS charts are so named because they were used in the “Early Treatment of Diabetic Retinopathy Study)6 funded by the National Eye Institute in the United states. The ETDRS employed the logMAR design to measure VA in the most reliable, standardized and reproducible way. Today, the ETDRS chart is preferred to the traditional Snellen chart (in use since 1862) whenever precise measurement of visual acuity is necessary in research or clinical settings.

The Sloan letters are equally discriminable, similar to Landolt “Cs”. Each row of the ETDRS chart uses a combination of letters with a global difficulty similar to the other rows. In total, 28 combinations of letters of equal difficulty are used.6 This distribution of letters was based on a study carried out by the designers of the optotypes using data from 234 eyes. This study gave rise to the original ETDRS optotypes that are currently used internationally.6

Subsequently, the authors themselves reviewed the difficulty of the Sloan letters and used the results from 7,420 eyes to slightly adjust the combinations of Sloan letters in each row, resulting in “revised” ETDRS letters 7. In theory, these new combinations are slightly more balanced in difficulty. The vision unit allows you to choose between “original” or “revised” Sloan letters (using the configuration menu).




This Sloan letters were selected by the International Council of Ophthalmology8 as the reference optotype for VA measurement, and have since been used as a standard in research studies.

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