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Snellen chart, chart used to measure visual acuity by determining the level of visual detail that a person can discriminate.
It was developed by the Dutch ophthalmologist Herman Snellen in 1862 and was adopted by medical professionals in many countries who have used it for more than 100 years.
The Snellen chart is a familiar sight in physician and optometrist offices. It consists of 11 lines of block letters, also known as “optotypes,” which are constructed according to strict geometric rules and whose size decreases on each lower line of the chart. In the traditional chart, the first line traditionally consists of the single letter E, and only nine letters are used: C, D, E, F, L, O, P, T, and Z. From a distance of 20 feet (6 metres), subjects read each line of the chart, using only one eye, until they can no longer decipher the letter shapes. Each row of letters is assigned a ratio which indicates the visual acuity required to read it, and the ratio for the lowest line a person can read represents the individual’s visual acuity for that eye.
