Snellen's Chart

Snellen's chart is 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 1854 and was adopted by medical professionals in many countries who have used it for over 100 years. Snellen's chart has several advantages, including sensitivity to common sources of visual impairment, but it has also come under criticism and several new types of eye chart have been proposed.

Snellen's chart is a familiar sight in physician and optometrist offices. It consists of eleven 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. The person whose vision is being tested stands at a specified distance from the chart (20 feet in the United States, 6 meters in countries that use the metric system) and reads each line, using only one eye, until he or she cannot 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 is his or her visual acuity for that eye. In the United States, normal vision is defined as 20/20; in countries that use the metric system, it is 6/6. A ratio less than 1 (for instance, 6/10) indicates worse-than-normal vision; a ratio greater than 1 (for instance, 6/5) indicates better than normal vision.

Several criticisms have been made of Snellen's chart. One is that the number of letters presented on each line differs, so the difficulty of differentiating the letters due to size is confounded with difficulties due to visual crowding caused by the nearness of other letters: it has been established that letters are more easily read when presented on their own. In addition, the spacing between rows as well as spacing between letters varies on Snellen's chart, introducing a third factor which further confounds the measurements. Another criticism is that the progression of ratios among the lines of letters is irregular and somewhat arbitrary, with particularly large gaps at the lower end of the acuity scale. Finally, repeatability of measurements taken with Snellen's chart are poor, complicating any effort to measure changes in vision over time. Alternatives to Snellen's chart have been developed by Edmund Landolt (the Landolt C), Sergei Solovin (using Cyrillic letters), Louise Sloan, Ian Bailey and Jan Lovie, Lea Hyvärinen (the Lea chart, for preschool children), and Hugh Taylor (the Tumbling E chart, for illiterates).

Repeatability of measurements taken with Snellen's chart are poor, complicating any effort to measure changes in vision over time.