What is a Snellen Chart?

A Snellen chart is a tool for measuring visual acuity, the ability to resolve fine details at a distance. The chart consists of rows of individual black characters printed on a white background. The first row is often a single large letter, with letters becoming more numerous and successively smaller with each additional row. Acuity is determined by having a subject stand at a standard distance from the chart and read out letters until they are unable to accurately identify the letters on a given row.

The familiarity of the Snellen chart, with its characteristic black letter "E" on the top row, is a testament to the success of Dutch ophthalmologist Hermann Snellen's 1862 invention. It is a standard fixture in the offices of most eye care professionals. The charts are inexpensive and available from most medical suppliers. Printable Snellen charts can be found online for free, but care should be taken to ensure the chart is printed at the correct size.

Corrective lenses such as glasses or contact lenses should be removed prior to using the chart as part of a visual acuity exam. The test is administered by having the patient stand 20 feet (6 meters) from the Snellen chart. Covering one eye, they should begin at the top row and read until they are unable to continue. The eye care professional will then record the smallest row which was read accurately. A guide, often printed in small type on the chart itself, will indicate the acuity score for each row. The test is then repeated for the other eye.

Each row on the test is assigned a distance, in feet or meters, at which a person with "normal" vision should be able to read it. The results of the test are given in a fraction for each eye. The top number indicates the distance that the patient was made to stand from the chart. The bottom number is the distance assigned to the last row they were able to read. Normal acuity is thus described as 20/20 in the United States and 6/6 in countries that use the metric system. A person with roughly half the normal amount of visual acuity would have a result of 20/40. It's also possible to score better than 20/20; a person with roughly double the normal amount of acuity would score 20/10.

Rather than using an existing typeface for his chart, Snellen opted to design his own letters, called optotypes, to ensure that test results would be standardized. His optotypes are block letters in which the width of the space between the strokes is equal to the width of the strokes themselves. The large "E" commonly seen at the top of the chart was also part of Snellen's original design. Eye charts often begin with a large "E" to provide a common reference point to establish chart size, but there is no requirement that they do so.

Since some letters are harder to tell apart than others, eye charts almost always use a subset of the alphabet, usually the letters C, D, E, F, L, N, O, P, T, and Z. These are called Sloan letters after vision researcher Louise Sloan, who specified them in the 1950s. Sloan also designed a new set of optotypes which have replaced the traditional designs in many Snellen eye charts. In addition to the Sloan letters, the letters A, H, K, R, U and V are also commonly used.

The Snellen chart has spread beyond the Western world, requiring translations for readers unfamiliar with the Latin alphabet used in most European languages. Some are printed with Indian or Arabic numerals. Others use Greek, Cyrillic, Arabic, or Hebrew characters. Another approach to internationalization has been to use charts featuring the letter "E" or "C" rotated at 90° intervals. Rather than reading the letter, the patient indicates which direction it is pointing. These tests are known as Snellen E and Landolt C tests, respectively, and can also be used for patients who are unable to read.

As a result of its instant recognizability, the chart makes regular appearances in popular culture. In illustrations and dramatic sets, it can be used as visual shorthand to instantly establish a setting as the office of a doctor or eye doctor. It is also often featured as a visual device in comics and posters. One such use is to modify the chart so that the optotypes spell out a phrase that becomes increasingly difficult to read, for either dramatic or comic effect. The World Wildlife Fund has published posters featuring the silhouettes of rare animals arranged in a Snellen chart with the tagline "Save endangered animals before they disappear in front of your eyes."