The Snellen chart is a tool for measuring visual acuity, invented by Dutch ophthalmologist Hermann Snellen in 1862. It consists of lines of black characters on a white background, and is used to determine a person’s ability to read letters at a distance. The chart is a standard device in eye care offices and is administered by having the patient stand 20 feet away and read the letters until they can no longer accurately identify them. The test results are reported in a fraction, with normal acuity being described as 20/20 in the US and 6/6 in countries using the metric system. The chart is also used in popular culture, such as in comics and posters.
A Snellen chart is a tool for measuring visual acuity, the ability to resolve fine detail at a distance. The graphic consists of lines of single black characters printed on a white background. The first line is often a single large letter, with letters becoming more numerous and successively smaller with each additional line. Acuity is determined by a subject standing a standard distance from the graph and reading the letters until they can accurately identify the letters on a given line.
The familiarity of the Snellen chart, with its distinctive black letter “E” on the top line, is a testament to the success of Dutch ophthalmologist Hermann Snellen’s 1862 invention. It is a standard device in the offices of most eye care professionals. Charts are inexpensive and available from most medical providers. Printable Snellen charts can be found online for free, but care must be taken to ensure that the chart prints at the correct size.
Corrective lenses such as eyeglasses or contact lenses should be removed before 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 start at the top row and read until they are able to continue. The ophthalmologist will then record the smallest line that was accurately read. A guide, often printed in small print on the chart itself, will indicate the acuity score for each line. The test is then repeated for the other eye.
Each line of the test is given a distance, in feet or meters, at which a person with “normal” vision should be able to read it. The test results are reported in a fraction for each eye. The top number indicates the distance the patient was made to stand from the graph. The bottom number is the distance assigned to the last line they were able to read. Normal acuity is therefore described as 20/20 in the United States and 6/6 in countries using the metric system. A person with about half normal visual acuity would have a result of 20/40. It is also possible to score better than 20/20; a person with about twice normal acuity would score 20/10.
Rather than use an existing typeface for his graph, Snellen chose to design his own letters, called optotypes, to ensure test results were standardized. The charts of him are capital 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 graph was also part of Snellen’s original design. Eye charts often start with a large “E” to provide a common reference point for establishing chart size, but they don’t have to.
Because some letters are more difficult to distinguish than others, eye maps 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 charts that replaced 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.
Snellen’s paper 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 graphs with the letter “E” or “C” rotated at 90° intervals. Rather than reading the letter, the patient indicates which direction he is pointing. These tests are known as the 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 graph makes regular appearances in popular culture. In illustrations and dramatic sets, it can be used as a visual shorthand to instantly establish an environment such as a doctor’s or eye doctor’s office. It is also frequently featured as a visual device in comics and posters. One such use is to modify the graph so that the optotypes fill in a sentence that becomes increasingly difficult to read, for dramatic or comedic effect. The World Wildlife Fund has released posters featuring the silhouettes of rare animals arranged in a Snellen chart with the tagline “Save endangered animals before they disappear before your eyes.”
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