A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862. Vision scientists now use a variation of this chart, designed by Ian Bailey and Jan Lovie (see "visual acuity").
The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, or N. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that eye.
The symbols on an acuity chart are formally known as "optotypes". In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:
- the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
- the height and width of the optotype (letter) is five times the thickness of the line.
Only the ten Sloan letters C, D, E, F, L, N, O, P, T, Z are used in the traditional Snellen chart. The perception of five out of six letters (or similar ratio) is judged to be the Snellen fraction.
Wall-mounted Snellen charts are inexpensive and are sometimes used for approximate assessment of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns.
BS 4274-1:1968(British Standards Institution) "Specification for test charts for determining distance visual acuity" was replaced by BS 4274-1:2003 "Test charts for clinical determination of distance visual acuity — Specification". It states that "the luminance of the presentation shall be uniform and not less than 120 cd/m2. Any variation across the test chart shall not exceed 20 %."
According to BS 4274-1:2003 only the letters C, D, E, F, H, K, N, P, R, U, V, and Z should be used for the testing of vision based upon equal legibility of the letters.
Snellen fraction 
"20/20" (or "6/6") vision 
Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc. Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.
In the most familiar acuity test, a Snellen chart is placed at a standard distance: 20 ft in the US, or 6 metres in the rest of the world. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 20/20 (US) or 6/6 (rest of world), is the smallest line that a person with normal acuity can read at a distance of 20 ft.
Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of 20 ft, but a person with normal acuity could be expected to read these letters at a distance of 40 ft. This line is designated by the ratio 20/40. If this is the smallest line a person can read, the person's acuity is "20/40," meaning in a very rough kind of way that this person needs to approach to a distance of 20 ft to read letters that a person with normal acuity could read at 40 ft. In an even more approximate manner, this person could be said to have "half" the normal acuity.
At 20 ft, the letters on the 20/20 line should subtend 5 minutes of arc (such that the limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.87 mm tall and the topmost "E" should be 88.7 mm tall.
Outside of the US, the standard chart distance is 6 m, normal acuity is designated 6/6, and other acuities are expressed as ratios with a numerator of 6. Many rooms do not have 6 m available, and either a half-size chart subtending the same angles at 3 m, or a reversed chart projected and viewed by a mirror is used.
Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. During such examinations, acuity ratios are never mentioned.
The largest letter on an eye chart often represents an acuity of 20/200 (6/60), the value that is considered "legally blind." Some individuals with moderate myopia may not be able to read the large E without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. By contrast, legally blind individuals have a visual acuity of 20/200 (6/60) or less when using the best corrective lens.
See also 
- H. Snellen, Probebuchstaben zur Bestimmung der Sehschärfe, Utrecht 1862.
- "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
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