Snellen defined "standard vision" as the ability to recognize one of his optotypes when it subtended 5 minutes of arc. "6/6"(m) or "20/20"(ft) vision įurther information: Visual acuity § Expression Visual acuity is the distance at which test is made / distance at which the smallest optotype identified subtends an angle of five arcminutes and the critical distinguishing features of the optotype subtend an angle of one arcminute. Any variation across the test chart shall not exceed 20 %."Īccording 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. It states that "the luminance of the presentation shall be uniform and not less than 120 cd/m 2. 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.īS 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". Wall-mounted Snellen charts are inexpensive and are sometimes used for approximate assessment of vision, e.g. The perception of five out of six letters (or similar ratio) is judged to be the Snellen fraction. Only the nine letters C, D, E, F, L, O, P, T, Z are used in the common Snellen chart. the height and width of the optotype (letter) is five times the thickness of the line.the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C".They have a particular, simple geometry in which: They are not, however, letters from any ordinary typographer's font. 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. Variation of Snellen chart with another letter sequence. The symbols on an acuity chart are formally known as " optotypes". The smallest row that can be read accurately indicates the visual acuity in that specific eye. A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top. Subsequent rows have increasing numbers of letters that decrease in size. The first lineĬonsists of one very large letter, which may be one of several letters, for example E, H, or N. The normal Snellen chart is printed with eleven lines of block letters. The original chart shows A, C, E, G, L, N, P, R, T, 5, V, Z, B, D, 4, F, H, K, O, S, 3, U, Y, A, C, E, G, L, 2. Snellen's charts published in 1862 used alphanumeric capitals in the 5×5 grid. The experimental charts developed in 1861 used abstract symbols. Snellen developed charts using symbols based in a 5×5 unit grid. Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders. Snellen chart is used to estimate visual acuity (last three rows are 20/15, 20/13 and 20/10)Ī Snellen chart is an eye chart that can be used to measure visual acuity.
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