It states that "the luminance of the presentation shall be uniform and not less than 120 cd/m 2.
#2020 vision chart license#
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 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.