The Snellen Eye Chart measures how well a person can see at various distances. The chart is imprinted with block letters that line-by-line decrease in size, corresponding to the distance at which that line of letters is normally visible. The Snellen Eye Chart is used to test visual acuity with the patient positioned 20 feet from the chart. Visual acuity refers to the clarity or clearness of the vision, a measure of how well a person sees. Only logged in customers who have purchased this product may leave a review.
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A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen , who developed the chart in The experimental charts developed in used abstract symbols. The normal 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 from 6 metres or 20 feet away, 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 specific 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 perception of five out of six letters or similar ratio is judged to be the Snellen fraction. 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 British Standards Institution "Specification for test charts for determining distance visual acuity" was replaced by BS "Test charts for clinical determination of distance visual acuity — Specification". Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of one minute of arc.
Other acuities are expressed as ratios with a numerator of 6. Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres 9.
In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres. 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. Putting it another way, the eye should be at a distance The formula is.
Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. To ensure adequate illumination of the Snellen charts, various medical device manufacturers had developed Snellen chart products with backlight or projection. Commonly digital chart products support randomizing optotypes displayed to prevent patients from cheating through memorizing the chart.
From Wikipedia, the free encyclopedia. Snellen chart Purpose Snellen chart is used to estimate visual acuity A Snellen chart is an eye chart that can be used to measure visual acuity. Pfoff Laser and Eye Home. Retrieved May 10, Retrieved 10 May Government Publishing Office. United States Department of Commerce. National Bureau of Standards. Archived from the original PDF on 18 June Retrieved 11 December Craig Blackwell, MD. Hidden categories: All articles with unsourced statements Articles with unsourced statements from February Commons category link is on Wikidata.
Snellen Eye Chart
3m DVLA Snellen Chart