Snellen Chart

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 Hermann Snellen who developed the chart in 1862.
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Snel·len chart n. A chart for testing visual acuity, usually consisting of letters, numbers, or pictures printed in lines of decreasing size which a [more]Snellen chart - Community and Resources

Noun
1. display consisting of a printed card with letters and numbers in lines of decreasing size; used to test visual acuity
(hypernym) display, presentation

the standard tool for the measurement of visual acuity, displaying letters of progressively smaller size. Normal vision is 20/20 which means that you are able to discriminate characters on the eye chart at 20 feet that a person with normal acuity can see at 20 feet. 20/30 vision means you are able to discriminate characters on an eye chart at 20 feet that a person with normal acuity can see at 30 feet, and so on. (See legal blindness). View a Snellen Chart here.