Definition of Snellen Chart

Snellen Chart is the familiar eye chart, featuring lines of letters and numbers that progressively decrease in size, to assess visual acuity. Dutch ophthalmologist Herman Snellen (1834–1908) developed the chart in the 1860s.

The top letter is typically E. A person with normal vision can read a one-inch letter from 20 feet away, designated as 20/20 vision.

Each line on the chart represents a ratio of normal vision. A person who can see at 20 feet what someone who has normal vision could see at 60 feet has 20/60 vision. A refraction test then determines the precise correction necessary to bring visual acuity as close to 20/20 as possible.

To take a Snellen test, a person reads the smallest line possible with each eye separately (one eye covered) and both eyes together.


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