Encyclopedia - Snellen Eye Chart

What is the Snellen Eye Chart?
How is the chart used?

The Snellen Eye Chart is the standard used in measuring the eye's ability to distinguish detail and shapes (visual acuity). The chart is based on the work of a Dutch ophthalmologist, Dr. Hermann Snellen, who in 1862 designed this system for describing human vision.

What is the Snellen Eye Chart?

The Snellen Chart is made up of a series of letters, numbers or symbols of progressively smaller size, the largest at the top. "Normal" vision is 20/20. A person who can clearly read a one-inch letter at a distance of 20 feet is considered to have normal vision. All measurements obtained from use of the Snellen Chart are a comparison to that standard. Persons who have 20/40 vision, for example, can read at 20 feet what people with normal vision can read at 40 feet.

How is the chart used?

Visual acuity is normally checked before proceeding with any other part of an eye examination, including administering any medications to the patient's eyes. The patient is allowed to keep on any eyeglasses or contact lens and is positioned 20 feet in front of the Snellen Eye Chart. Covering one eye at a time, the patient is asked to read progressively smaller letters until it is no longer possible to read the small letters. Then the test is repeated with the other eye.

The Snellen Eye Chart measures only visual acuity. It does not prove the absence of an eye disease or any other eye problem.