Snellen Test (Chart)

Ophthalmic Procedures
Labelled Diagram of the Eye

A Snellen Test (Chart) is one of the most common and widely recognised tools for assessing patient eyesight. It is used by medical doctors, including ophthalmologists, as well as High Street opticians.

A Snellen Chart is used for testing visual acuity, which is a measure of sharpness of vision, i.e. a person's ability to distinguish and resolve visual detail. Ideally this is tested in a controlled environment with appropriate light level, each eye being tested separately.

The Snellen Chart itself consists of rows of capital letters in a standard simple font. The largest character forms the first "line", the remaining characters, which are also all 'capital' or 'upper-case' letters, being on the lines beneath it and progressively smaller in size.

The 'patient' (in medical/clinical situations) or 'subject' (if / when the test is carried out for the purposes of non patient-specific reseach), is positioned a fixed distance from the Chart, generally 6 metres (6m) in Europe, or 20 feet in the United States - those distances being similar. He or she then reads the series of letters beginning at the top of the Snellen Chart and continuing to read along each line then down to the left-hand-side of the next line below until he or she is unable to read any further because the characters on the chart are so small that they appear blurred, hence indistinguishable, to the viewer.

People who are able to distinguish every letter down to the bottom line of the Snellen Chart are generally deemed to have "perfect" visual acuity. In all other cases the person's visual acuity is defined in terms of the lowest line on the Snellen Chart whose characters they are able to identify correctly.

See also the definition of the Sheridan-Gardiner Test.

More about Ophthalmology:

This section includes short definitions of many diseases, disorders, and conditions of the eyes and visual system.
For definitions of other terms in this category, choose from the list to the left (but note that this is not a complete/exhaustive list).

Other related pages include

For further information see also our pages of books about ophthalmology.

The following books may also be of interest:

The Aging Eye (Harvard Medical School)
MCQ Companion to the Eye (Textbook)
The Secret of Perfect Vision: How You Can Prevent and Reverse Nearsightedness
What Your Doctor May Not Tell You About(TM) Glaucoma: The Essential Treatments and Advances That Could Save Your Sight

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