Snellen chart

The Snellen chart is a commonly used tool for measuring visual acuity, typically in the context of an eye examination. It consists of rows of block letters that decrease in size from top to bottom, enabling the identification of a person's vision clarity at a standardized distance.

File:Snellen chart in North American standard.svg

Snellen chart in North American standard

Origin and Design

The Snellen chart was designed by Dutch ophthalmologist Hermann Snellen in 1862. The chart consists of 11 rows of capital letters, referred to as "optotypes". The topmost row contains one letter, usually an "E", which is the largest on the chart. Each row beneath it has progressively smaller letters.

Testing Procedure

During a visual acuity test, the Snellen chart is typically placed 20 feet (or 6 meters in metric measurements) away from the person's eyes. The individual is asked to read aloud the letters of each row, starting from the top, until the letters become too difficult to distinguish. The smallest row that can be read accurately indicates the individual's visual acuity in the tested eye.

Understanding the Results

The results of a Snellen chart test are expressed as a fraction, such as 20/20. The numerator (the top number) refers to the testing distance (in feet or meters), and the denominator (the bottom number) corresponds to the distance at which a person with normal vision could read the same line that the individual was able to read. For example, a visual acuity of 20/40 indicates that the test subject sees at 20 feet what a person with normal vision could see at 40 feet.

Variations and Adaptations

The Snellen chart has been adapted to suit different languages and alphabets. There are also variations for children or adults who can't read, which use symbols or pictures instead of letters.

Limitations

While the Snellen chart is effective in assessing central visual acuity, it does not test other aspects of vision such as peripheral vision, color perception, depth perception, or contrast sensitivity.

See Also

• Visual acuity
• Eye examination
• Ophthalmology
• Hermann Snellen