Snellen

The Snellen chart, named after Dutch ophthalmologist Hermann Snellen, is an eye chart used to measure visual acuity. Introduced in 1862, it presents rows of letters that decrease in size, with each line corresponding to a different level of acuity. The letters are designed to be read at a specific distance, helping clinicians assess how clearly a person sees at distance.

Testing is typically conducted at a standardized distance of 6 meters (20 feet) in many countries, though some equipment uses shorter distances. Visual acuity on the chart is expressed as a Snellen fraction, such as 20/20 or 6/6. The numerator indicates the testing distance, and the denominator indicates the distance at which a person with normal vision can read the same line. For example, 20/40 means the patient can read at 20 feet what a person with normal vision can read at 40 feet.

During the procedure, the patient covers one eye and attempts to read the smallest line of letters they can discern, usually starting from the top and moving downward. Each line typically contains multiple letters. The test may also be performed for near vision using a chart held at about 40 centimeters.

The Snellen chart remains widely used in clinical eye exams, school screenings, and licensing tests due to its simplicity and familiarity. However, it has limitations: results can be influenced by the specific letters used, crowding effects, lighting, and testing distance. Modern practice often supplements or replaces Snellen with LogMAR charts, which provide more consistent and precise measurements of visual acuity.