An eye chart is a chart used to measure visual acuity. Types of eye charts include the logMAR chart, Snellen chart, Landolt C, E chart, Lea test, etc.
Charts usually display several rows of optotypes (test symbols), each row in a different size. An optotype is a standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols.
The person is asked to identify the optotype on the chart, usually starting with larger rows and continuing to smaller rows until the optotypes cannot be reliably identified anymore.
Technically speaking, testing visual acuity with an eye chart is a psychophysical measurement that attempts to determine a sensory threshold (see also psychometric function).
Charts are available for very young children or illiterate adults that do not require letter recognition. One version uses simple pictures or patterns. Others are printed with the block letter "E" turned in different orientations, the so-called Tumbling E. The patient simply indicates which direction each "E" is facing. The Landolt C chart is similar: rows have circles with different segments missing, and the test-taker describes where each broken piece is located. The last two kinds of charts also reduce the possibility of the patient guessing the images.
Computer-based semi-automatic alternatives to the eye chart have several advantages, such as a more precise measurement and less examiner-induced bias and are also well suited for children.