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Eye charts are used to measure visual acuity. Different kinds of eye charts include the Lea test, the Snellen chart and Landolt C.
Eye charts commonly display several rows of test symbols, with each row of symbols being a different size. The patient must attempt to identify the symbols on the eye chart, typically starting with the larger rows and continuing onto smaller and smaller rows until the symbols cannot be identified anymore.
Technically, testing visual acuity with eye charts is psychophysical, and tries to determine a sensory threshold.
Eye charts which do not require letter recognition are available for young children or illiterate adults. Some only use basic pictures or patterns. Other eye charts have the block letter "E" turned in multiple orientations, otherwise known as the Tumbling E. The patient must indicate in which orientation each "E" is facing. The Landolt C eye chart is similar with rows that have circles with segments missing, and the patient attempts to describe where each broken piece is. Another two kinds of eye charts also reduce the possibility of patients guessing what the images are.
Eye charts example: