Procedure
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 large 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).
Read more about this topic: Eye Chart
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