Definition
The Ishihara test is the most widely used screening test for red-green colour vision deficiencies. The test consists of a series of plates (pseudoisochromatic plates) on which coloured dots form a number or line. Persons with normal colour vision see the number clearly; persons with a red-green deficiency see a different number or nothing at all.
Background
The test was developed in 1917 by Shinobu Ishihara, professor at the University of Tokyo, originally commissioned by the Japanese army for military selection. The full test consists of 38 plates; in practice a shorter version of 14 or 24 plates is often used.
The Ishihara test is quick and easy to administer but measures only whether a red-green deficiency exists — not the severity, not the type, and not the discrimination ability. For professional colour assessors the Ishihara test is a necessary but insufficient selection step. Persons who pass the Ishihara test are subsequently tested with the FM100 for discrimination level.
In the ASTM E1499-16 standard, the Ishihara test, the anomaloscope and the FM100 are all listed as relevant instruments for colour assessor selection and qualification.