There are various tests which can be used, however the most commonly used is the Ishihara test. In this test the optometrist shows the patient a number of plates which are printed with dots of different colours, brightness, density and sizes, arranged so that the dots of similar colour form a figure (for example a number) among a background of dots in another colour.

If a person has defective colour vision, they will be unable to identify some, or all, of the figures hidden in the dots depending upon what type of colour blindness that they suffer from.