Ishihara colour test

It consists of a number of coloured plates, on each plate is printed a circle made of of many different sized dots of slightly different colours, spread in a random manner. Within the dot pattern, and differentiated only by colour, is a number. What, or even if, a number is visible indicates if and what form of colour blindness the viewer has. The full test consists of thirty-eight plates, but the existence of a deficiency is usually clear after no more than four plates.

Common plates include a circle of dots in shades of green and light blues with a figure differentiated in shades of brown or a circle of dots in shades of red, orange and yellow with a figure in shades of green. The first testing for protanopia[?] and the second for deuteranopia[?].