An Ishihara test uses colored plates to test the type and seriousness of color vision deficiencies. These deficiencies may range from difficulty discerning differences between particular colors to complete color blindness. A plate in an Ishihara test has a circle filled with colored dots. Usually there are background dots of one color and a figure made out of dots of another color. If the person taking the test cannot make out the figure, he probably has a vision deficiency.
The Ishihara test was developed in 1918 by Japanese surgeon and ophthalmologist Shinobu Ishihara. He undertook the task of creating a color vision chart during his employment with the Japanese Military Medical School, which wanted a way to test for color vision problems in military recruits. The charts are still commonly used in eye exams throughout the world.
Color vision deficiencies are usually inherited and are more often passed on to males, but disease, trauma, or old age can also cause color blindness. On the retina, a tissue located in the inner eye, behind the pupil, has two types of cells on it that receive light. Rod cells do not recognize color but work better at night and cone cells recognize color and work better in the day. There are three types of cone cells in a healthy eye: S, which is sensitive to shorter wavelengths, M, which is sensitive to medium wavelengths, and L, which is sensitive to longer wavelengths. The cones pick up the colors that fall into their range of wavelengths, with S cones peaking at violet, M cones peaking at green, and L cones peaking at yellow.
The range of shades between these colors depends on differences between the stimulation between different cones. For example, when an L cone is very stimulated and M cones are receiving less stimulation, the eye perceives red. If one of these cone systems is damaged or missing, it causes a deficiency called dichromacy, in which it may be difficult to differentiate some colors. If two of these cone systems are non-functional, it causes monochromacy, a rare condition in which the patient can only see shades of gray. Both dichromacy and monochromacy are commonly called color blindness, though the term technically only refers to monochromacy.
The Ishihara test has 38 plates in total, but only a few of them are used in a typical eye exam. If, after a few plates, the patient is unable to distinguish the figure set into the plate of colored dots, the evidence will usually be enough to diagnose a color vision deficiency. A full Ishihara test may then be conducted to find the degree to which the patient’s vision is impaired and the cone system that is damaged.
If the patient has dichromacy, it can be further diagnosed as protanopia, deuteranopia, or, rarely, tritanopia. People with protanopia have deficient L cones and limited sensitivity to red and those with deuteranopia have deficient M cones and limited sensitivity to green. Tritanopia is caused by deficient S cones and results in difficulty differentiating blue from yellow. Ishihara plates that test for red-green color blindness or deficiency might have a number composed of red dots set in with many green dots, or a green figure set in with red dots. Patients with blue-yellow blindness or deficiency might have trouble finding a yellow figure set in with blue dots or a blue figure set in with yellow dots in an Ishihara test.