Color blindness is used in colloquial terms to refer to the difficulty in telling colors apart but a more correct term would be color vision defect. Color blindness is a misnomer because only a small percentage of people are unable to see any color.
Color vision is important in many everyday tasks, such a driving a car (does that traffic light mean “stop” or “go”?). Persons with color vision defects may be at a disadvantage in school or at work. In fact, lack of normal color vision may limit career opportunities. For example, normal color vision is vital in such jobs as color printing and color photography. Abnormal color vision can even be dangerous in certain situations, such as in rail and water navigation. For this reason, persons with color vision defects are excluded by law from certain occupations.
What causes color vision defects?
Color vision depends on the absorption of light by visual pigments contained within specialized cells in the eye called photoreceptors. There are two types of photoreceptors: rods and cones. Rods, which provide vision in dim light, have no ability to distinguish between colors. Cones are responsible for color vision. There are three different types of pigment in the normal eye. Color vision occurs within the visual part of the brain compares electrical signals from the different types of cones.
Defects in color vision are either inherited or acquired. Inherited defects result mainly from missing or incorrect visual pigments. There are different types of inherited defects, with different levels of severity. Color vision defects sometimes can be acquired, as a result of eye disease or normal aging or as a side effect of certain medications. In acquired defects, other parts of the eye besides cones and cone pigments may be affected.
What are the types of inherited color vision defects?
There are three groups of inherited color vision defects: monochromacy, dichromacy and anomalous trichromacy. The last two groups are subdivided into red-green and blue-yellow types of defects. Inherited red-green color vision defects are more common in males (1 to 8 percent depending on race) than in females (about 0.4 percent). Inherited blue-yellow defects are rare in either sex.
Monochromacy—Rod monochromats, or complete achromats, are truly “color blind” since they cannot distinguish any hues (e.g., blue, green, yellow and red). They see only different degrees of lightness. For them, the world appears to be shades of gray, black and white. They also have poor visual acuity, aversion to bright light and nystagmus (an involuntary, rapid movement of the eyes).
To have rod monochromacy, someone must inherit a gene for the disorder from both parents. This condition occurs in approximately 1 in 30,000 of the population.
Dichromacy is a less severe form of color defect than monochromacy. Dichromats can tell some hues apart. Dichromacy is divided into three types: protanopia, deuteranopia and tritanopia.
Protanopia and deuteranopia are red-green defects. Persons with red-green defects have difficulty distinguishing between reds, greens and yellows but can discriminate between blues and yellows. Protanopes often can name red and green correctly because green looks lighter to them than red.
Males have red-green defects if they inherit a defective gene from their mother. Affected males pass the defective gene to all of their daughters but none of their sons. Females who inherit only one defective gene are carriers of that gene. Females who inherit the gene for red-green defect from both parents are affected.
Hereditary tritanopia is a blue-yellow defect. Persons with blue-yellow defects cannot see the difference between blues and yellows but can distinguish between reds and greens. Tritanopia is somewhat rare (affecting between 1 and 15,000 and 1 in 50,000) and occurs equally in both sexes. Triatnopes usually have fewer problems in performing everyday tasks than do those with red-green dichromacy.
Anomalous Trichromacy—The ability of anomalous trichromats to distinguish between hues is better than dichromats but still not normal. Red-green anomalous trichromacy is subdivided into protanomaly and deuteranomaly. Both types are inherited in the same way as for red-green dichromacy. The severity can range from mild to extreme. Some persons with the mildest forms may not even realize their color vision is abnormal.
A third type of anomalous trichromacy is tritanomaly. This condition is more often acquired than inherited.
How are color vision defects diagnosed?
Specialized color vision tests can easily detect color vision defects. Pseudoisochromatic plate tests are commonly used to screen for inherited color vision defects. In this group of tests, a pattern of colored dots forms a number or letter against a background of other colored dots. Persons with normal color vision can discern these patterns but those with color defects cannot.
Arrangement tests are a second type of color vision test. They are used to measure the severity of inherited color vision defects or to test for acquired color defects. The person being tested must arrange color chips in order of similarity.
Can color vision defects be cured?
No cure exists for inherited color vision defects since they are caused by missing or incorrect visual pigments. Acquired color vision defects can be corrected sometimes if the underlying cause can be treated.
Special aids have been developed to help persons with color vision defects distinguish some of the colors that cause them trouble. These devices include specially tinted contact lenses and eyeglasses. However, these aids do not provide normal color vision and therefore should be used with caution.
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