A dichromat has only two populations of cone cells. Typically, either the red or green population is absent in the retina. There are three types of dichromacy: protanopia (red is absent), deuteranopia (green is absent), and tritanopia (blue is absent).
In protanopia, also called "red weakness," the cone cells sensitive to red light are absent. A protanope is able to see blue, short wavelengths, but as light waves approach green in the spectrum, the perception becomes gray, and longer wavelengths, normally red, are seen as yellows. This type of color blindness affects 1 percent of men and 0.02 percent of women.
In deuteranopia the perception of color is very similar to what a person with protanopia sees. In this type, also called "green weakness," cone cells sensitive to green light are absent. It affects 1 percent of men and 0.01 percent of women.
The rare dichromatic disorder called tritanopia, also known as blue-yellow deficiency, results from the absence of cones that are sensitive to blue light. A tritanope sees shades of green at short (normally blue) and middle wavelengths and can normally detect red at the longer wavelengths. It affects about 0.002 percent of men and 0.001 percent of women.
Most people with color vision deficiency have anomalous trichromacy. They are able to see colors in all three ranges because their eyes have all three types of cone cells. An anomalous trichomat, however, uses the three cones in different proportions from the normal trichomat; therefore, they are not as capable of discriminating between similar colors or detecting slightly different shades. About 5 percent of the male population has deuteranomaly (green cone malfunction).
Achromatopsia is the congenital, or inherited, inability to see color, resulting from either the complete absence of cone cells or severe defects in those that are present. A person with achromatopsia perceives the visual world in shades of gray, black, and white. In the United States, about one person in 33,000 is affected by achromatopsia.
In complete achromatopsia, the complete absence or dysfunction of cones, vision lacks color and detail. Persons with achromatopsia are usually completely color deficient and have poor visual acuity.
They also demonstrate a condition called nystagmus, an involuntary, rapidly repeated, side-to-side jerking of the eyes. Without normally functioning cones, the eyes are hypersensitive to light (photophobic) and must be protected by dark or colored lenses, which give the person some relief from the pain and blindness caused by bright light and glare.