Normal color vision is trichromatic—that is, capable of perceiving all three primary colors of light: red (#FF0000), green (#00FF00), and blue (#0000FF). However, approximately four percent of people of European descent either have color-deficient vision or are color-blind—specifically, about eight percent of men, but only 0.4 percent of women.
Color-deficient vision results from the malfunction or absence of certain classes of retinal cones—the photoreceptors that provide color vision and are sensitive primarily to either red, green, or blue light. With color-deficient vision, it is more difficult to discriminate all three dimensions of color—hue, value, and chroma. The severity of color-vision deficiencies ranges from a little difficulty in distinguishing similar hues to the inability to perceive any color at all. Color blindness is the complete inability to perceive one or more of the three primary colors of light. There are several different types of color-deficient vision, as follows:
- anomalous trichromacy—Mildly deficient trichromatic vision in which a shift in the peak sensitivity of light-sensitive pigments in the retinal cones results in reduced sensitivity to red (#FF0000) or green (#00FF00). Anomalous trichromats have some difficulty in distinguishing similar hues in the red/orange/yellow/green region of the spectrum. There are two forms of anomalous trichromacy and both are inherited:
- protanomaly—One percent of men have this form of anomalous trichromacy, commonly called red weakness. With protanomaly, a shift in the receptivity of red-sensitive cones toward the green part of the spectrum results in some degree of reduced sensitivity to red. Thus, all hues containing red appear to have reduced levels of chroma. The red content in magenta and violet hues appears so weak that it may be imperceptible, causing anomalous trichromats to perceive magenta and violet as hues of blue. Red, orange, yellow, and yellow-green hues appear shifted toward green.
- deuteranomaly—This is by far the most common form of color-deficient vision. Five percent of men and 0.38 percent of women have this form of anomalous trichromacy, commonly called green weakness. Deuteranomaly causes about 62 percent of all color-deficient vision in men; 95 percent, in women. With deuteranomaly, a shift in the receptivity of green-sensitive cones toward the red part of the spectrum results in some degree of reduced sensitivity to green. Green, yellow-green, yellow, and orange hues appear shifted toward red, but none of these hues containing green appears to have reduced levels of chroma.
- dichromacy—Seriously deficient color vision that is capable of perceiving only two of the three primary colors of light: either blue (#0000FF) and red (#FF0000) or blue (#0000FF) and green (#00FF00). Dichromacy is also inherited, and there are three forms:
- protanopia—One percent of men and 0.02 percent of women have this form of dichromacy and are unable to distinguish between red and green. Thus, protanopia is commonly called red-green color blindness. While many hues are mixtures of red and green or red and blue, protanopes perceive all hues as mixtures of blue and green, because they lack red-sensitive cones. Thus, protanopes see all hues of red, orange, yellow, and green as hues of ochre or yellow and cannot distinguish among them, except by their differences in value.
For protanopes, all hues containing red appear to have reduced levels of chroma and value, and hues that contain a lot of red appear much darker. Normally, sixty-four percent of all cones are red-sensitive cones, so because protanopes have reduced sensitivity to red light, they receive less light overall. Therefore, protanopes can read neither red text on a black or gray background nor black or gray text on a red background. The red content in magenta and violet hues appears so weak that it is imperceptible to protanopes. Thus, they perceive all hues of magenta, violet, and blue as hues of blue and cannot distinguish among them, except by their values. Protanopes see white as a mixture of blue and green and cannot distinguish white from light blue-green.
- deuteranopia—One percent of men and 0.01 percent of women have this form of dichromacy and are unable to perceive the green part of the color spectrum. Deuteranopia is also commonly called red-green color blindness. While many hues are mixtures of red and green or blue and green, deuteranopes perceive all hues as mixtures of red and blue, because they lack green-sensitive cones. Normally, thirty-two percent of all cones are green-sensitive cones.
The deuteranope’s visible spectrum and receptivity to light are less limited than those of either the protanope or tritanope, because green is at the center of the color spectrum, and the sensitivities of both other classes of cones overlap with and, to some degree, compensate for the missing green-sensitive cones. Deuteranopes see all hues of green, yellow, orange, and red as hues of ochre or yellow and cannot distinguish among them, except by their values. They see all the hues of magenta, violet, and blue as hues of blue and cannot distinguish among them, except by their values. Deuteranopes see white as a mixture of red and blue and cannot distinguish white from light blue-violet.
- tritanopia—Among dichromats, loss of sensitivity to hue across the spectrum is most severe for tritanopes. Fortunately, this form of color blindness is rare. Only about 0.002 percent of men and 0.001 percent of women have this form of dichromacy. Because tritanopes are unable to distinguish between blue and yellow, tritanopia is commonly called blue-yellow color blindness. While many hues are mixtures of blue and red or blue and green, tritanopes perceive all colors as mixtures of red and green, because they lack blue-sensitive cones. Thus, the tritanope’s color discrimination is best in the red/orange/yellow/green region of the spectrum, where that of the protanope and deuteranope is worst. Normally, only about two percent of all cones are blue-sensitive cones.
Tritanopes see all hues of yellow, orange, red, and magenta as hues of red and are unable to distinguish among them, except by their values. They see white and all hues of blue, green, and violet as hues of blue-green and cannot distinguish among them, except by their values.
- monochromacy or achromatopsia—Monochromatic vision. These severe forms of color blindness are rare. Only about .005 percent of people—both men and women—have monochromacy and can see either no color at all or only one color. Most forms of monochromatic vision are inherited, but one form, cerebral achromatopsia, is acquired. Forms of monochromatic vision include:
- typical or complete rod monochromacy—Total color blindness. Typical rod monochromats completely lack cone vision due to the absence or malfunction of retinal cones; rely on only rod photoreceptors for vision; are extremely hypersensitive to light; and have very poor visual acuity, making them unable to perceive detail, especially in bright light. Rods provide vision in the dark or in dim light and are sensitive to light; they perceive only differences in value and are incapable of distinguishing colors. Therefore, rod monochromats cannot perceive color and see only shades of gray, black, and white. Among congenital monochromats, the vision of complete rod monochromats is the most severely impaired.
- atypical or incomplete monochromacy—Severe color blindness in which some classes of cones are absent or defective. Atypical monochromats have few functioning cones, so lack normal cone vision, but do have a very limited ability to perceive color. They are less sensitive to light and have better visual acuity than complete rod monochromats.
- blue cone monochromacy—A type of atypical monochromacy. Blue cone monochromats lack both red-sensitive and green-sensitive cones; rely on only blue-sensitive cones and rods for vision; are hypersensitive to light; have fairly poor visual acuity, especially in bright light; and can distinguish only a small range of blue hues.
- cerebral achromatopsia—Total color blindness, generally caused by trauma or disease, resulting in some disruption in the neural pathways between the eyes and the vision centers of the brain. People with cerebral achromatopsia cannot perceive color and see only shades of gray, black, and white.
All people with color-deficient vision can perceive differences in value and most can see a fairly wide range of hues—though many different hues appear identical for color-blind people. All anomalous trichromats experience greater problems with color confusion among hues with mid-range or dark values.
The hues that protanopes and deuteranopes see are actually quite similar, though there is a slightly green cast to the hues perceived by protanopes and a slightly orange cast to those perceived by deuteranopes. Neither can perceive green, orange, red, magenta, or violet. Protanopes and deuteranopes see hues that contain more red than blue as ochre or yellow; hues that contain more blue than red as blue; and hues that contain approximately equal amounts of green and blue as neutrals.