Overview of Color Blindness
Color vision deficiency, commonly called color blindness, results from a malfunction or absence of cone cells in the retina. The degree of deficiency ranges from a slight difficulty distinguishing among different shades of one color to the inability to see any color. There are millions of photoreceptor cells in the human eye's retina that contain photosensitive pigments. There are two types of these specialized cells: cones and rods. Each retina contains approximately 6 million cones and over 100 million rods.
Incidence and Prevalence of Color Blindness
Congenital color vision deficiency overwhelmingly affects more men than women. About 10 million men in the United States (7% of the male population) have a color vision deficiency compared to 0.4% of women. Caucasian men experience the highest prevalence of this disorder.
Objects we see reflect different wavelengths of light and give rise to the perception of color. Seeing color is a function of cone cells, which are stimulated by and responsive to wavelengths within the visible spectrum. There are three populations of cone cells, each having its own sensitivity range: blue, green, and red. When they are stimulated to different degrees, a match can be made to any color in the visible spectrum. The human eye is capable of matching over 7 million colors. In normal color perception, all three populations of cones are present (trichromacy) and function normally.
Cone cells are responsible for color vision and function only in moderate illumination and daylight or bright light. Each cone contains photopigments that make it sensitive to red, green, and blue wavelengths of light, but is most sensitive to one color of light. People with color vision disorders usually have a deficiency or absence of cone cells sensitive to red or green wavelengths. The inability to see blue light is rare. People who perceive colors normally are known as trichromats, because they are using three populations of cones, one sensitive to red, one to blue, and one to green, to match any color in the environment. Cone cells also provide sharp visual perception and high resolution. Cone cells are concentrated in the fovea (see eye anatomy) and become more sparse in the area immediately surrounding the fovea.
Rod cells function in the dark or in very dim light. When they are completely dark adapted, which takes about 30 minutes for humans, rods are actually more sensitive to light than cones. They contain only one photopigment and primarily detect shades of gray, thus color is perceived as black, white, and gray in dim lighting. Rods are located in the peripheral retina, not in the fovea. They cannot produce sharp visual acuity.