The human eye sees by light stimulating the retina (a neuro-membrane lining the inside back of the eye). The retina is made up of what are called Rods and Cones. The rods, located in the peripheral retina, give us our night vision, but can not distinguish color. Cones, located in the center of the retina (called the macula), are not much good at night but do let us perceive color during daylight conditions.
Many people think anyone labeled as “colorblind” only sees black and white – like watching a black and white movie or television. This is a big misconception and not true. It is extremely rare to be totally color blind. There are many different types and degrees of colorblindness, really they are “color deficiencies” since virtually no one is truly blind to all colors.
People with normal cones and color vision are able to see all the different colors and subtle mixtures of them by using cones sensitive to one of three wavelength of light – red, green, and blue.
A mild color deficiency is present when one or more of the three cones functions “poorly”. A more severe color deficiency is present when one of the cones does not function at “all” or is missing.
Protanomaly (one out of 100 males): Protanomaly is referred to as “red-weakness”, an apt description of this form of color deficiency. Any redness seen in a color by a normal observer is seen more weakly by the protanomalous viewer, both in terms of its “coloring power” (saturation, or depth of color) and its brightness. Red, orange, yellow, yellow-green, and green, appear somewhat shifted in hue (“hue” is just another word for “color”) towards green, and all appear paler than they do to the normal observer. The redness component that a normal observer sees in a violet or lavender color is so weakened for the protanomalous observer that he may fail to detect it, and therefore sees only the blue component. Hence, to him the color that normals call “violet” may look only like another shade of blue.
Under poor viewing conditions, such as when driving in dazzling sunlight or in rainy or foggy weather, it is easily possible for protanomalous individuals to mistake a blinking red traffic light from a blinking yellow or amber one, or to fail to distinguish a green traffic light from the various “white” lights in store fronts, signs, and street lights that line our streets. Do not let them adjust the color on the television, because it will look far to redish or violet for the rest of the family members.
Deuteranomaly (five out of 100 of males):
Let the deuteranomalous person adjust your television and he would add more green and subtract red. He is considered “green weak”. Similar to the protanomalous person, he is poor at discriminating small differences in hues in the red, orange, yellow, green region of the spectrum. He makes errors in the naming of hues in this region because they appear somewhat shifted towards red for him – difficulty in distinguishing violet from blue. From a practical stand point though, many protanomalous and deuteranomalous people breeze through life with very little difficulty doing tasks that require normal color vision. Some may not even be aware that their color perception is in any way different from normal. The only problem they have is passing a color vision test.
Dicromasy – can be divided into protanopia and deuteranopia (two out of 100 males): These individuals normally know they have a color vision problem and it can effect their lives on a daily basis. They see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population.
Protanopia (one out of 100 males): For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows and from greens primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible e.g. Pink flowers, reflecting both red light and blue light, may appear just blue to the protanope.
Deuteranopia (one out of 100 males): The deuteranope suffers the same hue discrimination problems as the protanope, but without the abnormal dimming. The names red, orange, yellow, and green really mean very little to him aside from being different names that every one else around him seems to be able to agree on. Similarly, violet, lavender, purple, and blue, seem to be too many names to use logically for hues that all look alike to him.
Some of the information in these pages was taken from:
An article by: Diana H. Heath, M.D., a member of the Morton Plant medical staff, specializing in ophthalmology. (formerly at http://www.zipmall.com/mpm-art-colorbl.htm)
A webpage written by Terrace L. Waggoner, O.D., Staff Naval Hospital, Pensacola (formerly at http://members.aol.com/nocolorvsn/color2.htm