Color Variant Vision

I hope you found this section if you are interested in "color blindness" or "color defective vision." I hesitate to use this terminology because in some instances it is just plain wrong and in others not quite correct. Let me explain.

If a person can see, but not very well, we would not call him or her blind. A blind person is someone who simply can not see. It is true that legally blindness is defined differently. Legally a person is considered blind if they have 20/200 (or worse) vision in their best corrected eye.

What does any of this have to do with color vision? We often refer to people who misperceive colors as being color blind. That may or may not be correct. For example, if a person has no cone receptors they are truly color blind. Such people are called achromats and are very rare. Some people have rods and one kind of cone. These people are also unable to make any discriminations based on color and are called monochromats.

In the section dealing with the trichromatic theory I explained that most people have three classes of cone receptors in their retina. These are the short wavelength, middle wavelength and long wavelength receptors. Such observers are called trichromats. Some people are born with only two classes of cone receptors. They are called dichromats. It follows, as noted above, that a person who has only one class of cones is called a monochromat.

In addition to trichromats there is a variation of color vision called anomalous trichromats. These people have three classes of cone receptors but do not perceive color precisely as trichromats do. It is believed that the photopigments that trichromats have in the outer segments of their receptors are not the same as those which anomalous trichromats have.

There are three classes of dichromats: 1. Those who are missing the long wavelength sensitive cones. These observers are called protanopes. 2. Those who are missing the middle wavelength sensitive cones. These observers are called deuteranopes. 3. Those who are missing the short wavelength sensitive cones. These observers are called tritanopes. Prot-, duet- and trit- refer to classes of the "first", "second" and "third" kind.

There is yet another class of dichromats called anomalous dichromats. There are, as you might guess three classes: 1. protanomalous, 2. deuteranomalous, and 3. tritanomalous. These observers have somewhat better chromatic discrimination characteristics than dichromats.

Now you may be thinking what is wrong with the term color defective vision? Can that name not apply to all who do not have normal color vision? The answer is yes, it could if normal vision was precisely defined. The problem is that there is considerable variation among those who test normal for color vision. Similarly those who test as protanopes are not carbon copies of one another. They also exhibit considerable variation as a class. The same holds true for all the other categories. Hence, it is probably more accurate to speak of color variant vision when describing color vision characteristics rather than color blindness or color defective vision.

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