It is well established that about 2 percent of otherwise normal human males are from birth confusers of red and green. There is considerable interest in the question: What do red-green confusers see? From a knowledge of the normal color perceptions corresponding to deuteranopic and protanopic red and green, we may not only understand better why colorblindness tests sometimes fail, and so be in a position to develop improved tests, but also the color-deficient observer may understand better the nature of his color-confusions and be aided to avoid their consequences. If an observer has normal trichromatic vision over a portion of his total retinal area and dichromatic vision over another portion, he may give valid testimony regarding the color perceptions characteristic of the particular form of dichromatic vision possessed by him. Preeminent among such observers are those born with one normal eye and one dichromatic eye. A review of the rather considerable literature on this subject shows that the color perceptions of both protanopic and deuteranopic observers are confined to two hues, yellow and blue, closely like those perceived under usual conditions in the spectrum at 575 mμ and 470 mμ, respectively, by normal observers. By combining this result with standard response functions recently derived for protanopic and deuteranopic vision, it has been possible to give quantitative estimates of the color perceptions typical of these observers for the whole range of colors in the Munsell Book of Color. These estimates take the form of protanopic and deuteranopic Munsell notations, and by using them it is possible not only to arrange the Munsell papers in ways which presumably appear orderly to red-green confusing dichromats but also to get immediately from the notations an accurate idea of the colors usually perceived in these arrangements by deuteranopes and protanopes much as the ordinary Munsell notations serve to describe the visual color perceptions of a normal observer.
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