TALKING ABOUT COLOR

I think it is fair to say that one of the most talked-about topics among the color fraternity right now is color con- stancy. Why is it that the colors around us change so little when the illumination changes?

I have said something about this already in an earlier column, but this time I want to ask myself a different ques- tion. Why do the colors of familiar objects look exactly the same to me now as they did when I was a boy? That was a long, long time ago and the more I think about it, the more remarkable it seems to be.

As we know all too well, our faculties degenerate with age, and the visual process is no exception. R. A. Weale has described in some detail the structural and biochemi- cal changes that occur in the successive stages of the de- velopment, growth, and aging of the eye,’ and from the point of view of color perception, the yellowing of the crystalline lens is one of the most significant changes as we age. At my age of 81, the lens has become so yellow that its optical density for a wavelength of 460 nm is likely to be about 0.7 or 0.8. There is also a marked increase in light scatter with age from both the cornea and the crystalline lens, and the level of retinal illumination di- minishes with age since the pupil diameter gets smaller as the years go by.

Even more significant are the changes that result from senescence in the neural retina, especially the disruption of the outer limbs of the retinal photo-receptors. This appears to be part of the normal aging process and must certainly affect the photochemical response of the receptors. It must presumably also affect the color response of the retina and possibly lead to some degree of color vision defect. If this were the case, though, it would hardly seem possible for colors to look essentially the same to me now as the colors I saw 60 or 70 years ago.

If 1 were asked how 1 knew that the colors appeared the same, 1 would have to admit that I could not prove it. This is a judgement 1 have to make purely on the subjective perceptions of my color sensations, which I cannot measure, and on my ability to remember them. Yet, what I am claim- ing is that green grass still looks like green grass to me, that yellow bananas still look like yellow bananas, that Dutch cheese still looks like Dutch cheese, and that I still see the full range of hues through the spectrum. I think these are not unreasonable claims.

If we try to explain this long-term stability of color ap- pearance, we come up against the ultimate mystery of just what color perception really is. We can think of the retina

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and brain as a neural network analogous to a microprocessor, and we can suppose with some confidence that the penul- timate stage of the visual process is an incredibly complex pattern of signals within the brain that contains all the in- formation required to generate the image that we see, in- cluding the color information. Yet the signals are not the colors themselves. Somewhere, somehow, there is a sub- jective color palette in the brain waiting to be stimulated, a palette with a color gamut that must, I believe, embrace Hering’s six unique perceptual primaries, namely red, yel- low, green, blue, black and white.

If this concept is correct, the range of colors that we see will be determined by the intensities or vividness of the Hering primaries and of their subjective mixture. Further, if I am correct in claiming that the colors that we see do not degenerate with age, then this implies that these sub- jective primaries show no sign of fading as we get older. I find this very significant.

I am not suggesting that there is anything unusual about my color vision, as I assume that other octogenarians ex- perience the same color constancy with age as I do. I should welcome confirmation of this. Of course, there will be some individuals who do suffer some loss in the range of hues that they can see should they develop an acquired color vision defect as the result of some pathological con- dition.

This whole subject raises some very interesting ge- netic questions. For example, some 2 or 3% of the male population are dichromats, that is, they can do all their color matching with only two controls instead of the three needed by the normal trichromat. It is assumed from this that they have only two types of color receptor in their retinas instead of the normal three. Does this mean that their subjective color palette is similarly restricted, per- haps with only four Hering primaries in their palette in- stead of six? For an observer who has only one type of cone receptor-the very rare cone monochromat-his vi- sion is achromatic, ranging from black to white, so that he needs only two Hering primaries in his color palette. It is interesting to speculate on just how the retina and the cerebral color palette develop in harmony, so that the color discriminating capacity of the retina is matched by the appropriate number of Hering primaries in the brain.

There is so much to wonder at, but let me finish with one final question. How does the color perception process in the human brain differ from that of a robot’s brain? Our computer experts can now design electronic brains of ever-increasing complexity, so that there is no great dif- ficulty in programming a robot so that he will “Stop” at a red light and “Go” at a green one. Yet, since the robot

COLOR research and application

cannot be provided with a subjective color palette, he will not actually see the redness or the greenness of the sig- nals. In other words, he does not have a perceptive mind like ours.

I am rather relieved about this and for myself I am just

W. D. WRIGHT, 68 Newberries Avenue, Radlett, Herts WD7 7EP, England.

thankful that there is no sign of my Hering primaries fading into monochrome greyness. At least, not yet! 1. R. A. Weale, A Biography of the Eye: Development, Growth, Age,

Lewis, London, 1982.

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