rcd@opus.UUCP (Dick Dunn) (08/16/84)
J Abeles: >I do take issue with those who have claimed in this newsgroup that a >simplistic interpretation of color is possible, based on the biological >evidence that we are only sensitive to three color ranges... We should be careful to keep in mind that the eye's perception comes from three receptors which have somewhat broad, overlapping sensitivities. Also, the sensitivities of the three receptors are not linear with respect to light levels, nor are they nonlinear in the same way. (Sounds like it was designed by a large US computer company, doesn't it?:-) >If it is true that our eyes only can collect data on three color components >of the color spectrum, then is it true that any three color components are >adequate for "simulating" for our protoplasmic optical receptors (eyes) the >sensation that would be caused by any visible wavelength monochromatic source? >Clearly, the answer is "no", since the three components could be chosen so that >none of them contained any red, for example, and red would not then be >possible. But that wasn't the real argument, I think (hope)...it would be reasonable to think that we would want to simulate any wavelength (for presenting to the eye) by three components chosen to lie at the maximum sensitivities of the three types of receptors in the eye. >Artists using oil paints identify many pigments by their origin, as in >"titanium white", as do at least some house paints. I assume that artistic >tastes in such matters are determined from experience, and could not be >justified entirely deductively but rather somewhat subjectively. One >of the reasons, however, for specifying titanium dioxide as a pigment could >be its color--maybe artists feel that it is not quite the same as other >white pigments!...I believe that my argument above does indicate that >the mixing "theory" is, in fact, incomplete. There is a good, objective reason behind what Abeles alludes to here: The reflectivity of a pigmented substance can be quite a complicated function of wavelength. This means that some pigments look quite different in different light sources. There are pathological sorts of pigments, which change their appearances radically when illuminated with sunlight vs. incandescent light. There are also pathological sorts of light sources-- the standard old mercury-vapor streetlight is a good example which looks fairly white (tending to bluish) but turns fleshtones into hideous hues. The choice of a "good" pigment must at least entail choosing one which will look "right" under various light sources, which entails not having any big anomalies in reflectance. -- Dick Dunn {hao,ucbvax,allegra}!nbires!rcd (303)444-5710 x3086 ...Never attribute to malice what can be explained by stupidity.
mike@hou2e.UUCP (M.LUKACS) (08/22/84)
<........> Although the physical basis of normal human color perception consists of three overlaping sensitivity curves corresponding roughly to the "red", "green", and "blue" areas of the visual spectrum, the psychophysical color sense is much more complex and depends on a lot of post-processing, both automatic and cognitive (relying on memory and expectation). Early experiments by Land (the founder of Polaroid corp. originally Polaroid-Land) showed that under some conditions of intensity and composition most people could see a full color image projected in only two primaries. Later experiments at Bell Labs by C.B.Rubinstein, D.E.Pearson, and myself (~1969->1972) which removed any memory cues by using randomly generated quilts of color patches, proved that normal human observers would perceive near perfect renditions of color (all colors being present and identifiable) from images projected by red and white primaries only. The mechanism which causes this is the adaptation of vision to the average color content of a scene. This same mechanism can be seen operating when you look at a scene under different illuminations such as flourescent and incandescent lights or bright sun at noon vs. late afternoon sun vs. blue sky shade. We tend to see almost the same colors in all these situations (excepting pathologic pigments and/or lightings, which tend to have emitted or reflected spectrums with most of the energy concentrated in a few narrow frequency bands), but any photographer will tell you that the senes do not look at all similar to film or other nonadaptive media. Modern color television cameras have circuitry to mimic this effect of the human visual system, the camera operator must push a button to "white balance" the camera while looking at a portion of the scene that looks neutral in color to the operator. Michael E. Lukacs Room 2C-426 Bell Communications Research Holmdel, N. J. 07733 houxm!houxe!mike