Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site opus.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!zehntel!hplabs!hao!cires!nbires!opus!rcd
From: rcd@opus.UUCP (Dick Dunn)
Newsgroups: net.physics
Subject: Re: color vision: not a fourier transform...
Message-ID: <698@opus.UUCP>
Date: Wed, 15-Aug-84 21:33:28 EDT
Article-I.D.: opus.698
Posted: Wed Aug 15 21:33:28 1984
Date-Received: Sat, 18-Aug-84 01:26:15 EDT
References: <188@mhuxm.UUCP>
Organization: NBI, Boulder
Lines: 47

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.