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From: rickc@iddic.UUCP
Newsgroups: net.physics
Subject: Colors: one more time
Message-ID: <1791@iddic.UUCP>
Date: Mon, 6-Aug-84 11:00:37 EDT
Article-I.D.: iddic.1791
Posted: Mon Aug  6 11:00:37 1984
Date-Received: Wed, 8-Aug-84 00:47:52 EDT
Organization: Tektronix, Beaverton OR
Lines: 65

The nattering nit-nits of the net are at it again. :-) (A distinct species
from the flamers, the nit-nits pick everything to death).

>>Human eyes cannot generally see "chords" of light. 
>>Our ears are capable of doing a spectral analysis of sound, but we cannot
>>do the same with light.  The ears can simultaneously detect a large range of
>>frequencies, our eyes can only detect (!) three.  However, we are able to see
>>one "chord", the combination of blue and red which makes magenta.
>(Exclamation point mine)
>
>
>The eyes also can detect (and discriminate) a large range of light frequencies
>(from about 400 to 700 nm), not just three.  Three light absorbing pigments 
>with different spectral sensitivities (broad, overlapping, but not identical 
>spectral sensitivities) report to the visual system the quantum catches 
>they receive from any light.  The visual system compares their three outputs 
>to determine the "color" of the light.  So the visual system
>can discriminate any two single wavelengths in the visible spectrum 
>(except a pair very close together), but any two mixtures of wavelengths 
>that produce the same quantum absorption in the three pigments look identical. 
>

Well, what I should have said is detect three frequencies SIMULTANEOUSLY.
Obviously, I didn't mean that human eyes can only detect three different 
frequencies of light. I used the word simultaneously earlier in the sentence,
and since I was drawing an analogy assumed that the reader would realize that
I meant three SIMULTANEOUS frequencies.

>I have no idea what to make of your use of the word "chord" with respect 
>to vision.  If a chord is simply a mixture of wavelengths, than of
>course we can _see_ them.  Can we tell what the constituent wavelengths
>are?  No.  In the case of magenta, we can tell approximately what two
>wavelengths must be mixed to produce it, but only really on the basis of
>empirical color mixture experience.  It's not at all like what the ear
>does, where the components of a chord are experienced as distinct in the 
>mixture.  Magenta looks like a single color, not like the superposition 
>of two colors, red and blue.

I'm truly sorry that you don't know what to make of my use of the word 
chord.  I didn't mean a section of a circle (:-)). I tried to make an
analogy to music, where a human can discriminate the the individual
frequencies that exist within a chord.

>I'm not at all certain that the author of the quoted lines
>misunderstands color vision,

As a matter of fact, I do understand color vision. Is this kind of
comment necessary?

>but I think that one could easily read what
>he said and come away with some mistaken ideas of vision.  I hope this
>makes some of the processes clearer.

I really can't say that 'report to the visual system the quantum catches they
receive' is any clearer than what I said. My intention was not to go into the
detail of human vision, but to make the point that I find to be an interesting
realization: that the eyes can only detect three frequencies simultaneously,
and what this implies.  What would vision be like of we could see
seven 'primary' colors? (that is, color receptors for those wavelengths
or ranges of wavelengths) A hundred?



							Rick Coates
							tektronix!iddic!rickc