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From: tso@rocky2.rockefeller.edu (Daniels Tso(Wiesel))
Newsgroups: sci.electronics,rec.audio,comp.graphics
Subject: Re: Perceptual Color (was: Re: Looking for Blue LEDs)
Message-ID: <207@rocky8.rockefeller.edu>
Date: 5 Oct 88 08:12:35 GMT
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In article <6101@watcgl.waterloo.edu> awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes:
>A BRIEF OVERVIEW OF "COLOR"
>
>You (I'm assuming that you not color blind) have three cone types which give
>rise to color vision. One is sensitive to short wavelengths, one to medium and
>one to long, but their coverages overlap somewhat.

	Actually the overlap is very significant.

>Spectral red light (eg, from
>a red LED or HeNe laser) stimulates mainly your long wavelength (low frequency)
>cone. This gives a psycophysical response/feeling we call "red". Ditto for a
>spectral green light and your mid-range cone (I refuse to call the other two
>cones "tweeters" and "woofers"). Now spectral yellow light happens to trigger
>both the long and mid-range cones simultaneously; we call this sensation
>"yellow". If you mix spectral red and green light in just the right amount
>so that the cones generate the same signals, then you have an identical cone
>response and therefore identical sensation -- the color is indistinguisable.

	This discussion neglects color opponency. Basically, (perhaps) since
the overlap of the cone spectral sensivitites is large, the "brain" (actually
the process begins in the retina), encodes wavelength (as opposed to color)
by the DIFFERENTIAL responses of cones. The two color opponency systems are
red vs. green and blue vs. yellow. Thus cells in the retina are excited by
input from the red cones and inhibited by input from the green cones, or
vice versa, and are a part of the red vs. green system. Similarly, another
class of cells are excited by the blue cones and inhibited by a combination
of input from the red and green cones. (This neglects the contribution of the
rods to color).
	Thus, wavelength sensitivity is only relative. We only know yellow
by what isn't blue and what is red by what isn't green. That is why, except
for unusual circumstances, it doesn't generally make sense to talk about a
"reddish-green" color, or a "bluish-yellow" color, although a reddish
yellow color seems perfectly reasonable.

>The deeper truth is much more elusive: this explanation is simplistic in that
>it doesn't take the observer's accomodation into account -- a bright yellow
>shirt under sunset illumination reflects orange light, if you were to pick a
>close match to a spectral color. However, the human perceptual system (brain)
>nicely hides this fact from us, and we conclude "nice yellow shirt; wow, nice
>sunset, too". Color film lacks such brains and this helps explain the need for
>"Tungsten-indoor" vs "outdoor-daylight" balanced color films.

	Indeed, this is the major difference between wavelength sensitivity
and color perception. This properties you refer to is termed "color constancy".

>Clearly our understanding of color is incomplete: we cannot model all the
>processes which take place in the brain, let alone fit a unified theory to
>all the optical illusions and other "anomalous" perceptual phenomena that
>have been discovered, but we're getting there.

	Maybe we are... For a real "eye-opener" (teehee), try hunting down an
example of the McCollough effect. It is similar in idea to the standard color
afterimages, but with the surprising feature that this afterimags can last
for days, weeks or months!