Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!mit-eddie!uw-beaver!tektronix!reed!psu-cs!omepd!uoregon!gary
From: gary@uoregon.UUCP
Newsgroups: comp.graphics
Subject: Re: Colour vision
Message-ID: <506@uoregon.UUCP>
Date: Sat, 14-Mar-87 13:05:14 EST
Article-I.D.: uoregon.506
Posted: Sat Mar 14 13:05:14 1987
Date-Received: Sun, 15-Mar-87 04:16:41 EST
References: <505@ubu.warwick.UUCP>
Reply-To: gary@fog.UUCP (Gary Meyer)
Organization: University of Oregon, Computer Science, Eugene OR
Lines: 41

In article <505@ubu.warwick.UUCP> rolf@ubu.UUCP (Rolf Howarth) writes:
>
>I thought I'd ask if anyone could explain to me how the human brain
>perceives colour.
>
The retina of the human eye contains receptors with spectral
sensitivities that peak in the short, medium, and long wavelength
regions of the visible spectrum.  Any spectral energy distribution has
therefore been reduced to three pieces of information by the time it
passes this initial stage of the visual system.  If two spectral
energy distributions produce the same three signals then the visual
system will not be able to tell them apart.  This is the principle
upon which all color reproduction techniques are based.

Any visual system with spectral sensitivities different from those
used to perform the color calculations will not be satisfied with the
color reproduction (although two separate images would not be seen as
you suggest).  All animals and some color defective humans fall into
this category.  However people whose color deficiency stems only from
the lack of one of the spectral sensitivity functions will still find
the color reproduction adequate.  These indviduals are known as
dichromats.

I have become convinced that most of the confusion which surrounds
this topic is caused by the use of perceptual terms (red, yellow,
green, etc.) to label physical quantities (wavelength, spectral
sensitivity).  Hue is a term of identification and its use in this
context leads to the misconception that this is a color naming task
when it is really the much simpler problem of color matching that is
involved.  I pass along the following quote which helped to
"enlighten" me.

     And if at any time I speak of Light and Rays as coloured
     or endued with Colours, I would be understood to speak
     not philosophically and properly, but grossly, and
     accordingly to such Conceptions as vulgar People in
     seeing all these Experiments would be apt to frame. For
     the Rays to speak properly are not coloured.

                                          Sir Isaac Newton
                                          Opticks, Book One, Part II