Path: utzoo!mnetor!uunet!mcvax!prlb2!ronse
From: ronse@prlb2.UUCP (Christian Ronse)
Newsgroups: sci.misc
Subject: Re: Color
Message-ID: <386@prlb2.UUCP>
Date: 3 Dec 87 14:00:50 GMT
References: <162300002@uiucdcsb>
Organization: Philips Research Laboratory, Brussels
Lines: 59
Keywords: cone receptors
Summary: this follows from the physiology of human retina

In article <162300002@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes:
> I've got two miscellaneous science questions. 
> 
> 1) On TV's and computers screens, why is it RGB (red, green, blue)
> instead of RYB (red, yellow, blue) the primary colors?

Color perception in humans is achieved by the cone receptor cells (opposed to
rod receptor cells, which respond to dim light). There are three types of
cones: B, G, and R, having each a response curve, which indicates their
sensitivity to photon energy as a function of wavelength (for monochromatic
light). These response curves have peaks at roughly 430nm (somewhat blue),
530nm (somewhat green), and 560nm (somewhat yellow). But these peaks do not
correspond to the colour percept associated to each type. Indeed, a light of a
given colour will stimulate a response from each type of cones, and the
perceived colour depends thus on the ratio of responses between the three
types. Thus the colour percept associated to a type of cones corresponds to
the light wavelength where the ratio of response between this type of cones
and the two other types is highest. This gives:

indigo-blue (around 430nm) for the B cones.
green (around 530nm) for the G cones.
red (around 600nm) for the R cones.

Reference: 
J.L. Schnapf, D.A. Baylor: How Photoreceptor Cells respond to Light.
Scientific American Vol. 256 no. 4, April 1987, pp. 32-39

This is why B,G,R are the primary colours in TV. The combinations B+G gives
the same colour perception as cyan-blue (around 480nm), G+R gives the same
colour perception as yellow, R+B gives the colour compound magenta (which
corresponds to no monochromatic colour in the light spectrum), and B+G+R gives
the same colour perception as white, that is a light having uniform wavelength
distribution in the visible spectrum.

Reference: Any book on colour perception, starting from H. von Helmholtz.

These colour mixtures are called additive. There are also subtractive colour
mixtures, as in painting or printing, where each colour absorbs certain
wavelengths, and a compound of colours absorbs wavelengths absorbed by any of
them. Here the primary colours are cyan (absorbs red), yellow (absorbs
indigo-blue), and magenta (absorbs green). Of course, this is a very
simplified situation, and in practice the primary colours in painting will be
a cyanish blue, yellow, and a reddish magenta.

This issue was discussed in sci.physics a few months ago.

> 2) Some light wave length produces the color green. A mixture of
> the wave lengths of blue and yellow also produces green.
> Even though these two greens are indistinguishable to our eyes, are there
> (could there be) instruments that distinguish them?

Yes. This is called a spectrometer. It is used in astronomy to analyse the
light coming from stars. Or you can refract the light through a prism, as
Newton did.

Christian Ronse		maldoror@prlb2.UUCP
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