Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!mailrus!ames!oliveb!tymix!antares!jms
From: jms@antares.UUCP (joe smith)
Newsgroups: comp.graphics
Subject: Re: Perceptual Color
Message-ID: <172@antares.UUCP>
Date: 2 Oct 88 08:48:28 GMT
References: <1138@nmtsun.nmt.edu> <862@ritcv.UUCP> <255@rna.UUCP> <4422@lynx.UUCP> <871@ritcv.UUCP> <870@dlhpedg.co.uk> <6101@watcgl.waterloo.edu>
Reply-To: jms@antares.UUCP (joe smith)
Organization: Tymnet QSATS, San Jose CA
Lines: 26

In article <6101@watcgl.waterloo.edu> awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes:
>Now to really complicate matters: there *ARE* "yellow" LEDs in the sense that
>you can buy one of those "two LEDS, wired back to back, one red and one green,
>potted in a clear compound" and then drive them with alternating polarity at a
>high perceptual rate, and you'll get the "blended" version of yellow, which
>looks like (has the same CIE chromaticity coordinates/provides the same
>"detector response" to the eye as) the spectral yellow LED.
> ...
<"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.
<When the color sensation is the same but the spectral response of the light is
<not, the colors are called "metamers".

Very nice article.  One thing you did not mention is that it is possible for
the human eye to distinguish between a spectral yellow point source and a
metamer due to chromatic aberration.  When the lens is set so that the green
light is focusses on the retina, the red is slightly out of focus, and vice
versa.  A spectral yellow, consisting of light of a single wavelength, can
be seen as in focus by both red and green cones.  That's one reason why
people consider green or amber phosphors to be easier on the eyes than white.
-- 
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