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From: awpaeth@watcgl.waterloo.edu (Alan Wm Paeth)
Newsgroups: sci.electronics,rec.audio,comp.graphics
Subject: Perceptual Color (was: Re: Looking for Blue LEDs)
Message-ID: <6101@watcgl.waterloo.edu>
Date: 30 Sep 88 20:49:43 GMT
References: <1138@nmtsun.nmt.edu> <862@ritcv.UUCP> <255@rna.UUCP> <4422@lynx.UUCP> <871@ritcv.UUCP> <870@dlhpedg.co.uk>
Reply-To: awpaeth@watcgl.waterloo.edu (Alan Wm Paeth)
Organization: U. of Waterloo, Ontario
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In article <870@dlhpedg.co.uk> cl@datlog.co.uk (Charles Lambert) writes:
>
>I guess that a yellow LED is really a red and a green LED in the same capsule:
>correct?

Nope, most yellow LEDs give a fairly spectrally pure yellow; this is not the
same as the yellow formed by mixing red and green -- but they "look" the same.

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.


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 [the audiophiles might wish
to think of a 3-way speaker with crossovers :-)]. 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.
When the color sensation is the same but the spectral response of the light is
not, the colors are called "metamers".

To prove that the spectral curves are clearly not the same, hold a narrow-
bandpass "spectral green" transmission filter before the yellow LED and you
get black -- all light is blocked. On the other hand, this same filter will
pass the green light of the metameric yellow from the combined source (such as
the yellow formed on a TV when both the green and red phosphors are excited).

In practice one can compute the three values which represent the cone response
to any light source (to within a linear change of basis). These are called
"chromaticity coordinates". The tristimulus tables used to calculate these
were published by the CIE in 1931 and form the basis of modern colorimetry.

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. Similarly, most
chocolate bars are quite "orange", but at illumination levels that make them
appear "brown". This can be demonstrated scientifically by using a colorimeter
or spectroradiometer. Less scientific but a lot more fun: view the chocolate
bar from within a darkened room using spot illumination and it will be orange.
Then make *sure* to eat the sample before it melts all over the floor.

Another perceptual shortcut here is that the red and green cones in the LED
example are NOT being stimulated simultaneously -- the colors are presented
alternately at fast enough perceptual rate to stop flicker and fuse the colors,
giving rise to "yellow". This property cannot be treated as self-evident: as a
non-intuitive counterexample, a flashing b/w light can take on the appearance
of color.

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.

    /Alan Paeth
    Computer Graphics Laboratory
    University of Waterloo