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From: throopw@xyzzy.UUCP (Wayne A. Throop)
Newsgroups: sci.misc
Subject: Re: Color
Message-ID: <426@xyzzy.UUCP>
Date: Fri, 4-Dec-87 13:59:06 EST
Article-I.D.: xyzzy.426
Posted: Fri Dec  4 13:59:06 1987
Date-Received: Wed, 9-Dec-87 07:09:32 EST
References: <162300002@uiucdcsb>
Organization: Data General, RTP NC.
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> kadie@uiucdcsb.cs.uiuc.edu (Carl Kadie)
> 1) On TV's and computers screens, why is it RGB (red, green, blue)
> instead of RYB (red, yellow, blue) the primary colors?

RYB are the so-called "subtractive" primary colors.  Red, green, and
blue are the so-called "additive" primary colors.  When a color
sensation is produced by removing selected components of white light, as
in light reflected from pigments for example, the process is
subtractive.  When a color sensation is produced by adding selected
components to "no light" (or, "blackness"), as in television for
example, the process is called additive.

To oversimplify, the human eye has sensors for red, green, and blue.
Thus, starting with nothing, and adding these three components, any
color perception can be reproduced.  When subtracting from white light,
the "primary subtractive colors" are those gotten by removing one
element of white light at a time.  For example, removing red from white
light gives a bluish color called "cyan".

In fact, the subtractive primary colors are often called red, yellow,
and blue, but to photographers and others who need to be a little more
precise, they are called magenta, yellow, and cyan.

To sum up, there are two "kinds" of primary colors.  In each case, there
are three of them, to a large extent because there are three types of
receptors in the human eye.  In an additive mixing process, these colors
are red, green and blue.  This corresponds to the human eye's receptors.
In a subtractive mixing process, these colors are magenta, yellow and cyan.

The two processes can be related by noting that the additive primaries
can be produced from the subtractive primaries like so:

        red = magenta - yellow
        green = yellow - cyan
        blue = magenta - cyan

And the subtractive primaries can be produced from the addative
primaries like so:

        magenta = red + blue
        yellow = red + green
        cyan = green + blue

Again note, this is an oversimplification.

    (The "green = yellow - cyan" is what corresponds to the over-cute
     "ye-low an' bloo make green" of the ziplock comercials)

> 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 and no.  Subtracting cyan and yellow gives green, which is not the
same as "mixing the wave lengths of blue and yellow".  In fact, if you
add blue and yellow *light* (as opposed to adding blue and yellow
*pigment* which subtracts light), you'll get white, not green.

With that slight nit out of the way, and translating the question into
additive terms from subtractive, the rest is correct.  One can add red
to green to get a yellow that the eye couldn't tell from a pure
monochromatic yellow.  But the difference would be obvious to a
spectrograph.  In short, the one could be decomposed by a prism, and the
other could not.

--
Another interesting facet of ninja was the use of magic. They had
a reputation as sorcerors and wizards who could fly and hypnotize, and
walk through walls, and get away with huge deductions on their taxes.
    --- Bruce Israel martial-arts-request@brillig
-- 
Wayne Throop      !mcnc!rti!xyzzy!throopw