🚀 go-pugleaf

                                     The Answers

          I've gotten answers to my questions about color. Here is a summary.

Q1.       On TV's and computers screens, why is it RGB (red, green, blue)
          instead of RYB (red, yellow, blue) the primary colors?

A.        There are two methods of mixing color, one the RYB for pigments,
          and one the RGB for light. If I remember the explanation correct-
          ly, the light combinations are truly combinations of wavelengths;
          the pigments combinations are really combinations of light wave-
          lengths filtered out by selective reflection and absorption. Thus
          the terms "additive" primaries for the light RGB group and "sub-
          tractive" primaries for the pigment RYB group.

          This whole business of "primary" colors is as much a function of
          how the human eye works as it is of the physics.  We have four
          kinds of light-gathering cells in our retinas.  One kind (rods?
          cones?) is sensitive to low levels of illumination and does not
          distinguish colors.  That's why, at night, everything looks grey.
          The other three kinds have pigments in them, so that one bunch
          responds most to long waves (red), another to intermediate wave-
          lengths (yellow, green), and the third to shorter waves (blue,
          violet).  The sense of vision somehow puts these three signals
          together to make the subjective experience of color.

Q2.       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?

A.        Yes, I think a spectroscope would do it.  It pulls apart the
          different frequencies; the mixed light would show up as a line of
          yellow and a line of blue.

          It is actually, quite hard to get a system to recognize color 
          the same way as your eye (mixing, etc.) It is very easy, however, 
          to get a system to discern the difference between "real" yellow 
          light and green/red mix.

ALSO      Other Information

i.        For more information look up the Retinex theory of color
          vision. There was an article in Scientific American within the
          last seven or eight years.

ii.       There is "no such color" as purple! Mixing red and blue ink 
          causes your eye to react in a way which is not reproducible 
	  by any single wavelength of light.

THANKS to          
          Barry Hayes
          Joe Beckenbach
          Christopher J. Henrich
          John M. Pantone



Carl Kadie
Inductive Learning Group
University of Illinois at Urbana-Champaign
UUCP: {ihnp4,pur-ee,convex}!uiucdcs!kadie
CSNET: kadie@UIUC.CSNET
ARPA: kadie@M.CS.UIUC.EDU (kadie@UIUC.ARPA)