Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!iuvax!pur-ee!uiucdcs!uiucdcsb!kadie From: kadie@uiucdcsb.UUCP Newsgroups: sci.misc Subject: Re: Color Message-ID: <162300004@uiucdcsb> Date: Thu, 3-Dec-87 21:26:00 EST Article-I.D.: uiucdcsb.162300004 Posted: Thu Dec 3 21:26:00 1987 Date-Received: Sun, 6-Dec-87 21:58:50 EST References: <162300002@uiucdcsb> Lines: 66 Nf-ID: #R:uiucdcsb:162300002:uiucdcsb:162300004:000:2946 Nf-From: uiucdcsb.cs.uiuc.edu!kadie Dec 3 20:26:00 1987 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)