kadie@uiucdcsb.cs.uiuc.edu (12/02/87)
I've got two miscellaneous science questions.
1) On TV's and computers screens, why is it RGB (red, green, blue)
instead of RYB (red, yellow, blue) the primary colors?
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?
Carl Kadie
UUCP: {ihnp4,pur-ee,convex}!uiucdcs!kadie
CSNET: kadie@UIUC.CSNET
ARPA: kadie@M.CS.UIUC.EDU (kadie@UIUC.ARPA)davel@whuts.UUCP (12/03/87)
In article <162300002@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes: > 1) On TV's and computers screens, why is it RGB (red, green, blue) > instead of RYB (red, yellow, blue) the primary colors? Green, not yellow, is the primary color. The three types of cones in our eyes have peak sensitivities to red, green, and blue. Red+green "add" to appear "yellow", because yellow light stimulates both green and red cones much more than blue cones. When you mix paints, you are using "subtraction", which is more complicated. Yellow paint absorbs blue light well, green and red less well. Blue paint absorbs red and yellow well, green and violet less well. When you mix the paints, you get therefore get a paint which absorbs red very well, yellow fairly well, blue fairly well, but green only somewhat, so it appears green. > > 2) Some light wave length produces the color green. A mixture of > the wave lengths of blue and yellow also produces green. Not true (see above). Recasting the question, substituting "yellow" for "green"... > Even though these two greens are indistinguishable to our eyes, are there > (could there be) instruments that distinguish them? Yes. Absolutely. They exist. -- David Loewenstern Bangpath: {rutgers...}!moss!whuts!davel The above are not my opinions. My lawyer has advised me not to tell you what my opinions really are.
ronse@prlb2.UUCP (Christian Ronse) (12/03/87)
In article <162300002@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes: > I've got two miscellaneous science questions. > > 1) On TV's and computers screens, why is it RGB (red, green, blue) > instead of RYB (red, yellow, blue) the primary colors? Color perception in humans is achieved by the cone receptor cells (opposed to rod receptor cells, which respond to dim light). There are three types of cones: B, G, and R, having each a response curve, which indicates their sensitivity to photon energy as a function of wavelength (for monochromatic light). These response curves have peaks at roughly 430nm (somewhat blue), 530nm (somewhat green), and 560nm (somewhat yellow). But these peaks do not correspond to the colour percept associated to each type. Indeed, a light of a given colour will stimulate a response from each type of cones, and the perceived colour depends thus on the ratio of responses between the three types. Thus the colour percept associated to a type of cones corresponds to the light wavelength where the ratio of response between this type of cones and the two other types is highest. This gives: indigo-blue (around 430nm) for the B cones. green (around 530nm) for the G cones. red (around 600nm) for the R cones. Reference: J.L. Schnapf, D.A. Baylor: How Photoreceptor Cells respond to Light. Scientific American Vol. 256 no. 4, April 1987, pp. 32-39 This is why B,G,R are the primary colours in TV. The combinations B+G gives the same colour perception as cyan-blue (around 480nm), G+R gives the same colour perception as yellow, R+B gives the colour compound magenta (which corresponds to no monochromatic colour in the light spectrum), and B+G+R gives the same colour perception as white, that is a light having uniform wavelength distribution in the visible spectrum. Reference: Any book on colour perception, starting from H. von Helmholtz. These colour mixtures are called additive. There are also subtractive colour mixtures, as in painting or printing, where each colour absorbs certain wavelengths, and a compound of colours absorbs wavelengths absorbed by any of them. Here the primary colours are cyan (absorbs red), yellow (absorbs indigo-blue), and magenta (absorbs green). Of course, this is a very simplified situation, and in practice the primary colours in painting will be a cyanish blue, yellow, and a reddish magenta. This issue was discussed in sci.physics a few months ago. > 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. This is called a spectrometer. It is used in astronomy to analyse the light coming from stars. Or you can refract the light through a prism, as Newton did. Christian Ronse maldoror@prlb2.UUCP {uunet|philabs|mcvax|...}!prlb2!{maldoror|ronse} STAT ROSA PRISTINA NOMINE, NOMINA NUDA TENEMUS
kadie@uiucdcsb.UUCP (12/04/87)
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)hansen@mips.UUCP (Craig Hansen) (12/04/87)
In article <162300002@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes: > 1) On TV's and computers screens, why is it RGB (red, green, blue) > instead of RYB (red, yellow, blue) the primary colors? > > 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? 1) RGB are primary colors for mixing light (eg. red light plus green light gives "yellow" light). When you are mixing paint, remember that paint absorbs light, so mixing red & green paint makes this icky brownish grey, because the mixture absorbs both red and green colors. 2) Simple instruments that will distinguish them are prisms, diffraction gratings, and fog (rainbows). -- Craig Hansen Manager, Architecture Development MIPS Computer Systems, Inc. ...{ames,decwrl,prls}!mips!hansen or hansen@mips.com
bill@videovax.Tek.COM (William K. McFadden) (12/04/87)
In article <162300002@uiucdcsb> kadie@uiucdcsb.cs.uiuc.edu writes: >I've got two miscellaneous science questions. >1) On TV's and computers screens, why is it RGB (red, green, blue) >instead of RYB (red, yellow, blue) the primary colors? >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? (1) You're talking about two different media here. A TV monitor produces color by mixing red, green, and blue light together in varying proportions. This is called additive color because you start out with black (no light) and add light to make color. On the other hand, when you produce color by mixing paints, it is called subtractive color because you start out with white (all colors) and remove light to make color. For example, red paint absorbes all colors except red so that when you shine a white light on it, it reflects the red component and absorbes the rest. If you shine a blue light on it, it looks black because there is no red component to reflect. It simply absorbes all the blue light and reflects nothing. Since subtractive systems work differently, they need different primary colors than those used in additive systems. For example, color ink jet printers use primary colors of magenta (reddish), yellow, and cyan (bluish). (2) Be careful, you are talking about subtractive colors, so to say you are mixing blue and yellow wavelengths to produce green is not correct (see (1)). Let's try a different example. If you mix red light with green light (as in a CRT) you get yellow. If you look at this light spectrally (e.g., using a prism), you will see a peak in the red wavelengths and another in the green. But it looks yellow to our eyes because our eyes are fooled (don't ask me why our eyes do this). In other words, the mixture of two light frequencies causes our eyes to perceive a new frequency (yellow). Consider us lucky our eyes do this, because otherwise color TV would have been impossible (or at least terribly difficult). -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 UUCP: ...{hplabs,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "How can I prove I am not crazy to people who are?"
throopw@xyzzy.UUCP (Wayne A. Throop) (12/04/87)
> 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 <the-known-world>!mcnc!rti!xyzzy!throopw
john@frog.UUCP (12/05/87)
In article <162300002@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes: > I've got two miscellaneous science questions. > 1) On TV's and computers screens, why is it RGB (red, green, blue) > instead of RYB (red, yellow, blue) the primary colors? Red, yellow, and blue are the primary *pigments*, which work by absorbing the contrasting primary light colors. > 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? A prism. Real green light in, green light out. Apparent green light in, blue and yellow lines out. -- John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101 ...!decvax!frog!john, ...!mit-eddie!jfw, jfw@eddie.mit.edu "Cutting the space budget really restores my faith in humanity. It eliminates dreams, goals, and ideals and lets us get straight to the business of hate, debauchery, and self-annihilation." -- Johnny Hart
dfc@hpindda.HP.COM (Don Coolidge) (12/05/87)
> I've got two miscellaneous science questions. >1) On TV's and computers screens, why is it RGB (red, green, blue) >instead of RYB (red, yellow, blue) the primary colors? >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? ---------- Your two questions are actually one...red, blue, and yellow are the primary PIGMENTS, but red, blue, and green are the primary LIGHT COLORS. Red pigment absorbs blue and green light; yellow absorbs blue and red; blue absorbs red and green. Mix blue and yellow pigment and everything is absorbed except green wavelengths, which are reflected. Green light does not come about from mixing blue and yellow light, though; its wavelengths are intermediate in frequency between blue and yellow. Shine a white light through a blue filter and a red filter in sequence, and only the green light will make it through. (No, I'm not talking about a blue- or red-COLORED filter - I'm talking about one that filters out blue or red light.) So, in pigments, blue plus yellow makes green. In filters, white minus blue minus red makes green. Don Coolidge
dfc@hpindda.UUCP (12/05/87)
Did I actually write this? That should teach me to not post a response late on a Friday afternoon...:-) > Your two questions are actually one...red, blue, and yellow are the primary >PIGMENTS, but red, blue, and green are the primary LIGHT COLORS. Red pigment >absorbs blue and green light; yellow absorbs blue and red; blue absorbs red >and green. Mix blue and yellow pigment and everything is absorbed except >green wavelengths, which are reflected. Ok, if the green is absorbed, it's clearly not reflected. I can't remember exactly how to get there from here, though...help, someone!
hildum@iris.ucdavis.edu (Eric Hildum) (12/06/87)
In article <162300002@uiucdcsb> kadie@uiucdcsb.cs.uiuc.edu writes: > > >I've got two miscellaneous science questions. > >1) On TV's and computers screens, why is it RGB (red, green, blue) >instead of RYB (red, yellow, blue) the primary colors? While RYB are commonly called primary colors, there is no real basis for this distinction. In fact, conceptually, any three distinct colors could be used for the purpose of creating color images, if we could subtract light. In practice, we can only add light sources, thus the RGB combination is used because it is practical from a cost, benefit, coverage point of view. This combination creates one of the larger gamuts of colors (but not all the chroma that the eye is capable of perceiving). > >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 - but you might want to consider what you mean by "green." What is a "green" light? The following are a set of references into color vision, the reporduction of color, and color standards. If you are interested, I would suggest that you start with chapter 35 (I think - I am not certain, but it is somewhere near there) of the Feynman physics text. It is the most readable of the descriptions of the visual process that I have found. I particular, it will answer the question of what is a "green" light. @article{kn:faugeras, author = "Faugeras, Oliver D.", title = "Digital Color Image Processing Within the Framework of a Human Visual Model", institution = "IEEE", journal = "IEEE Transactions on Acoustics, Speech, and Signal Processing", volume = "ASSP-27", number = 4, pages = "380--393", month = "August", year = 1979 } @article{kn:horn, author = "Horn, Berthold K. P.", title = "Exact Reproduction of Colored Images", journal = "Computer Vision, Graphics, and Image Processing", volume = 26, pages = "135--167", year = 1984 } @article{kn:joblove, author = "Joblove, George H. and Greenberg, Donald", title = "Color Spaces for Computer Graphics", institution = "ACM", journal = "Computer Graphics", volume = 12, number = 3, pages = "20--25", month = "August", year = 1978 } @article{kn:pritchard, author = "Pritchard, D. H.", title = "{US} Color Television Fundamentals-A Review", institution = "IEEE", journal = "IEEE Transactions on Consumer Electronics", volume = "CE-23", pages = "467--478", month = "November", year = 1977 } @article{kn:smith, author = "Smith, Alvy Ray", title = "Color Gamut Transform Pairs", institution = "ACM", journal = "Computer Graphics", volume = 12, number = 3, pages = "12--19", month = "August", year = 1978 } @book{kn:boynton, author = "Boynton, Robert M.", title = "Human Color Vision", publisher = "Holt, Rinehart and Winston", address = "New York", year = 1979 } @book{kn:graphics, editor = "Beatty, John C. and Booth Kellogg S.", title = "Tutorial: Computer Graphics", institution = "IEEE Computer Society", publisher = "IEEE Computer Society Press", address = "Maryland", year = 1982, edition = 2 } ************************************************************* START HERE!!!!!!!!! @book{kn:feynman, author = "Feynman, Richard P. and Leighton, Robert B. and Sands, Matthew", title = "The Feynman Lectures on Physics", volume = 1, publisher = "Addison-Welsey Publishing Company", address = "Massachusetts", month = "July", year = 1964 } ************************************************************* @book{kn:foley, author = "Foley, J. D. and van Dam, A.", title = "Fundamentals of Interactive Computer Graphics", publisher = "Addison-Wesley Publishing Company", address = "Massachussetts", month = "March", year = 1983 } @book{kn:hunt, author = "Hunt, Robert William Gainer", title = "The Reproduction of Colour", publisher = "John Wiley \& Sons", address = "New York", year = 1975, edition = 3 } @book{kn:colortv, editor = "Rzeszewski, Ted", title = "Color Television", institution = "IEEE", publisher = "IEEE Press", address = "New York", year = 1983 } @techreport{kn:cie1, author = "{CIE International Commission on Illumination}", title = "Recommendations on Uniform Color Spaces, Color Difference Equations, Psychometric Color Terms", institution = "CIE International Commission on Illumination", year = 1978 } @techreport{kn:cie2, author = "{CIE International Commission on Illumination}", title = "Colorimetry-Official Recommendations of the International Commission on Illumination", institution = "CIE International Commission on Illumination", year = 1971 } @techreport{kn:cie3, author = "{CIE International Commission on Illumination}", title = "International Lighting Vocabulary", institution = "CIE International Commission on Illumination", volume = 1, year = 1957 } > >Carl Kadie >UUCP: {ihnp4,pur-ee,convex}!uiucdcs!kadie >CSNET: kadie@UIUC.CSNET >ARPA: kadie@M.CS.UIUC.EDU (kadie@UIUC.ARPA) Eric Hildum dehildum@ucdavis.edu (Internet) dehildum@ucdavis.bitnet (BITNET) ucbvax!ucdavis!dehildum (uucp)
jk3k+@andrew.cmu.edu.UUCP (12/07/87)
Some more information about color perception: When you add two colors of light, you get something that looks in-between but less pure. So if i show you a mixture of yellow and blue light, it will indeed look green. Then if show you monochromatic green light, it will look more green. Thus with any finite set of colors, you can't generate all the others, although you can get pretty close. The exception to this is purple, which, as pointed out before, is really only a mixture of red and violet. So if you mix together red and violet lights, you get something which is _really_ purple. --Joe
rice@swatsun (Dan Rice) (12/08/87)
In article <3290002@hpindda.HP.COM>, dfc@hpindda.HP.COM (Don Coolidge) writes: > Did I actually write this? That should teach me to not post a response > late on a Friday afternoon...:-) > > > Your two questions are actually one...red, blue, and yellow are the primary > >PIGMENTS, but red, blue, and green are the primary LIGHT COLORS. Red pigment > >absorbs blue and green light; yellow absorbs blue and red; blue absorbs red > >and green. Mix blue and yellow pigment and everything is absorbed except > >green wavelengths, which are reflected. > > Ok, if the green is absorbed, it's clearly not reflected. I can't remember > exactly how to get there from here, though...help, someone! O.K. The conventional ``primaries'' for light are red, green, and blue. A pigment that absorbs red will reflect green and blue (what actually happens is more complex, involving integrals of energy per wavelength and visual pigment sensitivities, but it winds up like this), the combination of which is usually known as cyan. A green-absorber reflects red and blue, so appears magenta, and a blue-absorber reflects red and green, and thus is yellow. Cyan is a greenish blue and magenta a bluish red, so the red, yellow, and blue pigments you used in kindergarten are not far off, and in fact can be mixed to form a decent set of colors. Notice that they add together into murky brown, not true black, since they don't absorb all of the light incident on them; in color printing, even with cyan, magenta, and yellow, it is customary to use black ink for the darkest areas. In fact, the concept of a primary color is meaningless; any three colors (excepting some degenerate cases) will serve to create a more-or-less wide range of colors. One standard color chart (the C.I.E diagram) looks like this, in a crude rendition: |---- Green v /-\ | | | | | | | . | . = white | | | | ---------- <---- Blue ^ | Red All the colors of a given brightness fall within a horseshoe-shaped boundary, where the monochromatic colors (colors formed by a single wavelength of light) make up the boundary. This diagram has the nice property that a mix of X% of color A and (100-X)% of color B falls X% along the line between A and B. Thus, arbitrary mixes of two colors span a line, and mixes of three non- collinear colors (the degenerate case mentioned above) span a triangle. You can see from the diagram that it's advantageous to span the triangle with vertices red, green, blue, since that triangle has the greatest area, and also is the only triangle that includes the purples (the bottom line between blue and red). In reality, though, there are no reproduction processes that even come close to this ideal triangle; instead they span smaller triangles in the middle. The moral is that everyone pretends that their color system is based completely on logic and necessity, whereas in fact all are compromises, and none truly create real (spectral) colors. A photograph can never create the sensation of a rainbow, for instance. Well, enough lecture. I hope this clears up a few things. -- - Dan Rice, Swarthmore College, Swarthmore PA 19081 ``I hear you're mad about Brubeck... I like your eyes, I like him too...'' UUCP: ...!seismo!bpa!swatsun!rice, ...!sun!liberty!swatsun!rice CSNET: rice%swatsun.swarthmore.edu@relay.cs.net
throopw@xyzzy.UUCP (Wayne A. Throop) (12/08/87)
> jk3k+@andrew.cmu.edu (Joseph G. Keane) > Some more information about color perception: When you add two colors of > light, you get something that looks in-between but less pure. Simply not true, as Joseph's own example of "purple" shows. Adding red and violet doesn't give a yellowish-green as you might expect, but purple. Again, the above claim is simply not true in general. > So if i show > you a mixture of yellow and blue light, it will indeed look green. Then if > show you monochromatic green light, it will look more green. I take it you haven't actually tried this. If you mix yellow and blue *light*, you normally get something very like white. (This depends strongly on the exact spectrums (spectra? whatever) and intensity of "yellow" and "blue" used of course.) Now, granted, yellow and blue *pigments*, when mixed, normally yield a green *pigment*, but that's a horse of a... well, it's a different kettle of fish, anyhow. -- "Trust me. I know what I'm doing." --- Sledge Hammer -- Wayne Throop <the-known-world>!mcnc!rti!xyzzy!throopw
wcalvin@well.UUCP (William Calvin) (12/09/87)
Color hues are basically a function of the relative activity in two cone types, e.g., the ratio of "red" cone to "green" cone activity gives yellows whether it is accomplished by a 540 nm pure wavelength or a mixture of 500 and 600. But red and blue cone absorbance curves don't overlap enough so as to produce some ratios with any single wavelength stimulus. The ratios produced by a 400 and 700 nm mixture are not unique but they cannot be matched by a single wavelength. We call it purple. William H. Calvin University of Washington NJ-15, Seattle WA 98195 wcalvin@well.uucp 206/328-1192
cdwf@root.co.uk (Clive D.W. Feather) (12/09/87)
Carl Kadie Inductive Learning Group University of Illinois at Urbana-Champaign writes: >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. The eye can see colours (for example, in afterimages) that cannot be reproduced by any combination of wavelengths of light ! There was an article in Scientific American c.1970 entitled "Phosphenes" that went into this.
jk3k+@andrew.cmu.edu (Joseph G. Keane) (12/10/87)
OK, right. As the colors you're mixing get farther apart, your mixture will look less like the in-between color. At some point you'll get white and after that the opposite of what you had before. `yellow+blue=green' is a bad example because this starts happening. I was going to use `red+yellow=orange' but tried to be consistent :-(. --Joe