cmb@ihima.UUCP (Christine M. Buss) (08/02/84)
>Human eyes cannot generally see "chords" of light. >Our ears are capable of doing a spectral analysis of sound, but we cannot >do the same with light. The ears can simultaneously detect a large range of >frequencies, our eyes can only detect (!) three. However, we are able to see >one "chord", the combination of blue and red which makes magenta. (Exclamation point mine) As someone who studies human color vision, I find these statements rather misleading. As you say, the ears can detect a large range of sound frequencies. They do a kind of Fourier analysis on the sound and retain information about each of the constituent frequencies in the sound. The eyes also can detect (and discriminate) a large range of light frequencies (from about 400 to 700 nm), not just three. Three light absorbing pigments with different spectral sensitivities (broad, overlapping, but not identical spectral sensitivities) report to the visual system the quantum catches they receive from any light. The visual system compares their three outputs to determine the "color" of the light. So the visual system can discriminate any two single wavelengths in the visible spectrum (except a pair very close together), but any two mixtures of wavelengths that produce the same quantum absorption in the three pigments look identical. I have no idea what to make of your use of the word "chord" with respect to vision. If a chord is simply a mixture of wavelengths, than of course we can _see_ them. Can we tell what the constituent wavelengths are? No. In the case of magenta, we can tell approximately what two wavelengths must be mixed to produce it, but only really on the basis of empirical color mixture experience. It's not at all like what the ear does, where the components of a chord are experienced as distinct in the mixture. Magenta looks like a single color, not like the superposition of two colors, red and blue. I'm not at all certain that the author of the quoted lines misunderstands color vision, but I think that one could easily read what he said and come away with some mistaken ideas of vision. I hope this makes some of the processes clearer.
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (08/02/84)
Then there are the fluorescent colors, which emit more light at a certain frequency that is impingent upon the object.
dgary@ecsvax.UUCP (08/02/84)
<>
Rick Coates' aaarticle on color is very informative and I would like
to add just one note:
The cones of the eye are actually sensitive to three somewhat overlapping
frequency ranges, not three specific frequencies of light. I believe
this may be why additive color "works".
That is, if our blue-sensitive and green-sensitive cones are excited to
some degree, our vision system interprets this as a color in between,
even though it could be a mixture of blue light and green light.
By the way, this article introduces a new term, "phenomenae," which I
take to be a plural of "phenomena." However, "phenomena" is itself
the plural of "phenomenon," so this must make "phenomenae" a super-plural.
Wow! What a concept!! :-)
(Please - no anti-spellingflame flames! I'm just kidding!)
D Gary Grady
Duke University Computation Center, Durham, NC 27706
(919) 684-4146
USENET: {decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgarygwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (08/04/84)
There is a pure green spectral line, so it is not a mixture. By the way, mixing blue and yellow light is more likely to give white, light blue, or light yellow than green for most people.
marcus@pyuxt.UUCP (M. G. Hand) (08/07/84)
> There is a pure green spectral line, so it is not a mixture. > By the way, mixing blue and yellow light is more likely to > give white, light blue, or light yellow than green for most > people. No, no, no! The hue green can be produced by a single specral emission, by a narrow band spectrum, a broad band spectrum or by a mixture of wavelengths which don't include the green spectral line. The eye and brain are not sophisticated enough to determine the components of the hue in isolation, although the purer the light the more intense it appears (Not "bright", intense - this is a saturation effect because it is not watered down by other specral emissions.) The addition of the two coloured light sources blue and green probably will produce a whitish hue because most light sources which people have access to have fairly broad band spectral outputs; ie, its yellow because of the absence of the blue end of the spectrum rather than any strong yellow spectral content. Any book on colour photographic printing will give lots of good info. Marcus Hand (pyuxt!marcus)
martin@ism780.UUCP (08/10/84)
When I see green, is it always a combination of blue and yellow wavelengths, or is there a green wavelength too? martin smith
tjr@ihnet.UUCP (Tom Roberts) (08/20/84)
I am puzzled by the recent discussions of color vision, and the following: A few years ago I attended a lecture/demonstration by Dr Land, inventor of the Polaroid camera. During this demonstration he displayed two (monochrome) slide photographs of the same image, one taken with a red (I think) filter, one with a green (I think) filter. He displayed the two images side-by-side, using red and white projectors. When he caused the two images to EXACTLY superimpose, a FULL COLOR image appeared (suddenly!). I find this striking demonstration hard to understand using a theory that color vision is strictly frequency dependent - I have not made any effort to do so (e.g. measuring the frequency spectrum of his white projector). Superficially, it seems as though some sort of intereference effect is being interpreted by the eye as "color" (though both projectors were incoherent incandescent sources). Does anyone know what is really going on? Tom Roberts ihnp4!ihnet!tjr
stekas@hou2g.UUCP (J.STEKAS) (08/20/84)
Colors: Land had a very nice Scientific American article a few years back. In it, he explained the how color perception REALLY worked, as op- posed to how people thought it worked. What he found was that color is not absolute but relative. The color we perceive an object to be has nothing to do with the absolute spectrum of light reflected from it, but of the difference between its reflected spectrum and those of other objects in the field of view. The effect is to subtract out the effect of the spectrum of illuminating light. That's why a peice of white paper looks just the same to our eye whether illuminated by incadescent (red), sunlight (white), or flourescent (blue) light. Color film records the abolute spectrum, so the proper combinations of films and filters are needed for different lightings. Jim
BILLW%SRI-KL@sri-unix.UUCP (08/22/84)
this is true only to a certain extent. You can "correct" for the color tempeture of the light source, as long as it contains at least some of a variety of frequencies. If you start dealing with nearly monochromatic light sources, this is not possible. For example, we have some orange exhaust stacks here that appear quite grey under the ilumination of sodium arc lamps (also orange, but not quite the same frequency). BillW
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (08/22/84)
Funny, everything looks red in a darkroom with the red safety light on.
dgary@ecsvax.UUCP (08/31/84)
<>
The Exploratorium in San Francisco has a number of fascinating color
exhibits, including one that creates an impression of multiple colors
by shining monochromatic light on a black and white print! Color
perception is indeed a complicated area and very few people are doing
work in it. I find in interesting that one of the best known
researchers, Dr. Edwin Land, is a college dropout (his doctorates are
honorary, like Franklin's). One wonders if he could get a research
job at his own company if he had to go through personnel....
D Gary Grady
Duke University Computation Center, Durham, NC 27706
(919) 684-4146
USENET: {decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgary