awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) (10/01/88)
In article <870@dlhpedg.co.uk> cl@datlog.co.uk (Charles Lambert) writes: > >I guess that a yellow LED is really a red and a green LED in the same capsule: >correct? Nope, most yellow LEDs give a fairly spectrally pure yellow; this is not the same as the yellow formed by mixing red and green -- but they "look" the same. Now to really complicate matters: there *ARE* "yellow" LEDs in the sense that you can buy one of those "two LEDS, wired back to back, one red and one green, potted in a clear compound" and then drive them with alternating polarity at a high perceptual rate, and you'll get the "blended" version of yellow, which looks like (has the same CIE chromaticity coordinates/provides the same "detector response" to the eye as) the spectral yellow LED. A BRIEF OVERVIEW OF "COLOR" You (I'm assuming that you not color blind) have three cone types which give rise to color vision. One is sensitive to short wavelengths, one to medium and one to long, but their coverages overlap somewhat [the audiophiles might wish to think of a 3-way speaker with crossovers :-)]. Spectral red light (eg, from a red LED or HeNe laser) stimulates mainly your long wavelength (low frequency) cone. This gives a psycophysical response/feeling we call "red". Ditto for a spectral green light and your mid-range cone (I refuse to call the other two cones "tweeters" and "woofers"). Now spectral yellow light happens to trigger both the long and mid-range cones simultaneously; we call this sensation "yellow". If you mix spectral red and green light in just the right amount so that the cones generate the same signals, then you have an identical cone response and therefore identical sensation -- the color is indistinguisable. When the color sensation is the same but the spectral response of the light is not, the colors are called "metamers". To prove that the spectral curves are clearly not the same, hold a narrow- bandpass "spectral green" transmission filter before the yellow LED and you get black -- all light is blocked. On the other hand, this same filter will pass the green light of the metameric yellow from the combined source (such as the yellow formed on a TV when both the green and red phosphors are excited). In practice one can compute the three values which represent the cone response to any light source (to within a linear change of basis). These are called "chromaticity coordinates". The tristimulus tables used to calculate these were published by the CIE in 1931 and form the basis of modern colorimetry. The deeper truth is much more elusive: this explanation is simplistic in that it doesn't take the observer's accomodation into account -- a bright yellow shirt under sunset illumination reflects orange light, if you were to pick a close match to a spectral color. However, the human perceptual system (brain) nicely hides this fact from us, and we conclude "nice yellow shirt; wow, nice sunset, too". Color film lacks such brains and this helps explain the need for "Tungsten-indoor" vs "outdoor-daylight" balanced color films. Similarly, most chocolate bars are quite "orange", but at illumination levels that make them appear "brown". This can be demonstrated scientifically by using a colorimeter or spectroradiometer. Less scientific but a lot more fun: view the chocolate bar from within a darkened room using spot illumination and it will be orange. Then make *sure* to eat the sample before it melts all over the floor. Another perceptual shortcut here is that the red and green cones in the LED example are NOT being stimulated simultaneously -- the colors are presented alternately at fast enough perceptual rate to stop flicker and fuse the colors, giving rise to "yellow". This property cannot be treated as self-evident: as a non-intuitive counterexample, a flashing b/w light can take on the appearance of color. Clearly our understanding of color is incomplete: we cannot model all the processes which take place in the brain, let alone fit a unified theory to all the optical illusions and other "anomalous" perceptual phenomena that have been discovered, but we're getting there. /Alan Paeth Computer Graphics Laboratory University of Waterloo
cook@stout.ucar.edu (Forrest Cook) (10/03/88)
In article <6101@watcgl.waterloo.edu> awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes: >a flashing b/w light can take on the appearance of color. This reminds me of one of those toys that consists of a spinning shutter over a mask: you spin the shutter, close your eyes, stare towards a bright light source such as the sun, and VOILA: colored stripes. Some kind of aliasing I imagine. So, if you take a fast response white light source and modulate its duty cycle and frequency, will you fake the eyes into seeing colors? Incandescent's probably switch too slow, that leaves such things as Xenon strobes and LEDs. White LEDs are probably possible: red, green and BLUE LEDs could be put in close proximity and ~mixed~ with a diffuser to yield what would be percieved as white light. The three LED currents would probably need tweaking to get true white. [Adrift in a sea of ideas.]
u-jmolse%sunset.utah.edu@utah-gr.UUCP (John M. Olsen) (10/04/88)
In article <790@ncar.ucar.edu> cook@stout.UCAR.EDU (Forrest Cook) writes: >awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes: >>a flashing b/w light can take on the appearance of color. >... >White LEDs are probably possible: red, green and BLUE LEDs could be >put in close proximity and ~mixed~ with a diffuser to yield what would be >percieved as white light. The three LED currents would probably need tweaking >to get true white. Interesting. Use RGB LED's to generate white light with which to trick the eye into seeing color. There seems to be an unnecessary step in there if you want to generate color with LED's. (Sorry, but .... well, no I'm not.) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) /\/\ /| | /||| /\| | John M. Olsen, 1547 Jamestown Drive /\/\ \/\/ \|()|\|\_ |||.\/|/)@|\_ | Salt Lake City, UT 84121-2051 \/\/ /\/\ | u-jmolse%ug@cs.utah.edu or ...!utah-cs!utah-ug!u-jmolse /\/\ "Really stupid people use comptuer programs every day." Chuck McManis
cook@stout.ucar.edu (Forrest Cook) (10/04/88)
In article <2903@utah-gr.UUCP> u-jmolse%sunset.utah.edu.UUCP@utah-gr.UUCP (John M. Olsen) writes: >Interesting. Use RGB LED's to generate white light with which to trick the >eye into seeing color. There seems to be an unnecessary step in there if >you want to generate color with LED's. (Sorry, but .... well, no I'm not.) >:^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) :^) Sorry, I did not miss the obvious, I just forgot to mention it. Consider the possible advantage of this approach: ALL DIGITAL (I.E. Pulse Width/Frequency) color modulation. No messy DACs or analog amps needed. While we're on the subject, would someone out there in LED manufacturing land please hurry up and make a simple color pixel consisting of RGB LEDs in one diffuse package? I could find quite a few uses for such a device! :-) An imbedded controller would be nice, but I can wait for that. Forrest Cook {husc6 | rutgers | ames | gatech}!ncar!stout!cook {uunet | ucbvax | allegra | cbosgd}!nbires!stout!cook
henry@utzoo.uucp (Henry Spencer) (10/04/88)
In article <790@ncar.ucar.edu> cook@stout.UCAR.EDU (Forrest Cook) writes: >>a flashing b/w light can take on the appearance of color. > >So, if you take a fast response white light source and modulate its duty >cycle and frequency, will you fake the eyes into seeing colors? It helps if you can modulate not only the duty cycle but the repetition pattern, since my recollection of this stuff is that simple periodic flashes are not quite right for the job. But yes, you can fake colors this way. It's not spectacular, and I think it doesn't work on everyone, but it does work. Last I heard, it's not clear exactly why. -- The meek can have the Earth; | Henry Spencer at U of Toronto Zoology the rest of us have other plans.|uunet!attcan!utzoo!henry henry@zoo.toronto.edu
tso@rocky2.rockefeller.edu (Daniels Tso(Wiesel)) (10/05/88)
In article <6101@watcgl.waterloo.edu> awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes: >A BRIEF OVERVIEW OF "COLOR" > >You (I'm assuming that you not color blind) have three cone types which give >rise to color vision. One is sensitive to short wavelengths, one to medium and >one to long, but their coverages overlap somewhat. Actually the overlap is very significant. >Spectral red light (eg, from >a red LED or HeNe laser) stimulates mainly your long wavelength (low frequency) >cone. This gives a psycophysical response/feeling we call "red". Ditto for a >spectral green light and your mid-range cone (I refuse to call the other two >cones "tweeters" and "woofers"). Now spectral yellow light happens to trigger >both the long and mid-range cones simultaneously; we call this sensation >"yellow". If you mix spectral red and green light in just the right amount >so that the cones generate the same signals, then you have an identical cone >response and therefore identical sensation -- the color is indistinguisable. This discussion neglects color opponency. Basically, (perhaps) since the overlap of the cone spectral sensivitites is large, the "brain" (actually the process begins in the retina), encodes wavelength (as opposed to color) by the DIFFERENTIAL responses of cones. The two color opponency systems are red vs. green and blue vs. yellow. Thus cells in the retina are excited by input from the red cones and inhibited by input from the green cones, or vice versa, and are a part of the red vs. green system. Similarly, another class of cells are excited by the blue cones and inhibited by a combination of input from the red and green cones. (This neglects the contribution of the rods to color). Thus, wavelength sensitivity is only relative. We only know yellow by what isn't blue and what is red by what isn't green. That is why, except for unusual circumstances, it doesn't generally make sense to talk about a "reddish-green" color, or a "bluish-yellow" color, although a reddish yellow color seems perfectly reasonable. >The deeper truth is much more elusive: this explanation is simplistic in that >it doesn't take the observer's accomodation into account -- a bright yellow >shirt under sunset illumination reflects orange light, if you were to pick a >close match to a spectral color. However, the human perceptual system (brain) >nicely hides this fact from us, and we conclude "nice yellow shirt; wow, nice >sunset, too". Color film lacks such brains and this helps explain the need for >"Tungsten-indoor" vs "outdoor-daylight" balanced color films. Indeed, this is the major difference between wavelength sensitivity and color perception. This properties you refer to is termed "color constancy". >Clearly our understanding of color is incomplete: we cannot model all the >processes which take place in the brain, let alone fit a unified theory to >all the optical illusions and other "anomalous" perceptual phenomena that >have been discovered, but we're getting there. Maybe we are... For a real "eye-opener" (teehee), try hunting down an example of the McCollough effect. It is similar in idea to the standard color afterimages, but with the surprising feature that this afterimags can last for days, weeks or months!
u-jmolse%sunset.utah.edu@utah-gr.UUCP (John M. Olsen) (10/06/88)
tso@rocky2.rockefeller.edu (Daniels Tso(Wiesel)) writes: >awpaeth@watcgl.waterloo.edu (Alan Wm Paeth) writes: >>A BRIEF OVERVIEW OF "COLOR" >> >For a real "eye-opener" (teehee), try hunting down an >example of the McCollough effect. It is similar in idea to the standard color >afterimages, but with the surprising feature that this afterimags can last >for days, weeks or months! This could lead to a new crib-sheet test cheating method! Just burn the key into your retina, then just copy down the afterimage answers. :^) Back to a more comp.graphics related topic, I seem to recall reading about a new manufacturing technique that will be used to make large color LED screens. Current models are about 5 inches, but the new ones can be much larger, and could be used for way-high-res screens. I can't remember which tech mag I read it in, though. Maybe EDN? I don't recall it mentioning whether they were analog or pulse driven. /\/\ /| | /||| /\| | John M. Olsen, 1547 Jamestown Drive /\/\ \/\/ \|()|\|\_ |||.\/|/)@|\_ | Salt Lake City, UT 84121-2051 \/\/ /\/\ | u-jmolse%ug@cs.utah.edu or ...!utah-cs!utah-ug!u-jmolse /\/\ "Really stupid people use computer programs every day." Chuck McManis