jbm@eos.UUCP (Jeffrey Mulligan) (09/22/88)
Pardon me for not editing the newsgroups line, but I couldn't decide to which newsgroup this digression was less relevant. From article <255@rna.UUCP>, by dan@rna.UUCP (Dan Ts'o): > As far as why your blue LED appears purplish. It probably is > psychophysical/physiological, rather than any red output. agreed > Just as the > extreme shortwavelength end of the rainbow appears violet/purple, the > blue LED probably is emitting light at, say 430nm. OK > Your red cones in the > retina actually have significant sensitivity in the deep blue region -- > more so than your green cones. certainly not (reference upon request) > Thus deep blue appears purplish or > actually bluish/red, an anomolous sensation. ????????? The shortest visible wavelengths are usually referred to as "violet" (that's why the slightly shorter invisible wavelengths are "ultraviolet"). Psychologically unique "blue" (that which appears neither reddish nor greenish) is evoked by somewhat longer wavelengths (without looking it up, I'd guess about 480 or 490nm). A light increment (away from white) affecting only the blue-sensitive cones will appear violet, while the corresponding decrement appears chartreuse (greenish-yellow). The commonly accepted physiological explanation for this is that the signal from the "blue" cones combines with signals from the "red" cones in the formation of a red-green opponent signal in the brain. -- Jeff Mulligan (jbm@aurora.arc.nasa.gov) NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035 (415) 694-6290
dan@rna.UUCP (Dan Ts'o) (09/23/88)
In article <1586@eos.UUCP> jbm@eos.UUCP (Jeffrey Mulligan) writes: >> Your red cones in the >> retina actually have significant sensitivity in the deep blue region -- >> more so than your green cones. > >certainly not (reference upon request) Please let me know your reference. I realize that this issue may be somewhat controversial, partly because there must be at least a dozen reported methods for determining cone spectral sensitivity. But look at Smith and Pokorny, Vision Research 15:161-171 (1975) or Boynton in Human Color Vision (Holt, Rinehart and Winston, 1979). The spectral sensitivity of human foveal cone mechanisms as obtained by high intensity heterochromatic flicker, in the region of 400-500nm, shows a crossover point of 450nm, beyond which the red cone mechanism is actually more sensitive than the green cone mechanism. At that point, coincidentally (or perhaps not), the blue cone mechanism also over takes both the red and the green (but this last point is just an aside, since the absolute relationship between blue and red+green doesn't matter, just the relative one). >The shortest visible wavelengths are usually referred to as "violet" >(that's why the slightly shorter invisible wavelengths are "ultraviolet"). >Psychologically unique "blue" (that which appears neither reddish >nor greenish) is evoked by somewhat longer wavelengths (without >looking it up, I'd guess about 480 or 490nm). A light increment >(away from white) affecting only the blue-sensitive cones >will appear violet, while the corresponding decrement appears >chartreuse (greenish-yellow). The commonly accepted >physiological explanation for this is that the signal from >the "blue" cones combines with signals from the "red" cones >in the formation of a red-green opponent signal in the brain. Right. So my crude understanding of this idea is that blue beyond 450nm adds red input anomalously because the red/green opponent system normally expects the green cones always to be more active than the red cones for wavelengths that are green and shorter, like blue (this is an oversimplication, but anyways...). However, at 450nm, all of a sudden the red cones are contributing more than expected, and this "fools" the visual system into thinking that there is actually red light coming in. (again, an oversimplication). Well, I guess this doesn't belong in rec.audio anymore. Why did it start here in the first place... But this just shows that not only do we not understand much about auditory perception, but we don't understand much about visual perception either.