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From: dan@rna.UUCP (Dan Ts'o)
Newsgroups: sci.electronics,rec.audio
Subject: Re: appearance of violet light (was: Blue LEDs)
Message-ID: <261@rna.UUCP>
Date: 23 Sep 88 03:17:31 GMT
References: <255@rna.UUCP> <1586@eos.UUCP>
Reply-To: dan@rna.UUCP (Dan Ts'o)
Organization: Rockefeller University Neurobiology
Lines: 45

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.