Re: Just How Blind is the Human Race?

From: Dennis M. Hammes (scrawlmark_at_arvig.net)
Date: 07/01/04 

Date: Thu, 01 Jul 2004 07:33:34 GMT

Ian Stirling wrote:
>
> In sci.space.policy Dennis M. Hammes <scrawlmark@arvig.net> wrote:
> > Ian Stirling wrote:
> >>
> >> In sci.space.policy Dennis M. Hammes <scrawlmark@arvig.net> wrote:
> >> > Ian Stirling wrote:
> >> >>
> >> >> In sci.space.policy Christopher James Huff <cjameshuff@earthlink.net> wrote:
> >> >> <snip>
> >> >> > Well, we really are practically blind. Our eyes are trichromatic
> >> >
> >> > Monochromatic-with-filters; or "quadrichromatic." But not "tri-."
> >> > 11-cis retinal is the /only/ photosensor we've got.
> >> > The three oil filters (assorted among the "cones") reduce the
> >> > incident light level considerably, not the sensitivity.
> >> > Same happens putting color filters on a camera or litho
> >> > separations.
> >> >
> >> >> > sensors, with limited resolution and only capable of giving very crude
> >> >> > estimates of color, lightness, and size. We can not see spectral
> >> >>
> >> >> Looking at the spectral sensitivities, it's amazing there is
> >> >> any vivid contrast between red and green at all.
> >> >
> >> > Heh. Some few people have none whatsoever.
> >> > Monochromatic contrast is /all/ in the filters.
> >> >
> >> >> The two sensors are so similar that the difference in sensitivity
> >> >> at any given wavelength between red and green is quite small.
> >> >
> >> > Actually not; we are rather more sensitive to green by at least
> >> > e=hf.
> >> > Why subs and planes are set "cockpit red" at night.
> >>
> >> That's to optimise the low-light sensitive cells, which are different
> >> to the ones that are used to sense colour vision, and very insensitive
> >> to red.
> >
> > Merely "less" by e=hf.
>
> True, quite a lot less though, especially if you'r working down near
> the tail end of red sensitivity, and comparing it to an equivalent
> white or green light.
>
> <snip>
> > They're "most sensitive" to "green" because (see your chart,
> > again) the "green" filter is the sloppiest, i.e., has the broadest
> > passband. Neural response is proportional to number of successful
> > (at 11-cis --> 11-trans-retinal) incident photons above the
> > infrared, and the green cone is passing a lot of red and blue.
> > Rods have no filter and a longer absorption path along which
> > /more/ photons will be successful at converting retinal, making the
> > /neuron/, not retinal, more sensitive with respect to incident
> > levels.
> > (N.B.: "Infrared" is /defined/ by retinal's e=hf threshold; it's
> > the color whose energy is too low to succeed at the transition.
>
> Which is in itself quite a fuzzy number, only hitting a millionth
> way out at over 800nm.
>
> > "Ultraviolet" is defined by the passband of the filters, humors,
> > lens, and cornea; water and window glass, e.g., are opaque to it.)
> >>
> >> Normalising the sensitivity, where 1 is the sensitivity peak.
> >> The differences are fairly small, compared to the differences between
> >> them and blue.
> >>
> >> red green blue
> >> 658nm .1 .085
> >> 600nm .8 .3
> >> 570nm 1 .8
> >> 554nm .96 .96
> >> 542nm .9 1
> >> 513nm .5 .7
> >> 503nm .32 .47 .1
> >> 442nm .04 .07 1
> >> 456nm .06 .1 .84
> >
> > This is a /filter/-sensitivity curve set. Be rather sharper and
> > essentially exclusive were they dichroic rather than dye filters,
> > too.
>
> True.
> Got a kit to upgrade the existing ones?

Sometimes I wish. But a dichroic filter is a /physical/ filter, the
surface scored like a Fresnel lens but with all prisms at the same
angle. Their passbands are all identical, and are as broad or
narrow as the sampling slit with respect to the total spread.
  Couldn't do that with wet or crystal chemistry (whose passbands
owe to bond resonances, many of which are broadened, even shifted,
by the presence and nature of adjacent bonds).
>
> I was referring only to the dramatic difference between the sharpness of
> the red/green filters, and the blue ones, which is really quite dramatic,
> and emphasises how much of the red/green distinction is not made by the
> filters, but by the processing of their output by the neurons and brain
> behind them.

Etc. Save that red-green color blindness (the most common kind) is
absolute, and is not solveable by reference to resulting
grayscales. (Ditto the other varieties.)
>
> Obvious reasons can be seen for the tradeoff. Compared to having R/G
> filters with similar sharpness to the B one, you gain significantly
> in lower light visual acuity, though the colour drops out as the
> signal-noise from the incoming photons becomes too poor to work out
> if something is red or green.

Be more interesting to know what that chart was calibrated against,
because simple inspection of a spectrum shows that green is the
narrowest filter; even including the yellow (R+G) fringe, it's the
/narrowest/ color band we see, and /that/ is definitive of the
passband. I.e., the green cone is fired by frequencies from *here*
to *here*; that's the passband, and it's observably the narrowest. 

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