[geeks] 720 Megapixel photo

[wa2egp at att.net]

> AIUI there are two different versions of one of the colour receptors,
> and the gene is on the X chromosome.  A woman can thus end up with four
> different colour receptors instead of the usual three.  It is
> apparently not a myth that some women have a richer experience of
> colour than men (or other women).  The difference in the response
> curves between the two versions is apparently slight but definite.

Could explain why my wife doesn't like my taste in clothes....:)


> > Well, potentially the eye may have a "digital" response to color (the
> > chemicals either react or not) but which has greater "color"
> > resolution (if I may use that term), the monitor or the eye, I have
> > no clue.
> 
> That's not what I'm talking about.
> 
> When you look at something, your colour experience (assuming you're a
> typical trichromat, which you personally apparently are - I'm speaking
> to the generic "you" here) can be reduced to three numbers, the
> responses of your three colour receptors to the particular frequency
> spectrum of light you're getting.
> 
> Now, when you look at a colour on a colour monitor, your receptors'
> responses are (effectively) the sums of their responses to the levels
> of the three primaries displayed.  Depending on how the three
> phosphors' (for a CRT, the analog for other technologies) emission
> spectra match your receptors' response curves, it may be impossible to,
> for example, get receptor C firing above 50% without receptor A firing
> above 10%, because each of the primaries has a spectrum that stimulates
> receptor A at least 1/5 as much as C.  Yet it's possible that the right
> spectrum of light could well achieve such a condition.  This would mean
> there is a colour you can see in the world but which you cannot see on
> a colour monitor.

I know there is an overlap of the spectral response for each of the
"primary" colors, that's how we see "yellow" on a TV or monitor that
actually only generates three colors.  Monitors (and graphics cards)
do have a limited number of color intensities for each primary that
can be produced which limit the number of colors.  The "real" world
easily can have more.

 > Now, as I say, I haven't looked up the response curves of human
> photoreceptors and typical CRT phosphors to see whether such a
> situation is actually possible.  But given how messy the real world is,
> it would rather surprise me if there weren't some such unreproducible
> colours somewhere in the space of all viewable colours.

Agreed.

> Then there are afterimages; there may be more "impossible" colours (ie,
> firing ratios not normally achievable) lurking there.
> 
> The above discussion is actually oversimplified, because it ignores the
> rods; most humans actually have four receptors, and a tetrachromat has
> five, but the extra receptor has a more or less flat response curve and
> does not give rise to perception of colour on its own.  I don't know
> enough about human visual processing to know whether it affects
> perception of colour or not; if it does, there's another response curve
> to consider.

There was a physics lab I would do with students where I would take them
to a dark corridor that leads to the outside.  I'd have them close their 
eyes and cover their eyes with one hand (Shut down the cones, keep the
rods working) for a few minutes.  Then I would lead them, chain gang 
style, outside to a sunlite, contrasty scene.  I would have them uncover
their eyes for only a fraction of a second, immediately cover them again
and then look at the afterimage.  It would always come out purple and 
black, 3D (no matter where they moved their eyes) and after a minute or so,
the colors would reverse.  If you're getting enough vitamin A, it works
everytime.
The human eye is really lousy in "seeing" individual colors in a mixture.
That's why we use diffraction gratings when looking at spectra.  Gets
too messy with all the interaction.

Bob