> It’s interesting to me because I lecture on vision as part of my job, and to
> day I’ve spoken only about rods and cones (and photo-sensitive ganglion
> cells), never about the 3-colour properties of most people’s cone cells. And
> now I know that there are people with 4-colour cells.
Several years ago I did quite a bit of research into this. The people
with four-color (not colour) cones are rare enough to not be
considered part of the normal human race. :) HOWEVER, there are
people who have rods with some pigment.
It's pretty fascinating that the human eye does not actually "see"
red. The red cones see a wide band of color from red down to yellow.
This band is overlapped by the green cones. To determine "red", the
human vision system sees the absence of green. So if the eye sees
green and red, it's actually yellow/orange. If it sees red but not
green, then it's actually red.
This is very different than most digital sensors that traditionally
have had pretty narrow-band filters with little overlap between blue,
green and red sensels. Canon sensors are very wide-band with a thin
pigment. The color accuracy is secondary to dynamic range. Color is
determined by the same method as the human eye - using "mix-minus"
calculations. (done on-chip at time of exposure, pre-raw). The Kodak
sensors used a very deep filter that preserved color accuracy, but at
the expense of bit-depth and sensitivity.
Not much of that matters to me at the moment. Everything is monochrome
in Alaska now.
AG Schnozz
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