miguel@aclcb.purdue.edu (Phillip) (03/25/91)
Two days ago I asked how, given that yellow/blue and red/green are the
primary colors, the color TV monitor could work since it uses only red,
green and blue phosphors. Thanks for all the responses. With them and a
little reading in a few text books I've got the idea.
In summary:
--There are no "physical" additive primaries; they are a product of
vision
--There are 3 types of cones in the retina and each produces only
one class of photopigments
--These photopigments are either most sensitive to long (red ~570 nm),
middle (green-yellow ~535 nm) or short (blue ~445 nm)
--These photopigments are made up of two parts: the retinal moiety
and the opsin (protein) moiety. Only the opsin moiety is
different among the three photopigments of cones (and the
photopigment of rods).
--Transduction of a stimulating photon into signal occurs by this
photon reacting with the photopigment to cause the "bleaching"
of the retinal from its bent, 11-cis, state to its
straightened, all-trans, conformation. Decrease in
concentration of cGMP is implicated as a messenger which
results in receptor potentials.
--Opponent processing, which produces the red&green and blue&yellow
primary complements, does not occur in the cones but in the
retinal ganglion and lateral geniculate nucleus.
--"Unique blue" is perceived at 475 nm where the red-green system
is balanced while the blue-yellow is "tilted" towards blue
--"Unique green" is percieved at 515 nm where the blue-yellow
is balanced while the red-green system is tilted towards
green
--"Unique yellow" occurs at another wavelength where red and green
are balanced-out, 580 nm; here the blue-yellow scale is
tilted towards yellow
--There is no single wavelength which causes the blue-yellow scale
to be balanced while the red-green scale is tilted towards
red. Thus, no single wavelength can produce the sensation
of pure unique red untinged by any other color.
--The red of a red, blue and green monitor is actually magenta and
tinged with yellow. Adding the right amount of green
cancels (balances) out the red, leaving yellow.
--So here is the additive "color wheel":
Red
Purple | Orange
\ | /
\ | /
\ | /
\ | /
\ | /
Blue---------------+---------------Yellow
/ | \
/ | \
/ | \
/ | \
/ | \
Blue-green | Yellow-green
Green
choosing judiciously, one can pretty much pick any three
colors (they will tend to be 120 degrees apart) and
generate all the other colors with them.
So thanks everyone for helping me understand this problem which had been
perplexing me for many days.
Take it easy,
_ _
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