"Dave Platt" wrote
...
Art Unwin wrote:
I don't know about waves but my understanding is that all colors come
from the mixing of the three basic colors, or is it four?

Your understanding is in error... at least, if you're referring to
colors in terms of actual photon behavior (energy and wavelength)
rather than to the human *perception* of color.

That's the RGB standard designed for fooling human
eyes into seeing more than just red, green, and blue.

Yup. And, the red/green/blue system is an artifact of the human
visual system... most of us happen to have three different types
of photo-sensitive molecules in the cone cells in our eyes, and these
three types of molecules have their peak receptivities at the
frequencies that we refer to as "red", "green", and "blue."

There seems to be some amount of genetic variation, among humans, in
the exact frequencies at which the peak sensitivies lie. And, some
people have are missing one or more of these types of photoreceptor,
and are referred to as "colorblind".

There are apparently some humans who have four different types of
photopigment, and thus may have an improved ability to perceive
distinctions between colors. Certain species of animal are known to
have four photopigments (one for e.g. UV sensitivity) and I wouldn't
be surprised if some species have five or more variants.

Photons in nature come in *all* EM frequencies.

Yup again. It's an interesting process:

- Light comes in a continuous range of frequencies.

- Our eyes "sample" this continous range, with three types of sensor
having different-but-overlapping sensitivities. Each sensor
generates a variable amplitude (or pulse train) based on the
intensity that it's detecting, within its sensitivity range.

- Our nervous system maps the three amplitudes back into a perception
of a continuous range of colors.

The process is far from perfect... information is lost during the
sampling process, and thus the perception of a continuous spectrum is
necessarily flawed and imperfect.

This is why a mixture of two different pure colors (e.g. red and
green) can look like a single pure color to our eyes (e.g. yellow or
amber)... it happens to excite the red and green photosensors in the
same proportion that a single, pure-yellow light would. Mixed
together, the colors look like one... split them apart with a prism
and you can easily distinguish them and see the trick.

Sometimes the screen on TV or cinema is perfectly white. This in cinema
reflect. This reflected light splitted with the prism has only three
frequences?

[Almost] All Is Illusion.
S*