Eye and color event

1. Eye structure:

(Image is from: http://www.schools.net.au/edu/lesson_ideas/optics/optics_wksht2_p1.html)

Of all these names I should know the most important ones as an optical scientist:

Cornea: focusing light to form an image (together with lens; but cornea plays the major role on focusing [1, page 16]).

Iris: aperture.

Lens: besides focusing adjustment, it also acts as a UV filter to protect the retina.

Retina: see next section.


2. Retina

There are two types of nerve cells, or receptors, in the retina:

Rods: provide vision at low light and has a peak absorption at 499nm. It is color blind.

Cones: provide color info. The three types of cones, RGB, have peak responses at 420nm, 530nm and 565nm respectively.

(Image is from: Eysenck, Cognitive Psychology: A Student's Handbook)

Therefore one can stimulate almost any colors by using just three well-chosen primary colors.

Additive primary colors - RGB:
starting from black (no wavelengths), adding R,G,B one by one, we obtain white light (all wavelengths).

Subtractive primary colors - CMYK:
starting from white,
subtracting cyan, we get red (cyan ink is "red-subtractor" or "long-wavelength subtractor");
subtracting magenta, we get green (magenta ink is "green-subtractor" or "medium-wavelength subtractor");
subtracting yellow, we get blue (yellow ink is "blue-subtractor" or "short-wavelength subtractor").

Opponent color pairs:
This is very mysteries to me. The opponent color pairs are:
Light-Dark;
Red-Green;
Yellow-Blue.

3. Color event

Strictly speaking, color is an event. It is a product of three things: light, object, and observer.

4. Metamerism

Metamerism is a phenomenon that two incident lights with different spectra produce the same color sensation by human eye. For example, a blend of R and G produces Y but this is different from a pure Y produced by a yellow laser, although both appear the same color, yellow, to our eye. Another example is that two clothes having the same color in store may become different colors viewed under sunlight or at home. This is because of the limitation of our eye as a spectrum analyzer. Our eye divides the incident light into only 3 components by the R,G,B receptors, whereas an optical spectrum analyzer is able to divide the incident light into many pieces. In other words, our eye's resolution bandwidth is very crude and this causes metamerism.

Metamerism is good! Why? Because if without metamerism, our printers would need many inks in all different colors, instead of just four (CMYK).

References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005