Opponent colors

There exist three opponent color pairs:
Light-Dark;
Red-Green;
Yellow-Blue.

We cannot have something that is both light and dark, or red and green, or yellow and blue (but we can have reddish-yellow (orange) and blueish-red (purple), etc.). The two colors in each pair are totally opposite or exclusive. This fact enables us to establish LAB, a 3-D color space, in which these three pairs are the three axes.

Structure below shows the relationship. The diagonal colors are opponent pairs:

     (O)
    R - Y
(M) |   |
    B - G
     (C)

       (橙)
      红--黄
(品红) |   |
      蓝--绿
       (青)

It seems that there should be four basic or root colors (RYBG), however Y can be produced by mixing R and G (this makes sense as Y resides between R and G in spectrum, so the overall response from eye cone receptors is yellow) so Y is not a primary color.

Anyway, opponent colors are very mysterious to me.

Opponent colors appear in our afterimage or ghost image, which refers to an image continuing to appear in one's vision after the exposure to the original image has ceased [2].

Are opponent color pairs the same as the inverse colors in the negative film? I don't think so. The three primary colors in negative film should be CMY.


References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005
2. www.absoluteastronomy.com/topics/Afterimage

Brightness, hue, and saturation

These are the three attributes of color.

Brightness is the achromatic component, i.e., light power or intensity that detected by our eye.

Hue and saturation are the chromatic components. Simply defining,
Hue = wavelength;
Saturation = spectral purity.

Hue:
The wavelength that appears most prevalent in a color sample determines its hue. The set of basic hues, for example, 赤橙黄绿青蓝紫, is very subjective and differ from culture to culture.

Saturation:
"Spectral purity" is enough to define saturation. Lasers produce the most saturated colors whereas white-gray-black are the least saturated colors.

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

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

Light source and color temperature

1. Hot source and cold source:

Light emission (I am talking about incoherent light, not laser light this time) is from two basic types of sources: heat source and non-heat (or cold) source. For the first source, light radiation is only from thermal energy and we call it blackbody radiator (this process is called incandescence). For the latter source, light is not caused by a rise in temperature and we call it luminescence process.

2. Color temperature:

This term is only for blackbody radiators. At 3200K, long wavelength dominates; light appears yellow and is from a typical incandescent light bulb. When temperature rises to 5000K, the blackbody emits relatively flat spectrum and is a very neutral white. At higher temperature, short wavelength starts to dominate and appears blueish.


Figure is from: www.techmind.org/colour/coltemp.html

So when we talk about color temperature, remember it is only for pure thermal source! But color temperature is used very often on all other sources, e.g., fluorescence light tubes. Strictly speaking, this should be called "correlated color temperature" because people are picking the closest blackbody temperature to quantize the color the source appears.

(In addition, color temperature is for white light. When we want to know or quantize how white a source is, we use color temperature so we know if it is neutral, blueish or yellowish. For monochromatic sources, color temperature is no use.)

3. Luminescence:

Luminescence is light that emitted at low temperatures without heat so it is "cold body radiation" (as compared to blackbody radiation). Fluorescence is only one mechanism of luminescence: the material absorb high-energy photons at UV or blue spectrum and re-emit photons (Stokes photon) in the visible spectrum. An example is UV brightener, a material that paper and ink manufacturers use to make an extra-white paper or extra-bright ink.

Atomic physics view of luminescence vs. incandescence:
Luminescence occurs when material absorbs external energy and electrons are pumped to excited states, then the radiative transition back to lower state produces luminescence emission. Incandescence emission is from the thermal vibration of heated atoms themselves. This temperature radiation is in the far IR spectrum region when material is at room temperature and shifts towards visible when material's temperature increases.

References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005.
2. Luminescence (physics) -- Britannica Online Encyclopedia. www.britannica.com