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Essay on Color, Para. 2, reality in physics (continued)

2.6 Surface Color and Colored Light
Talking about colors, in most cases we mean surface colors. The wall and the willow, the car and the cow, all bear surface properties that we call a color. White (or at least: broad-band) light impigning on such surfaces is not reflected equally at all wavelengths. At some wavelengths reflection is enhanced, at other wavelengths absorption prevails. The light that is reflected from these objects to our eyes will show a different distribution of intensities over wavelengths than the original illuminant (e.g. skylight).

The reflected light carries the information about surface color to our eyes. And so, strictly speaking we see only colors of light.

But in a narrower sense, "color of light" means either an illuminant (e.g. daylight / candlelight) or the light color of light emitters (painted lamp, low-pressure gas discharge tube, tv screen phosphor, laser, setting sun).

Look at fig. 2.6. Reddish light is reflected at a blue surface and carries color information to the eye. The incident light spectral characteristics is multiplied by surface reflectivity spectral characteristics, before it reaches the eye.

    fig.2.6: multiplying surface color and light color (15 kByte)

Though "received power" and "reflectivity" do not look very much alike, our eyes will anyway recognize the surface color as "blue" ... provided there is some obviously white object (or some object of known broad-band color) in the scene.

Most evident consequence: Spectral components that are missing in the illuminant cannot be regained by surface colors. (Except for some rare colorants which fluoresce.) And some surface colors may turn invisible (i.e. black) or ill-matched if the illuminant is not appropriate. You can read more about this problem in para. 3.4 "metamerism".

Continued: 3. technology / 3.1 mixing additively
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