Barry Rice

- doing science on a terrestrial planet -

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What is color?

Visible light, although occupying only a tiny range of the electromagnetic spectrum, is of particular interest for two reasons. First, our sun emits most of its electromagnetic radiation in the visible wavelengths. Second, while our atmosphere blocks many forms of radiation, it does not impede visible radiation. As a result of these two effects, the daylight surface of the Earth is bombarded with visible radiation. Organisms on the Earth's surface have exploited this by evolving sophisticated radiation-sensitive sensory organs (for example, eyes). Eyes are very sophisticated--they produce accurate, high resolution images at a wide range of illumination levels. The eyes of many organisms, humans included, can distinguish photons of different wavelengths within the visible light range. (Specifically, human eyes perform three-band multispectral photometry.) The different wavelengths in this range are called "colors." Ordered from long to short wavelengths, these colors are red, orange, yellow, green, blue, indigo, and violet, remembered with the peculiar mnemonic "Roy G. Biv."

The human eye-brain system responds in a peculiar way when it simultaneously detects photons of different colors. For example, if the eye receives red and green photons simultaneously, it reports that it is detecting "yellow." Only by analysis with a more reliable piece of equipment can a careful researcher determine if the "yellow" being detected is a result of a beam of yellow-wavelength photons, or a beam containing both red photons and green photons.

If the human eye is bombarded with a crowd of diverse photons with wavelengths sampling all the colors ROYGBIV, the overworked eyeball-brain system reports the detection of "white light." Conversely, "black" is the signal reported to indicate the lack of photon detection. The concepts of "white" and "black" are artifacts of our eye-brain's light sensing and interpretation limitations and do not have much in the way of counterparts in the real universe.

It is useful for the student of remote sensing to ponder color further. Consider white lilies in a field. These flowers are highly reflective at all optical wavelengths. As such, they reflect all the light that strikes them, and when they are illuminated by "white" sunlight, they appear "white" to our eyes. Now, consider a yellow lily. This flower may appear yellow for one of several reasons.

The human eye cannot distinguish between the three situations above, but many other remote sensing instruments can. This is an example of how remote sensing instruments can out-perform human sensory abilities.

For reasons of completeness, I am compelled to note that even a flower that uniformly reflects all colors equally (i.e. a "white" flower) would appear yellow if illuminated by a yellow beam of light.


4 February 2009