Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
For nearly a century, scientists argued about whether the trichromatic theory or the opponent process theory explained color vision. As it turned out, both theories were partly right. The trichromatic theory was upheld by the discovery of the three types of cones. The opponent-process theory was upheld by the discovery of red/green and yellow/blue pathways. However, neither theory fully explains human color perception. A series of demonstrations by Edwin Land, inventor of instant color photography, made this clear in the 1950s.
The perception of color is a psychological experience and a neural event. The same event can be activated in a variety of ways. When you look at the green part of a rainbow, you are looking at nearly pure frequencies around the middle of the visual spectrum (the "G" in Roy G. Biv). When you look at green paint, you are bombarded with all sorts of different frequencies of light. The two greens are not at all equivalent, yet somehow we see both as green.
What do humans have in common with goldfish, when it comes to color perception?
How can this be? Evidently the part of your nervous system that interprets something as green can be aroused in a variety of ways, with different sorts of physical events. This is a fancy trick, but it is widely shared in the animal kingdom. Even goldfish see colors the way humans do, able to discriminate the "same" color in many different ways, using stimuli composed of different mixes of frequencies (Ingle, 1985).
Edwin H. Land, the father of instant color photography, showed that the sensations we call color could be produced by combining images photographed under any two different frequencies of light, as long as they were a little bit different from each other and did not come from the farthest blue/violet end of the spectrum.
What was Land's demonstration?
In his demonstration, Land placed two photographic negatives of a scene with fruits and vegatables in front of projectors. One projector emitted yellowish-orange frequencies. The other emitted yellowish-green frequencies. Earlier, the same two colors had been used to take images of the scene, captured in the photographic negatives. The difference in the color of the two beams was not great, to a human observer. Both projectors seemed to be emitting yellow light. However, when the beams passed through the photographic negatives and combined onto a screen, a full color image appeared. As Land (1959) put it:
What is the "astonishing conclusion"?
In this experiment we are forced to the astonishing conclusion that the rays are not in themselves color-making. Rather they are bearers of information that the eye uses to assign appropriate colors to various objects in an image. (p.47)
Land says the two beams of light "are bearers of information." Evidently the visual system can use information gained simultaneously in two slightly different frequency ranges to calculate the colors in a scene.
How might the analogy to FM radio signals be helpful?
How can two beams of light that appear yellow carry information about all different colors? The light beam is just electromagnetic radiation carrying lots of potentially discoverable information. The frequency of an FM radio station does not determine the type of music you hear on the station. The FM signal is just a medium for coding the information received by an FM receiver. Evidently the same is true of light waves used to perceive color. If they carry information consistent with an object being blue, your brain tells you it is blue, even if the information is found in relationships between yellow wavelengths. As Land put it, "The rays in themselves are not color-making."
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Copyright © 2007-2011 Russ Dewey