### Color Vision

For centuries scientists wondered how humans perceived color. The scientific study of color vision began with a classic 1666 experiment by Isaac Newton.

Newton was fascinated by glass prisms he saw at the Stourbridge Fair, an international trade exposition. A triangular glass prism produces a rainbow of colors when struck by light from the sun. Newton realized this was a clue to the nature of color.

Newton observed how a prism split light into a rainbow of colors

Coming home with a prism he purchased at the fair, Newton set up a dark room where a small amount of light could be admitted into the room through a hole. Newton placed the prism in front of the beam of light, producing a rainbow of colors on the wall.

Contemplating this phenomenon, Newton decided the prism must bend each color by a different amount. The result was a rainbow display.

##### Color Mixing

Newton performed a simple but original experiment. Positioning a lens in front of the prism, he recombined the rainbow of colors, focusing them onto a single spot.

To his surprise, this mix of colors pro­duced a white light "not at all sensibly differing from the direct light of the sun." Intrigued, Newton kept experimenting.

Newton tried recombining only parts of the colored light by moving a slotted board in front of the prism. He found that if he allowed a band of yellow to project on the wall, and next to it a band of red, he saw a third color–orange–where they mixed. This third color was always one that appeared between the other two on the rainbow.

What experiments did Newton perform with a prism? Where did the concept of "primary colors" come from?

Newton had discovered additive color mixing. Newton found he could create all different colors by adding together wavelengths from three primary colors : red, green, and blue.

"But wait!" you might say. "I was taught in art class that red, yellow, and blue are primary colors because they can be mixed to produce any other color, but no other colors can be mixed to produce them. So how did green get into Newton's scheme?"

The solution is to realize there are two different types of color mixing. Adding different wavelengths together like Newton did with his prism experiment is called additive color mixing. It is used, for example, on computer displays, where red, green, and blue dots combine to produce color images.

The other way to produce different colors by mixing is through sub­tractive color mixing. In subtractive color mixing, wavelengths are removed from a mix.

When you mix yellow and blue paint to get green, you are actually carrying out a form of subtractive color mixing. The pigments of the paint absorb yellow and blue wavelengths, leaving the middle range strongest, producing green.

What is the difference between additive and subtractive color mixing?

When you are mixing paints the primary colors are red, yellow, and blue. They are called primary colors because these colors can be used to produce all the other colors, and no other colors of paints can be combined to produce red, yellow, or blue. But this is only true when subtractive color mixing is used.

##### Theories of Color Vision

In 1802, Thomas Young proposed that all human vision occurred through sensitivity to red, green, and blue. This theory, modified by Hermann von Helmholtz in 1852, came to be known as the Young-Helmholtz or trichromatic (three-color) theory of color vision.

The basic idea was that the eye responded to three primary colors. Combining the three primary colors, using additive color mixing, formed all the other colors.

The finding of three types of color-sensitive cone receptors in the retina supported the three-color theory. One set of receptors is sensitive to long wave­lengths such as red, one to medium wavelengths like green, and one is sensitive to short wavelengths like blue.

What is the trichromatic theory?

So there is some truth to the three-color theory. However, other aspects of color vision cannot be accounted for by the trichromatic theory. For example, it cannot explain the phenomenon of color afterimages.

If you stare at a red dot, then move your gaze to a white wall, you will see a green dot as an afterimage. If you stare at a green dot, you will see a red afterimage. The same thing happens with yellow and blue.

Based on the existence of color afterimages, Ewald Hering proposed the opponent process theory of color vision in 1878. Hering suggested that color vision occurred in three channels where "opposite" colors (called comple­mentary colors) are in a form of competition.

For example, red and green are comple­mentary colors. When you stare at something red, your redness detectors are worn out or fatigued. Their opponents, the green receptors, gain the upper hand, and you see a green afterimage after staring at a red dot.

What is the opponent-process theory? How are color afterimages explained?

The modern form of this theory assumes there are three basic channels for vision. One channel is the red/green channel; another is the yellow/blue channel.

A third channel, the black/white or brightness/darkness channel, may also provide information relevant to color vision. That is a complex issue being debated among researchers.

What are the three vision channels?

The yellow/blue channel may seem odd, because there are no yellow-sensitive cones in the retina. Yellow light stimulates a combination of long-wavelength (red-sensitive) and medium wavelength (green-sensitive) cones.

If there is more activity in blue receptors (compared to red plus green receptors) the brain interprets this as blue. If there is more red plus green activity (as com­pared to blue) the brain interprets this as yellow. The result is a yellow/blue channel.

Yellow and blue act as opponent pro­cesses just like red and green. If you stare at a blue image, you get a yellow afterimage; if you stare at a yellow dot, you get a blue afterimage.

What activity in color receptors does the brain interpret as yellow?

Color-blind people usually are missing one or more cone types: red-sensitive, green-sensitive, or blue sensitive. The result is a disorder in one or both color channels.

The most common type of colorblind­ness is red/green colorblindness. Genetic studies show this type of color-blindness is usually caused by a defective gene on the X chromosome.

If this gene is defective, women (having two X chromosomes) are "protected" by a duplicate copy of the gene on the other X chromo­some. Males (having one X and one Y chromosome) do not have the extra copy, so red/green colorblindness is about 20 times more common in men than in women.

What is the most common type of colorblindness? Why is it more common in men?

A person with no color-sensitive pigments, therefore no color vision, is called a monochromat (one-color person). To such a person, the world looks like a black-and-white TV picture. Colors are shades of gray.

A person with a defect in one channel–either the red/green or yellow/blue channel–is called a dichromat. Both colors in a channel are affected, so if the person cannot distinguish red that same person cannot distinguish green.

A person who cannot see blue as a distinct color will also not see yellow as a distinct color. People with normal color vision use all three channels (black/white, red/green, and yellow/blue). They are called trichromats.

##### Land's Demonstration

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. (Green is the "G" in Roy G. Biv).

When you look at green paint, you are bombarded with all sorts of different frequencies of light, not just those from the middle of the spectrum. These two greens are not at all equivalent, yet somehow we see both as green.

How can this be? Evidently the neurons that interpret something as green can be aroused in a variety of ways, with a variety 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 mixes of frequencies (Ingle, 1985).

What do humans have in common with goldfish, when it comes to color perception?

Edwin H. Land, the father of instant color photography, showed the sensations we call color could be produced by com­bining images photographed under any two different frequencies of light. The only requirement was that the two frequencies be a little bit different from each other and not come from the farthest blue/violet end of the spectrum.

To prepare his demonstrate, Land set up a scene of fruits and vegetables with bright colors. He photographed this table top scene under two slightly different types of light: yellowish-orange and yellowish-green.

The difference in the color was not great. To a human observer, both lighting arrangements involved yellow light. The two pictures were developed as photographic negatives.

In his demonstration, Land placed the two photographic negatives in front of two different projectors. One projector emitted yellowish-orange freq­uencies. The other emitted yellowish-green frequencies.

When the beams passed through the photographic negatives and were combined on a screen, a full color image appeared. As Land reported, in a classic Scientific American article:

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. (Land, 1959, p.47)

What was Land's demonstration? What is the "astonishing conclusion"?

Land says the two beams of light "are bearers of information." The visual system can use information from two slightly different frequency ranges to calculate the colors in a scene.

The calculations would be something like this: "If yellowish-green bounces off it this way, and yellowish-orange bounces off it that way, it can only be blue."

How can two beams of light that appear yellow carry information about all different colors? The light beam is just electromagnetic radiation carrying all sorts 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 the light (even a yellowish light) carries information consistent with an object being blue, your brain tells you it is blue.

As Land put it, "The rays in themselves are not color-making." However, the rays carry information that the brain can use to make calculations, and those calculations are color-making.

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References:

Ingle, D. J. (1985). The Goldfish as a Retinex Animal. Science, 227, 651-654.

Land, E. (1959, May) Experiments in color vision. Scientific American, pp.2-14.

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