The Color Circle and Color Vision
A color circle based on spectral wavelengths appears with red at one end of the spectrum and violet at the other. A wedge-shaped gap represents colors that have no unique spectral frequency. These extra-spectral colors, the purples, form from additive mixture of colors from the ends of the spectrum.
In normal human vision, wavelengths of between about 400 nm and 700 nm are represented by this incomplete circle, with the longer wavelengths equating to the red end of the spectrum. Complements are located directly opposite each other on this wheel. These complements are not identical to those in pigment mixing (such as are used in paint), but when lights are additively mixed in the correct proportions appear as a neutral grey or white.
The color circle is used for, among other purposes, illustrating additive color mixture. Combining two colored lights from different parts of the spectrum may produce a third color that appears like a light from another part of the spectrum, even though dissimilar wavelengths are involved. This type of color matching is known as metameric matching. Thus a combination of green and red light might produce a color close to yellow in apparent hue. The newly formed color lies between the two original colors on the color circle, but they are usually represented as being joined by a straight line on the circle, the location of the new color closer to the (white) centre of the circle indicating that the resulting hue is less saturated (i.e., paler) than either of the two source colors. The combination of any two colors in this way are always less saturated than the two pure spectral colors individually.
Objects may be viewed under a variety of different lighting conditions. The human visual system is able to adapt to these differences by chromatic adaptation. This aspect of the visual system is relatively easy to mislead, and optical illusions relating to color are therefore a common phenomenon. The color circle is a useful tool for examining these illusions.
Arranging spectral colors in a circle to predict admixture of light stems from work by Sir Isaac Newton. The psychophysical theory behind the color circle dates to the early color triangle of Thomas Young, whose work was later extended by James Clerk Maxwell and Hermann von Helmholtz. Young postulated that the eye contains receptors that respond to three different primary sensations, or spectra of light. As Maxwell showed, all hues, but not all colors, can be created from three primary colors such as red, green, and blue, if they are mixed in the right proportions. The Young–Helmholtz theory is still usually seen as the most effective in modeling human color vision, though the color vision system is far more complex than differences in the retina alone, with different cells in the lateral geniculate nucleus also responding in opponent fashion to complementary colors, and further color coding occurs in the visual cortex. A mnemonic for Newton's seven spectral colors is "Roy G. Biv" .
Read more about this topic: Color Wheel
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