Editing Color wheel (section)

==The color circle and color vision==
{{Spectral colors simple table}}
[[File:Opponent color circle 1917.png|thumb|A 1917 four-way color circle related to the color [[opponent process]].]]
{{See also|Visible spectrum|Spectral color}}
A color circle based on spectral wavelengths appears with [[red]] at one end of the spectrum and is 100% mixable [[Violet (color)|violet]] at the other. A wedge-shaped gap represents colors that have no unique spectral frequency. These extra-spectral colors, the [[purple]]s, form from an additive mixture of colors from the ends of the spectrum.

In normal human vision, wavelengths of between about 400&nbsp;nm and 700&nbsp;nm are represented by this incomplete circle, with the longer wavelengths equating to the red end of the spectrum. Complement colors are located directly opposite each other on this wheel. These complement colors are not identical to colors 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.<ref name="complements">Krech, D., Crutchfield, R.S., Livson, N., Wilson, W.A. jr., Parducci, A. (1982) ''Elements of psychology'' (4th ed.). New York: Alfred A. Knopf. pp. 108–109.</ref>

The color circle is used for, among other purposes, illustrating [[additive color]] mixture. Combining two colored lights from different parts of the spectrum will 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 [[Metamerism (color)|metameric]] matching.<ref name="Schiffman"/> Thus, a combination of [[green]] and red light will produce the color [[yellow]] in apparent hue. The newly formed color lies between the two original colors on the color circle.

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 illusion]]s relating to color are therefore a common phenomenon. The color circle is a useful tool for examining these illusions.

Arranging [[spectral color]]s in a circle to predict admixture of light stems from work by Sir [[Isaac Newton]]. Newton's calculation of the resulting color involves three steps: First, mark on the color circle the constituent colors according to their relative weight. Second, find the [[barycenter]] of these differently weighted colors. Third, interpret the radial distance (from the center of the circle to the [[barycenter]]) as the saturation of the color, and the [[Azimuth|azimuthal position]] on the circle as the hue of the color. Thus, Newton's color circle is a predecessor of the modern, horseshoe-shaped [[CIE 1931 color space|CIE color diagram.]]

The psychophysical theory behind the color circle dates to the early [[color triangle]] of [[Thomas Young (scientist)|Thomas Young]], whose work was later extended by [[James Clerk Maxwell]] and [[Hermann von Helmholtz]].