Color

Abstract

Abstract Two theories of color perception have been dominant. The trichromatic theory relates to blue‐, green‐, and red‐sensitive cones in the retina of the eye. It explains various forms of color blindness and is the basis of the 1931 CIE chromaticity diagram (color triangle) using x,y,Y and the 1976 CIELUV color space using L*, u*, v*. The opponent theory uses the opposites light/dark, red/green, and blue/yellow. It explains a variety of perception phenomena and is the basis of the 1976 CIELAB color space using L*, a*, b*. These apparently incompatible approaches apply to different parts of the color perception process. Modern electronic instruments have greatly simplified the measurement of color. In considering color mixtures, it is important to distinguish two types, having different sets of primary colors and having different results for color mixing. In subtractive color mixing, applying to paints and dyes, the addition of the three primary colors absorbs all light and produces black; here mixing blue and yellow paints produces green. In additive color mixing, applying to beams of light and video displays, the addition of the three primary colors reconstitutes the full spectrum and produces white; here mixing blue and yellow light beams produces white. Fifteen distinct chemical and physical mechanisms explain the various causes of color, all involving the behavior of electrons in matter. Vibrations and simple excitations explain the colors produced by incandescence (flames), gas excitation (vapor lamps), and vibrations and rotations (blue of water). Ligand‐field effects give the colors of transition‐metal compounds and impurities (turquoise and ruby, respectively). Molecular orbitals give the colors in organic compounds (dyes) and charge‐transfer compounds (blue sapphire). Energy bands lead to the colors of metals (copper), semiconductors (vermilion), doped semiconductors (blue diamond), and color centers (amethyst). Geometrical and physical optics explain colors from dispersive refraction (prism spectrum), scattering (blue sky), interference (soap bubble), and diffraction (opal).