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Color Mixing

Differently colored lights can be mixed together optically to produce lights of another color. Which lights and which mixtures perceptually match each other can be predicted from a few relatively simple rules. The simplicity of these rules, discussed in this entry, derives from the properties of the visual photoreceptors in our eyes that convert arriving photons into neural signals, rather than from physics.

Overview

The visible spectrum is that part of the electromagnetic spectrum that we can see. It covers wavelengths from about 390 to 730 nanometers (nm). When viewed alone, the appearance of monochromatic lights made up of single wavelengths varies across the spectrum from violet at short wavelengths through blue, blue-green or cyan, green, yellow-green, yellow, orange to red at long wavelengths. Sodium street lighting, which appears yellow, is a commonly encountered example of lights that are nearly monochromatic. Most lights that we encounter in the natural environment are broadband in the sense that they consist of lights of many different wavelengths covering broad regions of the spectrum.

Mixtures of monochromatic lights or mixtures of broadband lights can be perceived as identical even when the components in the mixtures are physically different. For example, a mixture of red and green monochromatic lights can appear identical to a yellow monochromatic light, and a mixture of blue and green monochromatic lights can appear identical to a cyan monochromatic light (see color insert, Figure 16, left panel). The relationship between the physical characteristics of lights or mixtures of lights and whether or not they appear to match can be investigated in simple color matching experiments.

Figure 16 Color Mixing—Additive and Subtractive Color Mixing (1)

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Note: The left panel simulates the additive combination of red, green, and blue lights. The right panel simulates the subtractive combination of yellow, purple, and cyan pigments. See the Color Mixing entry for additional information (pp. 262–264).

Properties of Color Mixing

In a typical color matching experiment, an observer looks at a circle, half of which is illuminated by a “test” light of variable wavelength (λ) and the other half by a mixture of three “primary” lights usually chosen to look red (R), green (G), and blue or violet (B). For each test light, the observer adjusts the intensities of the three primary lights, so that the test field is perfectly matched by the mixture of primary lights. With the proviso that sometimes one of the primaries must be added to the test light to complete the match and that the primaries must be independent (in the sense that none of the primaries can be matched by a mixture of the other two), only three primaries are required to match any test light. The upper panel of Figure 1 shows the mean red, green, and blue color matching functions (CMFs) for primary lights of 645, 526, and 444 nm (denoted by the vertical dashed lines) as a function of test wavelength, λ. Each function defines the amount of that primary required to match a monochromatic light of λ nm. That any light can be matched by a mixture of just three primary lights reflects a fundamental property of normal human vision: it is trichromatic. The operation of color television, computer monitors, and projection systems, which produce colors by mixing together red-, green-and blue-appearing lights in different proportions (as you can see for yourself by looking closely at the screens), relies on our color vision being trichromatic.

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