Light is electromagnetic energy within a narrow range of frequencies capable of causing the sensation of vision. The human eye/brain combination acts, in some respects, like a radio receiver tuned to receive this particular range of radio frequency energy. Other electromagnetic energy at frequencies lower than light include radio waves, microwaves, and infrared heat waves, and at frequencies higher than light are ultraviolet, x-ray, gamma, and cosmic rays.

The high frequency visible light range, or spectrum, of 400-750 terahertz is usually specified by wavelength rather than frequency. Each different wavelength of light energy (if seen alone) is perceived by the human eye/brain as a different color. The lowest frequency visible light energy (extreme red) has a wavelength of about 750 nanometers (billionths of a meter). The highest frequency visible light energy (extreme violet) has a wavelength of about 380 nm. Some perceived colors, such as purple, do not correspond to any one wavelength of light, and are called non-spectral colors. These colors are the result of a mixture of various proportions of spectral colors.

If the light energy from the sun is all equally reflected from some surface, the eye/brain sees the surface as white. If another surface absorbs all light energy and reflects none, the surface appears black. If yet a third surface absorbs all light energy except that of one wavelength, which it reflects, the surface appears to the eye/brain as a particular color. If only long wavelength visible light energy is reflected, the surface appears some shade of red; short wavelength visible light appears some shade of violet.

If more than one wavelength is reflected, the eye/brain performs a mixing of all light energy present and perceives a single color which is a result of the mixture. Paints, dyes, or pigments appear a particular color because they absorb light energy of all other wavelengths. The color of a solid object is formed by the scattering or reflection of only those colors in the illuminating light source which are not absorbed. This is a subtractive process.

When paints, dyes, or pigments of two different colors are mixed, a third color results. The resulting material absorbs all the light energy that was absorbed by each of the two original materials. The resulting color mixture is always darker than either of the two original colors since more light energy is being absorbed.

When viewing a mixture of light sources directly, however, the eye simply sees a summation of all the light energy present in the individual light sources. This is an additive process. When light sources of two different colors are mixed, a third color results. The resulting color is always brighter than either of the two original colors since more light energy is present in the light mixture.

Since the light sources mix by an additive process rather than a subtractive process, the resulting color is different than when mixing paints of the same colors. The figure illustrates the additive mixing process that results when mixing lights.

When mixing either paints or lights, a set of three primary colors is often chosen, which are then mixed in various combinations to reproduce the range of desired colors. Primary colors are simply a set of colors (usually three) chosen such that none of the primary colors can be duplicated by any combination of the other two.

There are infinite sets of primary colors. The choice of primary colors determines the range of mixed colors able to be reproduced. For mixing paints, pigments, and dyes, the primary colors of red, yellow, and blue are often chosen, although printers usually use cyan, yellow, and magenta.

For mixing lights, the primary colors of red, green, and blue are usually chosen because they correspond most closely to the response of the eye and provide the greatest range of color matches.

The human eye/brain combination responds to three different characteristics of emitted or reflected light energy. These three human sight characteristics are hue, saturation, and brightness.

Hue is the perceived color of visible light energy, expressed as violet, blue, green, yellow, orange, red, etc. This hue may be a result of a single wavelength of light energy or the dominant wavelength of multiple wavelengths of light energy. The term color is often interchanged for hue. Since white light is a combination of all colors of light, and has no dominant wavelength, it has no hue.

Saturation is the colorfulness of an area judged in proportion to its brightness, and is primarily the degree to which light energy is concentrated at a single wavelength. Light energy of a single wavelength, appearing to the eye/brain as the color red for example, is at maximum saturation.

Light of equal energy at all wavelengths appears to the eye/brain as white and is at zero saturation. As some white light is added to pure red light, the red desaturates somewhat to a pastel pink. As more light is added at wavelengths other than red, eventually the color reaches zero saturation, which is white light.

Brightness is the visual sensation in which an area appears to emit more or less light. Brightness is a characteristic of any light source, while hue and saturation are characteristics only of light sources that are not balanced white light.

White and shades of gray have different brightness levels, but have no hue and have zero saturation.