Color of Light

Abstract

Color can be seen only if there is light. Unlike the absolute sense of musical pitch, possessed by some persons, there is no absolute sense of color. All color perception is relative. For wide ranges of variation of spectral distributions and of location in the chromaticity diagram of the point that represents it, any light source looks white, as do all objects illuminated by it that have reflectances that are both high and independent of wavelength. Such objects are called white, almost regardless of the quality of their illumination. Chromatic objects are those that in these same illuminations, appear qualitatively different from white. The points that represent chromatic objects in the chromaticity diagram have locations different from the point that represents both the illumination and white objects. Chromatic qualities of common sources or illuminations can be sensed only by comparison with some reference illumination. For the colorimetry of reflecting or transmitting objects, illuminants A, C, or D₆₅ are commonly used for reference. They can also be used for assessing the color of light and often are so used to specify the colors of highly chromatic sources. However, to designate commonly encountered qualities of illumination, a continuous series of sources that span the expected range and that are determined by a single parameter is useful. Such a series is provided by the ideal thermal radiators commonly called blackbodies. Their parameter is temperature, expressed on the Kelvin scale.