The Primary Colors

The human eye is not a selective instrument; it cannot distinguish two superimposed colors as such.  Taking advantage of this fact, in 1801, Thomas Young, and later Hermann von Helmholtz, found that it was possible to match any given colored light using a combination of three primary light sources.  Occasionally one color was found that could not be matched by direct addition of the three primaries.

It was always found, however, that if one of the primaries was added to the given color, the other two primaries could produce a color match with the combination of the sample and the third primary. The amounts of the three primaries required to produce a given spectrum color as a function of wavelength are called tristimulus values. In determining these values, the use of negative values of the primary are occasionally required.

The selection of the three primaries is arbitrary, and the primaries need not be monochromatic sources. It is convenient, of course, to use primaries yielding tristimulus values that are positive throughout the spectral region, but no so such curves can be found experimentally. Artists choose red, blue, and yellow pigments as their three primary colors, but red, blue, and green light is used in color television.