[Etymology: Commission International de l'Eclairage (i.e. Illumination)] colorimetry The key scientific system for describing and specifying colour, based on the additive mixing of precisely defined coloured lights.
The three colours red, green, and blue are sufficient to produce, by appropriate mixing, all colours, including white, the ‘complete colour’. The three are therefore called the ‘primary’ colours, or, more particularly, the ‘additive primary colours’. (Their initials RBG grace many computer monitors.) The common unit for measuring the quantity of each primary, and of the composite product, is the lumen; however, the quantity of the mixture is not the sum of that of the constituents, and the three primaries are not equi-potent, i.e. the formulation of white is not one of equal lumens of each primary. If we divide the quantity of each primary in a mixture by the respective number of lumens necessary to produce white, we have coefficients rationalized to white, i.e. in a sense corrected for the differential power of the different primaries. If we divide each of the three new coefficients by their sum, we get what are called normalized coefficients, i.e. three coefficients that add up to 1. If we regard these values as quantities in lumens of the respective primaries, then the quantity of produced colour, in lumens, is called the trichromatic unit or T unit of the product. If the coefficients are now divided by the number of lumens in the T unit, then we have the composition for one lumen of mixture, and the coefficients are termed the trichromatic coefficients. The calculations depend on what precisely is meant by red, green, and blue; the adopted standards for these as cardinal illuminants are monochromatic radiations of 700.0 nm, 546.1 nm, and 435.8 nm wavelengths, respectively.
Any other three colours whose formulations are mathematically independent, i.e. such that no one can be obtained by multiplicative addition/subtraction of the other two, are equally viable as constructs for all colours. One standard set is:
| the X stimulus | = + 2.364 6 red - 0.515 1 green + 0.005 2 blue |
| the Y stimulus | = - 0.896 5 red + 1.426 4 green - 0.014 4 blue |
| the Z stimulus | = - 0.461 8 red + 0.088 7 green - 1.009 2 blue. |
[ISO/CIE 10526:1991 CIE Standard Illuminants]
The three coefficients required to match any given sample with a base trio of independent reference colours are called the tristimulus values. These can be obtained by direct experimental comparison, but are usually obtained indirectly, by applying established tables of the values for 5-nm increments of wavelength to spectrographic data, else, for X, Y, and Z, similarly from experimental comparison using the RBG trio.
[Hardy A. C. Handbook of Colorimetry (Cambridge, MA: Technology Press, 1936)]
