Color-Coordinate Systems.
CHROMATICITY
Instead of specifying a color by its tristimulus values R, G, B colorimetrists use normalized quantities called chromaticity coordinates. These are expressed by,
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and  |
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Of course since r+g+b=1, two chromaticity coordinates are sufficient. This however leaves us with only two pieces of information. The third dimension of color is called the luminance (Y) which may be obtained by a separate match. Luminance is an objective measure of that aspect of radiant energy that produces the sensation of ‘brightness'.
Radiation of different wavelengths contributes differently to the sensation of brightness. The relative contribution of monochromatic radiation of a given wavelength to luminance i.e. the brightness sensation is termed as the relative luminous efficiency y .
Since this is obtained by photometric matches i.e. matching of brightness, it is dependent on the condition of observations. Fig (2.11 ) shows the y vs 
curve for 20and 100 fields of view.
Figure (2.11)
Both these curves are normalized such that maximum 
is taken to be unity.
The luminance of any given spectral distribution
 is then taken to be
where Km=680 lumens/watt.As in color matches, a brightness match is observed between two spectral distributions, and  if,
It is easy to see that the luminance of the sum of two spectral distributions is the sum of their luminances.
A complete specification of color given by luminance and chromaticities is often used since it is very close to familiar concepts defining perceived color.
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