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Photometry is concerned with the measurement of optical radiation as it is perceived by the human eye.  The CIE 1931 Standard Observer established a standard based on the average human eye response under normal illumination with a 2° field of view.  The tristimulus values graphed below represent an attempt to describe human color recognition using three sensitivity curves.  The y(l) curve is identical to the CIE V(l) photopic vision function.  Using three tristimulus measurements, any color can be fully described.
 
Fig. 1.3 CIE spectral tristimulus values.

Color Models

Fig. 1.4 CIE L*a*b* color space.
Most models of perceived color contain three components: hue, saturation, and lightness.  In the CIE L*a*b* model, color is modeled as a sphere, with lightness comprising the linear transform from white to black, and hues modeled as opposing pairs, with saturation being the distance from the lightness axis. 

 


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