Tristimulus colorimetry
The concept for the XYZ tristimulus values is based on the three-component theory of color vision, which states that the eye possesses receptors for three primary colors (red, green, and blue) and that all colors are seen as mixtures of these three primary colors.
The concept is very simple. One needs a light source; three glass filters with transmittance spectra that duplicate the X. Y. and Z curves; and a photocell. With this arrangement. One can get an XYZ reading that represents the color of the sample.