XYZ tristimulus values and the associated Yxy color space form the foundation of present CIE color spaces.
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 XYZ tristimulus values are calculated using these Standard Observer color matching functions.
If we measure the apple using the Yxy color space, we obtain the values x=0.4832, y=0.3045
as the chromaticity coordinates, which correspond to point (A) on the diagram
in Figure 12; the Y value of 13.37 indicates that the apple has a reflectance of 13.37%.