XYZ tristimulus values and the associated  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%.

Yxy color spaces uses very few, and in general traffice road sign needs this Yxy color space, so 3nh design a special model NS808 spectrophotometer with a special software SQCT for traffice road sign.