Visually, yellowness is associated with scorching, soiling, and general product degradation by light, chemical exposure, and processing. Yellowness indices are used chiefly to measure these types of degradation.
Yellowness Index per ASTM Method E313 is calculated as follows:
YI=(100(CxX - CzZ))/Y
Where X, Y, and Z are the CIE Tristimulus values, and the coefficients depend on the illuminant and observer as indicated in the table below.