Yellowness - ASTM D1925
A measure of the yellowing of a plastic, such as might occur after long-term exposure to light. The deviation in chroma from whiteness or water-whiteness in the dominant wavelength range from 570 to 580 nm as compared to a magnesium oxide standard.
Yellowness is calculated using the three tristimulus values of a specimen as follows:
Yellowness = [100(1.28XCIE - 1.06ZCIE)]/YCIE
where XCIE is the tristimulus value for Red,
YCIE is the tristimulus value for Green,
and ZCIE is the tristimulus value for Blue. (CIE stands for the Commission International de l'Eclairage).
Positive yellowness values indicates presence and magnitude of yellowness, while a negative yellowness value indicates that a material appears bluish.
Yellowness is a function of thickness, so comparison between data should be between specimens of comparable thickness.