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(1.28X_CIE - 1.06Z_CIE)]/Y_CIE
where X_CIE is the tristimulus value for Red,
Y_CIE is the tristimulus value for Green,
and Z_CIE 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.