ASTM D1925 is an outdated yellowing index standard that specifies that only C/2 (light source C, 2 ° viewing angle) can be used for calculation. When using the ASTM D1925 standard, the coefficients for Cx and Cy are 1.28 and 1.06, respectively, which can be traced back to the 1940s when colorimeter users needed to calculate the yellowness index using paper and pen. These coefficients will result in the calculation of transparent samples not being exactly zero, but rather producing a deviation value of 0.303.

The ASTM E313 standard is applicable for calculating the whiteness index (WI) or yellowness index (YI) under various light source/observer combination conditions such as C/2, C/10, D65/2, D65/10, etc. If ASTM E313 standard is adopted, both the yellowing/whiteness index and the corresponding light source observer combination must be clearly specified.

It should be noted that the plastic industry often uses the yellowness index to characterize whether transparent plastics undergo yellowing. Under the C/2 condition of the E313 standard, the Cx and Cy coefficients were corrected to 1.2769 and 1.0592, respectively. The increased numerical accuracy eliminated intercept errors and accurately zeroed the yellowness index of the transparent sample. Therefore, as long as the sample has color characteristics, there will be no significant deviation between ASTM D1925 and E313 C/2 standards in specific applications.

The International Commission on Illumination (CIE) recommends that unless otherwise specified, D65/10 conditions should be preferred for color comparison and index calculation. This means that unless there are special requirements, the D65/10 yellowing index under the E313 standard should be used.

Due to the minimal difference in Cx and Cy coefficients under C/2 conditions, when the sample appears clearly yellow, the index results calculated by the two standards are basically consistent (as shown in the following example). But if D65/10 conditions are used, there will be differences.

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