FAQ: “We have discussed before how the EasyMatch software reports per the ASTM method D1925 and the difference to E313. However, we have legacy documents that imply that our instrumental color test, using the ColorQuest XE and ColorQuest II are compliant with USP method 1061. It is not clear to us how the calculations required by the USP method agree with either of the ASTM methods.”
USP 38 Monograph 1061 Color – Instrumental Measurement
USP - United States Pharmacopoeia Inc., Rockville, MD USA www.usp.org
USP1061 describes the CIE system of color measurement from measured spectral data through to CIE X, Y, Z tristimulus values, and subsequent calculation of CIE L*, a*, b* values as a complete descriptor of color.
USP1061 does not reference Yellowness Index, a specialized single number scale for quantifying trace amounts of yellowness in near colorless liquids or white opaque solid materials. Yellowness Index - YI D1925 C/2 per ASTM D1925 and the near identical YI E313 C/2 per ASTM E313, are both mathematically derived from the same CIE X, Y, Z tristimulus values as CIE L*, a*, b* values.
If you have a perfect clear sample of 100% transmission, or a perfect white sample of 100% reflectance, YI D1925 C/2 and YI E313 C/2 will calculate as 0 indicating no yellowness. If there is any trace yellowness, YI D1925 C/2 or YI E313 C/2 will rise in response. YI D1925 C/2 and YI E313 C/2 is more responsive than CIE L*, a*, b* values in discerning trace yellowness.
In summary, USP 1061 defines the calculation of CIE X, Y, Z tristimulus values which in turn is used to directly calculate YI D1925 C/2 and YI E313 C/2.