Visually, yellowness is associated with scorching, soiling, and general product degradation by light, chemical exposure, and processing. Yellowness indices are used chiefly to quantify these types of degradation with a single value. They can be used when measuring clear, near-colorless liquids or solids in transmission and near-white, opaque solids in reflectance.
Conditions for Measurement
Instrumental: Any HunterLab color measurement instrument
Illuminant: D65 (YI E313 only), C.
Standard Observer Function: 2° and 10° (YI E313 only).
Transmittance and/or Reflectance: Either
Yellowness Index per ASTM Method E313 is calculated as follows:
The tristimulus values of clear air (for CIE illuminant C and the 1931 CIE 2° standard observer) are X = 98.041, Y = 100.000, Z = 118.103. Using these values, the ASTM formula yields YI = 0.303 for clear air because the factors are truncated to three significant figures. In order to set the yellowness index for air equal to 0.0, the constant multipliers for XCIE and ZCIE have been expanded slightly.
The ASTM D1925 method was withdrawn in 1995, but this formula still provides useful information. This index is always calculated for C/2°, regardless what illuminant and observer are chosen. The focus of this index was on evaluation of transparent plastics.
These indices are often used by the textile, paint, and plastics industries, but may be used for measurement of any nearly white or nearly colorless object.
(See attached pdf file for the complete article with Formula and Illustrations)