Yellowness Index (YI) is a number calculated from spectrophotometric data to characterize the change in color of specimen from clear or white to yellow.
Testing standard
- ASTM E313
- ASTM D1925
By ASTM E313, YI is calculated by the following equation:
X, Y, Z are the CIE Tristimulus values; Cx, Cz can be obtained from the following table.
By ASTM D1925, YI is calculated by the following equation:
However, using this equation, the YI for clear air is 0.303. To set the value to 0.0, the equation is adjusted as the following.
Typically, E313 with D65/10 degree is preferred. D1925 is obsolete now, although the value provides certain meaningful information and is still mentioned for specific cases.
Sample
Samples need to be clear, near-colorless liquid or solids in transmission mode and near-white, opaque solids in reflection mode
Instrument
BYK Gardner Spectrophotometer or Konica Minolta tool should be suitable.