Z% Brightness is a single-number index that is used to measure the brightness of white materials that tend to yellow with age and/or degradation. This index is not recognized in a formal test method. It is most often used as an in-house test method by companies in the paper industry. It may be used in quantifying optical brightening effects when a sample is measured with the instrument's UV filter in and then its UV filter out. It is sometimes used to measure the effectiveness of bleaching.
Conditions for Measurement
Instrumental: Any HunterLab color measurement instrument
Illuminant: Usually C, but others may be used
Standard Observer Function: Usually 2 degree
Transmittance and/or Reflectance: Reflectance.
Z% is the CIE Z value divided by Z for the top of scale and multiplied by 100 to yield a percentage.
This index is used mainly in the paper industry, but it may be used on any white product that yellows.
(See attached pdf file for the complete article and formulas)