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Whiteness-blue reflection or Z% brightness

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.

The Z% brightness is calculated from the following equation:

where

ZCIE is the tristimulus value determined by the CIE 1976 L*a*b* color model and Z is the maximum value for Z on the scale.

The Z% brightness is just calculated for illuminant C!