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
ZCIEis 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!
