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Monday, September 21, 2015

Are brightness and tristimulus Z the same?

Both brightness and tristimulus Z are optical measurement results from the blue part of the spectrum.

According to TAPPI T 452, the brightness function is defined with the effective wavelength - 457.0 +/- 0.5 nm obtained by the combination of illuminant, glass optics, filters and photodetector as seen in the graph below.  It was defined in the 1930's to assist in measuring the bleaching process. The wavelength and function were built around a copper-sulfate filter which was used in the original GE Brightness instrument.

Tristimulus Z
According to CIE 15-2004, this comes from the 1931 standard observer experiment (2° cone angle) of the CIE which defines how the average person sees color. The Z value specifically relates to how the average person sees blue. It is centered around 445 nm.
Relative comparison of brightness and z-function
People over the years have used the tristimulus Z to estimate brightness. However, they are different functions and will result in different values.  The Z value was used in the newsprint industry for some time and was later replaced with brightness measurement in the 1980's.  Also, since measurements on the Zeiss Elrepho were complicated, many people used the Z value instead of brightness simply to save time. If the spectral curve of a sample is flat in the blue region the values of the two different measurements will be similar if not the same. However, if the sample has a lot of character (up slope or down slope) in the blue region, the brightness and tristimulus Z values can be very different.