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.
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|