Color measurement of anatase and rutile pigmentations using various color-mixture functions has shown that Judd’s 1949 modification of the standard observer provides the closest agreement with visual perception. The effect of Judd’s modification is to change the Z¯ function and the short-wavelength lobe of the X¯ function, so that for a tristimulus filter colorimeter it is necessary to change only the blue filter. Such a blue filter has been designed for use with the Colormaster differential colorimeter. With this new filter, accurate color grading (in agreement with visual perception) can be made for titanium dioxide.
L. S. WHITE and A. E. JACOBSEN, "New Blue Filter for the Colormaster Differential Colorimeter Based on Judd’s Modified Standard Observer," J. Opt. Soc. Am. 55, 177-184 (1965)