The invariance of unique white; a possible implication for normalizing cone action spectra.

The locus of unique white was measured for two observers over a wide range of stimulus intensities, using both direct matching and absolute judgements. Chromaticity coordinates of each observer's unique white were found to be invariant over a range of about 4 log units. This result is discussed in the context of the class of models that explain the intensity-evoked change in perceived color (Bezold-Brücke effect) on the basis of response compression in color channels. Invariance of unique white may be interpreted, then, as evidence for its non-polarizing effect on color channels, including the three classes of cones. On the basis of that assumption we have used unique white as criterion for normalizing cone action spectra. Assuming an equal-energy spectrum for the standard observer's unique-white stimulus, as we found to be consistent with literature data, we calculated the associated coefficients of the conversion formulae between CIE space and cone space (both for XYZ to LMS and LMS to XYZ) of the Vos-Walraven (1971 Vision Research, 11, 799-818) and Smith-Pokorny (1975 Vision Research, 15, 161-171) receptor fundamentals. The action spectra that thus result, intersect at the spectral neutral (achromatic) points of the three classes of dichromats.