Statistical confirmation of Thornton's zero‐crossing conjecture for metameric blacks

In an effort to use statistics to arbitrate the recent debate about the clustering of zero crossings of metameric blacks, the statistically‐most‐significant metameric black was determined as a linear combination of the mean daylight and the first three daylight phases. This spectrum, S4(Λ), is orthogonal to all the 1931 CIE color matching functions, and has zero crossings near 450, 540, and 610 nm. W. Thornton observed that these wavelengths tend to be crossover points for real reflectances that are metameric under some daylight. For the present work, a metameric black is defined as a difference between two matching spectral power distributions rather than between two matching spectral reflectances. The zero crossings of S4(Λ) amply confirm the zero‐crossing conjecture for metameric blacks.