Chromaticity of unique white in object mode

The chromaticity of unique white viewed in object mode and under dark adapted conditions was investigated for 3 luminance levels (200, 1000 and 2000 cd/m(2)) using two experimental methods: unique white setting and rating. The results of the two methods were found to agree well. Both showed quite large observer variation and an apparent shift of the average unique white (across observers) towards colder correlated color temperatures as the stimulus luminance was dropped from 2000 cd/m(2) to 200 cd/m(2), although no such trend was observable at the individual observer level. Unique white was shown to encompass a region in color space, mostly located below the blackbody locus at around 6000 K. The low and high color temperature ends of the CIE class A and B white regions tend to respectively over- and slightly underestimate the size of the chromaticity area perceived as white by the dark adapted average observer. However, the agreement along a direction approximately perpendicular to the blackbody locus was quite good. Finally, the unique white ratings were modeled by a bivariate Gaussian function, resulting in a simple empirical metric to predict the degree of neutrality of any object stimulus viewed under dark adapted conditions.