Evaluation of whiteness metrics

Whiteness is an important characteristic for surface colour, which is affected by the spectral power distribution of the illumination due to the fluorescent whitening agents contained in many white objects. The CIE whiteness metric has been widely used but is known to have limitations. For example, the boundary noted by the metric is not considered large enough, because, in many cases, a surface whose chromaticity lies outside the boundary can still be perceived as white. Further, the metric is only defined to characterize the whiteness of a surface under CIE Illuminant D65, and thus it should not be applied in other lighting conditions or to other light sources. This paper describes a psychophysical study, investigating the whiteness of 50 samples (12 paper and 38 textile samples) under 12 lighting conditions with different levels of correlated colour temperature and ultraviolet radiation. An optimized whiteness metric and a boundary of the white region based on the CIE whiteness metric were derived from the results, which can be used to precisely measure the whiteness of a surface under any lighting condition and to guide spectral engineering for LED sources.