Color metamerism and the structure of illuminant space

The colors of two surfaces might appear exactly alike under one illuminant while varying under others. This is due to the metamerism phenomenon in which physically distinct reflectance spectra result in identical cone photoreceptor excitations. The existence of such metameric pairs can potentially cause great ambiguities for our visual perception by challenging phenomena such as color constancy. We investigated frequency and magnitude of metamerism in a wide range of scenarios by studying a large set of surface reflectance spectra illuminated under numerous natural and artificial sources of light. Our results extend previous studies in the literature by demonstrating that metamers are indeed relatively infrequent. Potentially troublesome cases in which two surfaces with an identical color under one illuminant appear very differently under a second illuminant are exceedingly rare. We used the frequency of metameric pairs in combination with non-metric multidimensional scaling to establish a new representation of illuminants based on metamerism. This approach imposes a systematic structure onto the representation of illuminants and allows to better prognosticate the likelihood of metamers under new illuminants.