How many colours can you see? Real environmental lighting increases discriminability of surface colours

Color supports object identification. However, two objects that differ in color under one light can appear indiscriminable under a second light. This phenomenon, known as illuminant metamerism, underlies the difficulty faced by consumers of selecting matching fabric or paint colors in a store only to find that they appear not to match under home lighting. The frequency of illuminant metamerism has been evaluated only under single, uniform illuminants. However, in real world conditions, the spectral content of light falling on an object varies with direction (Morimoto et al. 2019), meaning that a surface will sample different spectra depending on its angle within the environment. Here we used computer-graphics techniques to simulate a pair of planar surfaces placed under newly measured hyperspectral illumination maps that quantify the directional variability of real-world lighting environments. We counted the instances of illuminant metamerism that can be solved simply by viewing surfaces tilted to a different direction. Results show that most instances of illuminant metamerism can in theory be resolved for both trichromatic and dichromatic observers. Color deficient observers benefit more than trichromats implying that the directional variability allows the recovery of the missing dimension in their colour vision systems. This study adds a new perspective to the classic trichromatic theory of human vision and emphasizes the importance of carefully considering the environments in which biological vision operates in daily life. It is striking that the physical directional variability available in natural lighting environments substantially mitigates the biological limitations of trichromacy or dichromacy.