Active manipulation of surface orientation under directional illumination improves color discrimination

Two surfaces that appear different colors under one illumination might appear the same color under another illumination. This phenomenon is known as illuminant metamerism and is thought to limit surface color discrimination. However, real-world lighting environments are not directionally uniform. In theory, surfaces that appear the same color at one tilt might not appear the same color at another tilt where they sample a different illumination spectrum. We tested whether human observers could use this directional variation to improve surface color discrimination. Our results confirm that human color discrimination is improved when observers are able to tilt surfaces to sample directional lighting. Observer sampling motions were spatiotemporally structured and appeared to be directed towards the general lighting geometry rather than systematically maximizing color discriminating information for specific surface pairs.