Abstract

Changes in the surface reflectance of an object and changes in the spectral content of the illumination both change the spectral composition of light reaching our eyes, but usually we do not have difficulty determining which change occurred. We investigated how specular reflections might allow an observer to distinguish changes in lighting and reflectance in scenes containing only one surface. Specular reflections make a surface look glossy and contain reflected light that has not been spectrally modified by the surface, so could provide information about the illuminant. Additionally, there are spatio-chromatic changes that differ systematically for changes in lighting and reflectance, and we investigate how observers extract information that is not available at any given instant. We presented hyperspectrally raytraced movies showing isolated objects undergoing gradual illuminant and reflectance changes. Observers were able to distinguish the two types of change at very low specularities (~1%). Performance was reduced, but not extinguished, when we distorted the images to weaken some of the spatial or chromatic relationships. The highlights on low-gloss surfaces are heavily contaminated by surface colour so they do not allow direct estimation of the illuminant. Yet the chromatic transitions under gradual changes in lighting and reflectance are perceptually discriminable.