The color of specular highlights

An integral part of computer graphics, machine vision and human vision understanding is modeling how a surface reflects light. There is a substantial body of work on models describing surface reflectance ranging from purely matte to purely specular. One of the advantages of diffuse reflectance is that the color and the intensity of the reflected light are separable for most materials. Color is determined by the chromophores of the material, while intensity depends on the scene geometry. In specular highlights the color and the intensity of a specularity depend on both the geometry and the index of refraction of the material, which in turn is a function of wavelength. The graphics and vision communities often employ the following simplifying assumption when modeling specular highlights: For non-conductive materials the color of the specularity is the color of the light source. We will show that in most cases this assumption is violated. Theoretical analysis demonstrates that even for non-metallic surfaces the reflectivity ratio at specularities varies with both wavelength and angle of incidence. Furthermore, our experiments with a multispectral sensor clearly show that the deviation of the color of the specularities from the color of the incident light can be consistently measured.