For inhomogeneous materials, the standard reflectance model suggests that under all viewing geometries surface reflectance functions can be described as the sum of a constant function of wavelength (specular) and a diffuse function that is characteristic of the material. As the viewing geometry varies, the relative contribution of these two terms varies. In a previous study [ J. Opt. Soc. Am. A 6, 576 ( 1989)] we described how to use light reflected from inhomogeneous materials, measured in different viewing geometries, to estimate the relative spectral power distribution of the ambient light. Here we show that two restrictions, that (a) surface reflectance functions are all nonnegative and (b) surface reflectance functions are the positive weighted sum of subsurface (diffuse) and interface (specular) components, may be used to estimate the subsurface component of the surface reflectance function. A band of surface spectral reflectances is recovered, as possible solutions for the subsurface estimates.
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