Physical models indicate that, in general, reflectance is a complicated function of wavelength and geometry. An analysis of general reflectance models, however, shows that approximate reflectance models that exist that preserve much of the structure of the more-detailed models. In particular, I show from general models that Shafer's dichromatic reflection model [Color Res. Appl. 10, 210 (1985)] is a reasonable approximation for a large class of inhomogeneous dielectrics. I also show that a unichromatic reflection model is a useful approximation for metals. The approximate color-reflectance model is the basis for two algorithms that use color information. The first algorithm uses normalized color to classify surfaces according to material composition and is insensitive to geometrical variation in the scene. The second algorithm is used to identify metal and dielectric materials from their images. Experimental results are presented.

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