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I read the MTL specification for materials and noticed that the specular exponent Ns must be a scalar value instead of a spectrum. Would a specular exponent with three color channels (i.e. RGB, XYZ, ...) make sense (i.e. could represent (the specular exponent is of course not physically based at all) something physically plausible)?

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Using independent settings for each colour channel is like using settings dependent on wavelength when doing spectral rendering. So the question could be re-formulated as:

Is a material which changes its roughness with wavelengths physically plausible?

With some kind of nano-fabrication process you could build a material surface using some microfacets reflecting certain parts of spectrum and distributing each type of microfacets in a different way. So not only it is physically plausible, but it is also physically realisable. On the other hand, you normally don't encounter such behaviour in "common" materials.

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