<p>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:</p>

<blockquote>
  <p>Is a material which changes its roughness with wavelengths physically plausible?</p>
</blockquote>

<p>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.</p>

CC BY-SA 3.0
2017-02-09T15:35:00.203Z
2017-02-09T15:35:00.203Z
ivokabel
<p>I read the <a href="http://paulbourke.net/dataformats/mtl/" rel="nofollow noreferrer">MTL</a> specification for materials and noticed that the specular exponent <strong>Ns</strong> 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 <em>represent</em> (the specular exponent is of course not physically based at all) something <em>physically plausible</em>)?</p>

<shading><specular>
Specular exponent spectrum
Matthias
# Specular exponent spectrum

(Asked by: username_0 on 2017-02-09)

I read the [MTL](http://paulbourke.net/dataformats/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*)?

**Tags:** shading, specular


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## 1 answers

### Answer by username_1 on 2017-02-09. Score: 2 | Accepted answer

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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