Eugene d'Eon

While it doesn't tick all of the artist-friendly checkboxes, I would note that it is possible to color correct, do spectral projections, and exchange materials between different 2-parameter Fresnel models...

\sigma is the cross section / extinction coefficient for a half space of participating media that simulates the rough surface. In the Smith model, such a medium can exactly describe...

Yes, surface scattering under Smith's assumptions can be formulated exactly as a homogeneous half space volume. See: https://eheitzresearch.wordpress.com/240-2/ and: http://onrendering.com/data/papers/ms16/ms16.pdf