I made the TrueFresnel function based on information from this article:

http://en.wikipedia.org/wiki/Fresnel_equations

The article mentions several interesting things that I conveniently ignore here:

A) Polarized light behaves differently depending on the angle of polarization.

Poser has no way to represent light polarization so we can't use this info. I used the unpolarized calculation. Polarized light is not a common real-life occurance, except light which has reflected off of water. This is why polarized sun-glasses work to reduce glare when on a boat. We'd only care if we were dealing with secondary and tertiary reflections in a water scene and we were specifically setting out to demonstrate a side-by-side difference. By itself, you would never notice that polarization was being ignored in a way that causes an obvious departure from reality.

B) In metals and semiconductors, the IOR is not a simple "real" number. It is a complex number (it has a non-zero imaginary component).

I'd love to see how this actually is done. I have not found any reference that clearly explains the details, and I suspect my math skills might be insufficient to exploit the info even if I had it. The article's mention of this is tantalizing, though.

C) For accurate modeling of thin films, you must also take into account the frequency of the light.

This is an important effect. The rainbow reflections from oily puddles (such as in a parking lot) are produced by this. I have faked this in the past. I think LuxRender can handle this kind of thing. I suppose I could make Poser do it with some improvement in accuracy over the hack, but why bother? It's not a common feature in your typical Poser render.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)