Good news - I have successfully rebuilt the light shader that can process IES-style sample information, at least for the axially symmetric forms.

Now I have to rewrite the IES file parser/interpreter.

The render shows 3 copies of the same light, but the back wall is turned so the distance to the light is different for each. It shows how complex this lighting can appear.

The splotches on the ground are IDL sample artifacts - I was using really cheesy settings for quick results.

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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)