Thanks for the links to pictures and to that caustic generator program. That sidetracked me a bit while I explored how best to use that effect.

It looks to me like the caustics are most pronounced near the surface, and less so as depth increases. So I changed the main light to behave that way.

Sorry about the delay on the "how to".

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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)