I think this has been covered in a few threads, over the years.  I'm pretty sure one or more techniques has been outlined in those threads.  Unfortunately, I'm not very adept with Rosity's Search function.  The closest I can find is a post where Ockham shows how to use the P shader variable to generate a displacement map.  He includes a pz3 file which shows his basic setup, which may help in some ways, but the example uses material ambience and no lighting.  Some other process would be needed to light the rendered material.

I'm pretty sure bagginsbill has outlined a process for this, in one of those threads, which is based on a mathematically correct application of optics.  Umm.  :unsure:

At any rate, here's Ockham's example:

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=4069505&ebot_calc_page

 

Scooped by BB himself!  :lol:

===========================sigline======================================================

Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.