modeling a fresnel lense diffuser


I have to model a diffuser which consists of an array of fresnel lenses, and I want to be able to use the pmap extension. Is there any other way then modeling the lenses' geometry (or its equivalent full lenses')? As far as I got it, I cannot model it in prism2, as the distance from the lense center is not available when calculating the outgoing ray. Generic brtf is not supported with pmap.

I guess the best way to determine the properties of the lense is a laser pointer, millimeter-gridded paper and moving the pointer parallel to the lense surface, right? Or is there another way recommended for those of us who do not have a full equipped lab at home?

I have that strange feeling that I am leaving Radiance's application focus right here, but this is something quite relevant. These diffusers are commonly used with ligh redirecting systems, and these systems can be (and are) modeled using Radiance+pmap.

CU... Lars.