Ok,
You have to enter the photon director, then add props to the light that will be casting the radiance light. Play with the number of photons for better rsults.
Once you do that go to its rendering property inside the photon director and check global illumination on.
Beware it is very very slow and time consuming!!!!
You can also check Caustics for that light and for caustics you have to turn the props for the objects too, like a whine glass or fish tank.
Another Tip:
This features only work in Extreme that means you need Mental ray 2.1 to work with them.
Use a small preview to change settings before final rendering, it is very very slow even with just a few objects in the scene.
Be sure to turn mental ray-on, on the preview/setup.
Cheers.
BTW I love the quality of the renderings it is almost enough to justify the time it takes.
Santiago Batiz-Benet
Z3D
Animation & Design
Seattle WA USA
