I have the following situation:
I have a scene lighted by a "sun" lamp that gives me good sharp shadows. But I also want to have a large plane with an emissive material placed above the scene to regulate the "general" brightness. My problem is that while this plane does emit light the way I want it to, it also blocks the sun and casts a shadow on my scene. So I want it to both emit light, and by invisible to the rays of the sun. How can I achieve this?