i modeled a very simple sene. the plane above is a light that will render like an area light.

its very important that the normal is in the right direction. i could use a cube. but luxrender renders every face like a seperate light source so this means that it would be like i would have 4 are arealights. since its not visible i just used one face.

p.s.:
on the outside i make a cube where the normals are inside.you think that we need to have this? i have been reading somewhere that its better to have ''walls'' so that rays know where to stop bouncing.