Is the exitant radiance the normalized integration of the incident radiance?
Edit : I see how that could be wrong since it doesn't account for the color of the surface itself.
@sickgraph: Exactly. The surface will absorb some light, and this absorption could be unequal in different wavelengths. So there will be some coloration of the outgoing radiance, and in general the total outgoing radiance should be no greater than the total incident radiance.