Quote - True area lights would be a lot faster than using IDL for this. And a word of caution is called for; the level of detail/calculation required to sample the environment well enough to do a perfect job of lighting via a relatively small, self-lit rectangle is extraordinary.

True, while indirect diffuse light can certainly reproduce certain area light effects, it's not a full replacement. Note on the other hand, that in my image you can also see how light from the box gets reflected off the ground and illuminates the back of the figure, which would be completely dark if there was just a plain area light.