> Quote - And how is the light fall-off from the light box treated? I'm going away to find out....

Excuse me quoting my own question, but not having an answer (yet) I experimented using a light meter material developed by... well, not me... guess who. Correct.

I'm not saying this is conclusive, but it certainly appears to demonstrate that the falloff for surface-emitted light is just as it should be - inverse square. I think.

Edited to add: this also demonstrates the rather random nature of IDL light calculations. I've been doing IDL renders on low to medium quality settings, and two consecutive renders with precisely the same settings can be quite noticeably different. Just something else to keep in mind...

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)