One more thing for possible discussion. I notice that the main and rim lights in the tutorial scene have fall-off set to inverse linear, rather than inverse square. I have not yet found any mention of this in the thread. Was this simply the default, or intended that way?

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

It probably doesn't make much difference in an empty environment with a single centre-positioned subject, but with surrounding or backdrop geometry it surely would.

Correction: I suppose it would also affect the bounced light from the left hand wall.

"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)