Lesbentley: nice idea, but I'd thought of that. Look up to the reduced pic at the top of this page, and trace out the light cones you can just see in the middle of the blue glow at about waist height. Those lights should fall on the floor just outside the raised lip of the well, but I could see virtually no detail until I placed that ceiling light.

All four lights were originally set with "angle start" at 90° and "angle end" at 110° for mostly even light coverage -- after I noticed the light cones were leaving the top and bottom wall corners dark (and remembered the cones were actually round...) I increased the angles a bit until the gaps filled in.

The effect I was aiming for was that the blue glow was the only light source in the scene, something that is a bit clearer in the original unlightened pic. Should my setup have worked as well as it did, or can anyone suggest improvements?