The information posted here about reflection of light from certain surfaces has been quite interesting. However, to return to the original question that prompted the thread, I agree with Mike Finley's first response, i.e. a coating of white paint on the inside of a UV bank of tubes would be more effecteive than mirrors, and probably about as effective as any other coating.

I have made and used a number of UV tube banks. Initially the issue of internal reflection was a subject of considerable interest for me. Eventually what I found was that it makes virtually no difference at all whether you provide internal reflection or not, and i f so plain white paint is as good as anything. However, even if you just leave the inside unpainted (assuming it is a light wood wuch as pine or fir) I seriously doubt that you would be able to detect any loss of printing speed compared to any other reflection method you devise.