Thanks for that and I think the answer is in your reply. I had a solid look into the works (as far as I could without disassembling the machine) and it all looks fine, but the bottom diffuser is, I think, not original. You note that yours has concentric circles - I bet it is a fresnel surface built into the diffuser to give the light box a focussing capacity in the same way that a condenser is focussed ( I seem to recall that tlr cameras often had a fresnel under the ground glass to brighten up the image too). I'll get a light meter and compare it with yours, but the use of a fresnel would certainly be logical and would concentrate the light. I'll work on that thesis for the moment.
Originally Posted by bluejeh