Your experiment sounds fascinating, and you are probably not the first one to try it. The focusing issue sounds like one of the problems you would encounter, but I have a feeling that that is not the main problem with this method. I think that the main problem encountered is getting enough UV to even expose the paper. The problem with UV bulbs I have heard is there is not enough UV to expose the print in a resonable amount of time. Your idea of pointing it at the sun is a good idea. Please post your results. I am very interested.