What I have done is, that after I have taken out the Xenon tube from a flash system I etched away the UV blocking metallic coating by bathing it for about 20-30 minutes in 30% HCL acid (Hydrochloric acid) using proper eye and hand protection, under a running exhaust, with cleaned water to rinse the tube while and after the procedure and making sure not to bump the Xenon tube anywhere, as it is under very high pressure and it may explode if not properly handled. Only the glass tube should be in contact with the acid, not the metallic conductors leading into the tube. Further, this only works, if the tube has a metallic ingnitor wire around it and NOT if the metallic coating also works as ignitor - etching that away renders the tube useless and it would most likely not fire anymore.
[click on image to see a larger one]
Transmission of a Xenon tube before/while/after/:
So what you see here is the transmission of my Xenon tube before (pink line), while (yellow line) and after the procedure (red line). The difference is quite significant, yielding a total 1.8 stop increase in transmission = output power at 365nm and even more importantly, a much deeper reach into the UV region beyond 300nm. I'm not predicting that this will work with any tube, but that yellowish tube I had, turned out to be perfectly clear afterwards. Another method is to erase that coating using Cerium Oxide abrasive powder, but personally I haven't done that.
AGAIN; DON'T DO THAT IF YOU ARE NOT WELL AWARE OF THE INHERENT DANGER!!
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos