[raymo]

I don't remember whether UV-C is at the lower or upper end of the UV
frequency range,[ I suspect upper because it is much more dangerous than lower] but collectors of the stamps of many countries use UV
lamps of both varieties to detect watermarks. The lamps are fluoros,
and from memory are about $25-35 on ebay under philatelic accessories
if I remember correctly. They are battery powered, but could easily be converted
to run off a 6v power supply from the mains.
raymo