Deep down on this geology website : you find: "On May 16, 1896 Edison filed a patent [US 865,367] for a fluorescent light source. It was basically an X-ray tube coated on the inside with calcium tungstate. The device was highly dangerous and impractical because it gave off X-rays, the dangerous effects of which were made clear when one of Edison's assistants died of X-ray exposure. It was also impractical because X-ray tubes require a very high voltage and convert most of the electric power supplied to them into heat. Edison patented it anyway but soon ended his X-ray work. Neither Roentgen nor Pupin patented their inventions, both of which have been of immense value to mankind over the last one hundred years. Now, what about the fluorescence of the natural mineral, scheelite? It was also found to fluoresce under X- rays."