Justintoxicated,
Tritium (H3) is a radioactive isotope of hydrogen. They fill a phosphor coated glass tube with it and as the tritium throws off particles the phosphor is charged and it glows. From what I've read the half-life of tritium is 12 years so the glow should last at least that long. They use it on gun sights , special exit signs , watches , mapreaders, flashlights (my Jets22 modified 3w arc should be here soon!), etc. Although it is radioactive, it really isn't dangerous. If you live in the U.S. you'll have a hard time tracking it down unless you scavenge it from a gun sight or something. My understanding is that the modders who use it on flashlights scavenge tritium viles from "Glow-Disks". Glow Disks are little bubble stickers with a little tritium vile in it. Tritium is also used in "Glow Rings", keychains that have a much larger (than glow disks) tritium vile in them. Glow Rings (not sure about the disks)are illegal to import into the U.S. so if you want one you'll have to get someone who lives in a much less anal country to send you one.