Yup, good ol' radioactive hydrogen isotope. Culled fresh from deep within the core of a nuclear reactor, worth more than any other material on Earth based on mass. Its beta radioactivity travels roughly a quarter inch in open air and cannot penetrate a Kleenex tissue, so the glass vial its contained in is quite safe.
I'm not sure about this but I think because of the long half-life it has? that means let's say 10 years is the half-life of a tritium marker, that means in 10 years time the amount of radiation (the light) coming from it is half as bright. In another 10 years it will be half of the previous one again. Somebody can probably explain this better than me
Out of curiosity, why is tritium instead of any of the other luminescent material used? In other words, what are the characteristics that make it commonly used?
Helios is correct, tritium glows with absolutely no energy input or interaction whatsoever. You can put a tritium vial inside a dark box in your basement for several years, and when you open the box it will still be glowing brightly, no change.
Tritium's radioactive half life is roughly 12 years, so after 12 years the vial would be only half as bright; this would be difficult to notice unless you had a newer vial to compare it to, and you'd be hard-pressed to find a flashaholic who is still using the same flashlight after 12 years..
Out of curiosity, why is tritium instead of any of the other luminescent material used
It is probably the safest suitable material. It has fairly low levels of energy, naturally degrades in the environment fairly quickly, and tends to remove itself from the human body if somehow injested.
The Tritium itself is not luminescent, it is emitting energy, that energy strikes the phosphors, the phosphors do the actual glowing. The phosphors would work fine with several other radioactive materials, but they tend to be things you don't want to have around the house.
Also, out of curiosity, since it is gas in a phosphor-coated glass tube, how are these tubes protected from damage from edcing? Say, if I drop my light and something hits the spot where the tritium is, what will happen to the tube?
The vial is usually placed within a counter-sunk slot in the flashlight body and then entombed in optical adhesive - it would be very difficult for anything external to affect the tritium at this point. The alternate option is to just put a vial on a keyring and attach it to your lanyard/ring/hole; these tritium fobs are usually either solid slabs of hard, clear epoxy with the vial inside them, or a plastic tube which has soft epoxy adhesive at each end holding the vial to work like a shock absorber.
I use a tritium fob that's not attached to the flashlight at all; it sits right in front of my EDC light on my nightstand so I can plainly see where the flashlight is in complete darkness.
if you do manage to crack your vial, though, most likely all you'd notice is that it would start dimming over the next few days. I had one crack with no other visible signs of damage other than the loss of light.