Tritium is a radioactive isotope of hydrogen. Tritium gas is inserted in a glass tube or vessel, the inside of which is coated with a phosporescent material, and then sealed. The electrons released by the beta decay of the tritium excites the phosphorescent material and causes it to glow. As the tritium decays, it releases decreasing amounts of electrons and the sights gradually dims. I believe most tritium sight makers guarantee their sights as being usable for a period of about 12 years. The Meprolight sights I have are sufficiently bright that I can read a book or a newspaper a few words at a time if I move the sights along the line of type once my eyes become sufficiently accustomed to the dark.