Tritium is a low-yield radioactive isotope commonly used in illuminated watch dials. It is also used extensively in the manufacture of low-light firearm sights, collectively known as Night Sights. This “radioluminescence” is a function of the Tritium’s decay which emits a “phosphors” glow in low light. Tritium decay cannot penetrate human skin, so it is safe for everyday use. But the illumination does generally reduce by half over about 10 years of use.