Having the property to emit light rays. In watchmaking, Tritium is the name given to a radioluminescent substance containing tritium. It was used to coat numerals, markers and hands so they could be read in the dark.
The most widespread system today is LumiNova® which comprises tiny phials of a tritium-based substance. It has replaced the phosphorescent dials for which the first patents were filed in 1878, and dials using radium-based substances which were first patented in 1907.