GTLS (Gaseous Tritium Light Source) Tritium watches are timepieces that use small, sealed vials filled with tritium gas to provide self-powered illumination for the hands, hour markers, and sometimes bezels of the watch. Tritium is a radioactive isotope of hydrogen, and when it decays, it emits beta particles. These particles interact with a phosphor coating inside the vials, causing them to glow without the need for an external power source like sunlight or battery-powered backlighting.