Tritium
Tritium lumination is the use of gaseous tritium, a radioactive isotope of hydrogen, to create visible light. Tritium emits electrons through beta decay, and, when they interact with a phosphor material, fluorescent light is created, a process called radioluminescence. As tritium illumination requires no electrical energy, it found wide use in applications such as emergency exit signs and illumination of wristwatches. More recently, many applications using radioactive materials have been replaced with photoluminescent materials.