Tritium is a self-illuminating material used in watchmaking to give glow effects to hands and hour markers for darker applications. It is made up of small glass tubes filled with radioactive tritium gas that interacts with a phosphorescent coating to create a constant glow. While other luminous materials require some light exposure to become charged and thus emit light, the tritium glows steadily in total darkness for decades. That makes it particularly suitable for the purposes of military, dive, and tactical watches for which low-light readability is crucial.