Strontium aluminate combined with europium
Radium was used initially but eliminated when its level of radioactivity was found to be dangerous. It was replaced with two other substances, promethium and tritium (still radioactive but less so and with a shorter half-life). Tritium gas-filled fluorescent tubes are still used on everything from cockpit instruments to gunsights; tritium luminous materials are versatile and still preferred by the military, security services or for outdoor activities. However, most domestic watch faces are illuminated now by a material which incorporates strontium aluminate combined with europium (non-toxic and, non-radioactive chemical). Unlike the radioactive materials, strontium aluminate needs to charged, like a battery, with light before it phosphoresces.
