Generally, radioactive paints are used to make watch surfaces visible in the dark. These paints, which do not require activation by an outside light source, will glow for several years. In the past, radium was often the active substance used in luminescent paints for watch faces. However, the practice was discontinued when it was found to emit dangerous gamma rays. The radioactive materials used today give off much lower emissions, which are easily blocked by the glass or plastic covering the watch face. These substances include tritium, krypton 85, promethium 147, and thallium 204.