Many watches have glow-in-the dark hands and hour markers. The substance used for this purpose has evolved over the years. Originally, Radium was used in the 1950’s but was found to be highly radioactive and was replaced with a substance called Tritium. Tritium had much lower levels of radiotoxicity and was considered a much safer alternative to Radium. You can tell if a watch has Radium or Tritium markers because it will have the letter ‘T’ or ‘R’ printed on the dial, usually flanking the country of origin(ex. T-Swiss Made-T, or R-Swiss Made-R).
Recently a new substance, Super- LumiNova®, has been introduced, which has no radioactive properties and is three timesas bright as Tritium. It also does not discolor as Tritium does when it ages.