The move away from radio active materials has prompted the development of new photo luminescent compounds. Italy is a watch market trend setter. When the Italian government announced that there would be strict controls on the import of radioactive materials from 1997, the watch industry had to find a replacement for Tritium-based luminescent systems.
Nothing can emit light without the input of energy in some form.
A naturally luminous substance like phosphorous emits light as it oxidizes. The energy is produced in the chemical reaction and when all the phosphorous has been oxidised no more light is emitted. In the radioactive systems the decay of the radioactive material provides the energy to excite a fluorescent compound which then emits light. Note that, especially with the radium systems, the radioactivity will still be present even if the luminosity has been lost as the relationship with the fluorescent compound may break down. In the new luminous systems ambient light energy is stored and re emitted, with gradually diminishing intensity, remaining visible for some hours in the dark.