Tritium vials
Tritium is a radioactive isotope of Hydrogen that emits Beta rays as it decays. Sealed inside a vial, the gas activates the luminous inner coating, making it glow 100 x brighter than conventional non-radioactive luminous materials.
In Europe, regulations on the use of isotopes like Tritium in commercial products is very strict. The radioactive Beta rays of Tritium are weak enough to be stopped by a watch crystal, as long as the vials are not broken or burned.
With the improvement of materials, Tritium vials boast a lifespan of 23 years. The atoms have a half-life of 12.3 years, which means that the brightness of the vials will drop by 50% after 12.3 years, by another 25% after 24.6 years, by another 7.5% after 36.6 years, and so on…