Using gaseous Tritium on the hour markers on watch dials was the way watchmakers used to make dials glow in the dark. Tritium emits electrons through beta decay and over time lose their luminescence and turn yellowish in colour. This is the ‘patina’ effect often referred to by watch aficionados.
Some of us feel this makes a watch look old but many prefer this look – especially when evenly spread – as it gives the watch a vintage effect that is hard to replicate on a new watch. This look also lends to the pedigree of the watch as it implies that the watch is “untouched” and therefore original.
As tritium illumination requires no electrical energy, it found wide use in wristwatches until the 80’s. You can identify a Swiss watch with a tritium dial by the words T Swiss T normally seen at the 6 o’clock mark on watches. Rolex, Tudor and Omega were big users of Tritium dials.
Gaseous tritium is a radioactive isotope of hydrogen. To create visible light there needs to be interaction with a phosphor material. the by-product is the creation of a fluorescent light through a process called radioluminescence. Tritium emits radiation and over the years have been replaced by photoluminescent materials.