Tritium is an isotope of hydrogen with a 12.3 year half-life and decay via electron emission. When mixed with phosphorescent compounds and encased, it makes for semi-permanent watch lume. (Luminox claims 25 years, which is more than most watches' useful lifetime.)
Tritium is rare and expensive, requiring permits and handling expertise, so is confined to just a few brands:
- Ball Watches makes mechanicals with tritium.
- Luminox is the top vendor for quartz with tritium
See superluminova for the compound used in virtually all luminous watches.
Here's one of Luminoxes:
Tubes are about 1mm across and varying in length.
Here's a PR picture of a stack of mixed color tubes:
(Image credit: Bonding Ltd)
This is called GTLS, or gaseous tritium light source. Primary vendor is mb microtec from Switzerland.