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

(Links below)

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.