Telling the time in the dark used to be the sole realm of minute repeaters until the early 20th century when radio-luminescent paint came onto the scene. This was the hazardous substance that was at the centre of the Radium Girls scandal. Closer to the 1930s, tritium lumination was introduced but rather than being painted directly on the dials, they were delivered in tiny vials and embedded onto dials and hands.
(Related: The Watch Expert's Guide: D for Decoration)
While tritium provided unequalled luminescence, it also had a lifespan. This is why vintage watches with tritium dials made in the 90s or earlier no longer glow in the dark today.
These days, watchmakers use non-radioactive luminova, the few exceptions being brands like Ball Watch or Luminox that continue to make watches with tritium tubes. Instead of radium or tritium, luminova is made of a substance called strontium aluminate, which is a biologically inert monoclinic crystalline powder that acts as a photoluminescent phosphor when activated with a suitable chemical agent.
When applied to the indexes or hands of a timepiece, it glows for a limited period of time in the dark and gets recharged by exposure to light, particularly UV rays. Unlike tritium, luminova does not have a limited lifespan, so it can emit light and then be recharged over and over again. Super-luminova is the brand name that’s used most ubiquitously across the luxury watch industry and sometimes is made in different colours than the default green.
Luminova may also be used creatively, as Franck Muller has shown with the Vanguard Krypton—obviously no “real” krypton had been used in the watch. Franck Muller achieved a one-of-a-kind carbon case and dial flecked with luminova such that the entire watch glows.