Persistent or self-luminous luminous colors are used for watch numerals, indexes and hands, to be read in the dark.
Previously these luminous colors consisted of radioactive substances, today only phosphorescent colors are used. Prevailed for the average everyday use has the absolutely harmless and non-radioactive Superluminova, which has a relatively long persistence and does not use up.
The luminous hands, dials and scales of older watches contained radioactive material with far reaching rays. This presented a danger particularly when the watches were constantly being worn on the body. This was true, for example, for radium, which is not used anymore.
Also tritium was used, which is recognizable by a dial marking with "T". This is usually found at the bottom edge of the dial or in connection with "Swiss", such as stating "T <25". The numerical value refers to millicurie as a measure of radioactivity.
Where a higher legibility of the dial in the dark, beyond the everyday usual, is needed (eg for military watches), today the lighting system Tritium H3 is customary. It was developed by the Swiss company mb-microtec (watch brand: Traser). Here tritium gas light sources ensure the illumination: a minimal amount of tritium gas - H3 - brings the lighting elements permanently to light, without external charging power supply or a button and 100 times stronger than any comparable lighting system.