Guido Panerai created the original luminous watches for use in Italy's navy. In the early part of the 20th century, he experimented with luminous substances and created a system based on a combination of radium bromide and zinc sulphide. He achieved a patent for his development in 1915 and then produced a watch called Radiomir, which became an iconic timepiece.
New technology for luminous watches uses small gas lights that are self-powered. Through the use of Swiss technology, a glass tube contains a gas called tritium. A luminous substance coats the glass tube. These gas lights glow more brightly than the previously used luminous paints and last a long time -- even up to 25 years. They carry no radiation risks because even if the tritium gas escapes, the amount of radiation that would result would be insignificant.
The radioactive threat of radium was not known at first. It is actually very radioactive in nature. Because companies were unaware of this threat, many things were made with it besides the self-luminous watch. Objects such as switches and instruments for readouts aboard aircraft were made with the material, which is far more powerful than uranium. The material will not decay for more than 16 centuries. In the 1940s, Panerai began the process towards safer watches and patented another substance called Luminor, which used tritium rather than the earlier mixture used in Radiomir watches. However, tritium still presented a toxic threat due to its radioactive particles but they do not enter through skin. It is dangerous only after eating food or drinking water that contains it but even then, it decays in the body within a couple of weeks.
Most of today's luminous watches use a substance called Super-LumiNova, which uses pigments that are luminescent. It utilizes strontium oxide aluminate chemistry, which makes the watches safely luminescent. The LumiNova absorbs daylight and in 30 minutes is fully charged; it can last all night. The safe Super-LumiNova substance is used by all the major watch brands.