Rolex Tritium Use
A luminescent material is needed on the hand and hour markers in order for a timepiece to be able to be read in the dark. For the most part, the emission of light on a watch is caused by either photo luminescent material, which is determined by an exciting luminous radiation, or a radio luminescent type, something determined by the radioactivity of the material.
From the 1950s to the late 90s, Rolex used the radioactive material Tritium, which refers to the chemical used on the hands and hour markers of the watches, which causes them to illuminate. To indicate the amount of the radioactive material, Rolex began marking a designation at the bottom of the dial:
- T SWISS MADE T: the radioactive material Tritium is present on the wristwatch. The amount of radioactive material emitted is limited to a maximum of 25 milliCurie.
- SWISS T < 25: indicates more specifically that the wristwatch emits an amount of Tritium that is less than the 25 milliCurie limit. This indication can be found on all Submariners from 1989 to 1997.
- SWISS T 25: the wristwatch emits the maximum allowable amount of Tritium (i.e. a full 25 milliCurie).
- SWISS (or) SWISS MADE: wristwatches produced after (around) 1998, indicates the presence of LumiNova as the luminous material. These terms were also the indication on wristwatches produced prior to the 1950s, when Radium was used as the luminous material. However, at that time "SWISS" or "SWISS MADE" simply indicated that the watch was made in Switzerland.
- T: deposits activated by tritium
- Pm: deposits activated by promethium
- T 25: indicated deposits activated by tritium on higher value watches
- Pm 0,5: deposits activated by promethium indicated on higher value watches.
Timepieces that feature radio luminescent emissions are, for the most part, designed for very specific uses: aviation watches such at the GMT-Master I & II or professional diver watches such as the Submariner. When a radioactive material is used, the ISO 3157 Standard has strictly defined guidelines that allows only low values of two types of radionucleides to be used: tritium (3H) and promethium (147 Pm), materials that emit a radiation of low energy.
It is important to mention that Tritium used for Rolex watches with luminescent parts are not harmful when sealed inside the watch. Around 1998, watchmakers such as Rolex changed the designation to read SWISS or SWISS MADE, when they replaced the Tritium with LumiNova, an organic, non-radioactive chemical, as the source of luminescence.