"T Swiss Made T" or "Swiss T 25" : what does it mean ?
In order for timepieces to be read in the dark, a luminescent material is laid on the dial indexes and hands. Generally speaking, the emission of light is either of photoluminescent type (determined by a exciting luminous radiation) or of radioluminescent type (determined by the radioactivity of the material).
Timepieces featuring radioluminescent emission are mostly designed for very specific uses : military watches, professional divers watches, etc.
In this case, the use of radioactive material is strictly defined by ISO 3157 Standard which allows only two types of radionucleides : tritium (3H) and promethium (147 Pm). It is important to specify that these radionucleides emit a radiation of low energy.
ISO 3157 Standard allows an optional marking for timepieces emitting less than a certain value. The marking may be made on the dial as follows :
deposits activated by tritium : T
deposits activated by promethium : Pm
On the other hand, timepieces with a higher value, such as divers' watches, must be marked as follows :
deposits activated by tritium : T 25
deposits activated by promethium : Pm 0,5
The indication "T Swiss made T" means that the watch is Swiss and contains a certain quantity of tritium that emits less than 227 MBq (7,5 mCi).The indication "Swiss T<25" means that the watch is Swiss and contains a certain quantity of tritium that emits less than 925 MBq (25 mCi).
Most of the Swiss watches use a light emission of photoluminescent type. Some of them bear the optional marking "L Swiss Made L" to indicate it.
