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 an 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 :
http://www.fhs.ch/images/t_sm_t2.gif
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 :
http://www.fhs.ch/images/swiss25.gif
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).
http://www.fhs.ch/images/swiss26.gif
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. |
