So that watches (and timekeeping devices in general) can be read in the dark, the numerals, hour markers and hands must be coated with a luminescent substance. As a general rule, the light emitted is either photoluminescent (using energy absorbed from a light source) or radioluminescent (using the radioactive property of the coating substance).
Radioluminescence is generally reserved for watches intended for professional use, such as military watches or diving watches, and is strictly regulated by ISO 3157 which allows only two types of radionuclides: tritium (3H) and promethium (147 Pm). Both these radionuclides emit very low-energy radiation.
ISO 3157 provides for optional marking of timekeeping devices that emit less than a certain value. This marking can be shown on the dial as follows:
- tritium-activated deposits: T
- promethium-activated deposits: Pm
In contrast, devices that emit above this value must be marked as follows:
- tritium-activated deposits: T 25
- promethium-activated deposits: Pm 0.5
"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). "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 Swiss watches use photoluminescent light emission, sometimes indicated by the optional marking, "L Swiss Made L".