The principle of using fluorescent coating to emit light on a watch is that when a substance is excited to absorb energy, it transitions to an excited state (unstable state) and then returns to the ground state, it releases energy in the form of light, that is, the fluorescent substance emits light after being excited.
Self-luminous fluorescent agents mostly rely on trace radioactive substances carried by themselves to release rays to excite fluorescent agents to emit light. However, the light storage fluorescent agent basically does not contain radioactive substances, but only needs to absorb and store enough external light in advance to make its own electrons jump from low energy level to high energy level and store them. When the surrounding environment is dark, it gradually and slowly releases the absorbed energy. At this time, the electrons jump from high energy level to low energy level, and the fluorescent agent begins to emit light.
The fluorescent coating of watches is generally a fluorescent agent containing tritium (3H or T), promethium (Pm) and radioactive radium sulfate (Ra), and the fluorescent agent is mostly zinc sulfide, calcium sulfide or strontium sulfide, etc. Tritium (3H or T) and promethium (Pm) are still radioactive, but they are basically harmless to human body.