How do activator ions allow crystal phosphors to emit visible light?

In crystal phosphors, activator ions facilitate the emission of visible light by introducing specific energy levels within the energy gap that exists between the valence band and the conduction band of the host crystal.1

When the phosphor is excited by energy—such as cathode rays, ultraviolet radiation, or other particles—electrons are raised from the valence band to higher energy states. Activator ions act as luminescing centres by providing intermediate energy levels that "bridge" the gap between the valence and conduction bands.12

Visible light is produced when an excited electron returns to the ground state of the activator centre. By capturing an electron that has been excited to the conduction band, the activator ion allows the electron to drop into these intermediate levels before finally transitioning back to the ground state, releasing the energy difference as a photon of visible light. The specific chemical nature and symmetry of the host crystal surrounding these activator ions determine the precise energy of the emitted light, thereby dictating its colour.1234