How does infrared light stimulate some phosphors to emit visible light?

Infrared light stimulates certain phosphors to emit visible light through a process involving traps within the crystal lattice of the material.1

In these specific phosphors, such as cubic strontium sulfide activated with samarium and europium ions, the material is first excited by higher-energy photons (such as blue light). This excitation causes electrons to be ejected from the active luminescent centres and captured by traps—imperfections in the crystal lattice where the electrons remain in a metastable state.1

Because the energy required to release these trapped electrons is higher than the available thermal energy at room temperature, the electrons remain stored for long periods. When the phosphor is subsequently irradiated with infrared photons, the absorbed infrared energy provides the necessary stimulus to eject the electrons from the traps. Once released, these electrons recombine with ionized activator centres, returning to their ground state and emitting the stored energy as visible light. This mechanism is utilized in devices designed for viewing scenes at night using reflected infrared light.1