Phosphorescence

Phosphorescence is a type of photoluminescence in which a material exposed to light continues to emit visible light after the stimulating source is removed. In phosphorescence, electrons are promoted to higher energy levels and become trapped in metastable states, typically involving defect sites or dopant ions in a solid. Because the transition back to the ground state is quantum mechanically forbidden or partially forbidden, the recombination proceeds slowly, producing a delayed glow that can last from milliseconds to hours.

Most common phosphors include zinc sulfide doped with copper (older glow materials) and strontium aluminate doped with europium and dysprosium (modern, long-lasting). The latter can emit green or blue-green light and persist after brief or prolonged exposure to light. Glowing duration and brightness depend on the material, the trap density, and temperature; higher temperatures generally shorten persistence.

Excitation is typically by ultraviolet or blue light, but can be by other wavelengths depending on material. The emitted light is often visible without any ongoing energy source.

Applications include safety signs, watch dials, novelty items, and emergency exit signs that require visibility after power loss. Natural phosphorescence occurs in minerals like zircon and certain rocks. Phosphorescence is distinct from fluorescence, which ends almost immediately after excitation, and from chemiluminescence and bioluminescence, which produce light by chemical reactions.