Postillumination

Postillumination refers to the phenomenon where a material continues to emit light after the external source of excitation has been removed. This luminescence is a result of energy absorbed by the material and then slowly released in the form of photons. The duration and intensity of postillumination can vary greatly depending on the material's properties.

There are several types of postillumination, with phosphorescence being the most common. Phosphorescence occurs when a substance absorbs energy, such as from ultraviolet light, and then re-emits this energy as visible light over a period of time, ranging from seconds to hours. This is distinct from fluorescence, which ceases almost immediately upon removal of the excitation source. Thermoluminescence is another form, where light is emitted when a previously irradiated material is heated. Chemiluminescence, while not strictly postillumination as it involves a chemical reaction, can sometimes exhibit a lingering glow.

Materials exhibiting postillumination are often referred to as phosphors or luminescent materials. They find applications in various fields. Glow-in-the-dark toys and watch dials are classic examples, utilizing phosphorescent pigments. In scientific research, postillumination properties are important for applications like dosimeters that measure radiation exposure (thermoluminescence) and in certain imaging techniques. The underlying mechanism involves trapped electrons within the material's crystal lattice that are excited to higher energy levels. When these electrons return to their ground state, they release energy as light. The rate of this release determines the persistence of the glow.