A luminescence phenomenon in which electron de-excitation occurs almost spontaneously, and in which emission from a luminescent substance ceases when the exciting source is removed. In fluorescent materials, the excited state has the same spin as the ground state. If denotes an excited state of a substance A, then fluorescence consists of the emission of a photon,

where h is Planck's constant and is the frequency of the photon.

The quantum yield of a fluorescent substance is defined by

Fluorescent Lamp, Luminescence, Opalescence, Phosphorescence