Define Luminescence, Fluorescence, and Phosphorescence?
Luminescence is a collective term for different phenomena where a substance emits light without being strongly heated, i.e., the emission is not simply thermal radiation. This definition is also reflected by the term “cold light”. Fluorescence and Phosphorescence fall under this category. Fluorescence refers to the light emission caused by irradiation with visible or UV light. The light emission occurs typically within nanoseconds to milliseconds after irradiation. It involves the excitation of electrons into states with a higher energy, from where radiative decay is possible. Typically, the emitted wavelengths are longer than the excitation wavelengths. Phosphorescence, on the other hand, refers to a light emission, which can occur over much longer times (sometimes hours) after irradiation. It involves storage of energy in metastable states and its release through relatively slow processes. This release of energy is usually enhanced when thermally activated.
