ABSTRACT
The term luminescence means the emission of radiation by some bodies; its cause is not simply a rise in their temperature as is true in the case of emission by incandescence. An electric field of sufficient intensity suitably applied to a body may excite it to emit light. In all cases, the colour of the light emitted can be produced by incandescence only at a temperature considerably higher than that of the emitter. In the spectral lamps with metallic vapours, the potential difference is a few tens of volts. Various chemical reactions, principally oxidations are accompanied by an emission of light. The bio-luminescence phenomena are linked to chemi-luminescence. The molecules of a vapour may be excited by the absorption of an electromagnetic radiation of suitable frequency.
