Light is a form of energy, and in accordance with the fundamental concept of energy conservation, energy must be supplied to every material system emitting light. There are two processes by which the material can become a generator of light after absorbing suitable energy . In one process, the absorbed energy is converted into heat. The thermal agitation of all molecules within the system increases, and simultaneously, more and more of the molecules are transfered into excited states. The higher the temperature, the greater is the number of excited molecules and the greater is the intensity of the emitted light. In the other process, the molecules are brought into excited states without increasing their average kinetic energy and without heating the system. An appreciable part of the absorbed energy is temporarily localized as excitation of atoms or small groups of atoms which then emit light; this process is called luminescence. Luminescence is characterised by emission of light in excess of the thermal radiation produced by heat in a given material. The basic rule for distinguishing between thermal and light radiations can be formulated as follows: If the intensity of the emitted light exceeds the intensity of the radiation of the same wavelength from a black body of the same temperature, the radiation is a case of luminescence .
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