Spontaneous emission is the process in which a quantum mechanical system transitions from an excited energy state to a lower energy state and emits a quantized amount of energy in the form of a photon. If the system in question is excited by some means other than heating, the spontaneous emission is called luminescence. There are different sub-categories of luminescence depending on how excited atoms are produced. If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Some systems have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission.