Spontaneous emission luminescent. And there are different forms of luminescence depending on how excited atoms are produced (electroluminescence, chemiluminescence etc.). If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission. Spontaneous emission cannot be explained by classical electromagnetic theory and is fundamentally a quantum process. The first person to derive the rate of
Stimulated emission to the number of atoms in the lower state, "N". Einstein showed that the coefficient for this transition must be identical to that for stimulated emission: Thus absorption and stimulated emission are reverse processes proceeding at somewhat different rates. Another way of viewing this is to look at the "net" stimulated emission or absorption viewing it as a single process. The net rate of transitions from "E" to "E" due to this combined process can be found by adding their respective rates, given above: Thus a net power is released into the electric field equal to the photon energy "hν"