“Emission” occurs during transition from a higher level to a lower level if energy is transferred to the radiation field. When no radiation is emitted the phenomenon is called “nonradiative decay.” This type of spectroscopy relies on the range of electromagnetic spectra in which a particular substance radiates. The substance first absorbs energy and then radiates (that is, emits) this energy as light. The excitement energy that is absorbed first can come from a number of different sources, including collision (from high temperatures or other means), chemical reactions or light.
It also should be noted that atoms or molecules once excited to high-energy levels then can decay to lower levels by emitting radiation. This is called emission or luminescence. When atoms are excited by a high-temperature energy source this light emission commonly is called atomic or optical emission, and for atoms excited with light, it is called atomic fluorescence.