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Atoms, molecules, or solids that are excited to high energy levels can decay to lower levels by emitting radiation (emission or luminescence). For atoms excited by a high-temperature energy source this light emission is commonly called atomic or optical emission (see atomic-emission spectroscopy) and for atoms excited with light it is called atomic fluorescence (see atomic-fluorescence spectroscopy). For molecules it is called fluorescence if the transition is between states of the same spin and phosphorescence if the transition occurs between states of different spin. Separate documents describe molecular fluorescence, which can be done with compact instruments, and laser-induced fluorescence.

The emission intensity of an emitting substance is linearly proportional to analyte concentration at low concentrations. Atomic emission and molecular fluorescence are therefore useful for quantitating emitting species.

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