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The law of conservation of energy states that the light transmitted through any sample, plus the sum of reflections by the sample, results in a factor of 1 of the introduced energy. The law does not regard the fact that transmitted light may be refracted, diffracted, or scattered by the sample. It also does not take into account that absorbed energies may appear in the beam as luminescent signals. Luminescence is the collective description for the emission of electromagnetic energies by an atomic or molecular sample, which may be caused by different reasons, such as chemical reactions, mechanical pressure, or the excitation by external electromagnetic radiation. The latter is the topic of this chapter. The energy that is absorbed by the sample is briefly stored in the electron’s orbits and re-emitted to a lower level of energy. Thus, a previous excitation by light with a shorter wavelength than that of the emission must have taken place. The optically excited luminescence is called photoluminescence.
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