"Raman scattering is a weak phenomenon and that is sometimes the reason for being unable to acquire a Raman spectrum, particularly at a low concentration or with a limited amount of sample. However, the most frequent reason for not being able to acquire a Raman spectrum is because a strong fluorescent background and the detector noise generated by it overwhelms the Raman signal. Therefore, it should be self-evident that a Raman spectrometer is ideal for performing laser-excited photoluminescence" . Absorbing a photon, an electron is promoted to an excited level of energy. It can decay emitting a photon. This "emission process is called photoluminescence", since "the excited state was reached through the absorption of a photon … Photoluminescence occurs in many bulk materials that are direct band gap semiconductors and also because of impurities that absorb and emit light in an otherwise transparent medium" .