Fluorescence is the spontaneous emission of light shortly after the excitation of a material by an electron transition. The fluorescence photons are usually lower in energy than the excitation photons (= Stokes shift). The fluorescence typically has a lifetime (or duration) of nanoseconds (10−9 s) per transition. A fluorescence spectrophotometer normally has an excitation monochromator that defines the excitation energy, and an emission monochromator that provides a full spectrum of the fluorescence emission. Note that fluorescence is the most common interference in Raman spectra and that fluorescence is a strong background interferent of Raman scattering. Often the much stronger fluorescence masks the weaker Raman signal. This is often true for resonance Raman and also for heterogenous biological or mineralogical samples. Also traces of impurities can lead to fluorescence masking the Raman signal.