The Raman effect is caused by inelastic scattering when light interacts with molecules. Most of the incident light scatters elastically making a broadband source impractical. Thus for most common Raman techniques, a monochromatic light source is used. The incident photon’s energy changes in accordance to quantum mechanical rules and can be detected as a shift in the wavelength from the excitation light. Energy transfers from the photon to the molecule cause rotational or vibrational transitions [Stokes] or transfer energy from the molecule to the photon [anti-Stokes]. Since the energy gained/released corresponds to a molecule’s energy levels, this leads to characteristic fingerprints by which molecules can be identified and quantified.
The spectral sensitivity of the measurement system is crucial for the applicability of this technique since only a small portion of the incident photons are Raman shifted. The sensitivity is defined by a number of parameters, which include the spectral efficiency of the polychromatic setup and the quantum efficiency of the detector.