A technique for measuring the elastic properties of single crystals in materials, providing data that allow the calculation of seismic velocities and changes in density that occur in materials under extreme pressure and/or temperature. It is based on the inelastic scattering of photons (light) by acoustic vibrations (phonons) induced by changes of temperature. This type of scattering was first described in 1922 by the French physicist Léon Nicolas Brillouin (1889–1969).

http://serc.carleton.edu/NAGTWorkshops/mineralogy/mineral_physics/brillouin.html Explains and describes Brillouin spectroscopy.