Inelastic scattering includes Brillouin scattering, Raman scattering, inelastic X-ray scattering and Compton scattering. Neutrons undergo many types of scattering, including both elastic and inelastic scattering. Many phonon dispersion curves have been measured by inelastic neutron scattering. Raman spectroscopy relies upon inelastic scattering of photons, known as Raman scattering. Scattering almost always presents both elastic and inelastic components. Furthermore, momentum is conserved in both elastic and inelastic scattering. Inelastic scattering is common in molecular collisions. During inelastic scattering processes, significant energy exchange happens. Alternatively, the same vibrational excitation can be produced by an inelastic scattering process. However, the difference between elastic and inelastic collision was not understood at the time. It is a form of inelastic light scattering, where a photon excites the sample. The structure functions are important in the study of deep inelastic scattering. Electrons which undergo inelastic interaction are then similar to non-monochromatic light. In fact this allows the calculation of electron inelastic mean free path (IMFP). However, kinetic energy is not conserved in cases of inelastic collision.