In physics, the Lamb–Mössbauer factor (LMF, after Willis Lamb and Rudolf Mössbauer) or elastic incoherent structure factor (EISF) is the ratio of elastic to total incoherent neutron scattering, or the ratio of recoil-free to total nuclear resonant absorption in Mössbauer spectroscopy. The corresponding factor for coherent neutron or X-ray scattering is the Debye–Waller factor ; often, that term is used in a more generic way to include the incoherent case as well.