The longwave absorption coefficient of diamond is composed of the two-phonon red wing and
multiphonon difference (hot) bands. As temperature increases the difference bands rapidly grow in strength and
dominate the absorption. A variation on a multiphonon sum band model is developed for difference bands and
applied to a temperature dependent data set on CVD polycrystalline diamond that are 1.54 mm thick. The model
can also be used to estimate multiphonon difference band contribution to the refractive index.