We report infrared ellipsometric spectra of bulk LIF from 50 to 1500 wavenumbers. The measurements were optimized to provide maximum precision and accuracy of the complex response functions. We find a generally valid rule for strongly polar lattice modes : while the best results for the transverse (TO) vibrations are obtained at high angles of incidence, the range of the longitudinal (LO) modes should be measured fairly close to the normal incidence. We also demonstrate the possibility of tuning the sensitivity in order to detect rather weak two-phonon bands in both ellipsometric and polarized reflectivity measurements. The measured data provide a firm basis for the discussion of the static dielectric constant, the low-energy electronic polarizability and the TO and LO scattering rates. Since both real and imaginary parts of the response functions are reliably measured, we are able to extract quantitatively the two-phonon summation bands. Finally, we address the behavior of a thin LiF film on a metallic substrate, especially in the LO mode region.