In this work we study the infrared (IR) optical properties of n-type PbSe samples doped with Cl exhibiting an electron carrier density in the range 0.75×1019 cm-3-3.8×1019 cm -3. The IR reflectivity spectra are dominated by free carriers giving rise to blue-shifted reflectivity minimum in the region > 700cm -1. The analysis of the experimental reflectivity spectra showed that the conventional Drude model with a constant carrier relaxation time failed to describe the behaviour in the whole frequency range. It was found that the lineshape of the reflectivity spectra can be better described by a frequency dependent electron damping parameter with resonant-like behaviour similar to the one observed in the case of resonant electron scattering off potential wells. The density of states effective mass was found to increase with increasing carrier concentration supporting the non-parabolicity of the band model for PbSe.