Clean single-walled carbon nanotubes (SWCNTs) synthesized using the HiPCO process and purified (purchased from Nanointegris Inc.) were chemically treated by two different methods: i) a microwave-assisted acidic oxidation process by a concentrated HNO3 solution (ox-CNTs), ii) a covalent grafting of a methoxyaryl groups by a thermal radical functionalization (MeOB-CNTs). Thin films of raw and chemically modified SWCNTs were made and analysed using ellipsometric spectroscopy from 0.6 to 4.96 eV using three incident angles (50°, 60° and 70°) and also in the infrared region from 0.07 eV to 0.6 eV with a unique incident angle (60°). The films were supposed to be thick and dense enough to model them as a homogeneous and isotropic semi-infinite medium. Under this hypothesis, we analytically calculated the complex dielectric function of the films and extracted the real (er) and imaginary (ei) parts of this function. Raman spectroscopy was also used to asses the number of covalent defects introduced in the graphitic structure of SWCNTs.