A procedure is proposed to determine the optical properties of commercial polymeric films used as solar control for building skins. The spectral variation (190-1100 nm) of the refractive index and extinction coefficient of the samples was obtained through the simulation of the experimental transmittance spectra of the samples. Polyvinyl chloride (PVC) and polyethylene terephthalate (PET) films were analyzed. Refractive index values between 1.1 and 2.5 at λ = 500 nm and extinction coefficient values of 104 cm-1 in the ultraviolet (UV) region were found. All these results show a good fit with those given in the literature for similar thin films. A new concept for the illumination and temperature control of buildings by the use of polymeric thin films as a transparent solar protection material is presented.