A homogeneous model for extracting the optical constants of a weakly absorbing film, which is based on the envelope of the transmittance spectrum, has been developed. Expressions are given to correct the spectrum measurements of the thin film to eliminate the effects due to the reflectance from the rear surface of the substrate. Moreover, a simple approach for determining the optical constants of the substrate is also described. For a LaF3 film deposited on CaF2 substrate by Mo-boat evaporation, the optical constants of the as-deposited and ultraviolet (UV)-treated 40 min LaF3 films and CaF2 substrate in the wavelength region from 160 to 340 nm are obtained. The experimental results indicate that UV-treatment can improve the optical properties (increase the refractive index and reduce the extinction coefficient) and reduce the physical thickness of the LaF3 film.