The transmittance and reflectance of superconductive YBa2Cu3O7-δ (YBCO) thin films deposited on Si substrates have been measured in the far-infrared frequency region from 10 to 100 cm−1 (wavelength from 1000 to 100 μm) at temperatures between 10 and 300 K. The effects of interference, optical resonance, and antireflection on the radiative properties of high-temperature superconducting (HTSC) films are observed and quantitatively analyzed. Furthermore, we have measured the reflectance of the HTSC film-substrate composites for radiation incident on the substrate side (backside reflectance) for the first time. The backside reflectance increases significantly from the normal state to the superconducting state at certain frequencies; this experimentally demonstrates that HTSC films can be used to build far-infrared intensity modulators. The complex refractive index of the YBCO films is determined from the measured transmittance using the Drude model in the normal state and a two-fluid model in the superconducting state. The complex refractive index obtained from this study is useful for various applications of YBCO films, including radiation modulators, detectors, and Fabry-Perot resonators.