Formulae describing the spatial distribution of radiation intensity in a semiconductor film in a multilayer structure are given for the case of non-normal incidence of radiation. Interference of radiation internally reflected in such a sample makes the radiation intensity very sensitive to geometrical and material parameters of the structure as well as to the wavelength, angle of incidence, and polarization of radiation. The influence of radiation wavelength range and uncertainties of film thickness and refractive index on the spatial distribution of radiation intensity is discussed. The case of a graded-gap semiconductor film is analyzed. The presented formulae are useful in photoelectric and photoelectromagnetic investigations of thin film semiconductors and are also useful for the optimization of photon detectors.