Formulae describing the spatial distribution of radiation intensity in a thin semiconductor film on a relatively thick, transparent, parallel-sided substrate are given far the case of non-normal incidence of radiation, internal reflection and interference of radiation in the film 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 presented formulae are useful in photoelectric and photoelectromagnetic investigations an thin-film semiconductors. They make the calculations about fifty times faster than those using the general equations.