Spectra of X- and γ-rays emitted by radioactive sources and of the radiation emitted by X-ray tubes, in the energy range 15–160 keV, have been measured with a silicon PIN photodiode at room temperature. The experimental X-ray tube spectra are compared with calculated ones on the basis of a semi-empirical model. Results indicate that the method utilized is adequate for the characterization of X-ray beams in the medical diagnostic energy region.