Abstract Films which are semi-transparent in the infrared are extensively used in solar energy applications and agriculture. For analysing the performance of such systems, it is important to knoe the thermal radiation properties of these films. Whereas transmissivity can be determined easily, the measurement of emissivity poses a problem. The intensity usually consists of radiation emitted from the film and transmitted background radiation. In the paper a method to separate these parts is presented. The measurement device, which has been built up, uses a black body as background. Special attention has been given to the temperature of the film. By a heated air stream the film temperature will be kept close to the black-body temperature. Therefore a simple, but nevertheless exact analysis can be used. The paper describes the measurement method and the experimental device. Results for transparent and non-transparent materials are presented and discussed.