The background radiation effect of the measurement for the size-of-source effect (SSE) of radiation thermometers must be considered at low and median temperature. The calculation model includes items of radiation thermometer outputs to measure an infinite-size radiation source. After analyzing the influence of the detector temperature on the models of temperature and SSE measurements, a novel calculation model, utilizing a suppositional detector temperature balanced with the background radiation for the SSE measurement is presented. This method prevents the need for an infinite-size radiation source and unavoidable approximation. The relations of the SSE and the increase of temperature reading are given. The measurement conditions of the source temperature, reading resolution and thermometer emissivity setting are analyzed to optimize the result of the SSE measurement.