System for the determination of the size-of-source effect of radiation thermometers with the direct reading of temperature

The paper describes a system for the determination of the size-of-source effect (SSE) of radiation thermometers with the direct reading of temperature, which form the majority of commercial radiation thermometers. Optics in every radiation thermometer (RT) gathers radiation from a larger area than is defined by the nominal target size. Thus, a measured temperature is more or less dependent on available target area. Every radiation thermometer suffers from the SSE. We developed a system, based on a water-cooled holder, for measuring the SSE by a direct method. The system could be placed in front of any blackbody. Manufacturing of such a system was relatively easy and could be important for users of commercial radiation thermometers because the majority of those thermometers have direct reading of temperature. We measured the SSE characteristics by the direct method for a radiation thermometer with a linearized signal and direct reading of temperature. The results of the SSE were analysed for the direct method without correction of the background radiation and with correction of the background radiation. Knowledge about the SSE characteristic of a radiation thermometer is one of the key elements for correct temperature measurement.