Radiation thermometers represent thermometers for realization of temperature above the freezing point of silver (961,78 °C). For calibrations and measurements with these thermometers, it is very important to consider the factors, which influence the uncertainty of measurements. One of these is an occurrence called the size of source effect. This occurs because of little irregularities in the optical system of the measuring instrument, which can cause a higher or lower measured temperature of an object than its true temperature. This happens due to the radiance, which originates outside of a nominal field of view. Due to numerous difficulties in determination of contributions that influence the size of source effect, the experimental method is the only way of confirmation the extent of the size of source effect. There are two most common methods to determine this effect, indirect and direct method. For radiation thermometers with lower resolution and with the output signal in temperature, the latter is much more suitable. This enables the determination of the size of source effect based on direct temperature readings. Calibration furnaces (blackbodies) are used as a source of radiation and the size of the measured area is reduced with particular aperture plates. That is how the response of the radiation thermometer to different sizes of measured areas is established. The nominal target size at a particular distance is a minimal area of measurement needed, with which it is still possible to measure temperature with the radiation thermometer. The knowledge of the source size effect for the particular radiation thermometer is very important to adjust the circumstances for the correct execution of calibrations and measurements.
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