Calibration and use of low-temperature direct-reading radiation thermometers

The calibration and use of low-temperature radiation thermometers with uncooled detectors require special consideration of the many influence variables affecting the readings, including target emissivity, background temperature and instrumental emissivity setting. The readings for instruments with fixed emissivity less than 1 depend on the detector temperature, which is generally not made available to the user. The readings also depend on the spectral responsivity of the thermometer, which is rarely provided in much detail. The effects of all these influence variables are somewhat obscured by the processing algorithm employed by the thermometer to convert the measured signal into a temperature reading. This paper outlines methods for determining expected thermometer readings during calibration against blackbody sources, and for correcting readings during use where large reflection errors are present. These methods are essential for ensuring traceability of measurements made with these instruments, and are easily implemented in spreadsheet applications.