A new approach to infrared thermometry

Surface temperature is a crucial variable linking surface–atmospheric energy exchange, but it is difficult to measure accurately. Remote measurement by infrared (IR) thermometry is often the only viable choice, but is plagued by problems that limit its absolute accuracy. Primary among these are calibration shifts and an inability to eliminate or properly account for the influence of detector temperature on the measurement. We have developed a new approach that avoids these and other difficulties by making the measurement differentially, essentially providing continuous calibration. The system uses a conventional infrared thermometer (IRT) coupled to a rotary actuator so that its field of view can be periodically switched from the target of interest to a blackbody cavity, whose temperature is controlled with a Peltier block/controller board assembly and measured with carefully calibrated thermocouples. The blackbody temperature is controlled so that the detector output is the same when viewing the blackbody as it is when viewing the target surface. When this condition is satisfied the blackbody temperature and the brightness temperature of the target surface are equal, i.e. the thermal radiation emanating from each is the same. A prototype instrument, using a conventional IRT as the detector, was built and tested in the laboratory by using it to measure the surface temperature of a mineral oil reservoir that was cycled over a range of temperatures and independently monitored with calibrated thermocouples. Over a 24°C temperature range, the mean absolute error of the instrument was 0.04°C, and a regression against thermocouple-measured oil temperature yielded a slope of 1.002, intercept of −0.015°C, and r2 of 0.99998, substantially better than the performance of a conventional IRT subjected to the same tests. A field instrument was also built, based on these principles but with smaller components for lower power consumption and lower cost. In an important departure, it uses two IR detectors and a modified switching/control algorithm that provides improved dynamic response while maintaining the accuracy of the prototype. We conclude that continuously-calibrated IR thermometry (CC-IRT) is a viable means for improving the accuracy of radiometric temperature measurement.