A new approach to infrared thermometry

Surface temperature is a crucial variable linking surface-atmospheric energ y exchange, but it is difficult to measure accurately. Remote measurement b y infrared (IR) thermometry is often the only viable choice, but is plagued by problems that limit its absolute accuracy. Primary among these are cali bration shifts and an inability to eliminate or properly account for the in fluence of detector temperature on the measurement. We have developed a new approach that avoids these and other difficulties by making the measuremen t 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 Pelt ier 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 ta rget surface. When this condition is satisfied the blackbody temperature an d the brightness temperature of the target surface are equal, i.e. the ther mal 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 temperatur e of a mineral oil reservoir that was cycled over a range of temperatures a nd independently monitored with calibrated thermocouples. Over a 24 degrees C temperature range, the mean absolute error of the instrument was 0.04 deg reesC, and a regression against thermocouple-measured oil temperature yield ed a slope of 1.002, intercept of -0.015 degreesC, and r(2) of 0.99998, sub stantially better than the performance of a conventional IRT subjected to t he 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 radio metric temperature measurement. (C) 2001 Published by Elsevier Science B.V.