A low temperature infrared pyrometer with optical probe

Abstract

The theory and considerations behind infrared pyrometry are presented. The principles are applied to the development of a remote temperature sensor for low temperatures ranging between zero and one hundred degrees Celsius. Due to the low level of radiation at these temperatures, lock-in amplification was required. A fibre optic cable was used as a probe to convey the infrared radiation from the object under inspection to the sensor, and its performance was analysed. In order to achieve this, a fibre optic cable transparent to infrared light was required, and a chalcogenide core fibre was chosen. The design of the sensor itself was cost effective, and except for the chalcogenide probe, expensive components were avoided. The sensor performed well, exhibiting good linearity and repeatability over the entire temperature range. The signal-to-noise ratio, and hence the resolution, could be increased by increasing the time constant, and consequently the response time of the sensor. The fibre optic probe, however, was discovered to be too fragile for this application as it is easily damaged while being handled. Furthermore, its small core diameter and inherent losses severely attenuated the radiation reaching the sensor, with the result that a high signal-to-noise ratio could be achieved only by using impractically long time constants.