INFRARED-THERMOGRAPHY THROUGH WINDOWS AT LOW-TEMPERATURES

Infrared thermography offers itself for non-intrusive measuring of local temperature distributions at internal surfaces of channels and hollow objects. The temperature distribution at an internal surface can only be measured through an IR-transparent window, if the IR-camera is positioned outside. Thermal radiation and reflection by this window may falsify the temperature distribution indicated by the IR-camera, the required correction is analyzed in this paper. For this study the radiation reaching the IR-camera within the detected range of wavelength is considered by a balance of radiative energy fluxes. The assumptions made to evaluate this balance are verified by comparing calculated and measured temperature values and the validity is confirmed. Results show, that linear calibration curves for measurements through a window may be applied with reasonable accuracy in a range of temperature 295 K less-than-or-equal-to T(O) less-than-or-equal-to 365 K of both window and environment. The influence of both temperatures on the calibration curve has been studied by considering the balance of radiative fluxes. In addition, the influences of different angles of view of the IR-camera and of different window orientations are examined.