Proceedings PaperFacility for automatic radiometric calibration of thermal imagers
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A facility is under development which largely automates the radiometric calibration of a pair of AGEMA Thermovision Series 780 radiometers or both channels of a dual waveband radiometer. The output of the radiometer is proportional to the photon flux incident on the detector. Calibration is performed by determining the relationship between the total radiant emittance of a black body and the electrical output of the detector. Measurements are made at a specified number of temperatures over the range 0 to 1000 °C and a least square method is used to determine the calibration constants. The radiometers, which are mounted on a motorised stage providing motion transverse to their principal optical axes, are positioned under the control of a computer so as to view one of two black body sources. The available video signal is dependent on range and level controls, and, to achieve full resolution and maximum accuracy, a mechanism is employed to adjust the level control automatically. The video signal is displayed on the computer screen for the purpose of alignment and for determining where the black bodies are located in the image. Once alignment and the initial setup has been completed, the process is put under the control of the computer. For each measurement point, the computer sets the black body controller and waits for the set temperature. After a specified delay to allow stabilisation, a predetermined number of video frames are averaged to determine the radiometric signal levels corresponding to the image of the black body. When measurements are complete, the data points are fitted to determine the calibration constants. Provision has also been made to accept digitised signals taken directly from the detector preamplifiers.