Abstract: An imaging sensor for scanning a thermal image is disclosed. The imaging sensor comprises a first set of elemental detectors operable to receive a thermal image during a first portion of the calibration cycle. A second set of elemental detector is provided which are operable to receive the image during a second portion of the calibration cycle. Means are provided for calibrating the first and second sets of elemental detectors by comparing the output of the first set of elemental detectors during the first portion of the calibration cycle with the outputs of the second set of elemental detectors during the second portion of the calibration cycle.