Abstract: In order to avoid the encumbrance of a rotary mirror scanner by large calating radiation sources for intermittently illuminating the detector element of the scanner, a supplementary radiometer responsive to the same scene as is scanned is absolutely calibrated by intermittent illumination from calibrating sources that are conveniently small. The calibrated radiometer signal is compared to portions of the scanner output responding to the same object field, for calibration of the scanner output. The radiometer may be scanned at right angles to the direction of scan of the scanner in order to provide calibration of both the forward cross lines and rear cross lines alternately scanned by a stereoscopic scanner, for stereoscopic observation of cloud formations, land masses, air traffic distribution or the like from a space craft.