Abstract: Conventional scanners used in thermal imagers scan the image along curved lines rather than straight lines, resulting in image curvature in the display. The invention eliminates this by processing the video signal from the scanner to compose an output video signal which represents scanning along straight lines (18) across the image. The output signal is composed of a number of successive portions (A to F) each derived from a different line or lines of the video signal provided by the scanner.

Abstract: Drive apparatus, particularly for a twin polygon thermal scanner, utilizes a stepper motor driven by pulses of variable width timed, relative to the rotational position of the motor, so that the pulses are substantially centred on the maximum torque position of the motor. This minimizes power consumption. The control circuitry for the motor performs a start sequence, which is aborted and reinitiated in the event that the motor fails properly to start and in which the frequency of the pulses applied to the stopper motor is increased as the motor accelerates. Any excessive increase in the rate of increase of frequency of the pulses is detected and corrected. When the motor is running up to speed, the power applied to the motor is controlled by a phase detector which detects variations in phase between a referenced pulse train and a pulse train generated by the rotation of the motor.

Abstract: A thermal imager is provided with a signal processing circuit which compensates for differences in the sensitivities of the thermal detectors. The compensating circuit produces a signal dependent on the mean output of each detector over a predetermined period, stores the signal in a storage device at a frequency of not more than once each field and applies a correction signal derived from the storage device to the detector.

Abstract: An anamorphic optical system for use in a thermal imager with a sprite detector includes two or more optical elements, preferably prisms, which are shaped and positioned so that change in their refractive index with temperature and/or wavelength results in a lateral shifting of the output energy substantially without any change in direction. The energy passes through a focussing lens to the detector. As a result, the lateral shifting of the energy output by the anamorphic system does not affect the position of the image on the detector. This compensates for the effect of variations in refractive index of the materials of which the prisms are made, as a function of wavelength and/or temperature.

Abstract: Thermal imaging apparatus is provided with a comparator and a digital counter for counting the number of pixels in the scene within certain defined brightness bands. Utilizing this data, a microprocessor controls gain, level, gamma correction and integration for noise reduction.

Abstract: A twin polygon thermal imager is provided with a compensator for compensating for horizontal distortion in the image produced on a CRT. The distortion compensator applies an amount of correction which is determined, in each horizontal line, to compensate for the particular magnitude of the distortion in that line. The amount of correction needed is determined as a function of the angle between the rotational axis of the polygons and the particular facets thereof producing the particular video line. Compensation is achieved by adjusting the timing and the frequency at which digital video data is read-out from a signal store.

Abstract: Thermal imaging apparatus particularly but not exclusively for use on an aircraft with a head-up display is provided with signal processing means which progressively expands or compresses the image in the vertical and/or horizontal direction to compensate for parallax arising from the difference in the positions of the observer and the scanner. The adjustment is performed, in the preferred embodiment, as a function of height of the aircraft.