Thermal imaging systems

In a passive infra-red thermal imaging system the imaging rays converge at the detector in two or more solid angles 13, 14, because of different magnifications in the imaging system. A single reflector 11, which is a mirror intended to reflect back into the window of the detector rays which emanate from the detector outside the solid angle of the imaging beam in use, would only be appropriate for one of the envelopes 13, 14. In accordance with the invention, the reflector is movable so that its aperture closely surrounds whichever of the two envelopes is in use. Alternatives are to provide two reflectors with different sized apertures on a turret or a plain apertured mirror on a turret in a part of the optical system in which the beam is parallel-sided.