Optical scanning system with unwanted reflections controlled

Claim 1

In a thermal imaging system comprising scanning means for effecting scanning of a distant scene, a first optical sub-system for collimating radiation from the distant scene and directing the resultant collimated beam on to the scanning means, a detector having a detecting surface, a second optical sub-system for directing radiation from the scanning means to be incident on the detecting surface of the detector effectively normal to said surface, and means defining an aperture stop for passing all the rays from the distant scene to be scanned within a defined field of view, the optical sub-systems comprising optical elements bearing at least one optical surface which gives rise to unwanted reflections of radiation within the system; the improvement wherein said at least one optical surface has its center of curvature or an image thereof in a fixed location at least approximately at the aperture stop means or an image thereof, thereby to cause the unwanted reflections incident on the detecting surface to be of substantially uniform magnitude throughout scanning within the defined field of view.