MULTIPLE RANGE PASSIVE INFRARED DETECTION SYSTEM

A passive infrared detection system which exhibits a consistent optical aperture and sensitivity over different operating ranges. The system includes a mirror assembly having a plurality of spherical segments arranged in two or more ranks, each corresponding to a different operating range. Each mirror segment is disposed at its respective focal length from a detector, and is of a size and configuration to provide an intended optical aperture to achieve uniform detection sensitivity irrespective of the range of the target.