Abstract: The invention relates to an intruder detector for use in a burglary in which infra-red radiation is imaged onto a sensor 16. The imaging system, which serves to divide the area of surveillance into monitored and unmonitored zones and to focus the radiation onto the sensor, consists of a window 12 constructed as a flat one-piece segmented Fresnel lens, each segment 12a, 12b and 12c focusing energy from a different respective monitored zone onto the sensor 16. The lens is double sided with one side acting as a convex lens and the other as a set of deflecting prisms.