An intrusion detector with a plurality of reflector segments that focus infrared energy from a corresponding plurality of detection zones onto a common sensor. Uniform coverage of a rectangular area with detection zones and detection sensitivity unaffected by distance is achieved by means of a reflector segment arrangement in which the reflector segments are mounted on supporting structures and in which the distance from the sensor and therefore the focal length of the reflector segments is substantially proportional to the detection distance. The reflector segments are staggered horizontally and vertically on the supporting structures such that the centrally positioned reflector segments are set lower and have a different shape than the laterally positioned segments, and whereby the number of reflector segments is reduced with decreasing detection distance such that the density of the detection zones is uniform throughout the retangular protected room.