Abstract: An intrusion detection system has an optical device for focusing radiation energy onto radiation sensing elements. Connected to the sensing elements is a circuit which produces intrusion signals from detected radiation energy for controlling the operation of an alarm. The optical device has a sensing mirror with a concave reflective surface that comprises a plurality of pairs of vertically disposed contiguous reflective segments. The reflective segments of the sensing mirror have the same vertical curvature, but the pairs of reflective segments differ from one another in horizontal curvature, in the area of reflective surface and in the horizontal focal lengths. The reflective segments of the sensing mirror will reflect radiation energy from a wide range of angles offset from the direction of the center line of the optical device and project the radiation energy onto a focusing mirror, which, in turn, casts the radiation energy onto the sensing elements.