Abstract: An improved infrared intrusion detector is provided which conveniently and expeditiously monitors two fields of view by use of a single sensor. In a preferred embodiment, the improved intrusion detector monitors fields of views which are substantially orthogonal in a nonobvious manner. It is nonobvious in that an aperture is provided in a housing which permits radiation from a second field of view to enter the housing. Internal optical elements are provided to direct this incident radiation from the second field of view onto a sensor used for the first field of view. The internal optical elements are protected from the environment and do not give advance warning that a second field of view is monitored. Generally, the second field of view is below the intrusion detector to detect unauthorized activity in a dead zone below the detector.