Abstract: An infrared intrusion detector has an optical system for gathering infrared radiation from discrete sensitive regions and focusing such received radiation onto an infrared radiation sensor. The detector includes a monitoring device to sound an alarm when radiation sensed reaches a predetermined level. The optical system comprises an optical collector for gathering radiation from a field which includes the sensitive regions and for converging such radiation onto a plurality of reflectors. Each reflector determines one of the discrete sensitive regions and, in turn, is oriented to reflect converging radiation gathered from a sensitive region on the detector. The invention provides a simple, efficient optic system for a broad field of gathering infrared radiation, concentrating and focusing it in a manner to provide sufficient levels of received radiation for purposes of detection.