Abstract: The present invention provides an improved intrusion detection system of the type in which interruption of a beam of infrared energy transmitted from a transmitter to a receiver generates an alarm signal. The receiver includes a light source which remains unlighted when the transmitter and receiver are properly aligned but will be continuously lighted if either the beam is interrupted or the receiver and transmitter are not aligned. In one aspect of the invention, flasher circuitry in the receiver causes the light source to flash when the transmitted beam is only marginally aligned with the receiver. In another aspect of the invention, a lens in the receiver for focusing infrared radiation on a photodetector is movably mounted and accessible from the outside of the receiver to properly align the transmitted beam with the receiver without dismounting the receiver or requiring the use of any special alignment tools.

Abstract: A passive infrared intrustion detector is provided with a lens which has a plurality of first lens segments, each for focusing infrared radiation from various beams of sensitivity onto an infrared detecting element. Associated with each of the first lens segments is a second lens segment, with a displaced lens center, which serves to focus light originating within the detector device into a radiated beam, which corresponds in space to the region of sensitivity of the infrared beams.

Abstract: A passive infrared intrusion sensor is provided with a cover which can be mounted in a closed or partially open position. In the partially open position a tamper switch is activated to illuminate a beam locator light within the detector.

Abstract: A passive infrared intrusion detector is provided of the type having an infrared sensor for detecting a change in temperature in the field of view of the sensor caused by an intruder moving therein, optical means for focusing radiation from the field of view on the sensor, and an alarm which is activated when the sensor detects a change in temperature. The improvements reside in mounting the passive intrusion detector in a generally C-shaped housing exposed to a field of view which may be shifted through an angle of at least 180.degree.. A bendable, translucent window is provided having slots in opposite ends thereof which engage projections on opposite sides of the housing. The window is very simply released by applying thumb pressure to the window in the proximity of the projections. The intrusion detector is also provided with a horn-like enclosure which surrounds the sensor. A flexible Fresnel lens may be rapidly mounted on the enclosure, permitting ready adaptation of the detector to local conditions.

Abstract: An improvement in a photoelectric intrusion detector unit of the type designed to resemble a duplex electrical outlet. An angled mirror and a collimating lens are supported on a common mount which is rotatable about an axis of rotation which is angularly displaced from the optical axis of the lens but passes through its focal point. The optical axis of the lens describes a conical surface about the axis of rotation.