Abstract: An intrusion detector for detecting an intrusion into a protected space overlying a predetermined surface is provided. In one embodiment, the intrusion detector includes a housing to be mounted over the surface and including an infrared radiation sensor for sensing moving infrared radiation sources, and a transparent window thereover, a plurality of lens elements formed in the transparent window each oriented to receive infrared radiation from a moving infrared radiation source within a predetermined viewing zone of the protected space and to transmit infrared radiation from said source to the sensor; an alarm; and a control system for receiving the electrical output of the sensor and for outputting an alarm signal to the alarm when an infrared radiation source is detected simultaneously within at least two of the predetermined viewing zones.

Abstract: A method of filtering an input signal which includes noise cyclically repeating at a known noise frequency, to substantially remove said noise from the input signal is provided. The method includes sampling the input signal at a frequency corresponding to a whole multiple “N” of the noise frequency; sequentially storing the samples in 0-N storage devices; sequentially subtracting the sample in each storage device from the sample previously stored in the Nth storage device preceding the respective storage device, to thereby produce for each sample, a difference sample in which the cyclically repeating noise is effectively cancelled from the respective sample; and sequentially outputting the difference samples to produce an output signal from which the cyclically repeating noise has been substantially removed.