Active electro-optical intrusion alarm system having automatic balancing means

Claim 1

An intrusion detection system comprising means for illuminating a preselected volume of space with amplitude-modulated radiant energy of a preselected carrier frequency such that said energy will be incident upon and reflected by objects entering, moving within and exiting said volume; means for receiving said reflected radiant energy having a field-of-view which is at least partially coincident with said illuminated volume of space, including means for collecting and focusing radiant energy reflected from said objects within said coincident portion of said field-of-view, and a single photodetector disposed in the focal plane of said collecting and focusing means, said photodetector producing an electrical output signal which is representative of the level of radiant energy received thereby; and signal processing means coupled to said photodetector, including an electrical band-pass filter coupled to said photodetector and tuned to said preselected frequency, a phase-sensitive demodulator coupled to the output of said band-pass filter, a low pass filter coupled to the output of said demodulator, a signal comparator coupled to the output of said low pass filter, and an automatic balancing means coupled between the output of said demodulator and the input of said band-pass filter whereby an inverted signal at said carrier frequency and of an amplitude controlled by the output level of said demodulator is coupled to the input of said band-pass filter to thereby substantially cancel time invariant background signals, and producing an output signal in response only to changes in the level of said received radiant energy.