Importance

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Abstract

An obstruction resistant alarm system for detecting the presence of an intruder includes a housing (2) having a selectively transmissive window (1) that transmits radiation within a first wavelength range and scatters radiation within a second wavelength range. An intrusion sensor positioned to receive from an intruder radiation of wavelengths within the first wavelength range that pass through the window produces a presence signal that indicates the presence of the intruder. A radiation source (20) is positioned to direct radiation within the second wavelength range to strike and then be scattered by the window. A radiation detector (21) is positioned to detect the radiation striking and scattered by the window. The radiation emitted by the radiation source and scattered by the window forms a normal radiation pattern having a normal intensity whenever the window is unobstructed and forms an abnormal radiation pattern with an abnormal intensity whenever the window is obstructed. An alarm circuit (19) produces an alarm signal whenever the radiation detector detects radiation intensity corresponding to an abnormal radiation pattern to indicate an obstruction of the window and a consequent compromise to the reliability of the presence signal produced by the intrusion sensor.

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First Claim