Intrusion alarm system

Claim 1

An obstruction resistant alarm system for detecting the presence of an intruder in a location being monitored, comprising: a housing including a selectively transmissive window 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 passing through the window, the intrusion sensor producing in response to the radiation within the first wavelength range a presence signal indicative of the presence of the intruder in proximal location to the intrusion sensor; a radiation source emitting and a radiation detector detecting radiation within the second wavelength range, the radiation source positioned to direct the radiation within the second wavelength range to strike and then be scattered by the window and the radiation detector positioned to detect the radiation within the second wavelength range striking and scattered by the window, such that the radiation within the second wavelength range emitted by the radiation source and scattered by the window forms a normal radiation pattern having a normal intensity detected by the radiation detector whenever the window is unobstructed and an abnormal radiation pattern having an abnormal intensity detected by the radiation detector whenever the window is obstructed; and an alarm circuit operatively connected to the radiation detector to produce an alarm signal in response to detection by the radiation detector of 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.