Abstract: A surveillance system comprising a motion detector is secured by a security system comprising a light emitter and light receiver optically coupled to light guides positioned, for example, along the circumference of a light-transmitting window that covers the motion detector. In this manner the window is secured against damage or attack on its integrity, for example, in the form of a film covering or sprayed on the window. The security system delivers an alarm signal when the detected light intensity level is too low or too high.

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.