Patent ID: 7807970

Claim:
An obstruction detection device for an infrared intruder detection system, comprising: a first light path ( 10 ), wherein a first ending of said first light path ( 10 ) forms at least one transparent facet ( 103 , 104 , 105 ) having a roughed surface; a light emitter ( 2 ) arranged for emitting light, wherein said light is guided along the first light path ( 10 ) towards the roughened surface; a light detector ( 4 ) arranged for detecting the intensity of light, which is reflected by the roughened surface back into a second light path ( 16 ); and an output device for outputting an alarm-signal, when an absolute difference between the intensity of the detected light and a reference value exceeds a threshold value, wherein the roughened surface of the at least one facet is arranged adiacent to an entrance window of the infrared intruder detection system and is exposed such that a spray applied to the entrance window is necessarily applied to the roughened surface.