Bewakingsinrichting

An intrusion detector provided with a passive sensor for detecting an object entering a room to be monitored, said intrusion detector comprising a housing provided with a window for said passive sensor, a light source for emitting electromagnetic radiation onto the window and in the vicinity thereof, a light detector for generating a signal with a signal value for detected electromagnetic radiation from the light source, and alarm means connected to the light detector for generating an alarm in case the electromagnetic radiation from the light source being detected by the light detector corresponds to a signal value that exceeds a maximum level or falls below a minimum level, a special feature being the fact that the light detector generates at least n signals, n being an integer greater than or equal to 2, wherein the alarm means generate an alarm in dependence on a predetermined correlation between (i) two signal values if n=2, (ii) at least n/2 signal values if n=4,6,8,... and (iii) at least (n+l)/2 signal values if n=3,5,7,...