Another type of perimeter security is based on infra red
light beams. The transmitter and the receiver are mounted on the wall facing each other.
Whenever there is an interruption between the transmitter and the receiver, this activates
The disadvantage of such a system is that it may get activated by a cat walking on the
wall or a large bird sitting on the wall. Infrared beams are not advisable for farm
houses or houses which are in the open as in the morning peacocks and large birds would
descend on the walls.
To get over such problems, these days there are two beams integrated in a single unit.
When both the beams are cut simultaneously then only the alarm sounds. The beams are so
far apart that they both will get cut by a human being but not by a bird.
The typical distance for such beams is 100 meters indoors and around 60 meters outdoors.