The alarm systems based on Passive Infra-Red detectors catch the intruder, which is inside the room already. The system based on Infra-Red barriers is intended to cause an alarm when intruder only intends to invade the premises, and there is still outside the building.
The curtain is composed of several active IR barriers (beams detectors), which create an invisible "grill" inside the window or door bay. Tampering with this "grill" area causes an immediate start alarm.
IR curtains can be also used to protect the controlled crossing, gates, fences, precious objects at open areas, etc. In museums the IR curtains can be successfully used as conflict-free, discreet separation the viewers from the valuable exhibits.
During the constant interruption of the beam (by placing an obstacle between the transmitter and receiver), the barrier does not return to its original state. This behavior of the barrier is important for control gates, ramps, etc.
Transmitter and receiver (after remove the cover):
In the kit: