An Infrared Beam is a beam of light that is invisible to the naked eye. Infrared beams are connected to a security alarm system. When the beam is broken by someone passing through it, it can set off an alarm or trigger a trap.
Dumas Industrial Enterprises set up a series of rooms leading to their Safe located within the Dumas Towers in England. These rooms contained a variety of infrared beams that would scan across the room. If one of the beams was tripped, the doors would close to the room and Poison Gas was released, killing the intruder.
- Infrared on Wikipedia