Editing Security alarm (section)

===Photoelectric beams===
Photoelectric beam systems detect the presence of an intruder by transmitting invisible infrared light beams across an area, where these beams may be obstructed. To improve the detection surface area, the beams are often employed in stacks of two or more. However, if an intruder is aware of the technology's presence, it can be avoided. The technology can be an effective long-range detection system, if installed in stacks of three or more where the transmitters and receivers are staggered to create a fence-like barrier. To prevent a clandestine attack using a secondary light source being used to hold the detector in a sealed condition whilst an intruder passes through, most systems use and detect a modulated light source. These sensors are low cost, easy to install, and require very little sterile clearance area to operate. However, it may be affected by fog or very high luminosity, and the position of the transmitter can be located with cameras.