How Do Laser Security Systems Work?
By Richard Gaughan
You've probably seen an old Western movie where the good guys settle down and run a string at ankle height around their camp, tying it to a can filled with rocks. When the bad guys try to sneak up in the middle of the night, they kick the wire and pull the can over, making a rattle that awakens the sleeping good guys, who win the day. A laser security system works along the same principle. Instead of a string, there's a beam of light surrounding the area, and instead of a can of rocks, there's an alarm of one sort or another.
The Principles of Laser Security Systems
There are three essential components to a laser security system: a laser, a detector and sensing electronics. The laser is a concentrated light source that puts out a straight line "pencil beam" of light of a single color. The detector is sensitive to light and puts out a voltage when the laser light hits it. The detector is connected to the sensing electronics. When the laser beam is interrupted and can't reach the detector, its voltage output changes, and the electronics sense the change and put out a warning signal.
Lasers differ from other light sources in a few significant ways. There are two features that are most important for security systems. Unlike a lightbulb or flashlight, laser light doesn't spread out, it stays in a narrow beam. And laser light is essentially a single color. Because laser light doesn't spread much, you can send it a long way and still have enough energy in a small area to trigger the security system detector. Because it's a single wavelength, you can put a blocking filter on the detector to let laser light through without letting background light onto the detector.
Laser light travels in a straight line. If you just wanted to protect the front of your yard, putting the laser at one corner and the detector at the other corner would do the job. That's not a very practical configuration, though. More typically, you'll want to protect the perimeter of a room, or at least the entrances. So laser security systems start with a laser pointing to a small mirror. The first mirror is angled to direct the beam to a second small mirror, and so on until the final mirror directs the beam to the detector. If the beam is interrupted anywhere between the laser and the detector, the electronics will put out the warning signal.
The Warning Signal
Laser security systems are available in many configurations, with many levels of sophistication. There are do-it-yourself kits that will buzz or ring when the beam is interrupted. The electronics also can be set to trigger an auto-dialer that contacts local law enforcement or a monitoring company. The electronics can also trigger the exterior lights of the house to flash, helping police locate the house where the alarm has been triggered.
Movies and Reality
We all know that movies present exaggerated images of reality, and that's especially true of the way laser security systems are presented by Hollywood. The typical image will show a pattern of criss-crossing red or green shafts of light filling or outlining a room. The hero (or villain) then carefully steps through the maze, avoiding those lines of light and making it to the diamond. In reality, laser beams are not visible as they travel through the air, unless there is dust or moisture in the air. On the other hand, there will be some scattered light from the mirrors and the detector that can be a giveaway that a laser security system is installed.
First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.