How Infrared Detector Work in Security Systems
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Infrared Detectors Use Infrared Light
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In order to fully understand how infrared motion detectors work in security systems, users must first understand the function of infrared light. According to the National Aeronautics and Space Administration (NASA), infrared light exists in the electromagnetic light spectrum, and operates in a range of frequencies not visible to the human eye. The blanket term "infrared light" encompasses two frequency ranges, "near infrared," which operates at a frequency closer to that of visible light, and "far infrared," a range of frequencies closer to microwaves than to the visible spectrum. Because of the higher frequency range of far infrared, the infrared light possesses thermal properties that allow it to sense and detect very subtle temperature changes, and this temperature sensory ability is what helps detect movement in a home security system.
Infrared Detectors Monitor a Room
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In a typical home or office security environment, infrared motion detectors mount on a wall or the ceiling. Once in place, the motion detectors use far infrared light beams to create a thermal image of the monitored area. Because every component in a home or office---including furniture, floors, walls and fabric---has a certain heat signature, the infrared motion detector quickly develops a "normal" image of the room and its fixtures. If this image changes, as it does when a person, animal or object enters the area under infrared monitoring, the detector's built-in circuitry identifies the image change and sends a signal to the security system's control panel.
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Heat Can Affect Infrared Detectors
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Due to the low cost and reliability of infrared detectors, according to Alarm System Reviews, the devices account for the majority of motion detectors in home security applications. Despite the reliability of the devices, though, they are far from infallible. Manufacturers of infrared detectors ship the devices with explicit mounting instructions that include strong warnings against placing the detectors near a heat source. Because the devices rely on thermal imaging, a heat source like a radiator or heat pump vent can quickly change the thermal appearance of a monitored area and trigger a false alarm. Likewise, an infrared detector mounted to face a window may sound an alarm when the sun sets or rises, or when other environmental factors cause a sudden change in ambient temperature.
Some Detectors Use Dual Technology
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Although infrared detectors are fairly reliable and relatively inexpensive, Alarm System Reviews reports that an increasing number of infrared detectors also employ microwave technology as a secondary security measure. In addition to the thermal properties of infrared light, these sensors employ microwaves to quickly and accurately detect movement in a room. Every few seconds, the detector sends a burst of microwaves out across the monitored room and measures the amount of time the waves take to reflect back to the detector; if the amount of time changes between one burst and the next, the detector registers a change in the room and triggers an alarm. By combining the far reach of microwaves with the relative reliability of infrared, these "Dual Technology" detectors offer greater reliability than their single-technology cousins.
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