Passive Infrared Technology

How An Infrared Motion Detector Works

Infrared radiation exists in an electromagnum spectrum at wavelengths that are longer than visible light. The infrared cannot be seen, yet it can be detected. Objects that generate heat also generate infrared radiation. The human body and animals are objects which are detectable with an infrared motion detector. In effect we as warm bodied mammals have a heat signature that this technology sees. 

The technology built into infrared motion detectors are specifically designed to detect these heat signatures that human beings and animals have as part of being a warm bodied object.  All mammals are warm bodied objects.  In other words,  cats, dogs, mice, hamsters, etc

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Pets have the same infrared heat signatures as human beings.  Pictured below you see how a infrared motion detector sees two dogs.  Because these dogs are physically touching the motion detector would view this image as a alarm condition.  Pet immune motion detectors are effective in reducting false alarms however they have weight limitations.  Here the alarm system would definitely go into an alarm condition.  Its best to have a pet immune infrared motion detector installed by a competent alarm professional that understands this technology.

How Infrared motion detectors can be used to prevent false alarms. Passive infrared detectors can be programmed to ignore the first movement detected, as in when the intruder moves from one detection zone to another, and to sound the alarm only when the movement passes through two or more detection zones within a specified period of time. In this way, an insect landing on the detector's lens, or a sudden rise in background temperature caused by an activated furnace, is ignored.

Dual Technology

Another means of preventing false alarms is the dual-technology motion detector. This is probably the most common type of detector used in more sophisticated burglar alarm systems. A dual-technology detector combines a passive infrared device and a microwave device in one small unit. The passive infrared device sees many detection zones and measures the change in background temperature as a target moves across them. At the same time, the detector projects microwaves and measures the Doppler shift when a target moves through the protected space.

An infrared motion detector will detect movement regardless of whether the target is moving across the field of view or toward the detector. But such a detector is more sensitive to movement across its field of view. Thus, it is more prone to false alarms caused by disturbances such as a mouse or rat moving across its field of view than by movement toward it. Microwave detectors are just the opposite: more sensitive to targets moving toward them than they are to targets moving across their field of view. If a large leaf falls off a plant in a room, a microwave detector is more likely to detect the motion than is an infrared detector. But if there is movement outside a window, a microwave detector might detect it when an infrared detector probably would not.