What is a Passive Infrared Motion Detector?

A passive infrared motion detector is a device that detects moving objects by detecting the infrared light they emit. They take advantage of the fact that all objects emit infrared radiation, in amounts that vary according to their temperatures. Like all passive detectors, a passive infrared motion detector works solely by detecting emissions produced from other objects and does not use emissions of its own for this purpose, unlike active motion detectors based on microwaves or ultrasonic pulses. These detectors are frequently used in security systems to detect intruders.

The infrared region of the electromagnetic spectrum encompasses radiation with wavelengths between 0.7 and 300 micrometers, shorter than the wavelength of visible light but longer than microwaves. Infrared light with a relatively short wavelength close to that of visible light, called “near infrared,” cannot be detected by human senses. “Far infrared,” with longer wavelengths, cannot be seen by the human eye but is felt as heat.

An object's heat causes it to emit part of that energy in the form of electromagnetic radiation, with higher temperatures producing greater emissions. At high temperatures, some of these emissions are in the visible spectrum, as can be seen in the filament of an incandescent light bulb or the glow of metal being worked by a blacksmith. All objects, however, emit at least some thermal radiation at infrared wavelengths, and since no matter has a temperature of absolute zero, even extremely cold objects have infrared emissions.

When objects in the vicinity of a passive infrared motion detector move relative to the detector, the infrared radiation reaching the detector from the direction of the motion changes. If an intruder enters the detector's range, the change in emissions caused by the presence of his or her body will be noticed. Since the detector is triggered by changes in infrared light reaching it from a particular direction rather than its absolute amount, a moving, low-emissions object will set off the detector just as a moving, high-emissions object would. Thus, concealing the intruder's body heat will not conceal the intruder.

The sensor of a passive infrared motion detector is built from pyroelectric crystals, which are substances that produce a temporary electrical voltage when their temperatures change due to the subtle shifts the changes cause in their crystalline structures. Commonly used pyroelectric materials include cesium nitrate (CsNO₃) and gallium nitride (GaN). Changes in incoming infrared light caused by moving objects changes the pyroelectric crystals' temperatures, producing electric signals that are received and interpreted by the sensor's electronics to trigger the alarm. These components are placed in a housing with a plastic window that keeps most visible light out but is transparent to infrared light. The sensor can also contain devices, such as parabolic mirrors or specialized lenses called Fresnel lenses that focus the infrared light on its way to the pyroelectric crystal.