passive infrared (PIR)
Passive infrared motion detectors (PIR) are infrared sensors made of lithium tantalate(LiTaO3) that detect the movement of people or other heat-emitting objects to within a few meters and trigger electrical, optical or acoustic detection signals. These motion detectors work with temperature-dependent pyrosensors that react to infrared light of a specific wavelength, such as human body temperature or heat radiation from motor vehicles.
The infrared sensors themselves detect a wide range of wavelengths and generate a voltage of a few microvolts (µV). This infrared range is at wavelengths between 1 µm and 20 µm. From this spectrum, one filters out the wavelength range of human radiation, which is about 10 µm. A focusing lens - Fresnel lens or Selfoc lens - is mounted in front of the pyrosensor, which focuses the infrared light incident from different directions on the sensor.
Since it is possible to filter out other wavelength ranges using appropriate pyrotechnic crystals, PIR heat detectors can also be used as fire detectors. If wavelength-dependent materials are used, certain wavelength ranges can be filtered out.
The voltage generated by PIR sensors depends on the intensity of the heat radiation, the temperature, size and humidity of the air, the temperature difference between the heat source and the ambient temperature, and the movement of the heat source. Since PIR sensors cover only a certain monitoring angle due to the focusing of infrared radiation with optical lenses, multiple infrared sensors are used for larger detection fields. The sensors are often used with network cameras and webcams. One reason for this is that they contain a twilight switch via which infrared LEDs can be controlled.