1 Infrared radiation

All objects in nature emit infrared radiation. The higher the temperature of the object, the stronger the infrared radiation. The wavelength and distance of infrared radiation generated by objects due to their different physical and chemical properties, as well as their different temperatures, are usually divided into three bands.

• Near infrared: wavelength range 0.75-3 μ m
• Mid infrared: wavelength range of 3-25 μ m
• Far infrared: wavelength range of 25-1000 μ m

The infrared light emitted by the human body has a wavelength of 3-50 μ m, of which 8-14 μ m accounts for 46%, with a peak wavelength of 9.5 μ m. Human beings are warm blooded animals, and infrared radiation is also the most stable.

2 PIR motion detector

PIR motion detectors themselves do not emit any energy but only passively receive and detect infrared radiation from the environment. After the installation of PIR motion detectors, they have adapted to the environment for a few seconds. When no one or animals enter the detection area, the infrared radiation on site remains stable. Once human infrared radiation enters, it is focused by the optical system, causing the pyroelectric device to produce a sudden electrical signal and issue an alarm. The warning line formed by PIR motion detectors can generally reach tens of meters.

PIR motion detectors are mainly composed of optical systems, thermal sensors (also known as infrared sensors), and alarm controllers. The core of it is the infrared detection device, which can detect changes in thermal radiation within a three-dimensional defense space through the cooperation of the system. The detection wavelength range of infrared sensors is 8-14 μ m, and the infrared peak wavelength of human radiation is about 10 μ m, which is exactly within the range.

3 Classification of PIR motion detectors

PIR motion detectors can be roughly divided into two types: single beam and multi beam based on their different structures, warning ranges, and detection distances.

• The single beam PIR motion detector adopts a reflective focusing optical system, which uses curved mirrors to converge infrared radiation from the target onto the infrared sensor. This type of detector has a narrow field of view, generally below 5 °, but has a longer operating distance, up to 100 meters. Therefore, it is suitable for protecting narrow corridors, passages, and blocking doors, windows, and walls.
• The multi beam PIR motion detector adopts a lens focusing optical system, and currently most of them use infrared plastic lenses – Fresnel lenses with multi-layer beam structures. This type of lens is formed in one go using special plastic, with several small lenses arranged on a curved surface. The alert range presents multiple single beam states in different directions, forming a three-dimensional fan-shaped thermal sensing area and constituting a three-dimensional alert. Fresnel lenses are divided into several rows from top to bottom, with more lenses on the top and fewer on the bottom. Because the infrared radiation on the human face, knees, and arms is strong and directly faces the lens above. There are fewer lenses below, firstly because the infrared radiation from the lower part of the human body is weaker, and secondly to prevent interference from infrared radiation from small animals on the ground. The warning field of view of the multi beam PIR motion detector is much larger than that of the single beam PIR motion detector, with a horizontal field of view greater than 90 ° and a maximum vertical field of view of up to 90 °, but with a shorter operating distance. All lenses focus on the internally installed pyroelectric devices, resulting in high sensitivity. As long as someone walks within the lens field of view, an alarm will be triggered.

4 The use of PIR motion detectors

Infrared light has poor penetration, and there should be no tall objects in the prevention area. Otherwise, if someone walks in the shaded area, the alarm cannot be triggered. Do not face heat sources and strong light sources, especially air conditioning and heating. Otherwise, the constantly changing hot air flow will cause false alarms. In order to solve the problem of object obstruction, a ceiling mounted PIR motion detector was invented. Installed on the roof within a 360 ° downward range for surveillance, as long as it is within the protection range, the alarm will be triggered from any direction of intrusion. It is widely used in large spaces such as bank business halls and public activity areas in shopping malls.

PIR motion detectors are widely used due to their good detection performance, ease of deployment, and low cost. Its disadvantage is that it has a higher false alarm rate compared to active motion detectors.