The influence of the focusing system on the sensitivity of the PIR sensor

Abstract

Passive infrared motion sensors (hereinafter referred to as PIR sensors) are the most important components of modern security and automation systems. These sensors are de-signed to detect the presence of people by detecting infrared radiation emitted by their bodies. The technical documentation for PIR sensors does not describe how it works without a focus-ing lens, or if it can work without one at all. This article describes the study of the possibility of using a PIR motion sensor without a lens.
A study was conducted on the possibility of using a sensor without a lens and with a conventional glass lens. The experiment showed that without a lens in front of the sensor, the latter has extremely low sensitivity. Without a lens, the output signal is 10 times weaker in amplitude. When a glass lens was used, the signal completely disappeared from the sensor output. This is due to the fact that conventional glass does not transmit infrared radiation of the corresponding range. On the other hand, when using a sensor without a lens, the reaction to objects located in the near zone was greater. Based on both a priori information and the data obtained as a result of experiments, the following conclusions can be drawn:
1. The design and materials of the PIR sensors that were studied allow for the successful detection of the movement of bodies whose temperature differs from the surrounding environment by tens of degrees. 
2. Without the use of a Fresnel lens, detection of warm objects is possible only in the near zone. With the use of an appropriate lens, the detection zone is significantly expanded. 
3. Using conventional optical range lenses to successfully work with these sensors is practically impossible.