Infrared radiation is invisible to the human eye, so it possesses all the characteristics of light. All matter above absolute zero (-273°C) emits infrared radiation. Based on these characteristics, infrared radiation is used in various sensors, such as infrared temperature and humidity sensors and infrared human body detectors. Infrared sensors are also categorized into different types based on their emission and energy conversion modes. Below, we will examine in detail the working principles and characteristics of different infrared sensors.
Depending on the method of transmission, infrared sensors can be divided into active and passive types.
1. Active infrared sensor
The active infrared sensor emits a modulated infrared beam, thus forming an alarm line composed of infrared beams. It should not alarm if obstructed by leaves, rain, small animals, snow, dust, fog, etc., but will alarm if obstructed by a person or object of considerable size.
Active infrared detector technology primarily employs a single transmitter and receiver, representing linear detection. It has since evolved from single-beam to multi-beam systems, and can even support dual transmitters and dual receivers, reducing false alarm rates and thus enhancing product stability and reliability.
Because infrared light is a good detection medium for environmental factors (sound, lightning, vibration, various artificial light sources in the environment, electromagnetic interference sources, etc.) and has good independence; at the same time, it is also a product with good coherence with target factors (only targets that block the infrared beam will trigger an alarm). Therefore, active infrared sensors will be further promoted and applied.
2. Passive infrared sensor
Passive infrared sensors work by detecting infrared radiation emitted by the human body. The sensor collects external infrared radiation and then focuses it onto the infrared sensor. Pyroelectric elements are commonly used in infrared sensors; after receiving infrared radiation, they release an electrical charge when the temperature changes, which, after detection and processing, generates an alarm.
This sensor is designed to detect radiation from the human body. Therefore, the radiation-sensitive element is highly sensitive to infrared radiation with a wavelength of around 10 μm. In order to be sensitive to infrared radiation from the human body, its radiation surface is usually covered with a special filter, which can significantly control environmental interference.
A passive infrared sensor consists of two thermoelectric elements connected in series or parallel. Furthermore, the two elements are connected in opposite directions, and the ambient background radiation has almost the same effect on both pyroelectric elements, causing the discharge effects to cancel each other out, so the detector has no signal output.