The principle and classification of infrared detectors

Infrared is not unfamiliar to us. Scientific research shows that any object that has a temperature above absolute 0°C can generate thermal radiation. When the object temperature is lower than 1725°C, the heat radiation spectrum generated by the object is concentrated in the infrared region, so the natural world All objects can radiate infrared light outwards. The wavelengths and distances of infrared radiation generated by natural objects due to their own physical and chemical properties and their own temperature are not the same.

Usually we divide these infrared waves into the following three bands:

1, near infrared: wavelength range 0.75 ~ 3μm
2, in the infrared: wavelength range 3 ~ 25μm
3, far infrared: wavelength range 25 ~ 1000μm

The wavelength of infrared radiation emitted by the human body is 3 to 50 μm, of which 8 to 14 μm account for 46% and the peak wavelength is 9.5 μm. Infrared radiation is an electromagnetic wave whose wavelength is between visible light and microwave. It cannot be detected by the human eye. To perceive the existence of such radiation and measure its strength, it must be converted into other physical quantities that can be detected and measured. Infrared Detector is a device that converts incident infrared radiation signals into electrical signals.

Infrared detector classification:

According to whether infrared detector emits infrared, infrared detectors can be divided into passive infrared detectors and active infrared detectors.

(1) Passive infrared detector

The passive infrared detector itself does not emit any energy but only passively receives and detects infrared radiation from the environment. A few seconds after installation, the detector has been adapted to the environment. When nobody or animals enter the detection area, the infrared radiation at the scene remains stable. Once the human body's infrared radiation comes in, the pyroelectric device generates an abrupt electrical signal when focused by the optical system. And send an alert. The warning line formed by passive infrared intrusion detectors can generally reach tens of meters.

Passive infrared detectors are mainly composed of optical systems, thermal sensors (or infrared sensors) and alarm controllers. The core is that it is not a part of infrared detectors, and the change of thermal radiation within a stereoscopic space can be detected through the cooperation of the system.

The human body has a constant body temperature, usually at 37 degrees, so it emits infrared light with a specific wavelength of about 10 microns. A passive infrared sensor works by detecting infrared radiation emitted by the human body at about 10 microns. About 10 micrometers of infrared radiation emitted by the human body is enhanced by the Phillips Muller filter and collected on the infrared induction source. Infrared sensors usually use pyroelectric elements, which lose their charge balance when they receive a change in human infrared radiation temperature, and release charge outward. The subsequent circuit can generate an alarm signal after detection and processing.

(2) Active infrared detector

Active infrared detector consists of infrared transmitter, infrared receiver and alarm controller. The optical systems that are respectively placed at the receiving end and the emitting end are generally optical lenses that act to focus the infrared beam into relatively fine parallel beams, so that the infrared light energy can be transmitted in a concentrated manner. Infrared light is in the spectral range that is invisible to the human eye. Some people pass this invisible blockade line and must obstruct the infrared beam completely or partially. The intensity of the electrical signal output by the receiving end will change accordingly, thus starting the alarm controller to send an alarm signal. When the active infrared detector encounters small animals, leaves, dust, rain, snow, and fog, it shall not be alarmed, and an alarm will occur if a person or a considerable volume of objects is blocked.

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