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Infrared detectors Basics

  • Author:Snoppy Huang
  • Source:CPS encyclopedia
  • Release on :2016-07-21
Infrared detectors Basics 

    Any more than the absolute temperature 0 ℃ objects can produce thermal radiation, and thermal radiation spectrum of the object temperature below 1725 ℃ is concentrated in the infrared light region, and therefore all objects of nature can radiate infrared heat. The wavelength and the distance between any body itself due to the different physical and chemical properties of itself at different temperatures generated by infrared radiation are not the same, usually divided into three bands.


Near-infrared: a wavelength range of 0.75 ~ 3μm
Mid-infrared: wavelength range of 3 ~ 25μm
Far-infrared: a wavelength range of 25 ~ 1000μm
Human radiation wavelength infrared light 3 ~ 50μm, which accounted for 46% of 8 ~ 14μm peak wavelength at 9.5μm.

First, passive infrared alarm detector 
     At room temperature, the items have any radiation. The higher the temperature of the object, the stronger infrared radiation. People are warm-blooded animals, the infrared radiation is also the most stable. The reason why we called passive infrared, that does not emit any energy detector itself but only passively receive, detect infrared radiation from the environment. After a few seconds to install the detector has been adapted to the environment, animals or when no one enters the detection area, the infrared radiation field of steady, if there is human infrared radiation coming through the optical system focus on making Pyroelectric mutated electrical signal, The alert. Passive infrared intrusion detectors cordon formed generally up to tens of meters.
     Passive infrared detector consists of an optical system, a heat sensor (or infrared sensor) and alarm controller. Its core is missing is an infrared detector, through complexation off school system can detect changes in the thermal radiation of a three-dimensional guard space. Detection wavelength range of the infrared sensor is 8 ~ 14μm, the peak wavelength of the infrared radiation the human body is about 10μm, just within the scope of
     Passive infrared detectors (PIR) depending on its structure, scope and alert detection range is also different, it can be divided into single-beam and multi-beam type two. Single beam PIR employing a reflective focusing optics system, the use of curved mirrors will converge infrared radiation from the target in the infrared sensor. In this way the state detector viewing angle is narrow, typically 5 ° or less, but the role of distance, up to hundred meters. So-called straight red remote control - passive detector, for the protection of narrow corridors, passageways and blocked doors and walls. Multi-beam type with the focusing lens optical system, currently used mostly plastic lens infrared - multi-beam Fresnel lens structure. This lens is a special plastic molding, a number of small lenses arranged on a curved surface. Alert zone in different directions form a plurality of single-beam state, consisting of three-dimensional thermal sector region, constituting three-dimensional guard. Fresnel lens from top to bottom is divided into a few rows above the lens more, less below. Because people face, knees, arms, strong infrared radiation, just in front of the top of the lens. Below the lens is less, the lower part of the body first, because infrared radiation is weak, the second is to prevent small animals, ground infrared radiation interference. Alert viewing angle multi-beam type PIR larger than the single-beam type, the level may be greater than 90 °, the vertical angle of the maximum field of view can also reach 90 °, but the role of the short distance. All lenses are to Pyroelectric set internally focused, and therefore higher sensitivity, as long as people move around the field of view of the lens will alarm.
     Infrared light penetration is poor in the region should not prevent tall objects, otherwise the shadow will not be part of someone walking alarm, not being on heat and strong light sources, especially air conditioning and heating. Otherwise, changing the hot air will cause false alarms. To solve the problem of shelter materials, he invented the ceiling passive infrared intrusion detectors. Installation on the roof guard down within the range of 360 °, as long as the scope of protection, regardless of the direction from which the invasion would trigger an alarm in the banking hall, shopping areas and other public events where a large space is widely used.
    Passive detection alarm detectors because of good performance, easy deployment, cheap and widely used. The disadvantage is that the active phase high false alarm rate for probing.

Second, active infrared detectors
    Active infrared detector consists of an infrared transmitter, an infrared receiver and alarm controller. Were placed in income, the originator of the optical system is generally used in an optical lens serves to focus the infrared light beam into a parallel beam smaller role in the energy infrared light to focus on delivery. Infrared light is invisible to the human eye spectral range, it was through this invisible cordon, blocking all or part of the inevitable infrared beam. The strength of the receiver output electrical signal will therefore produce changes, which initiates alarm controller alarm. Active infrared detectors meet small animals, leaves, dust, rain, snow, fog obscured should not alarm people or the equivalent volume of goods tampering occurs. Since the beam is narrow, send and receive ends of the installation should be strong and reliable, should not be ground shaking impact, and displacement caused by false positives, the optical system should be kept clean, pay attention to maintenance. So active is detected by the detector point, rather than a range of surfaces. Its characteristics are very high detection reliability. However, if a space arm, you need to have multiple active detectors are expensive. Active detectors commonly used in museum exhibits artifacts monomer valuable arm and factory warehouse doors and windows blocked, shopping channel blockade, export blockade parking, balcony home blockade and so on.
    Active infrared detectors have single-beam, dual-beam, four beams of the points. The position transmitter and receiver set is divided into different infrared beam to the type of installation and reflective Installed by the way, a reflection-type receiver installation is not directly received transmitter sent, but received by mirrors or appropriate reflector (e.g. lime wall, door smooth surface paint layer) of the infrared beam reflected back. An alarm signal when the position and direction of the reflecting surface is changed or infrared emitted light beam and the reflected beam is blocked one leaving the infrared receiver can not receive the reflected beam. When using more detectors to prevent the layout should be taken to remove the cross beam mistakenly shot.