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- SECURITY
- Lighting appliances
- Household or other appliances
Thermal infrared sensors
Thermal infrared sensors which are also called radiation detectors undergo a change in temperature due to absorption of infrared radiation and convert this temperature change into an electric output signal. Their working principle varies compared to semiconductor based photon or quantum sensors which works on principle that due to different photoelectric effects the photons of the radiation generate charge carriers. For the sensors which work on photoelectric effects, to detect the low-energy infrared radiation, sensors have to be cooled below ambient temperature. In contrary to this, thermal infrared sensors can be operated at ambient temperature. The radiation noise that limits temperature resolution in thermal sensors has a √T dependence meaning that the cooling doesn't consequently improve detection rate. Hence they are more suitable for small, light and portable applications.
Working Principles:
Thermal radiation sensors convert radiant flux ᶲs into an electric signal (Voltage VS or Current IS). Figure represents the measuring chain of radiation sensors. Even though there are various operating principles the basic structure of all the thermal IR are identical. The incident IR radiant flux is absorbed by a thermally isolated detector element (pixel) and it is converted into heat. The temperature of the pixel is directly proportional to the power of the absorbed IR radiation. This makes the responsivity of thermal sensors independent of wavelength. However, in many scenarios there is a wavelength dependence of responsivity due to absorption characteristics of the pixel. If required IR radiation can be temporarily modulated with a chopper. The conversion of the pixel temperature into an electrical signal depends on the sensor type.
Figure: Measuring chain for determining infrared sensors.
IR sensors consist of a thin well isolated chip usually called as detector. The temperature of the detector depends on the absorbed radiant flux ᶲA and the radiant flux ᶲS emitted by the detector element. The figure below illustrates the radiation exchange at detector element, object and detector surroundings. The possible heat flow between detector element, object and detector surroundings due to the thermal conduction is neglected.
Figure:Radiation exchange at detector element.
References
- Hamamtsu Infrared detectors/technical information
- http://www.thesis.bilkent.edu.tr/0003853.pdf
- http://www.glolab.com/pirparts/infrared.html
- http://www.sbf1.sbfisica.org.br/procs/2006/pdfs%20optics/Interferometry,%20Holography%20and%20Applied%20Optics/604%20-2.pdf
- http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/DM00096551.pdf
- http://www.hitechnic.com/cgi-bin/commerce.cgi?preadd=action&key=NIS1070
- http://www.cypress.com/?docID=3317
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