IR Sensors | 2 Important Types | Applications



What is an infrared wave?

An infrared wave is electromagnetic radiation having a frequency between 300 GHz and 400 THz [wavelengths ranging from 10-3 – 0.7 x 10-6m]. IR radiations can not be seen by human eyes. Infrared waves can be used to heat food and produce television remotes, fiber optic cables, thermal imaging cameras, medical applications, etc.

What is an IR sensor?

An infrared sensor or IR sensor is an instrument that detects and analyses infrared waves. IR sensors can be of two types: thermal IR sensors and photoelectric IR sensors. Photoelectric IR sensors detect and then convert the incident light energy(in the form of infrared waves) into an electrical (electrons) signal. The photoelectric infrared sensor can produce electrical output signals possessing an energy that corresponds to the input or incident light energy. Thermal IR detectors sense and analyze IR radiations based on several temperature-dependent phenomena. Usually, thermal IR sensors vary output resistance based on incident temperature variations due to IR radiations. IR detectors are used in IR spectrometers, bolometers, microbolometers,  thermopiles, thermocouples, etc.

IR sensor
Prototype of high-speed infrared detector. Image source: ESO, RAPID A High-speed Infrared DetectorCC BY 4.0

Thermal IR sensors vs photoelectric IR sensors:

Thermal IR DetectorsPhotoelectric IR Detectors
Thermal IR sensors detect and analyze IR radiations based on several temperature-dependent phenomena.Photoelectric IR sensors detect and then convert the incident light energy(in the form of infrared waves) into an electrical (electrons) signal.
Thermal IR detector vary output resistance/voltage based on incident temperature variations due to IR radiations.The photoelectric infrared sensor generates an electrical output signal having an energy corresponding to the input light energy.
The sensitivity of Thermal detectors is lower.The sensitivity of photonic detectors is higher.
The photoelectric infrared sensor has a quicker response time.The photoelectric infrared sensors have a higher response time.
The photoelectric infrared sensor does not need to be cooled down.The photoelectric infrared sensor needs to be cooled down.

What are IR flame sensors?

IR flame sensors capable to sense and analyze the infrared spectral band for detecting specific patterns presents in hot gas and This patterns are captured with the help of thermographic or thermal imaging cameras. Near-Infrared flame detectors use flame recognition features present in a CCD device. IR sensors can get significantly influenced by water vapor as water can absorb most of the incident radiation. This makes infrared sensors unsuitable for an outdoor environment.

IR3 flame detector might be utilized for the comparison of radiation patterns in-between 3-different infra-red spectral bands and the ratio of the radiation band w.r.t each-others. Typically, these sensors are programmed for detecting one radiation band in the 4.4-micrometer range and the other two bands in the range succeeding and preceding the 4.4-micrometer spectrum. This range-specific detection allows the sensor to differentiate in-between the actual flame and the nonflame radiation that impacted the output. Ignoring the background radiations allows the detectors to provide a more accurate and error-free result.

Thermal image of a flame.
Image source: Hugues CREPINAllumette img ptCC BY-SA 3.0

What is the application of Infrared sensors in thermal imaging?

Thermal infrared sensors are specifically designed for detecting IR radiations and forming images and these images are dependent on the heat energy radiated by that selected object and the variation of temperature between objects in the foreground and background environment and these instruments serve a wide range of applications in various different fields. The process of analyzing thermal imaging data is known as thermography. to know more about this click here.

Night vision glasses used by militaries Image source: AlexPlank at English WikipediaNightvision, marked as public domain, more details on Wikimedia Commons

What are the materials used in IR sensors?

IR sensors can be made up of different materials based on the requirements of the form of information it has to provide. Some commonly used materials in IR detectors are:

  • Mercury cadmium telluride (Known as MCT, HgCdTe)
  • Lead(II) sulfide (PbS)
  • Indium antimonide (InSb)
  • Indium gallium arsenide
  • Indium arsenide
  • Lithium tantalate (LiTaO3)
  • Lead selenide
  • Quantum Well Infrared Photodetector (QWIP)
  • Triglycine sulfate (TGS)
  • Platinum Silicite (PtSi)

What type of circuits are used in IR detectors?

IR detectors are generally compatible with a Readout integrated circuit (ROIC) which will first accumulates the photo currents from the pixels and then will direct this processed signal to the o/p for the observations. A ROIC transmits pixel data outside of the IC with the help of high-speed analog outputs.

There are 2 types of Read-out integrated circuits.

  1. Digital pixel readout integrated circuit (DPROIC).
  2. Digital readout integrated circuit (DROIC).
Digital pixel readout integrated circuit (DPROIC) block diagram. image source: RbannatyneDigital pixel readout integrated circuitCC BY-SA 4.0
Digital readout integrated circuit (DROIC) block diagram Image source: RbannatyneDigital readout integrated circuitCC BY-SA 4.0

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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