Working of IR Imaging Devices
Most infrared imaging devices can work on about 30 scans per second sensing temperatures between -20 degrees and 2000 degrees with a gradient resolution of about 0.2 degrees. Quantum detectors with semiconductors are employed in high precision detection of radiation. Higher the wavelength of the radiation, the smaller the band gap of the semiconductors that are utilized in detecting it. However, if the band gaps are small, the intrinsic noise of the semiconductor itself would be higher than the incident signal, and the detector would start detecting its own radiation which is certainly highly undesirable. Thus such detectors need to be cooled to levels where intrinsic noise reduces to acceptable levels. Other room temperature detection methods have also been developed which entail loss of quality and resolution, but also reduce costs at the same time. This results in two types of thermal infrared imagers:
1. Cryogenically Cooled IR Imagers: These are vacuum sealed cryogenically cooled cases with cooling temperatures between 4 Kelvin to a few Kelvin below room temperatures. These are costly devices both to buy and to run and need sufficient time for cooling down before operating them after turning on. But, the quality and precision of these devices is the best on offer, along with greater sensitivity.
Cryogenically cooled Quantum IR Detectors (Dewar type)
Materials like indium antimonide, indium arsenide, MCT, lead sulfide etc. are used as narrow band semiconductors. Superconducting tunnel junction infrared detectors offer sensitivity of registering even single photons but the use is limited to research labs like ESA’s SCAM.
2. Uncooled IR Imagers: This is a less precise albeit the cheaper technology which is popular in industry. It is based on the principles of change in some property of the material such as resistance, voltage or current when exposed to IR radiation. These devices are mainly made of pyroelectric or ferroelectric materials or are based on bolometric techniques. These don’t actually detect photons, but the heating effect of the IR rays causes a change in electrical polarisation of the material. If compared to Cooled devices, they offer a resolution varying from 70-80mK in ferroelectric devices to 20mK in Silicon based bolometers. Research is going on in the field of Uncooled Focal Plane Arrays for increased sensitivity and pixel densities.
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