Technology
An infrared detector is a device that senses infrared radiation, typically in the wavelength range from about 0.75 to 14 micrometers, and converts it into an electrical signal. Infrared detectors are used in a wide range of applications, including thermal imaging, night vision, spectroscopy, remote sensing, and medical diagnostics. They are broadly classified into two categories: thermal detectors, which respond to the heating effect of radiation, and photon (or quantum) detectors, which respond to the absorption of individual photons.
Infrared detectors operate by converting incoming infrared radiation into a measurable electrical signal. The two main classes are thermal detectors and photon detectors. Thermal detectors, such as thermopiles, bolometers, and pyroelectric detectors, rely on the temperature change induced by absorbed radiation; they respond to all wavelengths but are relatively slow and less sensitive. Photon detectors, including photoconductors and photovoltaic devices, directly generate charge carriers when photons are absorbed; they offer higher sensitivity and faster response but often require cryogenic cooling to reduce thermal noise.1
Key performance metrics include responsivity, noise equivalent power (NEP), and specific detectivity (D*), which normalize sensitivity to detector area and bandwidth. The choice of material and operating temperature depends on the target wavelength band: short-wave infrared (SWIR) uses InGaAs, mid-wave (MWIR) uses InSb or HgCdTe, and long-wave (LWIR) often uses HgCdTe or microbolometers.
Photon detectors are made from narrow-bandgap semiconductors. Mercury cadmium telluride (HgCdTe) is the most widely used material for high-performance infrared detectors, offering tunable bandgap across the SWIR to very long-wave infrared (VLWIR) range. Indium antimonide (InSb) is common for MWIR, while indium gallium arsenide (InGaAs) is preferred for SWIR. Quantum well infrared photodetectors (QWIPs) and quantum dot infrared photodetectors (QDIPs) are emerging technologies based on intersubband transitions.2
Thermal detectors, especially uncooled microbolometers, have become dominant in commercial thermal imaging due to their low cost and room-temperature operation. They are fabricated using microelectromechanical systems (MEMS) technology, with vanadium oxide (VOx) or amorphous silicon (a-Si) as the sensing material. Recent advances include the use of carbon nanotubes and metamaterials to enhance absorption.3
Infrared detectors are essential in military and defense for night vision, surveillance, and missile guidance. In civilian life, they are used in thermal imaging cameras for building inspection, firefighting, and predictive maintenance. In medicine, infrared thermography helps detect inflammation and circulatory issues. In astronomy, infrared detectors on telescopes like the James Webb Space Telescope (JWST) observe cool objects and distant galaxies.4
In spectroscopy, infrared detectors enable chemical analysis by measuring absorption bands. In environmental monitoring, they track gas emissions and atmospheric composition. In automotive industry, infrared sensors are used for night vision systems and pedestrian detection.
One overlooked aspect is the role of infrared detectors in art conservation: they reveal underdrawings and pentimenti in paintings without damaging the artwork. Another niche application is in the detection of methane leaks from pipelines using hyperspectral imaging.5
Historically, the first infrared detector was a thermometer used by William Herschel in 1800 to discover infrared radiation. During World War II, German scientists developed primitive infrared night vision devices. Modern detectors have also been used in space missions to map the thermal inertia of asteroids and to study the atmospheres of exoplanets.6
Infrared detectors are a cornerstone of modern sensing technology, enabling both scientific discovery and practical applications across many fields.
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