What Are Infrared Detectors?

Infrared detectors are devices that sense electromagnetic radiation in the infrared portion of the spectrum, spanning wavelengths from approximately 0.75 micrometers to 1 millimeter, and convert that radiation into an electrical signal. They differ from visible-light sensors in both their material requirements and their operating mechanisms, because infrared photons carry less energy than visible photons and therefore require semiconductors with narrower bandgaps or thermal transduction mechanisms to produce a measurable response. The field divides primarily into two families: photon detectors, which respond directly to the quantum absorption of individual photons, and thermal detectors, which respond to the heat deposited by absorbed radiation.

Infrared detection has roots in the early 19th century when William Herschel discovered infrared radiation in 1800, but practical electronic detectors emerged in the 20th century alongside advances in semiconductor physics and cryogenic engineering. Today, infrared detectors are foundational to remote sensing, thermal imaging, spectroscopy, astronomy, and defense systems.

Photon Detectors

Photon detectors generate an electrical output directly from the quantum absorption of infrared photons by electrons in a semiconductor material. The dominant material systems include mercury cadmium telluride (HgCdTe, also written MCT), indium antimonide (InSb), and type-II superlattice structures based on InAs and GaSb. HgCdTe is particularly versatile because its bandgap can be tuned across a wide infrared range by adjusting the mercury-to-cadmium ratio, allowing detectors optimized for the short-wave (SWIR), mid-wave (MWIR, 3–5 µm), or long-wave (LWIR, 8–12 µm) atmospheric transmission windows. Most photon detectors require cooling, often to 77 K using liquid nitrogen or a Stirling-cycle cryocooler, to suppress thermally generated charge carriers that would otherwise mask the photon-induced signal. The NIST spectroradiometric detector measurement program provides calibration standards and characterization methods for infrared photon detectors used in precision radiometry.

Thermal Detectors and Bolometers

Thermal detectors respond to the temperature rise caused by absorbed infrared radiation rather than to individual photon events. A bolometer is a thermal detector in which the temperature-sensitive element is a resistive material whose electrical resistance changes with temperature, typically vanadium oxide (VOx) or amorphous silicon for microbolometers. The absorbed radiation heats the sensing element, and the change in resistance is read out as a voltage. Microbolometer focal plane arrays (FPAs) can operate at room temperature because they do not require the narrow bandgap semiconductors that must be cooled to function, which makes them substantially less expensive and more portable than cooled photon detector arrays. Pyroelectric detectors are another thermal detector type, producing a charge output proportional to the rate of temperature change rather than absolute temperature. NIST researchers have developed chip-scale bolometer devices for calibrating continuous-wave lasers in the 500-microwatt to 1-watt range with measurement uncertainties below 0.3%.

Superconducting and Quantum-Limited Detectors

At the frontiers of infrared detection sensitivity, superconducting devices offer performance approaching the fundamental quantum noise limit. Superconducting nanowire single-photon detectors (SNSPDs) consist of a thin superconducting wire, typically niobium nitride (NbN) or tungsten silicide (WSi), biased just below its critical current. When a single photon is absorbed, it locally disrupts the superconducting state, producing a measurable voltage pulse. SNSPDs achieve detection efficiencies exceeding 90% and timing jitter below 20 picoseconds, making them useful for quantum key distribution, time-resolved spectroscopy, and deep-space optical communications. These devices operate at temperatures of 1–4 K and are reviewed in the literature of the IEEE Transactions on Applied Superconductivity.

Applications

Infrared detectors have applications in a wide range of disciplines, including:

• Thermal imaging cameras for building inspection, medical diagnostics, and industrial process monitoring
• Infrared surveillance systems for perimeter security and border monitoring
• Astronomical observations of cool stars, exoplanet atmospheres, and dust-obscured galaxies
• Environmental remote sensing for vegetation health, sea surface temperature, and wildfire detection
• Spectroscopy for chemical analysis, gas sensing, and pharmaceutical quality control