Infrared radiation (IR) can be detected by thermal IR and quantum IR sensors.
A thermal IR sensor uses infrared energy as heat for detection and its sensitivity is independent of wavelengths. Therefore, it can detect objects in the near IR, mid IR, and far IR bands. The non-contact detectors do not require cooling after exposure but have slow response times and low resolution.
Melexis’ MLX90614 IR Thermometer
Only detecting objects in the 3 – 20 μm IR range, quantum well IR sensors are limited to mid IR and far IR bands. Compared to thermal IR sensors, Quantum sensors detect photons and are much more sensitive than sensing heat but require cooling to operate properly. The signal output of a quantum detector, typically a gallium arsenide (GaAs) semiconductor chip, is very small and easily swamped by noise generated internally to the sensing device at room temperatures. To overcome this problem, quantum sensors are often operated at cryogenic temperatures such as 77 K (liquid nitrogen) or 4 K (liquid helium)] to minimize the noise. This significantly increases their cost. Consequently, they are only used if high sensitivity, high resolution and/or short response times are required at a specific wavelength.
For really far imaging, NASA’s Thermal Infrared Sensor (TIRS) on the Landsat 8 satellite measures land surface temperatures using Quantum Well Infrared Photodetectors (QWIPs) to detect long wavelengths of light emitted by the Earth. This quantum IR sensing employs GaAs semiconductor chips that trap electrons in an energy well until the electrons are elevated sufficiently to a higher state by thermal infrared light of a certain wavelength. The measurements help track how land and water are being used on Earth.
