midIR

MidIR, or MidIR, denotes the portion of the infrared spectrum roughly from 3 to 8 micrometers, although some references extend the definition toward 12 micrometers or beyond. It sits between near-infrared and thermal infrared and includes many fundamental molecular vibrational transitions, making it especially useful for spectroscopy and chemical sensing. Atmospheric transmission in the MidIR is wavelength dependent; there are transmission windows near 3 to 5 micrometers and 8 to 12 micrometers under dry air, while wavelengths outside these windows are largely absorbed by water vapor and carbon dioxide.

Instrumentation in the MidIR relies on specialized sources and detectors. Broadband and tunable light sources include thermal emitters and quantum cascade lasers (QCLs). Detectors commonly used are mercury cadmium telluride (HgCdTe, MCT) and indium antimonide (InSb) photodetectors, typically cryogenically cooled to reduce noise; uncooled bolometers and microbolometer arrays serve imaging applications in parts of the MidIR. Optical materials used in MidIR components include chalcogenide glasses, germanium, zinc selenide, and zinc sulfide.

Applications span spectroscopy, chemical sensing, and imaging. MidIR is used to identify molecular species via vibrational spectroscopy, monitor environmental and industrial processes, and study astronomical objects shrouded by dust. In defense and security, MidIR imaging and spectroscopy support surveillance and countermeasure systems. Challenges include limited atmospheric transmission in many bands, the need for low-noise detectors and cooling, and material losses at longer wavelengths.