midinfrared

Midinfrared, or the midinfrared region (MIR), is a part of the infrared spectrum approximately from 3 to 8 micrometers in wavelength, lying between the near-infrared and the longer-wavelength thermal infrared. Some conventions extend this range up to about 12 micrometers, reflecting overlaps with adjacent bands.

Many molecular vibrational transitions lie in the midinfrared, giving the region its characteristic spectral fingerprints. This makes MIR spectroscopy a primary tool for chemical identification and quantification in chemistry, biology, and environmental science. In addition, objects at typical temperatures emit strongly in the MIR, enabling thermal imaging and scene radiometry in this band.

Detectors for the midinfrared are built from narrow-bandgap semiconductors such as mercury cadmium telluride (HgCdTe, MCT), indium antimonide (InSb), lead sulfide (PbS), and quantum well infrared photodetectors (QWIPs). Most high-performance MIR instruments require cooling. Fourier-transform infrared (FTIR) spectrometers and dispersive spectrometers commonly operate in the MIR with sources such as midinfrared lamps or lasers.

Common midinfrared laser and source technologies include quantum cascade lasers (QCLs), interband cascade lasers, and CO2 lasers emitting near 9 to 11 micrometers. These sources provide narrow, tunable lines for high-resolution spectroscopy and sensing.

Applications span chemical sensing, environmental monitoring, industrial process control, medical diagnostics, and defense. The region faces challenges from strong atmospheric absorption by water vapor and CO2, and from the need for cooled detectors, which adds cost and complexity. Designs exploit atmospheric windows near 3–5 micrometers and 8–12 micrometers to enable measurements from ground or airborne platforms.