far infrared

Far infrared (FIR) is electromagnetic radiation at the opposite end of infrared from visible light. The term is not consistently used, but often taken as EMR with a wavelength between 15 μm and 1 mm. In astronomy (far-infrared astronomy), the term is typically used for the wavelength range, 30-450 μm. It can indicate cold clouds of gas and dust, cold matter of 140 kelvins or less. The term submillimeter is used for the range from 450 μm to 1 mm and perhaps sometimes is used to cover far infrared astronomy as well.

The shorter-wavelength end of FIR has been somewhat observable by infrared telescopes such as Spitzer Space Telescope, IRAS, and the Infrared Space Observatory (all three defunct). A few submillimeter telescopes reach the very longest wavelengths, the Caltech Submillimeter Observatory (CSO, defunct) and the Atacama Large Millimeter Array (ALMA). Telescopes covering the intervening range are scarce: Herschel Space Observatory and SOFIA are both defunct. Thus very little observation can be carried out now, all that remains being the 61-cm High Elevation Antarctic Terahertz Telescope (HEAT) in Antarctica and some balloon-borne telescope projects.