Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (µm) to 1 mm (corresponding to a range of about 20 THz to 300 GHz), which places far infrared radiation within the CIE IR-B and IR-C bands. Different sources use different boundaries for the far infrared spectrum; for example, astronomers sometimes define far infrared as wavelengths between 25 µm and 350 µm.
Objects with temperatures between about 5 K and 340 K will emit radiation in the far infrared range (see Wien's displacement law and Black-body radiation). This property is sometimes used to observe interstellar gases where new stars are often formed.
For example, the center of the Milky Way Galaxy is very bright in far infrared images because the dense concentration of stars there heats the surrounding dust and causes it to emit radiation in this part of the spectrum. Disregarding the center of our own galaxy, the brightest far infrared object in the sky is the galaxy M82, which radiates as much far infrared light from its central region as all of the stars in the Milky Way combined. This is due to the dust at the center of M82 being heated by an unknown source.
Several studies have examined the use of infrared saunas in the treatment of chronic health problems, such as high blood pressure, congestive heart failure and rheumatoid arthritis, and found some evidence of benefit. However, larger and more-rigorous studies are needed to confirm these results.