Far infrared (FIR) is a region in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
spectrum of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
. Far infrared is often defined as any radiation with a
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
of 15
micrometer Micrometer can mean:
* Micrometer (device), used for accurate measurements by means of a calibrated screw
* American spelling of micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
s (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.
The long-wave side of the FIR spectrum overlaps with so named
terahertz radiation
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency
(THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fr ...
.
[ A.Glagoleva-Arkadiewa. (1924). "Short Electromagnetic Waves of wave-length up to 82 Microns". ''Nature'' 2844 113. do]
10.1038/113640a0
/ref> Different sources use different boundaries for the far infrared; for example, astronomers sometimes define far infrared as wavelengths between 25 μm and 350 μm.Visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
includes radiation with wavelengths between 400 nm and 700 nm, meaning that far infrared photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s have tens to hundreds of times less energy than visible light photons.
Applications
Astronomy
Due to black-body radiation
Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spe ...
, objects with temperatures between about 5 K and 340 K will emit radiation in the far infrared range according to Wien's displacement law. This property is sometimes used to observe interstellar gases where new stars are often formed.
For example, the center of the Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
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.
Human body detection
Some human proximity sensors use passive infrared sensing in the far infrared wavelength to detect both static and/or moving human bodies.
Therapeutic modality
Although all radiant heat is electromagnetic radiation, researchers have noted that only the far-infrared radiation band transfers energy purely in the form of heat that can be perceived by the human body as such. They report that this radiant heat can penetrate up to 1.5 inches (almost 4 cm) beneath the skin. Biomedical researchers have experimented with the use of FIR-emitting ceramics which are embedded into various fibers and woven into the fabric of garments. These researchers noted in subjects a "delay" in the "onset of fatigue induced by muscle contractions."
References
External links
Far infrared radiation (FIR): its biological effects and medical applications
Challenging a Myth and Misconception: Red-Light Vision in Rats
{{Library resources box
Infrared