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Far infrared (FIR) refers to a specific range within the infrared spectrum of electromagnetic radiation. It encompasses radiation with wavelengths ranging from 15 micrometers (μm) to 1 mm, which corresponds to a frequency range of approximately 20 THz to 300 GHz. This places far infrared radiation within the CIE IR-B and IR-C bands. The longer wavelengths of the FIR spectrum overlap with a range known as terahertz radiation. Different sources may use different boundaries to define the far infrared range. For instance, astronomers often define it as wavelengths between 25 μm and 350 μm. It is important to note that far infrared photons possess significantly lower energy compared to visible light photons, with tens to hundreds of times less energy. 
Objects within a temperature range of approximately 5 K to 340 K emit radiation in the far infrared range as a result of black-body radiation, in accordance with Wien's displacement law. This characteristic is utilized in the observation of interstellar gases, which are frequently associated with the formation of new stars.
An illustrative case is the brightness observed in far infrared images of the center of the Milky Way galaxy. This brightness arises from the high density of stars in that region, which heats the surrounding dust and induces radiation emission in the far infrared spectrum. Excluding the center of our own galaxy, the galaxy M82 stands out as the most prominent far infrared object in the sky. Its central region emits an amount of far infrared light equivalent to the combined emission of all the stars in the Milky Way. The source responsible for heating the dust at the center of M82 remains unknown.
Human body detection
Certain human proximity sensors utilize passive infrared sensing within the far infrared wavelength range to detect the presence of stationary and/or moving human bodies.
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Researchers have observed that among all forms of radiant heat, only far-infrared radiation transfers energy solely in the form of heat that can be sensed by the human body. They have found that this type of radiant heat can penetrate the skin up to a depth of approximately 1.5 inches (almost 4 cm). In the field of biomedicine, experiments have been conducted using fabrics woven with FIR-emitting ceramics embedded in their fibers. These studies have indicated a potential delay in the onset of fatigue induced by muscle contractions in participants. The researchers suggest that the emission of far-infrared radiation by these ceramics (referred to as cFIR) could facilitate cellular repair.
Certain heating pads are marketed as providing "far infrared" therapy, which is claimed to offer deeper penetration. It should be noted, however, that the infrared radiation emitted by an object is determined by its temperature. Therefore, all heating pads emit the same type of infrared radiation if they are at the same temperature. Higher temperatures will result in greater infrared radiation, but caution must be exercised by the user to avoid burns.
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- Far infrared radiation (FIR): its biological effects and medical applications
- Challenging a Myth and Misconception: Red-Light Vision in Rats
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