In physics, infrared (IR) or infrared radiation (first discovered in 1800 by astronomer William Herschel) is the electromagnetic radiation with a frequency band of the electromagnetic spectrum lower than that of visible light but greater than that of radio waves, i.e. wavelength between 700 nm and 1 mm (infrared band). The range of Infrared region is 12800 ~ 10 cm-1 and can be divided into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1). This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz and includes most of the thermal radiation emitted by objects near room temperature. The term means “under the red” (from the Latin infra, “under”), because red is the visible color with the lowest frequency.

It is often associated with the concepts of “heat” and “thermal radiation”, since every object with a temperature above absolute zero spontaneously emits radiation in this band (according to Wien’s law increasing the temperature the peak of emission moves more and more towards the visible until the object becomes incandescent).

Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. Infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for the study of these energy states for molecules of the proper symmetry. Slightly more than half of the energy from the Sun arrives on Earth in the form of infrared radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth’s climate. Infrared light is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Incandescent bulbs convert only about 10% of their electrical energy input into visible light energy, while the other 90% is converted to infrared radiation, according to the Environmental Protection Agency.

Infrared rays, invisible to human eye, are emitted by all bodies, at any temperature, with emission that increases with temperature: it is therefore ray energy; they have also the characteristic property to produce considerable heat development when they are absorbed by bodies.

Infrared radiations are diffused by fog, smog, gaseous molecules and are selectively absorbed by many gases in the atmosphere, such as ozone, carbon dioxide and water vapor, to a negligible extent by oxygen and nitrogen. Since infrared radiation is of the same nature as light, the formation of images is altered by the phenomena of diffraction and aberration; in addition, optical systems can be used for them as for visible radiation: lenses, mirrors, prisms, sometimes diffraction gratings.

For the detection of infrared radiation are used different devices and apparatuses: receivers or detectors are transducers that convert the ray energy into electric current or voltage, exploiting the variation of certain physical properties of the detector, or the property of the radiation to impress photographic film. Thermal detectors exploit the variation of physical properties of the detector due to the heating produced by the radiation, such as bolometers, bismuth-silver or copper-constantan thermocouples, thermopiles (succession of several thermocouples with thermoelectric joints in series).

The detectors called photoelectric effect, more sensitive, can be photoelectronic cells, photomultipliers, photovoltaic cells, photoluminescent elements, ie based on a photoluminescent semiconductor crystal, on which the radiation, depending on the material, can have both the effect of stimulating the light emission, resulting in a bright image of the source on a dark background, and to accelerate the decay, resulting in a dark image of the source on a light background. Infrared rays are produced in nature by many sources, including warm-blooded animals, in the technique are obtained by lamps called infrared rays, consisting of an electrical resistance to a temperature of about 600 º C, and equipped with parabolic mirror reflection.

Technological applications

Industrially infrared rays are used to heat bodies that allow a good penetration of heat inside, for example to dry painted surfaces, or also to heat outdoor environments.

Other important applications include remote temperature measurements in chemical and metallurgical processes, surveys of air pollution or air turbulence, gas analysis, fire alarms, heat map surveys, industrial surveillance with examination of photographic film during manufacture.

In the scientific field, the most interesting applications concern the localization of satellites, devices for the orientation of solar battery instruments, the measurement of temperatures of the Moon, planets or satellites, the measurement of reflected solar radiation, the localization of storm fronts, the visualization of the temperature of land or sea surfaces or ocean currents, the search for oil fields, the measurement of thicknesses of epitaxial films, the determination of the composition of organic substances. The observation of the Universe in the infrared constitutes an entire branch of astronomy, infrared astronomy.

In the military field the most important applications concern the localization of aircraft, ships, submarines, missiles and ground vehicles, surveillance of military targets, night guidance devices.

In the photographic field are sometimes used emulsions sensitive to infrared rays to perform photographs even with haze taking advantage of the property of infrared rays, emitted by the subject to be photographed, to cross almost undisturbed haze and even fog.

The medical applications concern, in the field of diagnosis, the remote measurement of the temperature of the skin and the area below (thermograms), with the possibility of revealing inflammatory, or infectious, or tumor processes; the determination of the content of carbon dioxide in the blood; the measurement of eye size and control of eye movements; the detection of obstacles for the blind. In the therapeutic field is exploited the thermal effect of infrared radiation resulting in vasodilation, acceleration of metabolic exchange and other effects on the peripheral nervous system.

Infrared spectroscopy

Infrared spectroscopy is the study of the interaction of infrared light with matter; it is the analysis of infrared light interacting with a molecule. This can be analyzed in three ways by measuring absorption, emission, and reflection. The fundamental measurement obtained in infrared spectroscopy is an infrared spectrum, which is a plot of measured infrared intensity versus wavelength (or frequency) of light. The main use of this technique is in organic and inorganic chemistry. It is used by chemists to determine functional groups in molecules. Infrared spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups. Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wavenumber). Infrared spectroscopy is an analytical technique that takes advantage of the vibrational transitions of a molecule, has been of great significance to scientific researchers in many fields such as protein characterization, nanoscale semiconductor analysis, and space exploration.

Fourier transform infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. FTIR spectrometers (Fourier Transform Infrared Spectrometer) are widely used in organic synthesis, polymer science, petrochemical engineering, pharmaceutical industry, and food analysis. In addition, since FTIR spectrometers can be hyphenated to chromatography, the mechanism of chemical reactions and the detection of unstable substances can be investigated with such instruments. Up till FTIR spectrometers, there have been three generations of infrared radiation spectrometers:

  • the first generation infrared radiation spectrometer was invented in the late 1950s. It utilizes a prism optical splitting system. The prisms are made of NaCl. The requirement of the sample’s water content and particle size is extremely strict. Furthermore, the scan range is narrow. Additionally, the repeatability is fairly poor. As a result, the first generation infrared radiation spectrometer is no longer in use;
  • the second generation IR spectrometer was introduced to the world in the 1960s. It utilizes gratings as the monochrometer. The performance of the second generation infrared radiation spectrometer is much better compared with infrared radiation spectrometers with prism monochrometer, But there are still several prominent weaknesses such as low sensitivity, low scan speed and poor wavelength accuracy which rendered it out of date after the invention of the third generation infrared radiation spectrometer;
  • The invention of the third generation infrared radiation spectrometer, Fourier transform infrared spectrometer, marked the abdication of monochrometer and the prosperity of interferometer. With this replacement, infrared radiation spectrometers became exceptionally powerful. Consequently, various applications of infrared radiation spectrometers have been realized.

The molecules of chemical compounds selectively absorb infrared radiation of certain wavelengths. Therefore, by crossing a sample of the compound under examination successively by infrared radiation of different wavelengths and recording on a graph in which measure each of them is absorbed, it is obtained a trace that takes the name of absorption spectrum.

Infrared absorption spectra are called vibration and rotation spectra, because infrared radiations convey quantities of energy able to cause in molecules vibrations of atoms or rotational motions of an atom or a group of atoms around the axis that binds it to another atom. Vibrations can occur in the sense of temporarily varying both the distances between two atoms linked to each other, and the angle that two atoms form with a third atom to which they are both linked.

Spectra in ultraviolet or visible are generally very simple because they are due to vibrations corresponding to electrons of double and triple bonds. Infrared spectra are also originated by simple bonds; consequently any organic molecule presents an infrared spectrum much more complex with a large number of absorption bands. For these characteristics the infrared absorption spectrum can quickly provide a series of information on the structural elements present in a molecule of unknown structure, also because of its complication, is an absolutely typical property of the molecule of a compound: while two molecules also very different can present various physical properties and even a spectrum of ultraviolet absorption in practice identical, there are no two compounds that present equal absorption spectrum in the infrared. This can therefore be useful to ascertain the identity of a compound of which is available a quantity even in the order of a few milligrams by comparing the spectrum with those already known.

Infrared absorption spectra can be measured on substances in solid, liquid or solution state, or even on gaseous substances.

Infrared astronomy

Astronomers have found that infrared radiation is especially useful when trying to probe areas of our universe that are surrounded by clouds of gas and dust. Because of infrared’s longer wavelength, it can pass right through these clouds and reveal details invisible by observing other types of radiation. Especially interesting are areas were stars and planets are forming and the cores of galaxies where it is believed huge black holes might reside.

Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect objects such as planets, and view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.

Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes. The James Webb Space Telescope (JWST) has three infrared instruments to help study the origins of the universe and the formation of galaxies, stars, and planets.

One of the advantages of infrared radiation observation is that it can detect objects that are too cool to emit visible light. This has led to the discovery of previously unknown objects, including comets, asteroids and wispy interstellar dust clouds that seem to be prevalent throughout the galaxy.

Infrared astronomy is particularly useful for observing cold molecules of gas and for determining the chemical makeup of dust particles in the interstellar medium, said Robert Patterson, professor of astronomy at Missouri State University. These observations are conducted using specialized CCD detectors that are sensitive to IR photons. Another advantage of infrared radiation is that its longer wavelength means it doesn’t scatter as much as visible light, according to NASA. Whereas visible light can be absorbed or reflected by gas and dust particles, the longer IR waves simply go around these small obstructions. Because of this property, IR can be used to observe objects whose light is obscured by gas and dust. Such objects include newly forming stars imbedded in nebulas or the center of Earth’s galaxy.

The birth of infrared astronomy came quite after the discovery of infrared radiation, by W. Herschel, who realized that the spectral region of the Sun located beyond the red is home to thermal radiation. The reasons of the delay are related to the chemical-physical characteristics of the infrared radiation itself. In fact, on the one hand the infrared radiation, coming from celestial sources, is usually absorbed by water vapor and carbon dioxide of the Earth’s atmosphere, on the other hand the thermal component, emanating from the same instrumentation, produces strong disturbances on the infrared signal to be detected.

Only in relatively recent times, the development of appropriate cooling techniques and the production of infrared-sensitive materials (lead sulfide cells, germanium optical parts, special photographic material), together with the realization of sophisticated electronic equipment for analysis, have allowed the start and progress of a field of research, proved to be very fertile in a short time, thanks also to space missions carried out with several probes equipped with telescopes and instrumentation for infrared.

In astronomical technology infrared bands are distinguished in thermal or photographic (up to 1.1 µm wavelength, region where detection techniques similar to those used in the visible are still possible), near (up to 4 µm), intermediate (up to 40 µm), extreme (up to 1 mm, practically in contiguity with the millimeter radio frequencies).

The sources of interest to infrared astronomy are characterized by the modest intrinsic temperature and therefore concern mainly the cold matter (gas and dust) that is diffused in the Galaxy, especially around its dynamic center. The galactic cold matter is articulated in large molecular formations, giving rise to those interstellar clouds from which is often re-emitted, by thermal dissipation, the incident radiation from surrounding stars or young associations of stars undergoing condensation in their interior.

Typical infrared sources are then almost all the long period variable stars (red giants of the Mira Ceti type) and the regions of star formation, whose optical observation is particularly hampered by absorption due to the presence of protostellar clouds themselves. Other infrared sources are the so-called black stars and brown dwarfs, celestial bodies too small to be able to radiate anything but thermal radiation, or too far in age to still emit visible light.

Another field of application for infrared astronomy is the search for extrasolar planetary systems, already formed or in gestation, which can be identified by that excess of thermal component, diffuse or concentrated, which is manifested for example in some stars close to us. In the near infrared the sky does not appear very different from usual, except for the fact that red stars (such as Betelgeuse, Antares) appear brighter than white-blue stars (such as Rigel). Moving into the intermediate infrared, the diffuse light of atmospheric origin is extinguished, so that the celestial sources now visible – the clusters of diffuse dust where the stars “nest”, and where are immersed very young stars acting as thermal exciters – become detectable even in the presence of the Sun.

At longer wavelengths in the image of the firmament prevail molecular clouds (at the temperature of a few tens of kelvin). These hide the direct vision already beyond 2-3 thousand light years away from the Sun, but their emission in the infrared is a sign of the occurrence of an impressive complex of violent phenomenologies: vortices of matter flowing in intense gravitational centers, such as galactic bulbs, neutron stars, black holes; corpuscular winds projected by stars in phase of gigantism; dispersion of matter in the mechanisms of mutual phagocytosis established within extraordinary thickenings of stars; and so on.

Infrared astronomy from earth

The beginnings of astronomical investigation in the infrared can be traced back to S.P. Langley (1881) with the invention of the bolometer. In 1922 decisive improvements were introduced by the use, due to E. Pettit and S.B. Nicholson, of the thermocouple. In the forties, G.P. Kuiper adopted lead sulfide cells for the detectors to be sent to high altitudes, aboard balloons; detectors of which G. Neugebauer and B. Leighton, ten years later, will be the first. Leighton, ten years later, improved the performance with cooling, managing to compile a first catalog of 5612 sources (the Two Micron Survey).

The modern sensors, cooled with nitrogen and liquid helium at a few Kelvin, are ten thousand times more sensitive than their ancestors, and are able to work up to 20 µm. Mounted on rockets, balloons and airplanes (e.g. KAO, Kuiper Airborne Observatory, an aircraft equipped with a 90 cm aperture telescope), during the seventies they already allowed the scanning of about nine tenths of the sky. In this technological phase, we should not forget the TIRGO, a telescope of Italian design installed on the top of the Gornergrat, in Swiss territory.

At the end of the 20th century and at the beginning of the 21st century, the awareness of the importance of infrared astronomy has assumed such general and unconditional connotations that the scientific community has done its best to realize and propose new and increasingly sophisticated designs. For the ground survey, it is worth mentioning the installation on Mauna Kea – over 4000 m high – of 4 powerful structures: the British UKIRT telescope with a 3.8 m mirror; the NASA IRTF telescope, with a 3 m mirror; the twin KECK telescopes, with multiple mirrors for a 10 m aperture. For its part, NASA has built, as a replacement for the Kuiper airborne observatory, a second generation airborne observing facility, SOFIA (Stratospheric Observatory For Infrared Astronomy), to explore the sky over the full range of infrared frequencies thanks to its giant 2.5 m eye and the availability of a vast array of receptors.

Infrared astronomy from space

IRAS (InfraRed Astronomy Satellite) was the first orbiting celestial infrared observatory in which most of the serious technological problems that opposed a systematic investigation from space were solved. Similar to a giant cryostat orbiting at an altitude of 900 km, IRAS – with its 60 cm objective – in the late eighties produced a map containing 300,000 sources of different types, including galaxies thousands of times brighter than the Milky Way, but radiating exclusively in the infrared. It revealed the presence of protoplanetary disks around numerous stars (β Pictoris, Vega) and that of cirriform formations scattered throughout the galactic space.

Subsequently, it was the turn of COBE (Cosmic Background Explorer), a cosmological probe, to investigate in the infrared the fossil radiation in order to capture the thermal inhomogeneities related to the primitive genetic centers of the Universe (protogalaxies and early generations of stars). In the investigation, the probe has also provided a general view of the dust that thickens in the disk and in the bulb of our Galaxy, thus improving the knowledge of its evolutionary mechanisms.

Equally and perhaps more important than IRAS was the orbiting infrared observatory of the European Space Agency (ESA), namely ISO, (Infrared Space Observatory) with 60 cm optics, which worked in space from 17 November 1996 to 8 April 1998. In 2003, NASA has completed the complex of the 4 large space observatories of its scientific program with the infrared observatory SIRTF (Space InfraRed Telescope Facility, renamed Spitzer Space Telescope after the launch), equipped with a 90 cm mirror and a spectrograph, refrigerated with liquid helium.

On the European side, always in the field of infrared space astronomy, the European Space Agency has defined the projects for the analogue of the Spitzer Space Telescope, that is for FIRST (already renamed Herschel), which will operate in the far infrared and will be put into orbit in 2007. Thanks to their extraordinary qualities of sensitivity and spatial resolution, with these infrared space observatories astronomers promise to observe celestial infrared sources a thousand times fainter than those known so far, and to discover the invisible component – dead stars, brown dwarfs, cometary reservoirs, extra-solar planetary bodies, intergalactic halos, etc. – that could be a not negligible part of the universe. – that could constitute a non-negligible part of the so-called dark matter.


  • What are Infrared Waves? NASA science.
  • Fourier Transform Infrared Spectrometer. LibreTexts.

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