infrared radiation


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infrared radiation,

electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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 having a wavelength in the range from c.75 × 10−6 cm to c.100,000 × 10−6 cm (0.000075–0.1 cm). Infrared rays thus occupy that part of the electromagnetic spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 with a frequency less than that of visible lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
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 and greater than that of most radio waves, although there is some overlap. The name infrared means "below the red," i.e., beyond the red, or lower-frequency (longer wavelength), end of the visible spectrum. Infrared radiation is thermal, or heatheat,
nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system. Measures of Heat
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, radiation. It was first discovered in 1800 by Sir William Herschel, who was attempting to determine the part of the visible spectrum with the minimum associated heat in connection with astronomical observations he was making. In 1847, A. H. L. Fizeau and J. B. L. Foucault showed that infrared radiation has the same properties as visible light, being reflected, refracted, and capable of forming an interferenceinterference,
in physics, the effect produced by the combination or superposition of two systems of waves, in which these waves reinforce, neutralize, or in other ways interfere with each other.
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 pattern. Infrared radiation is typically produced by objects whose temperaturetemperature,
measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
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 is above 10°K;. There are many applications of infrared radiation. A number of these are analogous to similar uses of visible light. Thus, the spectrum of a substance in the infrared range can be used in chemical analysis much as the visible spectrum is used. Radiation at discrete wavelengths in the infrared range is characteristic of many molecules. The temperature of a distant object can also be determined by analysis of the infrared radiation from the object. Radiometers operating in the infrared range serve as the basis for many instruments, including heat-seeking devices in missiles and devices for spotting and photographing persons and objects in the dark or in fog. Medical uses of infrared radiation range from the simple heat lamp to the technique of thermal imaging, or thermographythermography
, contact photocopying process that produces a direct positive image and in which infrared rays are used to expose the copy paper. In a specially designed machine the original is placed in contact with a copy paper containing a heat-sensitive substance.
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. A thermograph of a person can show areas of the body where the temperature is much higher or lower than normal, thus indicating some medical problem. Thermography has also been used in industry and other applications. Some lasers produce infrared radiation. A recent development has been the expansion of research in infrared astronomyinfrared astronomy,
study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and buildings on the earth to distant galaxies, emit infrared (IR) radiation.
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; infrared sensors are sent aloft in balloons, rockets, and satellites to study the infrared radiation reaching the earth from other parts of the solar system and beyond.

Infrared radiation

Electromagnetic radiation in which wavelengths lie in the range from about 1 micrometer to 1 millimeter. This radiation therefore has wavelengths just a little longer than those of visible light and cannot be seen with the unaided eye. The radiation was discovered in 1800 by William Herschel.

An infrared source can be described by the spectral distribution of power emitted by an ideal body (a blackbody curve). This distribution is characteristic of the temperature of the body. A real body is related to it by a radiation efficiency factor or emissivity which is the ratio at every wavelength of the emission of a real body to that of a blackbody under identical conditions. The illustration shows curves for these ideal blackbodies radiating at a number of different temperatures. The higher the temperature, the greater the total amount of radiation. See Emissivity

Radiation from blackbodies at different temperatures, shown on a logarithmic scaleenlarge picture
Radiation from blackbodies at different temperatures, shown on a logarithmic scale

Infrared detectors are based either on the generation of a change in voltage due to a change in the detector temperature resulting from the power focused on it, or on the generation of a change in voltage due to some photon-electron interaction in the detector material. This latter effect is sometimes called the internal photoelectric effect.

Infrared techniques have been applied in military, medical, industrial, meteorological, ecological, forestry, agricultural, chemical, and other disciplines. Weather satellites use infrared imaging devices to map cloud patterns and provide the imagery seen in many weather reports. Infrared imaging devices have also been used for breast cancer screening and other medical diagnostic applications. In most of these applications, the underlying principle is that pathology produces inflammation, and these locations of increased temperature can be found with an infrared imager. Airborne infrared imagers have been used to locate the edge of burning areas in forest fires.

infrared radiation

Electromagnetic radiation lying between the radio and the visible bands of the electromagnetic spectrum. The wavelengths range from about 0.8 micrometer (μm) to about 1000 μm. The definitions of the regions of the infrared are a little arbitrary, but are roughly as follows:

near-infrared : 0.8 to 8 μm

mid-infrared : 8.0 to 30 μm

far-infrared : 30 to 300 μm

The region around 8–13 μm has been described as far-infrared by some astronomers. Radiation above 300 μm is now called submillimeter radiation.

Infrared Radiation

 

electromagnetic radiation that occupies the spectral region between the red end of visible light (with a wavelength of λ = 0.74 microns [μ]) and shortwave radiation (λ ∼ 1–2 mm). The infrared region of the spectrum is usually subdivided conditionally into the near (λ ranges from 0.74 to 2.5 μ), middle, or intermediate (2.5−50μ), and far (50—2,000 μ) infrared regions.

Infrared radiation was discovered in 1800 by the British scientist W. Herschel, who observed an increase in the temperature of a thermometer placed outside the red end of the sun’s spectrum obtained with the aid of a prism (see Figure 1); that is, in the invisible part of the spectrum. It was proved in the 19th century that infrared radiation conforms to the laws of optics and consequently is of the same nature as visible light. In 1923 the Soviet physicist A. A. Glagoleva-Arkad’eva obtained radio waves with λ ∼ 80 μ, that is, waves that corresponded to the infrared range of wavelengths. Thus it was proved experimentally that there exists a continuous transition from visible radiation to infrared and radio-wave radiation, and consequently all are electromagnetic in nature.

Figure 1. Herschel’s experiment. A thermometer placed beyond the red part of the sun’s spectrum showed a higher temperature than did the control thermometers placed to the side.

The spectrum of infrared radiation, like that of visible and ultraviolet radiation, may consist of individual lines or bands or may be continuous, depending on the nature of the source of the infrared radiation. Excited atoms or ions emit infrared line spectra. For example, mercury vapors on electrical discharge emit a number of narrow lines in the interval 1.014–2.326 μ, and hydrogen atoms emit a number of lines in the interval 0.95–7.40 μ. Excited molecules emit infrared band spectra owing to their oscillations and rotations. Vibration and vibration-rotation spectra lie primarily in the middle infrared region and pure rotation spectra lie in the far infrared region. Thus, for example, in the radiation spectrum of a gas flame, a band at about 2.7 μ, emitted by water molecules, and bands at λ ≈ 2.7 μ. and λ ≈ 4.2 μ, emitted by molecules of carbon dioxide, can be observed. Heated solid and liquid, bodies emit a continuous infrared spectrum. A heated solid radiates in a very broad range of wavelengths. At low temperatures (below 800°K) the radiation of a heated solid lies almost entirely in the infrared region, and such a body seems to be dark. As the temperature rises, the part of the radiation in the visible region increases and the body first appears to be dark red, then red, then yellow, and, finally, at high temperatures (above 5000°K), white. Here, both the total energy of radiation and the energy of the infrared radiation rise.

Figure 2. Curve of atmospheric transmission in the region 0.6−14 μ. The bands are transparent “windows”: 2.0−2.5 μ, 3.2−4.2 μ, 4.5−5.2 μ, 8.0−13.5 μ. The absorption bands with maximums at λ = 0.93, 1.13, 1.40, 1.87, and 2.74 μ belong to water vapor; at λ = 2.7 and 4.26 μ. to carbon dioxide gas; and at λ = 9.5 to ozone.

Optical properties. The optical properties of substances (transparency, reflection coefficient, index of refraction) in the infrared region of the spectrum generally differ significantly from the optical properties in the visible and ultraviolet regions. Many substances that are transparent in the visible region prove to be opaque in certain areas of infrared radiation, and vice versa. For example, a layer of water several centimeters thick is opaque to infrared radiation for λ > 1 μ (therefore water is often used as a heat filter), and wafers of germanium and silicon, which are opaque in the visible region, are transparent in the infrared region (germanium for λ > 1.8 μ. and silicon for λ > 1.0 μ Black paper is transparent in the far infrared region. Substances that are transparent to infrared radiation and opaque in the visible region are used as light filters to isolate infrared radiation. A number of substances are transparent, even in thick layers (of several centimeters), over quite large regions of the infrared spectrum. Various optical components (such as prisms, lenses, and windows) of infrared devices are manufactured from such substances. For example, glass is transparent up to 2.7 μ, quartz to 4.0 μ and from 100 μ to 1,000 μ, rock salt to 15 μ., and cesium iodide to 55 μ. Polyethylene, paraffin, teflon, and diamond are transparent for λ > 100 μ. In most metals, the reflectivity is much higher for infrared radiation than for visible light and increases with the wavelength of the infrared radiation. For example, the reflection coefficient of Al, Au, Ag, and Cu reaches 98 percent when λ = 10 μ. Liquid and solid nonmetallic substances have selective reflection of infrared radiation in such a way that the position of reflection maximums depends on the chemical composition of the substance.

On passing through the earth’s atmosphere, infrared radiation is attenuated as a result of scattering and absorption. The nitrogen and oxygen in the air do not absorb infrared radiation and attenuate it only as a result of scattering, which, however, is much less for infrared radiation than for visible light. Water vapor, carbon dioxide, ozone, and other impurities present in the atmosphere selectively absorb infrared radiation. Water vapor, whose absorption bands lie throughout virtually the entire infrared region of the spectrum, and carbon dioxide in the middle infrared region absorb infrared radiation with particular intensity. In the ground layers, there is only a small number of “windows” transparent to infrared radiation in the middle infrared region (Figure 2). The presence in the atmosphere of suspended particles—smoke, dust, and fine droplets of water (mist or fog)—leads to the additional attenuation of infrared radiation as a result of its scattering on these particles. The extent of scattering depends on the correlation between the size of the particles and the wavelength of the infrared radiation. When the particles are small (distance fog), infrared radiation is scattered less than visible radiation (this fact is utilized in infrared photography), but when the drops are large (as in the case of a dense fog), the infrared radiation is scattered just as strongly as the visible radiation.

Sources. The sun, about 50 percent of whose radiation lies in the infrared region, is a powerful source of infrared radiation. Infrared radiation accounts for a large part (70–80 percent) of the radiated energy of incandescent lamps with a tungsten filament (Figure 3). For photography in the dark and in some infrared sight equipment, the illuminating lamps are fitted with an infrared light filter, which passes only infrared radiation. A carbon electric arc with a temperature of ∼3900°K, whose radiation is close to the radiation of a blackbody, as well as various gas discharge tubes (pulse and continuous-action) are powerful sources of infrared radiation. Spirals of nichrome wire, heated to a temperature of ∼950°K, are used for radiative room heating. Such heaters are fitted with reflectors to provide better concentration of the infrared radiation. In scientific research, special sources of infrared radiation, such as tungsten ribbon filament lamps, Nernst lamps, globars, and high-pressure mercury lamps, are used, for example, to obtain infrared absorption spectra in different parts of the spectrum. The radiatioi of some optical quantum generators—lasers—also lies in the infrared region of the spectrum; for example, the radiation of a neodymium glass laser has a wavelength of 1.06 u.; the radiation of a neon-helium laser, 1.15μ. and 3.39μ; the radiation of a carbon-dioxide-gas laser, 10.6 μ; and the radiation of a InSb semiconductor laser, 5 μ.

Figure 3. Radiation curves of an ideal blackbody A and tungsten B at a temperature of 2450°K. Shaded area is tungsten’s radiation in the infrared region; the interval 0.40–0.74 μ is the visible region.

Infrared detectors are based on the conversion of the energy of infrared radiation into other types of energy that can be measured by ordinary methods. There are thermal and photoelectric infrared detectors. In the former, the absorbed infrared radiation effects an increase in the temperature of the heat-sensitive element of the detector, and this rise is registered. In photoelectric detectors, the absorbed infrared radiation leads to the appearance of or a change in an electric current or voltage. Photoelectric detectors, in contrast to thermal detectors, are selective, that is, sensitive only in a certain region of the spectrum. Special photographic films and plates—infrared plates—are also sensitive to infrared radiation (up to λ = 1.2 μ), and therefore photographs can be made in infrared radiation.

Applications. Infrared radiation finds broad application in scientific research, in the solution of a large number of practical problems, in the military, and elsewhere. The investigation of emission and absorption spectra in the infrared region is used in the study of the structure of the electron shell of atoms, for the determination of the structure of molecules, and for qualitative and quantitative analysis of mixtures of substances of complex molecular composition, such as engine fuel.

Because of the difference in the scattering, reflection, and transmission coefficients by substances in visible and infrared radiation, a photograph made by infrared radiation has a number of properties not possessed by an ordinary photograph. For example, details invisible in an ordinary photograph are often visible in infrared photographs.

In industry, infrared radiation is used to dry and heat materials and products upon irradiation and also to detect concealed flaws in products.

Special devices—image converters, in which the infrared image of an object at the photocathode, which is invisible to the eye, is converted into a visible image—have been developed on the basis of photocathodes that are sensitive to infrared radiation (for λ < 1.3 μ). Various infrared sight equipment (such as binoculars and sighting devices), which make it possible by irradiating the observed objects with infrared radiation from special sources to view or sight in total darkness, have been constructed on the basis of this principle. The development of highly sensitive infrared detectors has made it possible to build special devices—thermal direction finders for detecting and finding the direction to objects whose temperature is greater than the background temperature (such as the heated smokestacks of ships, aircraft engines, and the exhaust pipes of tanks) on the basis of their thermal infrared radiation. Guidance systems for projectiles and missiles have also been developed on the basis of the principle of thermal radiation. The special optical system and infrared detector located in the nose cone of a missile receive infrared radiation from a target whose temperature is higher than that of the surrounding medium (for example, the infrared radiation of aircraft, ships, plants, and thermal electric power stations), while the automatic servomechanism connected to the controls guides the missile precisely to the target. Infrared radars and range finders make it possible to detect any objects in the dark and to measure the distance to them.

Optical quantum generators, which radiate in the infrared region, are also used in ground and space communications.

REFERENCES

Lecomte, J. Infrakrasnoe izluchenie. Moscow, 1958. (Translated from French.)
Déribéré, M. Prakticheskie primeneniia infrakrasnykh luchei. Moscow-Leningrad, 1959. (Translated from French.)
Kozelkin, V. V., and I. F. Usol’tsev. Osnovy infrakrasnoi tekhniki. Moscow, 1967.
Solov’ev, S. M. Infrakrasnaia fotografiia. Moscow, 1960.
Lebedev, P. D. Sushka infrakrasnymi luchami. Moscow-Leningrad, 1955.

V. I. MALYSHEV

infrared radiation

[¦in·frə¦red ‚rād·ē′ā·shən]
(electromagnetism)
Electromagnetic radiation whose wavelengths lie in the range from 0.75 or 0.8 micrometer (the long-wavelength limit of visible red light) to 1000 micrometers (the shortest microwaves).
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