infrared adj : having or employing wavelengths longer than light but shorter than radio waves; lying outside the visible spectrum at its red end; "infrared radiation"; "infrared photography"

1 the infrared part of the electromagnetic spectrum; electromagnetic wave frequencies below the visible range; "they could sense radiation in the infrared" [syn: infrared frequency]

2 electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves [syn: infrared light, infrared radiation, infrared emission]

English

Etymology

Latin infra, below, + red

Noun

infrared

1. electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation, having a wavelength between 700 nm and 1 mm

Translations

Adjective

1. Having the wavelength in the infrared.
2. In the infrared spectrum.

Derived terms

• infrared astronomy
• infrared camera
• infrared detector
• infrared divergence
• infrared film
• infrared filter
• infrared fixed point
• infrared homing
• infrared imagery
• infrared lamp
• infrared microscope
• infrared motion detector
• infrared multiphoton dissociation
• infrared photography
• infrared pointer
• infrared port
• infrared radiation
• infrared remote control
• infrared repellor
• infrared sauna
• infrared spectroscopy
• infrared thermometer

Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer than that of visible light, but shorter than that of terahertz radiation and microwaves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light with the longest wavelength. Infrared radiation has wavelengths between about 750 nm and 1 mm, spanning three orders of magnitude. Humans at normal body temperature can radiate at a wavelength of 10 microns.

Infrared imaging is used extensively for both military and civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking. Non-military uses include thermal efficiency analysis, remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect cool objects such as planets, and to view highly red-shifted objects from the early days of the universe.

At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states. Infrared spectroscopy examines absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.

Origins of the term

The name means below red (from the Latin infra, "below"), red being the color of the longest wavelengths of visible light. IR light has a longer wavelength than that of red light. A longer wavelength means it has a lower frequency than red, hence below.

Different regions in the infrared

Objects generally emit infrared radiation across a spectrum of wavelengths, but only a specific region of the spectrum is of interest because sensors are usually designed only to collect radiation within a specific bandwidth. As a result, the infrared band is often subdivided into smaller sections.

The International Commission on Illumination (CIE) recommended the division of optical radiation into the following three bands:

• IR-A: 700 nm–1400 nm
• IR-B: 1400 nm–3000 nm
• IR-C: 3000 nm–1 mm

A commonly used sub-division scheme is:

• Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.

• Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications

• Mid-wavelength infrared (MWIR, IR-C DIN) also called intermediate infrared (IIR): 3-8 µm. In guided missile technology the 3-5 µm portion of this band is the atmospheric window in which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing on to the IR signature of the target aircraft, typically the jet engine exhaust plume.

• Long-wavelength infrared (LWIR, IR-C DIN): 8–15 µm. This is the "thermal imaging" region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external light or thermal source such as the sun, moon or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of the spectrum. Sometimes also called the "far infrared."

• Far infrared (FIR): 15-1,000 µm (see also far infrared laser)

NIR and SWIR is sometimes called reflected infrared while MWIR and LWIR is sometimes referred to as thermal infrared. Due to the nature of the blackbody radiation curves, typical 'hot' objects, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.

Astronomers typically divide the infrared spectrum as follows:

• near: (0.7-1) to 5 µm
• mid: 5 to (25-40) µm
• long: (25-40) to (200-350) µm

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, and hence different environments in space.

A third scheme divides up the band based on the response of various detectors:

• Near infrared (NIR): from 0.7 to 1.0 micrometers (from the approximate end of the response of the human eye to that of silicon)
• Short-wave infrared (SWIR): 1.0 to 3 micrometers (from the cut off of silicon to that of the MWIR atmospheric window. InGaAs covers to about 1.8 micrometers; the less sensitive lead salts cover this region
• Mid-wave infrared (MWIR): 3 to 5 micrometers (defined by the atmospheric window and covered by Indium antimonide [InSb] and HgCdTe and partially by lead selenide [PbSe])
• Long-wave infrared (LWIR): 8 to 12, or 7 to 14 micrometers: the atmospheric window (Covered by HgCdTe and microbolometers)
• Very-long wave infrared (VLWIR): 12 to about 30 micrometers, covered by doped silicon

These divisions are justified by the different human response to this radiation: near infrared is the region closest in wavelength to the radiation detectable by the human eye, mid and far infrared are progressively further from the visible regime. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (The common silicon detectors are sensitive to about 1,050 nm, while InGaAs' sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). Unfortunately, international standards for these specifications are not currently available.

The boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to light above 700 nm wavelength, so shorter frequencies make insignificant contributions to scenes illuminated by common light sources. But particularly intense light (e.g., from lasers, or from bright daylight with the visible light removed by colored gelshttp://amasci.com/amateur/irgoggl.html) can be detected up to approximately 780 nm, and will be perceived as red light. The onset of infrared is defined (according to different standards) at various values typically between 700 nm and 800 nm.

Telecommunication bands in the infrared

In optical communications, the part of the infrared spectrum that is used is divided into several bands based on availability of light sources, transmitting/absorbing materials (fibers) and detectors:

The C-band is the dominant band for long-distance telecommunication networks. The S and L bands are based on less well established technology, and are not as widely deployed.

Heat

Infrared radiation is popularly known as "heat" or sometimes "heat radiation", since many people attribute all radiant heating to infrared light and/or to all infrared radiation to being a result of heating. This is a widespread misconception, since light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun only accounts for 49% of the heating of the Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. It is true that objects at room temperature will emit radiation mostly concentrated in the 8 to 12 micrometer band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see black body and Wien's displacement law).

Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum.

The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body. To further explain, two objects at the same physical temperature will not 'appear' the same temperature in an infrared image if they have differing emissivities.

Applications

Infrared Filters

Infrared (IR) filters can be made from many different materials. One type is made of polysulphone plastic that blocks over 99% of the visible light spectrum from “white” light sources such as incandescent filament bulbs. Infrared filters allow a maximum of infrared output while maintaining extreme covertness. Currently in use around the world, infrared filters are used in Military, Law Enforcement, Industrial and Commercial applications. The unique makeup of the plastic allows for maximum durability and heat resistance. IR filters provide a more cost effective and time efficient solution over the standard bulb replacement alternative. All generations of night vision devices are greatly enhanced with the use of IR filters.

Night vision

Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs.

Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence the name).

Other imaging

Weather satellites equipped with scanning radiometers produce thermal or infrared images which can then enable a trained analyst to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features. The scanning is typically in the range 10.3-12.5 µm (IR4 and IR5 channels).

High, cold ice cloud such as Cirrus or Cumulonimbus show up bright white, lower warmer cloud such as Stratus or Stratocumulus show up as grey with intermediate clouds shaded accordingly. Hot land surfaces will show up as dark grey or black. One disadvantage of infrared imagery is that low cloud such as stratus or fog can be a similar temperature to the surrounding land or sea surface and does not show up. However, using the difference in brightness of the IR4 channel (10.3-11.5 µm) and the near-infrared channel (1.58-1.64 µm), low cloud can be distinguished, producing a fog satellite picture. The main advantage of infrared is that images can be produced at night, allowing a continuous sequence of weather to be studied.

These infrared pictures can depict ocean eddies or vortices and map currents such as the Gulf Stream which are valuable to the shipping industry. Fishermen and farmers are interested in knowing land and water temperatures to protect their crops against frost or increase their catch from the sea. Even El Niño phenomena can be spotted. Using color-digitized techniques, the gray shaded thermal images can be converted to color for easier identification of desired information.

Climatology

In the field of climatology, atmospheric infrared radiation is monitored to detect trends in the energy exchange between the earth and the atmosphere. These trends provide information on long term changes in the earth's climate. It is one of the primary parameters studied in research into global warming together with solar radiation.

A pyrgeometer is utilized in this field of research to perform continuous outdoor measurements. This is a broadband infrared radiometer with sensitivity for infrared radiation between approximately 4.5 µm and 50 µm.

Astronomy

Astronomers observe objects in the infrared portion of the electromagnetic spectrum using optical components, including mirrors, lenses and solid state digital detectors. For this reason it is classified as part of optical astronomy. To form an image, the components of an infrared telescope need to be carefully shielded from heat sources, and the detectors are chilled using liquid helium. The sensitivity of Earth-based infrared telescopes is significantly limited by water vapor in the atmosphere, which absorbs a portion of the infrared radiation arriving from space outside of selected atmospheric windows. This limitation can be partially alleviated by placing the telescope observatory at a high altitude, or by carrying the telescope aloft with a balloon or an aircraft. Space telescopes do not suffer from this handicap, and so outer space is considered the ideal location for infrared astronomy.

The infrared portion of the spectrum has several useful benefits for astronomers. Cold, dark molecular clouds of gas and dust in our galaxy will glow with radiated heat as they are irradiated by imbedded stars. Infrared can also be used to detect protostars before they begin to emit visible light. Stars emit a smaller portion of their energy in the infrared spectrum, so nearby cool objects such as planets can be more readily detected. (In the visible light spectrum, the glare from the star will drown out the reflected light from a planet.)

Infrared light is also useful for observing the cores of active galaxies which are often cloaked in gas and dust. Distant galaxies with a high redshift will have the peak portion of their spectrum shifted toward longer wavelengths, so they are more readily observed in the infrared.

Similar uses of infrared are made by historians on various types of objects, especially very old written documents such as the Dead Sea Scrolls, the Roman works in the Villa of the Papyri, and the Silk Road texts found in the Dunhuang Caves. Carbon black used in ink can show up extremely well.

Biological systems

The pit viper is known to have two infrared sensory pits on its head. There is controversy over the exact thermal sensitivity of this biological infrared detection system.

Other organisms that actively employ thermo-receptors are rattlesnakes (Crotalinae subfamily) and boas (Boidae family), the Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata), darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamantus plateni), and possibly blood-sucking bugs (Triatoma infestans).

Photobiomodulation

Near infrared light is currently used for treatment of chemotherapy induced oral ulceration as well as wound healing. There is some work relating to anti herpes virus treatment. Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.

The Earth as an infrared emitter

The Earth's surface and the clouds absorb visible and invisible radiation from the sun and re-emit much of the energy as infrared back to the atmosphere. Certain substances in the atmosphere, chiefly cloud droplets and water vapor, but also carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons, absorb this infrared, and re-radiate it in all directions including back to Earth. Thus the greenhouse effect keeps the atmosphere and surface much warmer than if the infrared absorbers were absent from the atmosphere.

History of infrared science

The discovery of infrared radiation is ascribed to William Herschel, the astronomer, in the early 19th century. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism to refract light from the sun and detected the infrared, beyond the red part of the spectrum, through an increase in the temperature recorded on a thermometer. He was surprised at the result and called them "Calorific Rays". The term 'Infrared' did not appear until late in the 19th century.

Other important dates include:

• 1835: Macedonio Melloni makes the first thermopile IR detector;
• 1860: Gustav Kirchhoff formulates the blackbody theorem E=J(T,n);
• 1873: Willoughby Smith discovers the photoconductivity of selenium;
• 1879: Stefan-Boltzmann law formulated empirically \omega_T^4
• 1880s & 1890s: Lord Rayleigh and Wilhelm Wien both solve part of the blackbody equation, but both solutions are approximations that "blow up" out of their useful ranges. This problem was called the "UV Catastrophe and Infrared Catastrophe".
• 1901: Max Planck published the blackbody equation and theorem. He solved the problem by quantizing the allowable energy transitions.
• Early 1900s: Albert Einstein develops the theory of the photoelectric effect, determining the photon. Also William Coblentz in spectroscopy and radiometry.
• 1917: Theodore Case develops thallous sulfide detector; British develop the first infra-red search and track (IRST) in World War I and detect aircraft at a range of one mile;
• 1935: Lead salts-early missile guidance in World War II;
• 1938: Teau Ta-predicted that the pyroelectric effect could be used to detect infrared radiation.
• 1952: H. Welker discovers InSb;
• 1950s: Paul Kruse (at Honeywell) and Texas Instruments form infrared images before 1955;
• 1950s and 1960s: Nomenclature and radiometric units defined by Fred Nicodemenus, G.J. Zissis and R. Clark, Jones defines D*;
• 1958: W.D. Lawson (Royal Radar Establishment in Malvern) discovers IR detection properties of HgCdTe;
• 1958: Falcon & Sidewinder missiles developed using infrared and the first textbook on infrared sensors appears by Paul Kruse, et al.
• 1961: J. Cooper demonstrated pyroelectric detection;
• 1962: Kruse and ? Rodat advance HgCdTe; Signal Element and Linear Arrays available;
• 1965: First IR Handbook; first commercial imagers (Barnes, Agema ; Richard Hudson's landmark text; F4 TRAM FLIR by Hughes; phenomenology pioneered by Fred Simmons and A.T. Stair; U.S. Army's night vision lab formed (now Night Vision and Electronic Sensors Directorate (NVESD), and Rachets develops detection, recognition and identification modeling there;
• 1970: Willard Boyle & George E. Smith propose CCD at Bell Labs for picture phone;
• 1972: Common module program started by NVESD;
• 1978: Pommernig & ? Francis fabricate IRCCDs; US Common Module leads to a proliferation of IR Sensors in the U.S. military; commercial IR companies formed (Inframetrics in Boston, MA and FLIR Systems Inc. in Portland OR); Infrared imaging astronomy comes of age, observatories planned, IRTF on Mauna Kea opened; 32 by 32 and 64 by 64 arrays are produced in InSb, HgCdTe and other materials.

See also

commons infrared

• Atmospheric window
• Black body radiation
• Infrared astronomy
• Infrared camera
• Infrared filter
• Infrared homing
• Infrared photography
• Infrared signature
• Infrared spectroscopy
• Infrared thermometer
• Night vision
• pyrgeometer
• Terahertz radiation
• Thermographic camera
• Thermography
• RIAS (Remote Infrared Audible Signage)

References

External links

Journals

• Infrared Physics and Technology (Elsevier) (last access June 2005).

Web sites

• Infrared Spectroscopy NASA Open Spectrum wiki site
• Infrared Waves Detailed explanation of infrared light.
• U.S. Navy - Electronic Warfare and Radar Systems Engineering Handbook Source of transmittance diagram and further information on electro-optics.
• Infrared a Historical Perspective