Infrared

Image of two people in mid-infrared ("thermal") light (false-color)

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 radiate chiefly at wavelengths around 10 micrometres.^[1]

Overview

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.^[2]

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.^[3]

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:^[4]

• 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:^[5]

• 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.

Plot of atmospheric transmittance in part of the infrared region.

A third scheme divides up the band based on the response of various detectors:^[6]

• 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 gels[1]) 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:^[7]

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

Main article: Thermal radiation

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%^[8] 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).^[9]

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

Main article: Night vision

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

Main article: Night vision

Active-infrared night vision. Despite a dark back-lit scene, active-infrared night vision delivers identifying details, as seen on the display monitor.

Infrared is used in night vision equipment when there is insufficient visible light to see.^[10] Night vision devices operate through a process involving the conversion of ambient light photons into electrons which are then amplified by a chemical and electrical process and then converted back into visible light.^[10] Infrared light sources can be used to augment the available ambient light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source.^[10]

The use of infrared light and night vision devices should not be confused with thermal imaging which creates images based on differences in surface temperature by detecting infrared radiation (heat) that emanates from objects and their surrounding environment^[11]

Thermography

Main article: Thermography

A thermographic image of a dog

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

Infrared light from the LED of an XBox 360 remote control as seen by a digital camera.

In infrared photography, infrared filters are used to capture the near-infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and camera phones have less effective filters and can "see" intense near-infrared, appearing as a bright purple-white color. This is especially pronounced when taking pictures of subjects near IR-bright areas (such as near a lamp), where the resulting infrared interference can wash out the image. There is also a technique called 'T-ray' imaging, which is imaging using far infrared or terahertz radiation. Lack of bright sources makes terahertz photography technically more challenging than most other infrared imaging techniques. Recently T-ray imaging has been of considerable interest due to a number of new developments such as terahertz time-domain spectroscopy.

Tracking

Main article: Infrared homing

Infrared tracking, also known as infrared homing, refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infra-red wavelengths of light compared to objects in the background.

Heating

Main article: Infrared heating

Infrared radiation can be used as a deliberate heating source. For example it is used in infrared saunas to heat the occupants, and also to remove ice from the wings of aircraft (de-icing). FIR is also gaining popularity as a safe method of natural health care & physiotherapy. Far infrared thermomedic therapy garments use thermal technology to provide compressive support and healing warmth to assist symptom control for arthritis, injury & pain. Infrared can be used in cooking and heating food as it predominantly heats the opaque, absorbent objects, rather than the air around them.

Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, print drying. In these applications, infrared heaters replace convection ovens and contact heating. Efficiency is achieved by matching the wavelength of the infrared heater to the absorption characteristics of the material.

Communications

IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms. Infrared is the most common way for remote controls to command appliances.

Free space optical communication using infrared lasers can be a relatively inexpensive way to install a communications link in an urban area operating at up to 4 gigabit/s, compared to the cost of burying fiber optic cable.

Infrared lasers are used to provide the light for optical fiber communications systems. Infrared light with a wavelength around 1,330 nm (least dispersion) or 1,550 nm (best transmission) are the best choices for standard silica fibers.

IR data transmission of encoded audio versions of printed signs is being researched as an aid for visually impaired people through the RIAS (Remote Infrared Audible Signage) project.

Spectroscopy

Infrared vibrational spectroscopy (see also near infrared spectroscopy) is a technique which can be used to identify molecules by analysis of their constituent bonds. Each chemical bond in a molecule vibrates at a frequency which is characteristic of that bond. A group of atoms in a molecule (e.g. CH₂) may have multiple modes of oscillation caused by the stretching and bending motions of the group as a whole. If an oscillation leads to a change in dipole in the molecule, then it will absorb a photon which has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light. Typically, the technique is used to study organic compounds using light radiation from 4000-400 cm^-1, the mid-infrared. A spectrum of all the frequencies of absorption in a sample is recorded. This can be used to gain information about the sample composition in terms of chemical groups present and also its purity (for example a wet sample will show a broad O-H absorption around 3200 cm^-1).

Meteorology

IR Satellite picture taken 1315 Z on 15th October 2006. A frontal system can be seen in the Gulf of Mexico with embedded Cumulonimbus cloud. Shallower Cumulus and Stratocumulus can be seen off the Eastern Seaboard.

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

The Spitzer Space Telescope is a dedicated infrared space observatory currently in orbit around the Sun. (Note the black side to the telescope, to maximize infrared radiation.) NASA image.

Main articles: infrared astronomy and far infrared 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.^[2]

Art history

The Arnolfini Portrait by Jan van Eyck, National Gallery, London

Infra-red (as art historians call them) reflectogramsare taken of paintings to reveal underlying layers, in particular the underdrawing or outline drawn by the artist as a guide. This often uses carbon black which shows up well in reflectograms, so long as it has not also been used in the ground underlying the whole painting. Art historians are looking to see if the visible layers of paint differ from the under-drawing or layers in between - such alterations are called pentimenti when made by the original artist. This is very useful information in deciding whether a painting is the prime version by the original artist or a copy, and whether it has been altered by over-enthusiastic restoration work. Generally the more pentimenti, the more likely a painting is to be the prime version. It also gives useful insights into working practices. [2]

Among many other changes in the Arnolfini Portrait of 1434 (right), his face was higher by about the height of his eye, hers was higher, and her eyes looked more to the front. Each of his feet was underdrawn in one position, painted in another, and then overpainted in a third. These alterations are seen in infra-red reflectograms.^[12]

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.^[13] Carbon black used in ink can show up extremely well.

Biological systems

Thermographic image of a snake eating a mouse

Thermographic image of a fruit bat.

Pitviper have a pair of infrared sensory pits on its head. There is uncertainty regarding the exact thermal sensitivity of this biological infrared detection system.^[14][15]

Other organisms that have thermoreceptive organs are pythons (family Pythonidae), some boas (family Boidae), the Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata),^[16] darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamantus plateni), and possibly blood-sucking bugs (Triatoma infestans).^[17]

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.^[18] Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.^[19]

Health hazard

Strong infrared radiation in certain industry high heat settings may constitute a health hazard to the eyes and the vision. More so, since the radiation is invisible. Therefore special IR proof protective eyeglasses must be worn in such places.

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,^[20] 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.^[21]

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.^[22]

Other important dates include:^[6]

• 1835: Macedonio Melloni makes the first thermopile IR detector;
• 1860: Gustav Kirchhoff formulates the blackbody theorem ;
• 1873: Willoughby Smith discovers the photoconductivity of selenium;
• 1879: Stefan-Boltzmann law formulated empirically
• 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 (1.6 km);
• 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 {now part of FLIR Systems Inc.}; 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

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