Infrared (
IR) radiation is
electromagnetic radiation of a
wavelength longer than that of
visible light, but shorter than that of
radio waves. 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 five
orders of magnitude.
Infrared imaging is used extensively for both military and civilian purposes. Military applications include
target acquisition, surveillance, 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
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.
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:
[{{cite web]
| last=Henderson
| first=Roy
| url=http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm
| title=Wavelength Considerations
| publisher=Instituts für Umform- und Hochleistungs
| accessdate=2007-10-18
}}
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 this is the 'heat seeking' region in which the homing heads of passive IR homing 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:
[{{cite web]
| author=IPAC Staff
| url = http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html
| title = Near, Mid and Far-Infrared
| publisher = NASA ipac
| accessdate = 2007-04-04
}}
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:
[Miller, ''Principles of Infrared Technology'' (Van Nostrand Reinhold, 1992), and Miller and Friedman, ''Photonic Rules of Thumb'', 2004.]
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 InSb and HgCdTe and partially 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
http://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:
|-
!Band
!Descriptor
!Wavelength range
|-
|O band
|Original
|1260–1360 nm
|-
|E band
|Extended
|1360–1460 nm
|-
|S band
|Short wavelength
|1460–1530 nm
|-
|C band
|Conventional
|1530–1565 nm
|-
|L band
|Long wavelength
|1565–1625 nm
|-
|U band
|Ultralong wavelength
|1625–1675 nm
|
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, 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).
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
blackbody. 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 are made of polysulphone plastic that blocks over 99% of the visible light spectrum from any “white” light source. 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 is used in night vision equipment when there is insufficient
visible light to see.
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.
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.
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
Thermography
Infrared thermography is a non-contact, non-destructive test method that utilizes a thermal imager to detect, display and record thermal patterns and temperatures across the surface of an object. Infrared thermography may be applied to any situation where knowledge of thermal profiles and temperatures will provide meaningful data about a system, object or process. Thermography is widely used in industry for
predictive maintenance, condition assessment,
quality assurance, and
forensic investigations of electrical, mechanical and structural systems. Other applications include, but are not limited to:
law enforcement,
firefighting,
search and rescue, and
medical and
veterinary sciences.
Aside from test equipment, training is the most important investment a company will make in an infrared inspection program. Advances in technology have provided infrared equipment that is user-friendly; however, infrared thermography is not a simply "
point and shoot" technology. In addition to understanding the object or system being inspected, thermographers must also understand common error sources that can influence observed thermal data. Typically, infrared training courses should cover the topics of infrared theory, heat transfer concepts, equipment selection and operation, how to eliminate or overcome common error sources, and specific applications. Training courses from independent training companies are preferred since they are not biased toward a single brand or type of equipment.
Other imaging
.
Tracking
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
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). It is also gaining popularity as a method of heating asphalt pavements in place during new construction or in repair of damaged asphalt. 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.
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. CH
2) 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 3200cm
-1).
Meteorology
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 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.
Art history and Archaeology
Infra-red (as art historians call them) reflectograms are taken of paintings to reveal underlying layers, in particular the
underdrawing or outline drawn to 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-enthusiatic 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.
http://www.clevelandart.org/exhibcef/ConsExhib/html/grien.html
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.
[National Gallery Catalogues: The Fifteenth Century Netherlandish Paintings by Lorne Campbell, 1998, ISBN 185709171]
Similar uses of infrared are made by archaeologists 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.
[International Dunhuang Project An Introduction to digital infrared photography and its application within IDP -paper pdf 6.4 MB ] Carbon black used in ink can show up extremely well.
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[{{cite web | title = Global Sources of Greenhouse Gases | work = Emissions of Greenhouse Gases in the United States 2000 | publisher = Energy Information Administration]
| date = 2002-05-02 | url = http://www.eia.doe.gov/oiaf/1605/gg01rpt/emission.html | accessdate = 2007-08-13 }}, 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.
[{{cite web | title = Clouds & Radiation]
| url = http://earthobservatory.nasa.gov/Library/Clouds/ | accessdate = 2007-08-12 }}
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 UK Royal Society. 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:
External links