Encyclopedia
Infrared 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" , red being the
color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750
nm and 1 mm.
Different regions in the infrared
The infrared band is often subdivided into smaller sections but the divisions are not precise, and are used differently by different authors. One such scheme is:
;near infrared : 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 SiO
2 glass medium.
;short wavelength IR : 1.4–3 µm, water absorption increases significantly at 1450 nm. The 1530 to 1560 nm range is the dominant spectral region for long-distance telecommunications.
;mid wavelength IR also intermediate-IR : 3–8 µm
;long wavelength IR : 8–15 µm
;far infrared : 15–1,000 µm
Another common scheme is:
- near: 0.75–5 µm
- mid: 5–30 µm
- long: 30–1,000 µm
A third scheme divides up the band based on the response of various detectors:
;Near IR : from 0.7 to 1.0 microns
;Short-wave infrared : 1.0 to 3 microns ;Mid-wave infrared : 3 to 5 microns
;Long-wave infrared : 8 to 12, or 7 to 14 microns: the atmospheric window
;Very-long wave infrared : 12 to about 30 microns, 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 and the newest follow technical reasons . Unfortunately the 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 longer frequencies make insignificant contributions to scenes illuminated by common light sources. But particularly intense light can be detected up to approximately 780 nm, and will be perceived as red light. The onset of infrared is defined at various values typically between 700 nm and 780 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 and detectors:
- O-band 1260–1360 nm
- E-band 1360–1460 nm
- S-band 1460–1530 nm
- C-band 1530–1565 nm
- L-band 1565–1625 nm
- U-band 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, a widespread misconception. Light and electromagnetic waves of any frequency will heat surfaces which absorb them. IR light from the sun only accounts for 50% of the heating of the Earth, the rest being caused by visible light. Green lasers can char paper, incandescently hot objects put out visible radiation, and ice cubes emit mostly microwaves. However, it is true that objects at room temperature will
emit radiation mostly concentrated in the 8-12 micron band . Unlike true heat, which exists only in matter, infrared radiation can propagate through a vacuum.
Applications
Night vision
Infrared is used in
night-vision equipment when there is insufficient
visible light to see an object. The radiation is detected and turned into an image on a screen, hotter objects showing up in different shades than cooler objects, enabling the
police and military to acquire warm targets, such as
human beings and
automobiles.
Also see Forward looking infrared. IR radiation is a secondary effect of heat; it is not heat itself. Heat itself is a measure of the translational energy of an amount of matter. "Thermal" detectors do not actually detect heat directly but the difference in IR radiation from objects. The device itself that attracts the radiation is known as a photocathode. Military gunnery ranges sometimes use special materials that reflect IR radiation to simulate enemy vehicles with running engines. The targets can be at the exact same temperature as the surrounding terrain, but they emit much more IR radiation. Different materials emit more or less IR radiation as temperature increases or decreases, depending on the composition of the material. Infrared imagery is usually formed as a result of the integrated inband intensity of the radiation, based on temperate and emissivity.
Simple infrared sensors were used by British, American and German forces in the
Second World War as night vision aids for
snipers.
Smoke is more transparent to infrared than to visible light, so
firefighters use infrared imaging equipment when working in smoke-filled areas.
Thermography
Infrared radiation can be used to remotely determine the temperature of objects . This is termed
thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography 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.
Other imaging
In
infrared photography, infrared filters are used to capture the near-infrared spectrum.
Digital cameras often use infrared blockers. Cheaper
digital cameras and some
camera phones which do not have appropriate filters can "see" near-infrared, appearing as a bright white colour . This is especially pronounced when taking pictures of subjects near IR-bright areas , where the resulting infrared interference can wash out the image. It is also worth mentioning '
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.
Heating
Infrared radiation is used in infrared saunas to heat the occupants, and to remove ice from the wings of
aircraft . 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 heats only opaque, absorbent objects and not the air around them, if there are no particles in it.
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 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 or 1,550 nm are the best choices for standard silica fibers.
Spectroscopy
Infrared radiation spectroscopy is the study of the composition of
organic compounds, finding out a compound's structure and composition based on the percentage transmittance of IR radiation through a sample. Different frequencies are absorbed by different stretches and bends in the
molecular bonds occurring inside the sample.
Carbon dioxide, for example, has a strong absorption band at 4.2 µm.
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.
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. Hershell 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 suprised at the result and called them "Calorific Rays". The term Infrared did not appear until late in the 18th century. Incidently, Hershell is buried in Westminster Abbey between Darwin and Newton.
Other important dates include Melloni makes first Thermopile IR detector;
1859: Gustov Kirchhoff
- Blackbody theorem ;
1873: Willoughboy Smith Photoconductivity of Selenium;
1879: Stephen Boltzman Law
- Stefan Empirical
1880s & 1890s: Lord Raleigh and Wilhelm Carl Werner Otto Fritz Franz Wein 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".
1900: Then along comes Planck and in 1900 published the blackbody equation and theorm. He solved the problem by quantizing the allowable engery transitions. This worked. It also lead to 20th century physics and quantum mechanics.
Early 1900s: Einstein develops the Photoelectric Effect, determining the "Photon". Also Coblentz-Spectroscopy and Radiometry
1917: Case Develops Thallous Sulfide Detector; British Develop First IRST in WW1 and Detect Aircraft at One Mile;
1935: Lead Salts—early missile guidance in
World War Two;
1938: Teau Ta—Predicted pyroelectric effect could be used to detect Infrared radiation.
1950's: Things really take off:
1952: H. Welker Discovers InSb;
1950's: Paul Kruse and Texas Instruments form Infrared Images Before 1955;
1950's and 1960's: Nomenclature & Radiometric Units defined by
- Fred Nicodemenus
- G.J. Zissis and
- R. Clark Jones D* and root Hertz;
1958: W.D. Larson Discovers IR detection properties of HgCdTe;
1958: Falcon & Sidewinder Missiles developed using infrared and the First Text on Infrared Sensors Appears by Paul Kruse, et al.
1962: J. Cooper Demonstrated Pyroelectric detection;
1962: Kruse & Rodat Advance HgCdTe;
Signal Element and Linear Arrays Available;
1965: First IR Handbook;
- First Commercial Imagers :Richard Hudson’s Landmark Text;
- F4 TRAM FLIR by Hughes;
- Phenomenology Pioneered by Fred Simmons and A.T Stair;
- Rachets develops detection, recognition and identification modeling at NVESD;
- US Army's night vision lab formed
1970: Boyle & Smith Propose CCD at Bell Labs for picture-phone;
1972: Common Module Program Started by NVESD;
1978: Pommernig & Francis Fabricate IRCCDs;
- Common Module Proliferates IR Sensors in Military;
- Commercial IR Companies Formed ;
- 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
Web sites
- NASA Open Spectrum wiki site.
- Organization that creates low cost infrared data interconnection standards.
- Detailed explanation of infrared light.
- [https://ewhdbks.mugu.navy.mil/ U.S. Navy - Electronic Warfare and Radar Systems Engineering Handbook] Source of transmittance diagram and further information on electro-optics.
