An alloy is a combination of metals and of a metal or another element. Alloys are defined by a metallic bonding character. An alloy may be a mixture of metallic phases. Intermetallic compounds are alloys with a defined crystal structure. Zintl phases are sometimes considered alloys depending on bond types. Alloys are used in a wide variety of applications. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, brass, duralumin and amalgams; the alloy constituents are measured by mass percentage for practical applications, in atomic fraction for basic science studies. Alloys are classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy, they can be heterogeneous or intermetallic. An alloy is a mixture of chemical elements, which forms an impure substance that retains the characteristics of a metal.
An alloy is distinct from an impure metal in that, with an alloy, the added elements are well controlled to produce desirable properties, while impure metals such as wrought iron are less controlled, but are considered useful. Alloys are made by mixing two or more elements, at least one of, a metal; this is called the primary metal or the base metal, the name of this metal may be the name of the alloy. The other constituents may or may not be metals but, when mixed with the molten base, they will be soluble and dissolve into the mixture; the mechanical properties of alloys will be quite different from those of its individual constituents. A metal, very soft, such as aluminium, can be altered by alloying it with another soft metal, such as copper. Although both metals are soft and ductile, the resulting aluminium alloy will have much greater strength. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength of an alloy called steel. Due to its very-high strength, but still substantial toughness, its ability to be altered by heat treatment, steel is one of the most useful and common alloys in modern use.
By adding chromium to steel, its resistance to corrosion can be enhanced, creating stainless steel, while adding silicon will alter its electrical characteristics, producing silicon steel. Like oil and water, a molten metal may not always mix with another element. For example, pure iron is completely insoluble with copper; when the constituents are soluble, each will have a saturation point, beyond which no more of the constituent can be added. Iron, for example, can hold a maximum of 6.67% carbon. Although the elements of an alloy must be soluble in the liquid state, they may not always be soluble in the solid state. If the metals remain soluble when solid, the alloy forms a solid solution, becoming a homogeneous structure consisting of identical crystals, called a phase. If as the mixture cools the constituents become insoluble, they may separate to form two or more different types of crystals, creating a heterogeneous microstructure of different phases, some with more of one constituent than the other phase has.
However, in other alloys, the insoluble elements may not separate until after crystallization occurs. If cooled quickly, they first crystallize as a homogeneous phase, but they are supersaturated with the secondary constituents; as time passes, the atoms of these supersaturated alloys can separate from the crystal lattice, becoming more stable, form a second phase that serve to reinforce the crystals internally. Some alloys, such as electrum, an alloy consisting of silver and gold, occur naturally. Meteorites are sometimes made of occurring alloys of iron and nickel, but are not native to the Earth. One of the first alloys made by humans was bronze, a mixture of the metals tin and copper. Bronze was an useful alloy to the ancients, because it is much stronger and harder than either of its components. Steel was another common alloy. However, in ancient times, it could only be created as an accidental byproduct from the heating of iron ore in fires during the manufacture of iron. Other ancient alloys include pewter and pig iron.
In the modern age, steel can be created in many forms. Carbon steel can be made by varying only the carbon content, producing soft alloys like mild steel or hard alloys like spring steel. Alloy steels can be made by adding other elements, such as chromium, vanadium or nickel, resulting in alloys such as high-speed steel or tool steel. Small amounts of manganese are alloyed with most modern steels because of its ability to remove unwanted impurities, like phosphorus and oxygen, which can have detrimental effects on the alloy. However, most alloys were not created until the 1900s, such as various aluminium, titanium and magnesium alloys; some modern superalloys, such as incoloy and hastelloy, may consist of a multitude of different elements. As a noun, the term alloy is used to describe a mixture of atoms in which the primary constituent is a metal; when used as a verb, the term refers to the act of mixing a metal with other elements. The primary metal is called the matrix, or the solvent; the secondary constituents are called s
A laser diode, injection laser diode, or diode laser is a semiconductor device similar to a light-emitting diode in which the laser beam is created at the diode's junction. Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated; this is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase and wavelength; the choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from infra-red to the UV spectrum. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning and light beam illumination.
A laser diode is electrically a PIN diode. The active region of the laser diode is in the intrinsic region, the carriers are pumped into that region from the N and P regions respectively. While initial diode laser research was conducted on simple P-N diodes, all modern lasers use the double-hetero-structure implementation, where the carriers and the photons are confined in order to maximize their chances for recombination and light generation. Unlike a regular diode, the goal for a laser diode is to recombine all carriers in the I region, produce light. Thus, laser diodes are fabricated using direct band-gap semiconductors; the laser diode epitaxial structure is grown using one of the crystal growth techniques starting from an N doped substrate, growing the I doped active layer, followed by the P doped cladding, a contact layer. The active layer most consists of quantum wells, which provide lower threshold current and higher efficiency. Laser diodes form a subset of the larger classification of semiconductor p-n junction diodes.
Forward electrical bias across the laser diode causes the two species of charge carrier – holes and electrons – to be "injected" from opposite sides of the p-n junction into the depletion region. Holes are injected from the p-doped, electrons from the n-doped, semiconductor. Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed "injection lasers," or "injection laser diode"; as diode lasers are semiconductor devices, they may be classified as semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers. Another method of powering some diode lasers is the use of optical pumping. Optically pumped semiconductor lasers use a III-V semiconductor chip as the gain medium, another laser as the pump source. OPSL offer several advantages over ILDs in wavelength selection and lack of interference from internal electrode structures. A further advantage of OPSLs is invariance of the beam parameters - divergence and pointing - as pump power is varied over a 10:1 output power ratio.
When an electron and a hole are present in the same region, they may recombine or "annihilate" producing a spontaneous emission — i.e. the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron's original state and hole's state. Spontaneous emission below the lasing threshold produces similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating; the difference between the photon-emitting semiconductor laser and a conventional phonon-emitting semiconductor junction diode lies in the type of semiconductor used, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors; the properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered "direct."
Other materials, the so-called compound semiconductors, have identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical "direct bandgap" property. Gallium arsenide, indium phosphide, gallium antimonide, gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light. In the absence of stimulated emission conditions and holes may coexist in proximity to one another, without recombining, for a certain time, termed the "upper-state lifetime" or "recombination time", before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission; this generates another photon of the same frequency and phase, travelling in the same direction as the first photon. This means that stimulated emission will cause gain in an optical wave (of the correct wavele
Red is the color at the end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of 625–740 nanometres, it is a primary color in the RGB color model and the CMYK color model, is the complementary color of cyan. Reds range from the brilliant yellow-tinged scarlet and vermillion to bluish-red crimson, vary in shade from the pale red pink to the dark red burgundy; the red sky at sunset results from Rayleigh scattering, while the red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. Iron oxide gives the red color to the planet Mars; the red colour of blood comes from protein hemoglobin, while ripe strawberries, red apples and reddish autumn leaves are colored by anthocyanins. Red pigment made from ochre was one of the first colors used in prehistoric art; the Ancient Egyptians and Mayans colored their faces red in ceremonies. It was an important color in China, where it was used to colour early pottery and the gates and walls of palaces.
In the Renaissance, the brilliant red costumes for the nobility and wealthy were dyed with kermes and cochineal. The 19th century brought the introduction of the first synthetic red dyes, which replaced the traditional dyes. Red became the color of revolution. Since red is the color of blood, it has been associated with sacrifice and courage. Modern surveys in Europe and the United States show red is the color most associated with heat, passion, anger and joy. In China and many other Asian countries it is the color of symbolizing happiness and good fortune. See below for shades of pink The human eye sees red when it looks at light with a wavelength between 625 and 740 nanometers, it is a primary color in the RGB color model and the light just past this range is called infrared, or below red, cannot be seen by human eyes, although it can be sensed as heat. In the language of optics, red is the color evoked by light that stimulates neither the S or the M cone cells of the retina, combined with a fading stimulation of the L cone cells.
Primates can distinguish the full range of the colors of the spectrum visible to humans, but many kinds of mammals, such as dogs and cattle, have dichromacy, which means they can see blues and yellows, but cannot distinguish red and green. Bulls, for instance, cannot see the red color of the cape of a bullfighter, but they are agitated by its movement.. One theory for why primates developed sensitivity to red is that it allowed ripe fruit to be distinguished from unripe fruit and inedible vegetation; this may have driven further adaptations by species taking advantage of this new ability, such as the emergence of red faces. Red light is used to help adapt night vision in low-light or night time, as the rod cells in the human eye are not sensitive to red. Red illumination was used as a safelight while working in a darkroom as it does not expose most photographic paper and some films. Today modern darkrooms use an amber safelight. On the color wheel long used by painters, in traditional color theory, red is one of the three primary colors, along with blue and yellow.
Painters in the Renaissance mixed red and blue to make violet: Cennino Cennini, in his 15th-century manual on painting, wrote, "If you want to make a lovely violet colour, take fine lac, ultramarine blue with a binder" he noted that it could be made by mixing blue indigo and red hematite. In modern color theory known as the RGB color model, red and blue are additive primary colors. Red and blue light combined together makes white light, these three colors, combined in different mixtures, can produce nearly any other color; this is the principle, used to make all of the colors on your computer screen and your television. For example, magenta on a computer screen is made by a similar formula to that used by Cennino Cennini in the Renaissance to make violet, but using additive colors and light instead of pigment: it is created by combining red and blue light at equal intensity on a black screen. Violet is made on a computer screen in a similar way, but with a greater amount of blue light and less red light.
So that the maximum number of colors can be reproduced on your computer screen, each color has been given a code number, or sRGB, which tells your computer the intensity of the red and blue components of that color. The intensity of each component is measured on a scale of zero to 255, which means the complete list includes 16,777,216 distinct colors and shades; the sRGB number of pure red, for example, is 255, 00, 00, which means the red component is at its maximum intensity, there is no green or blue. The sRGB number for crimson is 220, 20, 60, which means that the red is less intense and therefore darker, there is some green, which leans it toward orange; as a ray of white sunlight travels through the atmosphere to the eye, some of the colors are scattered out of the beam by air molecules and airborne particles due to Rayleigh scattering, changing the final color of the beam, seen. Colors with a shorter wavelength, such as blue and green, scatter more and are removed from the light that reaches the eye.
At sunrise and sunset, when the
The lattice constant, or lattice parameter, refers to the physical dimension of unit cells in a crystal lattice. Lattices in three dimensions have three lattice constants, referred to as a, b, c. However, in the special case of cubic crystal structures, all of the constants are equal referred to as a. In hexagonal crystal structures, the a and b constants are equal, we only refer to the a and c constants. A group of lattice constants could be referred to as lattice parameters. However, the full set of lattice parameters consist of the three lattice constants and the three angles between them. For example, the lattice constant for diamond is a = 3.57 Å at 300 K. The structure is equilateral although its actual shape cannot be determined from only the lattice constant. Furthermore, in real applications the average lattice constant is given. Near the crystal's surface, lattice constant is affected by the surface reconstruction that results in a deviation from its mean value; this deviation is important in nanocrystals since surface-to-nanocrystal core ratio is large.
As lattice constants have the dimension of length, their SI unit is the meter. Lattice constants are on the order of several ångströms. Lattice constants can be determined using techniques such as X-ray diffraction or with an atomic force microscope. Lattice constant of a crystal can be used as a natural length standard of nanometer range. In epitaxial growth, the lattice constant is a measure of the structural compatibility between different materials. Lattice constant matching is important for the growth of thin layers of materials on other materials; the volume of the unit cell can be calculated from angles. If the unit cell sides are represented as vectors the volume is the dot product of one vector with the cross product of the other two vectors; the volume is represented by the letter V. For the general unit cell V = a b c 1 + 2 cos α cos β cos γ − cos 2 α − cos 2 β − cos 2 γ. For monoclinic lattices with α = 90°, γ = 90°, this simplifies to V = a b c sin β. For orthorhombic and cubic lattices with β = 90° as well V = a b c.
Matching of lattice structures between two different semiconductor materials allows a region of band gap change to be formed in a material without introducing a change in crystal structure. This allows construction of diode lasers. For example, gallium arsenide, aluminium gallium arsenide, aluminium arsenide have equal lattice constants, making it possible to grow arbitrarily thick layers of one on the other one. Films of different materials grown on the previous film or substrate are chosen to match the lattice constant of the prior layer to minimize film stress. An alternative method is to grade the lattice constant from one value to another by a controlled altering of the alloy ratio during film growth; the beginning of the grading layer will have a ratio to match the underlying lattice and the alloy at the end of the layer growth will match the desired final lattice for the following layer to be deposited. The rate of change in the alloy must be determined by weighing the penalty of layer strain, hence defect density, against the cost of the time in the epitaxy tool.
For example, indium gallium phosphide layers with a band gap above 1.9 eV can be grown on gallium arsenide wafers with index grading
Metalorganic vapour-phase epitaxy
Metalorganic vapour-phase epitaxy known as organometallic vapour-phase epitaxy or metalorganic chemical vapour deposition, is a chemical vapour deposition method used to produce single- or polycrystalline thin films. It is a complex process for growing crystalline layers to create complex semiconductor multilayer structures. In contrast to molecular-beam epitaxy, the growth of crystals is by chemical reaction and not physical deposition; this takes place not from the gas phase at moderate pressures. As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, it has become a major process in the manufacture of optoelectronics, it was invented in 1968 by Harold M. Manasevit. In MOCVD ultrapure gases are injected into a reactor and finely dosed to deposit a thin layer of atoms onto a semiconductor wafer. Surface reaction of organic compounds or metalorganics and hydrides containing the required chemical elements creates conditions for crystalline growth – epitaxy of materials and compound semiconductors.
Unlike traditional silicon semiconductors, these semiconductors may contain combinations of group III and group V, group II and group VI, group IV, or group IV, V and VI elements. For example, indium phosphide could be grown in a reactor on a heated substrate by introducing trimethylindium and phosphine in a first step; the heated organic precursor molecules decompose in the absence of oxygen. Pyrolysis leaves the atoms on the substrate surface in the second step; the atoms bond to the substrate surface, a new crystalline layer is grown in the last step. Formation of this epitaxial layer occurs at the substrate surface. Required pyrolysis temperature increases with increasing chemical bond strength of the precursor; the more carbon atoms are attached to the central metal atom, the weaker the bond. The diffusion of atoms on the substrate surface is affected by atomic steps on the surface; the vapor pressure of the metal organic source is an important consideration in MOCVD, since it determines the concentration of the source material in the reaction and the deposition rate.
In the metal organic chemical vapor deposition technique, reactant gases are combined at elevated temperatures in the reactor to cause a chemical interaction, resulting in the deposition of materials on the substrate. A reactor is a chamber made of a material, it must withstand high temperatures. This chamber is composed by reactor walls, liner, a susceptor, gas injection units, temperature control units; the reactor walls are made from stainless steel or quartz. Ceramic or special glasses, such as quartz, are used as the liner in the reactor chamber between the reactor wall and the susceptor. To prevent overheating, cooling water must be flowing through the channels within the reactor walls. A substrate sits on a susceptor, at a controlled temperature; the susceptor is made from a material resistant to the metalorganic compounds used. For growing nitrides and related materials, a special coating of silicon nitride, on the graphite susceptor is necessary to prevent corrosion by ammonia gas. One type of reactor used to carry out MOCVD is a cold-wall reactor.
In a cold-wall reactor, the substrate is supported by a pedestal, which acts as a susceptor. The pedestal/susceptor is the primary origin of heat energy in the reaction chamber. Only the susceptor is heated, so gases do not react before they reach the hot wafer surface; the pedestal/susceptor is made of a radiation-absorbing material such as carbon. In contrast, the walls of the reaction chamber in a cold-wall reactor are made of quartz, transparent to the electromagnetic radiation; the reaction chamber walls in a cold-wall reactor, may be indirectly heated by heat radiating from the hot pedestal/susceptor, but will remain cooler than the pedestal/susceptor and the substrate the pedestal/susceptor supports. In hot-wall CVD, the entire chamber is heated; this may be necessary for some gases to be pre-cracked before reaching the wafer surface to allow them to stick to the wafer. Gas is introduced via devices known as'bubblers'. In a bubbler a carrier gas is bubbled through the metalorganic liquid, which picks up some metalorganic vapour and transports it to the reactor.
The amount of metalorganic vapour transported depends on the rate of carrier gas flow and the bubbler temperature, is controlled automatically and most by using an ultrasonic concentration measuring feedback gas control system. Allowance must be made for saturated vapors. Gas exhaust and cleaning system. Toxic waste products must be converted to solid wastes for recycling or disposal. Ideally processes will be designed to minimize the production of waste products. Aluminium Trimethylaluminium, Liquid Triethylaluminium, Liquid Gallium Trimethylgallium, Liquid Triethylgallium, Liquid Indium Trimethylindium, Solid Triethylindium, Liquid Di-isopropylmethylindium, Liquid Ethyldimethylindium, Liquid Germanium Isobutylgermane, Liquid Dimethylamino germanium trichloride, Liquid Tetramethylgermane, Liquid Tetraethylgermanium, Liquid Nitrogen Phenyl hydrazine, Liquid Dimethylhydrazine, Liquid Tertiarybutylamine, Liquid Ammonia NH3, Gas Phosphorus Phosphine PH3, Gas Tertiarybutyl phosphine, Liquid Bisphosphinoethane, Liquid Arsenic Arsine AsH3, Gas Tertiarybutyl arsine, Liquid
Gallium arsenide is a compound of the elements gallium and arsenic. It is a III-V direct bandgap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circuits, infrared light-emitting diodes, laser diodes, solar cells and optical windows. GaAs is used as a substrate material for the epitaxial growth of other III-V semiconductors including indium gallium arsenide, aluminum gallium arsenide and others. In the compound, gallium has a +3 oxidation state. Gallium arsenide single crystals can be prepared by three industrial processes: The vertical gradient freeze process. Crystal growth using a horizontal zone furnace in the Bridgman-Stockbarger technique, in which gallium and arsenic vapors react, free molecules deposit on a seed crystal at the cooler end of the furnace. Liquid encapsulated Czochralski growth is used for producing high-purity single crystals that can exhibit semi-insulating characteristics.
Alternative methods for producing films of GaAs include: VPE reaction of gaseous gallium metal and arsenic trichloride: 2 Ga + 2 AsCl3 → 2 GaAs + 3 Cl2 MOCVD reaction of trimethylgallium and arsine: Ga3 + AsH3 → GaAs + 3 CH4 Molecular beam epitaxy of gallium and arsenic: 4 Ga + As4 → 4 GaAs or 2 Ga + As2 → 2 GaAsOxidation of GaAs occurs in air and degrades performance of the semiconductor. The surface can be passivated by depositing a cubic gallium sulfide layer using a tert-butyl gallium sulfide compound such as 7. If a GaAs boule is grown with excess arsenic present, it gets certain defects, in particular arsenic antisite defects; the electronic properties of these defects cause the Fermi level to be pinned to near the center of the bandgap, so that this GaAs crystal has low concentration of electrons and holes. This low carrier concentration is similar to an intrinsic crystal, but much easier to achieve in practice; these crystals are called reflecting their high resistivity of 107 -- 109 Ω · cm.
Wet etching of GaAs industrially uses an oxidizing agent such as hydrogen peroxide or bromine water, the same strategy has been described in a patent relating to processing scrap components containing GaAs where the Ga3+ is complexed with a hydroxamic acid, for example: GaAs + H2O2 + "HA" → "GaA" complex + H3AsO4 + 4 H2OThis reaction produces arsenic acid. GaAs can be used for various transistor types: MESFET HEMT JFET Heterojunction bipolar transistor The HBT can be used in integrated injection logic; the earliest GaAs logic gate used Buffered FET Logic. From ~1975 to 1995 the main logic families used were: Source-coupled FET logic fastest and most complex, Capacitor–diode FET logic Direct-coupled FET logic simplest and lowest power Some electronic properties of gallium arsenide are superior to those of silicon, it has a higher saturated electron velocity and higher electron mobility, allowing gallium arsenide transistors to function at frequencies in excess of 250 GHz. GaAs devices are insensitive to overheating, owing to their wider energy bandgap, they tend to create less noise in electronic circuits than silicon devices at high frequencies.
This is a result of lower resistive device parasitics. These superior properties are compelling reasons to use GaAs circuitry in mobile phones, satellite communications, microwave point-to-point links and higher frequency radar systems, it is used in the manufacture of Gunn diodes for the generation of microwaves. Another advantage of GaAs is that it has a direct band gap, which means that it can be used to absorb and emit light efficiently. Silicon has an indirect bandgap and so is poor at emitting light; as a wide direct band gap material with resulting resistance to radiation damage, GaAs is an excellent material for outer space electronics and optical windows in high power applications. Because of its wide bandgap, pure GaAs is resistive. Combined with a high dielectric constant, this property makes GaAs a good substrate for Integrated circuits and unlike Si provides natural isolation between devices and circuits; this has made it an ideal material for monolithic microwave integrated circuits, MMICs, where active and essential passive components can be produced on a single slice of GaAs.
One of the first GaAs microprocessors was developed in the early 1980s by the RCA corporation and was considered for the Star Wars program of the United States Department of Defense. These processors were several times faster and several orders of magnitude more radiation proof than silicon counterparts, but were more expensive. Other GaAs processors were implemented by the supercomputer vendors Cray Computer Corporation and Alliant in an attempt to stay ahead of the ever-improving CMOS microprocessor. Cray built one GaAs-based machine in the early 1990s, the Cray-3, but the effort was not adequately capitalized, the company filed for bankruptcy in 1995. Complex layered structures of gallium arsenide in combination with aluminium arsenide or the alloy AlxGa1−xAs can be grown using molecular beam epitaxy or using metalorganic vapor phase epitaxy; because GaAs and AlAs have the same lattice constant, the layers have little induced strain, which allows them to be g
Infrared radiation, sometimes called infrared light, is electromagnetic radiation with longer wavelengths than those of visible light, is therefore invisible to the human eye, although IR at wavelengths up to 1050 nanometers s from specially pulsed lasers can be seen by humans under certain conditions. IR wavelengths extend from the nominal red edge of the visible spectrum at 700 nanometers, to 1 millimeter. Most of the thermal radiation emitted by objects near room temperature is infrared; as with all EMR, IR carries radiant energy and behaves both like a wave and like its quantum particle, the photon. Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. More than half of the total energy from the Sun was found to arrive on Earth in the form of infrared; the balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines transmission of photons in the infrared range. Infrared radiation is used in industrial, military, law enforcement, medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, detect objects such as planets, to view red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to detect overheating of electrical apparatus. Extensive uses for military and civilian applications include target acquisition, night vision and tracking.
Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication and weather forecasting. Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 millimeter; this range of wavelengths corresponds to a frequency range of 430 THz down to 300 GHz. Below infrared is the microwave portion of the electromagnetic spectrum. Sunlight, at an effective temperature of 5,780 kelvins, is composed of near-thermal-spectrum radiation, more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, 32 watts is ultraviolet radiation. Nearly all the infrared radiation in sunlight is shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight.
However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, fires produce far more infrared than visible-light energy. In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors collect radiation only within a specific bandwidth. Thermal infrared radiation has a maximum emission wavelength, inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. Therefore, the infrared band is subdivided into smaller sections. A used sub-division scheme is: NIR and SWIR is sometimes called "reflected infrared", whereas 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 appear brighter in the MW compared to the same object viewed in the LW.
The International Commission on Illumination recommended the division of infrared radiation into the following three bands: ISO 20473 specifies the following scheme: Astronomers divide the infrared spectrum as follows: These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, hence different environments in space; the most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used. These letters are understood in reference to atmospheric windows and appear, for instance, in the titles of many papers. A third scheme divides up the band based on the response of various detectors: Near-infrared: from 0.7 to 1.0 µm. Short-wave infrared: 1.0 to 3 µm. InGaAs covers to about 1.8 µm. Mid-wave infrared: 3 to 5 µm (defined by the atmospheric window and covered by indium antimonide and mercury cadmium telluride and by lead