Infrared was discovered in 1800 by William Herschel, an English astronomer of German origin. Herschel placed a mercury thermometer "Mercury (element)") in the spectrum obtained by a glass prism in order to measure the heat emitted by each color. He discovered that the heat was strongest on the red side of the spectrum and observed that there was no light there. This is the first experience to show that heat can be transmitted by an invisible form of light. Herschel called this radiation "caloric rays", a name quite popular throughout the century that eventually gave way to the more modern infrared radiation.

The first infrared radiation detectors were bolometers, instruments that capture radiation due to the increase in temperature produced in an absorbing detector.

Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nm to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz&action=edit&redlink=1 "Terahertz (unit) (not yet redacted)") to 300 GHz. Below the infrared is the microwave portion of the electromagnetic spectrum.

Sunlight, at an effective temperature of 5780 K, (5510 °C, 9940 °F), is composed of near-thermal spectrum radiation that is slightly above the mid-infrared. At the zenith, sunlight provides an irradiance of just over 1 kW per square meter at sea level. Of this energy, 527 W are infrared radiation, 445 W are visible light, and 32 W are ultraviolet radiation.[8] Almost all infrared radiation from sunlight is near infrared, less than 4 μm.

On the Earth's surface, at temperatures much lower than those of the Sun's surface, part of the thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. However, blackbody, or thermal, radiation is continuous: it emits radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce much more infrared energy than visible light.[9]​.

Infrared is classified, according to its wavelength, in this way:[10]​.

• - Near infrared 0.7 to 1.0 μm (from the approximate end of the human eye's response to that of silicon).

• - Shortwave infrared: 1.0 to 3 μm (from the silicon cutoff to the MWIR atmospheric window). InGaAs covers up to approximately 1.8 µm; less sensitive lead salts cover this region.

• - Mid-infrared 3 to 5 μm (defined by the atmospheric window and covered by indium antimonide [InSb] and mercury cadmium telluride [HgCdTe] and partially by lead selenide PbSe")).

• - Long wave infrared: 8 to 12 or 7 to 14 μm (this is the atmospheric window covered by HgCdTe and microbolometers).

• - Very long wave infrared (VLWIR) (12 to approximately 30 μm, covered by doped silicon).

• - Far infrared (from 50 µm to 1000 µm).

The near infrared is the region closest in wavelength to the radiation detectable by the human eye. The mid and far infrared progressively move away from the visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs bands, water absorption) and newer ones follow technical reasons (common silicon detectors are sensitive at about 1050 nm, while the sensitivity of InGaAs starts around 950 nm and ends between 1700 and 2600 nm, depending on the specific configuration). There are currently no international standards for these specifications.

Infrared occurrence is defined (according to different standards) at various values, typically between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is noticeably less sensitive to light above 700 nm wavelength, so longer wavelengths make negligible contributions to scenes illuminated by common light sources. However, particularly intense near-infrared light (e.g., from IR lasers, IR LED sources, or from bright daylight with visible light removed by colored gels) can be detected up to about 780 nm and will be perceived as red light. Intense light sources providing wavelengths up to 1050 nm can be seen as a dull red glow, causing some difficulty in near-infrared illumination of dark scenes (this practical problem is usually solved by indirect illumination). Leaves are particularly bright in the near infrared, and if all visible light leaks around an infrared filter are blocked and the eye has a moment to adjust to the extremely dim image that comes from a visually opaque infrared pass photographic filter, it is possible to see the Wood effect which consists of infrared glowing foliage.

Matter, due to its energetic characterization (see black body), emits thermal radiation. In general, the wavelength where a body emits the maximum radiation is inversely proportional to its temperature (Wien's Law). In this way, most objects at everyday temperatures have their maximum emission in the infrared. Living beings, especially mammals, emit a large proportion of radiation in the infrared part of the spectrum, due to their body heat.

The power emitted in the form of heat by a human body, for example, can be obtained from the surface of its skin (about 2 m) and its body temperature (about 37 °C, i.e. 310 K), by means of the Stefan-Boltzmann Law, and turns out to be around 100 W.[11]​.

This is closely related to the so-called "thermal sensation", according to which we can feel cold or hot regardless of the environmental temperature, depending on the radiation we receive (for example from the Sun or other nearby hot bodies): If we receive more than the 100 W that we emit, we will be hot, and if we receive less, we will be cold. In both cases the temperature of our body is constant (37 °C) and that of the air around us as well. Therefore, the thermal sensation in still air only has to do with the amount of radiation (usually infrared) that we receive and its balance with that which we constantly emit as hot bodies that we are. If, on the other hand, there is wind, the layer of air in contact with our skin can be replaced by air at a different temperature, which also alters the thermal balance and modifies the thermal sensation.

Contenido

En general, los objetos emiten radiación infrarroja en todo un espectro de longitudes de onda, pero a veces sólo interesa una región limitada del espectro porque los sensores suelen recoger la radiación sólo dentro de un ancho de banda específico. La radiación infrarroja térmica también tiene una longitud de onda de emisión máxima, que es inversamente proporcional a la temperatura absoluta del objeto, de acuerdo con la ley de desplazamiento de Wien. La banda de infrarrojos suele subdividirse en secciones más pequeñas, aunque la forma en que se divide el espectro de infrarrojos varía según las distintas áreas en las que se emplea el infrarrojo.

Visible limit

Infrared, as its name implies, is generally considered to begin with wavelengths longer than those visible to the human eye. However, there is no "hard" wavelength limit to what is visible, as the eye's sensitivity decreases rapidly, but gently, for wavelengths greater than about 700 nm. Therefore, longer wavelengths can be seen if they are bright enough, although they may still be classified as infrared according to common definitions. Thus, the light from a near-infrared laser may appear dim red and may pose a danger as it can be quite bright. And even infrared with wavelengths up to 1050 nm from pulsed lasers can be seen by humans under certain conditions.[12][13][14][15]​.

Commonly used subdivision scheme

A commonly used subdivision scheme is:[16]​.

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

CIE division scheme

The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:[20]​.

ISO 20473 Scheme

ISO 20473 specifies the following scheme:[21]​.

Outline of the astronomy division

Astronomers usually divide the infrared spectrum as follows:[22]​.

These divisions are not precise and may vary by publication. The three regions are used for the observation of different temperature ranges, and therefore different environments in space.

The most common photometric system used in astronomy assigns capital letters to different spectral regions depending on the filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N and Q refer to the mid-infrared region. These letters are commonly understood to refer to "atmospheric windows") and appear, for example, in the titles of many academic articles.

Division scheme according to sensor response

A third scheme divides the band based on the response of several detectors:[10]​.

• - Near infrared: 0.7 to 1.0 μm (from the approximate end of the human eye's response to that of silicon).

• - Shortwave infrared: From 1.0 to 3 μm (from the silicon cut to the MWIR atmospheric window). InGaAs covers up to approximately 1.8 μm; less sensitive lead salts cover this region.

• - Medium wave infrared: 3 to 5 μm (defined by the atmospheric window and covered by indium antimonide [InSb] and cadmium and mercury telluride") [HgCdTe] and partially by lead selenide [PbSe]).

• - Long wave infrared: 8 to 12, or 7 to 14 μm (it is the atmospheric window covered by HgCdTe and microbolometer).

• - Very long wave infrared (VLWIR) (approximately 12 to 30 μm, covered by doped silicon).

The near infrared is the region closest in wavelength to the radiation detectable by the human eye. The mid-infrared and far-infrared progressively move away from the visible spectrum. Other definitions obey different physical mechanisms (emission peaks, versus bands, water absorption) and the most recent ones obey technical reasons (common silicon detectors are sensitive up to about 1050 nm, while the sensitivity of InGaAs begins around 950 nm and ends between 1700 and 2600 nm, depending on the specific configuration). There are currently no international standards for these specifications.

The onset of infrared is defined (according to different standards) at various values, usually between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is noticeably less sensitive to light above the 700 nm wavelength, so longer wavelengths contribute insignificantly to scenes illuminated by common light sources. However, particularly intense near-infrared light (e.g., that from IR lasers, IR LED sources, or bright daylight with visible light removed by colored gels) can be detected up to about 780 nm, and will be perceived as red light. Intense light sources providing wavelengths up to 1050 nm can be seen as a dull red glow, causing some difficulty in illuminating near-infrared scenes in the dark (this practical problem is usually solved by indirect illumination). Leaves are particularly bright in the near IR, and if all visible light leaks around an IR filter are blocked, and the eye is given a moment to adjust to the extremely dim image coming through a visually opaque photographic filter passing through the IR, it is possible to see the Wood effect of foliage glowing in the IR.[23]​.

Infrared telecommunication bands

In optical communications, the part of the infrared spectrum used is divided into seven bands based on the availability of light sources that transmit/absorb materials (fibers) and detectors:[24]​.

“C-band” is the dominant band for long-distance telecommunications networks. The S and L bands are based on less established technology and are not as widespread.

Infrared radiation is popularly known as "thermal radiation",[25]​ but light and electromagnetic waves of any frequency heat the surfaces that absorb them. Infrared light from the Sun accounts for 49%[26]​ of Earth's warming, with the remainder caused by visible light being absorbed and then re-radiated at longer wavelengths. The visible or ultraviolet light emitted by the laser can carbonize paper, and incandescent objects emit visible radiation. Objects at room temperature will emit spontaneous emission of concentrated radiation especially in the 8 to 25 μm band, but this is no different from the emission of visible light by incandescent objects and ultraviolet light by even hotter objects (see black body and Wien's displacement law).[27]​.

Heat is the energy in transit that flows due to a temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, thermal radiation can propagate through a vacuum. Thermal radiation is characterized by a particular spectrum of many wavelengths that are associated with the emission of an object, due to the vibration of its molecules at a certain temperature. Thermal radiation can be emitted by objects at any wavelength, and at very high temperatures said radiation is associated with spectra much higher than the infrared, extending to the visible, ultraviolet and even X-ray regions (for example, the solar corona). Therefore, the popular association of infrared radiation with thermal radiation is just a coincidence based on the typical (comparatively low) temperatures usually found near the surface of planet Earth.

The concept of emissivity is important to understand infrared emissions from objects. This is a property of a surface that describes how its thermal emissions deviate from the idea of ​​a black body. To explain it better, two objects at the same physical temperature may not show the same infrared image if they have different emissivity. For example, for any preset emissivity value, objects with higher emissivity will appear hotter, and those with lower emissivity will appear colder (assuming, as is often the case, that the surrounding environment is colder than the objects being viewed). When an object does not have perfect emissivity, it acquires properties of reflectivity and/or transparency, so the temperature of the environment is partially reflected and/or transmitted through the object. If the object were in a hotter environment, then a lower emissivity object at the same temperature would probably appear hotter than a more emissive one. For this reason, incorrect selection of emissivity and failure to consider ambient temperatures will give inaccurate results when using infrared cameras and pyrometers.

Los infrarrojos se utilizan en los equipos de visión nocturna cuando la cantidad de luz visible es insuficiente para ver los objetos. La radiación se recibe y después se refleja en una pantalla. Los objetos más calientes se convierten en los más luminosos.

Un uso muy común es el que hacen los mandos a distancia (o telecomandos) que generalmente utilizan los infrarrojos en vez de ondas de radio ya que no interfieren con otras señales como las señales de televisión. Los infrarrojos también se utilizan para comunicar a corta distancia los ordenadores con sus periféricos "Periférico (informática)"). Los aparatos que utilizan este tipo de comunicación cumplen generalmente un estándar publicado por la Infrared Data Association.[28]​.

La luz utilizada en las fibras ópticas es generalmente de infrarrojos.

Near Infrared

The near infrared is the shortest wavelength region of the infrared spectrum, located between visible light and the mid-infrared, approximately between 800 and 2500 nm, although there is no universally accepted definition.

Astronomy

In astronomy, near-infrared spectroscopy is used to study the atmospheres of cold stars. In this range, lines of rotational and vibrational transitions of molecules such as titanium oxide "Titanium (IV) oxide"), cyanogen and carbon monoxide can be observed, which give information about the spectral type of the star. It is also used to study molecules in other astronomical objects, such as molecular clouds.

Industrial infrared emitters

Another of the many applications of infrared radiation is the use of infrared emitting equipment in the industrial sector. In this sector, the applications occupy an extensive list but its use can be highlighted in applications such as drying of paints or varnishes, drying of paper, thermosetting of plastics, preheating of welds, curvature, tempering and laminating of glass, among others. The irradiation on the material in question can be prolonged or momentary taking into account aspects such as the distance of the emitters from the material, the speed of passage of the material (in the case of production lines) and the temperature to be achieved.

Generally, when talking about infrared emitting equipment, four types are distinguished depending on the wavelength they use:

1. • Shortwave infrared emitters.

2. • Fast medium wave infrared emitters.

3. • Medium wave infrared emitters.

4. • Long wave infrared emitters.

Other industrial applications

Light in the SWIR range, specifically, is used in the industrial context for the automatic inspection of products. The way different materials interact with this portion of the spectrum makes it very useful for detecting product imperfections or contaminations. Digital processing and machine vision are necessary to automate this process. Advances in the sensor sector have allowed this progress, since currently, the sensors necessary to detect SWIR do not require the amount of power or cooling that they needed in the past in all cases. [29]​.

night vision

Infrared is used in night vision equipment when there is not enough visible light to see.[30]​ Night vision devices work through a process that involves converting photons from ambient light into electrons which are then amplified through a chemical and electrical process and converted back into visible light.[30]​ Infrared light sources can be used to increase the ambient light available for conversion by night vision devices, increasing visibility in the dark without actually using a light source. visible.[30]​.

The use of infrared light and night vision devices should not be confused with thermal imaging, which creates images based on surface temperature differences by detecting infrared radiation (heat) emanating from objects and their surroundings.[31]​.

Thermography

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

Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (approximately 9,000 to 14,000 nanometers or 9-14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects depending on their temperature, according to the law of black body radiation, thermography makes it possible to "see" the environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, so thermography allows us to see temperature variations (hence its name).

Hyperspectral imaging

A hyperspectral image is an "image" that contains a continuous spectrum across a wide spectral range in each pixel. Hyperspectral imaging is gaining importance in the field of applied spectroscopy, especially in the NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defense and industrial measurements.

Hyperspectral imaging in the thermal infrared can be performed in a similar way using a thermal imaging camera, with the fundamental difference that each pixel contains a complete LWIR spectrum. Consequently, chemical identification of the object can be carried out without the need for an external light source, such as the Sun or Moon. This type of cameras are usually applied for geological measurements, outdoor surveillance and UAV applications.[33]​.

Other images

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 photos of subjects near areas of infrared light (such as near a lamp), where the resulting infrared interference can wash out the image. There is also a technique called 'T-rays', which involves obtaining images using far infrared or terahertz radiation. The lack of bright sources can make terahertz photography more difficult than most other infrared imaging techniques. Recently, T-ray imaging has attracted great interest due to a number of new developments, such as “time-domain terahertz spectroscopy”.

Follow-up

Infrared tracking, also known as infrared homing, refers to a passive missile guidance system, which uses a target's emission of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often called "heat-seeking" since infrared (IR) is just below the visible spectrum of light in frequency and is strongly radiated by hot bodies. Many objects, such as people, vehicle engines, and aircraft, generate and retain heat and, as such, are especially visible in the infrared wavelengths of light compared to background objects.[34]​.

The discovery of infrared radiation is attributed to William Herschel, the astronomer, at the beginning of the century. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism&action=edit&redlink=1 "Triangular prism (optics) (not yet drafted)") to refract sunlight and detected infrared, beyond the red part of the spectrum, through a rise in temperature recorded on a thermometer. He was surprised by the result and called them "caloric rays".[35]​[36]​ The term "infrared" did not appear until the end of the century.[37]​.

Other important dates are:[10]​.

• - 1737: Émilie du Châtelet predicted what is known today as infrared radiation in Dissertation sur la nature et la propagation du feu.[38]​.

• - 1830: Leopoldo Nobili made the first thermopile infrared detector.[39]​.

• - 1840: John Herschel produces the first thermal image, called a thermogram.[40]​.

• - 1860: Gustav Kirchhoff formulated the black body theorem.[41]​.

• - 1873: Willoughby Smith discovered the photoconductivity of selenium.[42]​.

• - 1878: Samuel Pierpont Langley invents the first bolometer, a device capable of measuring small fluctuations in temperature and, therefore, the power of far infrared sources.[43]​.

• - 1879: The Stefan-Boltzmann Law empirically formulated that the power radiated by a black body is proportional to T.[44]​.

• - 1880s and 1890s: Lord Rayleigh and Wilhelm Wien solved part of the blackbody equation, but both solutions diverged in parts of the electromagnetic spectrum. This problem was called "ultraviolet catastrophe") and infrared catastrophe".[45]​.

• - 1892: Willem Henri Julius published the infrared spectra of 20 organic compounds measured with a bolometer in units of angular displacement.[46]​.

• - 1901: Max Planck published the black body equation and theorem. He solved the problem by quantifying the allowed energy transitions.[47]​.

• - 1905: Albert Einstein developed the theory of the photoelectric effect.[48]​.

• - 1905-1908: William Coblentz published infrared spectra in units of wavelength (micrometers) for various chemical compounds in Investigations of Infra-Red Spectra.[49]​[50][51]​.

• - 1917: Theodore Case") developed the thallium sulfide detector"); A British scientist built the first infrared search and tracker (IRST) capable of detecting aircraft at a distance of one mile (1.6 km).

• - 1935: Lead salts: first missile guide in World War II.

• - 1938: Yeou Ta") predicted that the pyroelectric effect could be used to detect infrared radiation.[52]​.

• - 1945: The Zielgerät 1229 "Vampir" was introduced as the first portable infrared device for military applications.

• - 1952: Heinrich Welker") cultivated synthetic InSb crystals").

• - 1950s and 1960s: Nomenclature and radiometric units defined by Fred Nicodemenus"), G. J. Zissis") and R. Clark"); Robert Clark Jones defined D.

• - 1958: W. D. Lawson") (Royal Radar Establishment") in Malvern) discovered the infrared sensing properties of mercury cadmium telluride") (HgCdTe).[53]​.

• - 1958: Falcon and Sidewinder missiles were developed with infrared technology.

• - 1960s: Paul Kruse&action=edit&redlink=1 "Paul Kruse (engineer) (not yet redacted)") and his colleagues at the Honeywell Research Center demonstrate the use of HgCdTe as an effective compound for infrared detection.[53]​.

• - 1962: J. Cooper") demonstrated pyroelectric detection.[54]​.

• - 1964: W. G. Evans discovered infrared thermoreceptors in a pyrophilous beetle.

• - 1965: First infrared manual; first commercial imagers (Barnes, Agema") (now part of FLIR Systems") Inc.); Richard Hudson's reference text&action=edit&redlink=1 "Richard Hudson (physicist) (not yet drafted)"); Hughes F4 TRAM FLIR; pioneering phenomenology of Fred Simmons&action=edit&redlink=1 "Fred Simmons (scientist) (not yet written)") and A. T. Stair"); the US Army Night Vision Laboratory (now Night Vision and Electronic Sensors Directorate") (NVESD)) was formed, and Rachets") develops detection, recognition and identification models there.

• - 1970: Willard Boyle and George E. Smith propose the CCD at Bell Laboratories for video chat.

• - 1973: Common module program initiated by the NVESD.[55]​.

• - 1978: Infrared imaging astronomy comes of age, observatories are planned, NASA's Infrared Telescope on Mauna Kea is inaugurated; 32 × 32 and 64 × 64 arrays are produced using InSb, HgCdTe and other materials.

• - 2013: On February 14, researchers developed a brain implant that gives rats the ability to perceive infrared light, providing living beings with new abilities for the first time, rather than simply replacing or augmenting existing abilities.[56]​.

• - Infrared spectroscopy.

• - Greenhouse effect.

• - Spectral Radiance.

• - Detection of counterfeit bills by infrared.

• - Infrared sensors.

• - Infrared reflectography.

• - Ultraviolet.

• - Infrared search.

• - Infrared search and tracking.

Infrared was discovered in 1800 by William Herschel, an English astronomer of German origin. Herschel placed a mercury thermometer "Mercury (element)") in the spectrum obtained by a glass prism in order to measure the heat emitted by each color. He discovered that the heat was strongest on the red side of the spectrum and observed that there was no light there. This is the first experience to show that heat can be transmitted by an invisible form of light. Herschel called this radiation "caloric rays", a name quite popular throughout the century that eventually gave way to the more modern infrared radiation.

The first infrared radiation detectors were bolometers, instruments that capture radiation due to the increase in temperature produced in an absorbing detector.

Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nm to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz&action=edit&redlink=1 "Terahertz (unit) (not yet redacted)") to 300 GHz. Below the infrared is the microwave portion of the electromagnetic spectrum.

Sunlight, at an effective temperature of 5780 K, (5510 °C, 9940 °F), is composed of near-thermal spectrum radiation that is slightly above the mid-infrared. At the zenith, sunlight provides an irradiance of just over 1 kW per square meter at sea level. Of this energy, 527 W are infrared radiation, 445 W are visible light, and 32 W are ultraviolet radiation.[8] Almost all infrared radiation from sunlight is near infrared, less than 4 μm.

On the Earth's surface, at temperatures much lower than those of the Sun's surface, part of the thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. However, blackbody, or thermal, radiation is continuous: it emits radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce much more infrared energy than visible light.[9]​.

Infrared is classified, according to its wavelength, in this way:[10]​.

• - Near infrared 0.7 to 1.0 μm (from the approximate end of the human eye's response to that of silicon).

• - Shortwave infrared: 1.0 to 3 μm (from the silicon cutoff to the MWIR atmospheric window). InGaAs covers up to approximately 1.8 µm; less sensitive lead salts cover this region.

• - Mid-infrared 3 to 5 μm (defined by the atmospheric window and covered by indium antimonide [InSb] and mercury cadmium telluride [HgCdTe] and partially by lead selenide PbSe")).

• - Long wave infrared: 8 to 12 or 7 to 14 μm (this is the atmospheric window covered by HgCdTe and microbolometers).

• - Very long wave infrared (VLWIR) (12 to approximately 30 μm, covered by doped silicon).

• - Far infrared (from 50 µm to 1000 µm).

The near infrared is the region closest in wavelength to the radiation detectable by the human eye. The mid and far infrared progressively move away from the visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs bands, water absorption) and newer ones follow technical reasons (common silicon detectors are sensitive at about 1050 nm, while the sensitivity of InGaAs starts around 950 nm and ends between 1700 and 2600 nm, depending on the specific configuration). There are currently no international standards for these specifications.

Infrared occurrence is defined (according to different standards) at various values, typically between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is noticeably less sensitive to light above 700 nm wavelength, so longer wavelengths make negligible contributions to scenes illuminated by common light sources. However, particularly intense near-infrared light (e.g., from IR lasers, IR LED sources, or from bright daylight with visible light removed by colored gels) can be detected up to about 780 nm and will be perceived as red light. Intense light sources providing wavelengths up to 1050 nm can be seen as a dull red glow, causing some difficulty in near-infrared illumination of dark scenes (this practical problem is usually solved by indirect illumination). Leaves are particularly bright in the near infrared, and if all visible light leaks around an infrared filter are blocked and the eye has a moment to adjust to the extremely dim image that comes from a visually opaque infrared pass photographic filter, it is possible to see the Wood effect which consists of infrared glowing foliage.

Matter, due to its energetic characterization (see black body), emits thermal radiation. In general, the wavelength where a body emits the maximum radiation is inversely proportional to its temperature (Wien's Law). In this way, most objects at everyday temperatures have their maximum emission in the infrared. Living beings, especially mammals, emit a large proportion of radiation in the infrared part of the spectrum, due to their body heat.

The power emitted in the form of heat by a human body, for example, can be obtained from the surface of its skin (about 2 m) and its body temperature (about 37 °C, i.e. 310 K), by means of the Stefan-Boltzmann Law, and turns out to be around 100 W.[11]​.

This is closely related to the so-called "thermal sensation", according to which we can feel cold or hot regardless of the environmental temperature, depending on the radiation we receive (for example from the Sun or other nearby hot bodies): If we receive more than the 100 W that we emit, we will be hot, and if we receive less, we will be cold. In both cases the temperature of our body is constant (37 °C) and that of the air around us as well. Therefore, the thermal sensation in still air only has to do with the amount of radiation (usually infrared) that we receive and its balance with that which we constantly emit as hot bodies that we are. If, on the other hand, there is wind, the layer of air in contact with our skin can be replaced by air at a different temperature, which also alters the thermal balance and modifies the thermal sensation.

Contenido

En general, los objetos emiten radiación infrarroja en todo un espectro de longitudes de onda, pero a veces sólo interesa una región limitada del espectro porque los sensores suelen recoger la radiación sólo dentro de un ancho de banda específico. La radiación infrarroja térmica también tiene una longitud de onda de emisión máxima, que es inversamente proporcional a la temperatura absoluta del objeto, de acuerdo con la ley de desplazamiento de Wien. La banda de infrarrojos suele subdividirse en secciones más pequeñas, aunque la forma en que se divide el espectro de infrarrojos varía según las distintas áreas en las que se emplea el infrarrojo.

Visible limit

Infrared, as its name implies, is generally considered to begin with wavelengths longer than those visible to the human eye. However, there is no "hard" wavelength limit to what is visible, as the eye's sensitivity decreases rapidly, but gently, for wavelengths greater than about 700 nm. Therefore, longer wavelengths can be seen if they are bright enough, although they may still be classified as infrared according to common definitions. Thus, the light from a near-infrared laser may appear dim red and may pose a danger as it can be quite bright. And even infrared with wavelengths up to 1050 nm from pulsed lasers can be seen by humans under certain conditions.[12][13][14][15]​.

Commonly used subdivision scheme

A commonly used subdivision scheme is:[16]​.

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

CIE division scheme

The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:[20]​.

ISO 20473 Scheme

ISO 20473 specifies the following scheme:[21]​.

Outline of the astronomy division

Astronomers usually divide the infrared spectrum as follows:[22]​.

These divisions are not precise and may vary by publication. The three regions are used for the observation of different temperature ranges, and therefore different environments in space.

The most common photometric system used in astronomy assigns capital letters to different spectral regions depending on the filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N and Q refer to the mid-infrared region. These letters are commonly understood to refer to "atmospheric windows") and appear, for example, in the titles of many academic articles.

Division scheme according to sensor response

A third scheme divides the band based on the response of several detectors:[10]​.

• - Near infrared: 0.7 to 1.0 μm (from the approximate end of the human eye's response to that of silicon).

• - Shortwave infrared: From 1.0 to 3 μm (from the silicon cut to the MWIR atmospheric window). InGaAs covers up to approximately 1.8 μm; less sensitive lead salts cover this region.

• - Medium wave infrared: 3 to 5 μm (defined by the atmospheric window and covered by indium antimonide [InSb] and cadmium and mercury telluride") [HgCdTe] and partially by lead selenide [PbSe]).

• - Long wave infrared: 8 to 12, or 7 to 14 μm (it is the atmospheric window covered by HgCdTe and microbolometer).

• - Very long wave infrared (VLWIR) (approximately 12 to 30 μm, covered by doped silicon).

The near infrared is the region closest in wavelength to the radiation detectable by the human eye. The mid-infrared and far-infrared progressively move away from the visible spectrum. Other definitions obey different physical mechanisms (emission peaks, versus bands, water absorption) and the most recent ones obey technical reasons (common silicon detectors are sensitive up to about 1050 nm, while the sensitivity of InGaAs begins around 950 nm and ends between 1700 and 2600 nm, depending on the specific configuration). There are currently no international standards for these specifications.

The onset of infrared is defined (according to different standards) at various values, usually between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is noticeably less sensitive to light above the 700 nm wavelength, so longer wavelengths contribute insignificantly to scenes illuminated by common light sources. However, particularly intense near-infrared light (e.g., that from IR lasers, IR LED sources, or bright daylight with visible light removed by colored gels) can be detected up to about 780 nm, and will be perceived as red light. Intense light sources providing wavelengths up to 1050 nm can be seen as a dull red glow, causing some difficulty in illuminating near-infrared scenes in the dark (this practical problem is usually solved by indirect illumination). Leaves are particularly bright in the near IR, and if all visible light leaks around an IR filter are blocked, and the eye is given a moment to adjust to the extremely dim image coming through a visually opaque photographic filter passing through the IR, it is possible to see the Wood effect of foliage glowing in the IR.[23]​.

Infrared telecommunication bands

In optical communications, the part of the infrared spectrum used is divided into seven bands based on the availability of light sources that transmit/absorb materials (fibers) and detectors:[24]​.

“C-band” is the dominant band for long-distance telecommunications networks. The S and L bands are based on less established technology and are not as widespread.

Infrared radiation is popularly known as "thermal radiation",[25]​ but light and electromagnetic waves of any frequency heat the surfaces that absorb them. Infrared light from the Sun accounts for 49%[26]​ of Earth's warming, with the remainder caused by visible light being absorbed and then re-radiated at longer wavelengths. The visible or ultraviolet light emitted by the laser can carbonize paper, and incandescent objects emit visible radiation. Objects at room temperature will emit spontaneous emission of concentrated radiation especially in the 8 to 25 μm band, but this is no different from the emission of visible light by incandescent objects and ultraviolet light by even hotter objects (see black body and Wien's displacement law).[27]​.

Heat is the energy in transit that flows due to a temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, thermal radiation can propagate through a vacuum. Thermal radiation is characterized by a particular spectrum of many wavelengths that are associated with the emission of an object, due to the vibration of its molecules at a certain temperature. Thermal radiation can be emitted by objects at any wavelength, and at very high temperatures said radiation is associated with spectra much higher than the infrared, extending to the visible, ultraviolet and even X-ray regions (for example, the solar corona). Therefore, the popular association of infrared radiation with thermal radiation is just a coincidence based on the typical (comparatively low) temperatures usually found near the surface of planet Earth.

The concept of emissivity is important to understand infrared emissions from objects. This is a property of a surface that describes how its thermal emissions deviate from the idea of ​​a black body. To explain it better, two objects at the same physical temperature may not show the same infrared image if they have different emissivity. For example, for any preset emissivity value, objects with higher emissivity will appear hotter, and those with lower emissivity will appear colder (assuming, as is often the case, that the surrounding environment is colder than the objects being viewed). When an object does not have perfect emissivity, it acquires properties of reflectivity and/or transparency, so the temperature of the environment is partially reflected and/or transmitted through the object. If the object were in a hotter environment, then a lower emissivity object at the same temperature would probably appear hotter than a more emissive one. For this reason, incorrect selection of emissivity and failure to consider ambient temperatures will give inaccurate results when using infrared cameras and pyrometers.

Los infrarrojos se utilizan en los equipos de visión nocturna cuando la cantidad de luz visible es insuficiente para ver los objetos. La radiación se recibe y después se refleja en una pantalla. Los objetos más calientes se convierten en los más luminosos.

Un uso muy común es el que hacen los mandos a distancia (o telecomandos) que generalmente utilizan los infrarrojos en vez de ondas de radio ya que no interfieren con otras señales como las señales de televisión. Los infrarrojos también se utilizan para comunicar a corta distancia los ordenadores con sus periféricos "Periférico (informática)"). Los aparatos que utilizan este tipo de comunicación cumplen generalmente un estándar publicado por la Infrared Data Association.[28]​.

La luz utilizada en las fibras ópticas es generalmente de infrarrojos.

Near Infrared

The near infrared is the shortest wavelength region of the infrared spectrum, located between visible light and the mid-infrared, approximately between 800 and 2500 nm, although there is no universally accepted definition.

Astronomy

In astronomy, near-infrared spectroscopy is used to study the atmospheres of cold stars. In this range, lines of rotational and vibrational transitions of molecules such as titanium oxide "Titanium (IV) oxide"), cyanogen and carbon monoxide can be observed, which give information about the spectral type of the star. It is also used to study molecules in other astronomical objects, such as molecular clouds.

Industrial infrared emitters

Another of the many applications of infrared radiation is the use of infrared emitting equipment in the industrial sector. In this sector, the applications occupy an extensive list but its use can be highlighted in applications such as drying of paints or varnishes, drying of paper, thermosetting of plastics, preheating of welds, curvature, tempering and laminating of glass, among others. The irradiation on the material in question can be prolonged or momentary taking into account aspects such as the distance of the emitters from the material, the speed of passage of the material (in the case of production lines) and the temperature to be achieved.

Generally, when talking about infrared emitting equipment, four types are distinguished depending on the wavelength they use:

1. • Shortwave infrared emitters.

2. • Fast medium wave infrared emitters.

3. • Medium wave infrared emitters.

4. • Long wave infrared emitters.

Other industrial applications

Light in the SWIR range, specifically, is used in the industrial context for the automatic inspection of products. The way different materials interact with this portion of the spectrum makes it very useful for detecting product imperfections or contaminations. Digital processing and machine vision are necessary to automate this process. Advances in the sensor sector have allowed this progress, since currently, the sensors necessary to detect SWIR do not require the amount of power or cooling that they needed in the past in all cases. [29]​.

night vision

Infrared is used in night vision equipment when there is not enough visible light to see.[30]​ Night vision devices work through a process that involves converting photons from ambient light into electrons which are then amplified through a chemical and electrical process and converted back into visible light.[30]​ Infrared light sources can be used to increase the ambient light available for conversion by night vision devices, increasing visibility in the dark without actually using a light source. visible.[30]​.

The use of infrared light and night vision devices should not be confused with thermal imaging, which creates images based on surface temperature differences by detecting infrared radiation (heat) emanating from objects and their surroundings.[31]​.

Thermography

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

Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (approximately 9,000 to 14,000 nanometers or 9-14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects depending on their temperature, according to the law of black body radiation, thermography makes it possible to "see" the environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, so thermography allows us to see temperature variations (hence its name).

Hyperspectral imaging

A hyperspectral image is an "image" that contains a continuous spectrum across a wide spectral range in each pixel. Hyperspectral imaging is gaining importance in the field of applied spectroscopy, especially in the NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defense and industrial measurements.

Hyperspectral imaging in the thermal infrared can be performed in a similar way using a thermal imaging camera, with the fundamental difference that each pixel contains a complete LWIR spectrum. Consequently, chemical identification of the object can be carried out without the need for an external light source, such as the Sun or Moon. This type of cameras are usually applied for geological measurements, outdoor surveillance and UAV applications.[33]​.

Other images

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 photos of subjects near areas of infrared light (such as near a lamp), where the resulting infrared interference can wash out the image. There is also a technique called 'T-rays', which involves obtaining images using far infrared or terahertz radiation. The lack of bright sources can make terahertz photography more difficult than most other infrared imaging techniques. Recently, T-ray imaging has attracted great interest due to a number of new developments, such as “time-domain terahertz spectroscopy”.

Follow-up

Infrared tracking, also known as infrared homing, refers to a passive missile guidance system, which uses a target's emission of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often called "heat-seeking" since infrared (IR) is just below the visible spectrum of light in frequency and is strongly radiated by hot bodies. Many objects, such as people, vehicle engines, and aircraft, generate and retain heat and, as such, are especially visible in the infrared wavelengths of light compared to background objects.[34]​.

The discovery of infrared radiation is attributed to William Herschel, the astronomer, at the beginning of the century. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism&action=edit&redlink=1 "Triangular prism (optics) (not yet drafted)") to refract sunlight and detected infrared, beyond the red part of the spectrum, through a rise in temperature recorded on a thermometer. He was surprised by the result and called them "caloric rays".[35]​[36]​ The term "infrared" did not appear until the end of the century.[37]​.

Other important dates are:[10]​.

• - 1737: Émilie du Châtelet predicted what is known today as infrared radiation in Dissertation sur la nature et la propagation du feu.[38]​.

• - 1830: Leopoldo Nobili made the first thermopile infrared detector.[39]​.

• - 1840: John Herschel produces the first thermal image, called a thermogram.[40]​.

• - 1860: Gustav Kirchhoff formulated the black body theorem.[41]​.

• - 1873: Willoughby Smith discovered the photoconductivity of selenium.[42]​.

• - 1878: Samuel Pierpont Langley invents the first bolometer, a device capable of measuring small fluctuations in temperature and, therefore, the power of far infrared sources.[43]​.

• - 1879: The Stefan-Boltzmann Law empirically formulated that the power radiated by a black body is proportional to T.[44]​.

• - 1880s and 1890s: Lord Rayleigh and Wilhelm Wien solved part of the blackbody equation, but both solutions diverged in parts of the electromagnetic spectrum. This problem was called "ultraviolet catastrophe") and infrared catastrophe".[45]​.

• - 1892: Willem Henri Julius published the infrared spectra of 20 organic compounds measured with a bolometer in units of angular displacement.[46]​.

• - 1901: Max Planck published the black body equation and theorem. He solved the problem by quantifying the allowed energy transitions.[47]​.

• - 1905: Albert Einstein developed the theory of the photoelectric effect.[48]​.

• - 1905-1908: William Coblentz published infrared spectra in units of wavelength (micrometers) for various chemical compounds in Investigations of Infra-Red Spectra.[49]​[50][51]​.

• - 1917: Theodore Case") developed the thallium sulfide detector"); A British scientist built the first infrared search and tracker (IRST) capable of detecting aircraft at a distance of one mile (1.6 km).

• - 1935: Lead salts: first missile guide in World War II.

• - 1938: Yeou Ta") predicted that the pyroelectric effect could be used to detect infrared radiation.[52]​.

• - 1945: The Zielgerät 1229 "Vampir" was introduced as the first portable infrared device for military applications.

• - 1952: Heinrich Welker") cultivated synthetic InSb crystals").

• - 1950s and 1960s: Nomenclature and radiometric units defined by Fred Nicodemenus"), G. J. Zissis") and R. Clark"); Robert Clark Jones defined D.

• - 1958: W. D. Lawson") (Royal Radar Establishment") in Malvern) discovered the infrared sensing properties of mercury cadmium telluride") (HgCdTe).[53]​.

• - 1958: Falcon and Sidewinder missiles were developed with infrared technology.

• - 1960s: Paul Kruse&action=edit&redlink=1 "Paul Kruse (engineer) (not yet redacted)") and his colleagues at the Honeywell Research Center demonstrate the use of HgCdTe as an effective compound for infrared detection.[53]​.

• - 1962: J. Cooper") demonstrated pyroelectric detection.[54]​.

• - 1964: W. G. Evans discovered infrared thermoreceptors in a pyrophilous beetle.

• - 1965: First infrared manual; first commercial imagers (Barnes, Agema") (now part of FLIR Systems") Inc.); Richard Hudson's reference text&action=edit&redlink=1 "Richard Hudson (physicist) (not yet drafted)"); Hughes F4 TRAM FLIR; pioneering phenomenology of Fred Simmons&action=edit&redlink=1 "Fred Simmons (scientist) (not yet written)") and A. T. Stair"); the US Army Night Vision Laboratory (now Night Vision and Electronic Sensors Directorate") (NVESD)) was formed, and Rachets") develops detection, recognition and identification models there.

• - 1970: Willard Boyle and George E. Smith propose the CCD at Bell Laboratories for video chat.

• - 1973: Common module program initiated by the NVESD.[55]​.

• - 1978: Infrared imaging astronomy comes of age, observatories are planned, NASA's Infrared Telescope on Mauna Kea is inaugurated; 32 × 32 and 64 × 64 arrays are produced using InSb, HgCdTe and other materials.

• - 2013: On February 14, researchers developed a brain implant that gives rats the ability to perceive infrared light, providing living beings with new abilities for the first time, rather than simply replacing or augmenting existing abilities.[56]​.

• - Infrared spectroscopy.

• - Greenhouse effect.

• - Spectral Radiance.

• - Detection of counterfeit bills by infrared.

• - Infrared sensors.

• - Infrared reflectography.

• - Ultraviolet.

• - Infrared search.

• - Infrared search and tracking.