Infrared ( IR ; sometimes called infrared light ) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves . The infrared spectral band begins with waves that are just longer than those of red light (the longest waves in the visible spectrum ), so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm (400 THz ) to 1 mm (300 GHz ). IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum . Longer IR wavelengths (30–100 μm) are sometimes included as part of the terahertz radiation band. Almost all black-body radiation from objects near room temperature is in the IR band. As a form of electromagnetic radiation, IR carries energy and momentum , exerts radiation pressure , and has properties corresponding to both those of a wave and of a particle , the photon .
61-663: Diviner , also referred to as the Diviner Lunar Radiometer Experiment ( DLRE ), is an infrared radiometer aboard NASA 's Lunar Reconnaissance Orbiter , part of the Lunar Precursor Robotic Program which is studying the Moon. It has been used to create temperature maps of the Moon's surface, as well as detect ice deposits and surface composition. The instrument has measured temperatures of −247 °C (−412.6 °F) in
122-471: A passive missile guidance system , which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers" since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines, and aircraft generate and retain heat, and as such, are especially visible in
183-473: A thermographic camera , with the fundamental difference that each pixel contains a full LWIR spectrum. Consequently, chemical identification of the object can be performed without a need for an external light source such as the Sun or the Moon. Such cameras are typically applied for geological measurements, outdoor surveillance and UAV applications. In infrared photography , infrared filters are used to capture
244-600: A chemical and electrical process and then converted back into visible light. Infrared light sources can be used to augment the available ambient light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source. The use of infrared light and night vision devices should not be confused with thermal imaging , which creates images based on differences in surface temperature by detecting infrared radiation ( heat ) that emanates from objects and their surrounding environment. Infrared radiation can be used to remotely determine
305-477: A circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles . A zenith telescope is a type of telescope designed to point straight up at or near the zenith, and used for precision measurement of star positions, to simplify telescope construction, or both. The NASA Orbital Debris Observatory and the Large Zenith Telescope are both zenith telescopes, since
366-482: A continuous sequence of weather to be studied. These infrared pictures can depict ocean eddies or vortices and map currents such as the Gulf Stream, which are valuable to the shipping industry. Fishermen and farmers are interested in knowing land and water temperatures to protect their crops against frost or increase their catch from the sea. Even El Niño phenomena can be spotted. Using color-digitized techniques,
427-556: A crater at the northern pole and −238 °C (−396.4 °F) in craters at the southern pole. On 9 October 2009, the Diviner team announced the detection of a hot spot on the Moon at the location of the LCROSS spacecraft impact site. Infrared It was long known that fires emit invisible heat ; in 1681 the pioneering experimenter Edme Mariotte showed that glass, though transparent to sunlight, obstructed radiant heat. In 1800
488-407: A given location during the course of a day, the Sun reaches not only its zenith but also its nadir , at the antipode of that location 12 hours from solar noon . In astronomy , the altitude in the horizontal coordinate system and the zenith angle are complementary angles , with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as
549-403: A molecule vibrates at a frequency characteristic of that bond. A group of atoms in a molecule (e.g., CH 2 ) may have multiple modes of oscillation caused by the stretching and bending motions of the group as a whole. If an oscillation leads to a change in dipole in the molecule then it will absorb a photon that has the same frequency. The vibrational frequencies of most molecules correspond to
610-431: A more emissive one. For that reason, incorrect selection of emissivity and not accounting for environmental temperatures will give inaccurate results when using infrared cameras and pyrometers. Infrared is used in night vision equipment when there is insufficient visible light to see. Night vision devices operate through a process involving the conversion of ambient light photons into electrons that are then amplified by
671-463: A near-IR laser may thus appear dim red and can present a hazard since it may actually be quite bright. Even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions. A commonly used subdivision scheme is: NIR and SWIR together is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared". The International Commission on Illumination (CIE) recommended
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#1732780345410732-459: A particular location. "Above" means in the vertical direction ( plumb line ) opposite to the gravity direction at that location ( nadir ). The zenith is the "highest" point on the celestial sphere. The word zenith derives from an inaccurate reading of the Arabic expression سمت الرأس ( samt al-raʾs ), meaning "direction of the head" or "path above the head", by Medieval Latin scribes in
793-427: A spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth. Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law . The infrared band is often subdivided into smaller sections, although how
854-405: A worldwide scale, this cooling method has been proposed as a way to slow and even reverse global warming , with some estimates proposing a global surface area coverage of 1-2% to balance global heat fluxes. 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 ,
915-411: Is a property of a surface that describes how its thermal emissions deviate from the ideal of a black body . To further explain, two objects at the same physical temperature may not show the same infrared image if they have differing emissivity. For example, for any pre-set emissivity value, objects with higher emissivity will appear hotter, and those with a lower emissivity will appear cooler (assuming, as
976-451: Is absorbed then re-radiated at longer wavelengths. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. Objects at room temperature will emit radiation concentrated mostly in the 8 to 25 μm band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see black body and Wien's displacement law ). Heat
1037-426: Is also a technique called ' T-ray ' imaging, which is imaging using far-infrared or terahertz radiation . Lack of bright sources can make terahertz photography 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 . Infrared tracking, also known as infrared homing, refers to
1098-430: Is associated with spectra far above the infrared, extending into visible, ultraviolet, and even X-ray regions (e.g. the solar corona ). Thus, the popular association of infrared radiation with thermal radiation is only a coincidence based on typical (comparatively low) temperatures often found near the surface of planet Earth. The concept of emissivity is important in understanding the infrared emissions of objects. This
1159-537: Is being researched as an aid for visually impaired people through the Remote infrared audible signage project. Transmitting IR data from one device to another is sometimes referred to as beaming . IR is sometimes used for assistive audio as an alternative to an audio induction loop . Infrared vibrational spectroscopy (see also near-infrared spectroscopy ) is a technique that can be used to identify molecules by analysis of their constituent bonds. Each chemical bond in
1220-485: Is classified as part of optical astronomy . To form an image, the components of an infrared telescope need to be carefully shielded from heat sources, and the detectors are chilled using liquid helium . The sensitivity of Earth-based infrared telescopes is significantly limited by water vapor in the atmosphere, which absorbs a portion of the infrared radiation arriving from space outside of selected atmospheric windows . This limitation can be partially alleviated by placing
1281-423: Is counted as part of the microwave band, not infrared, moving the band edge of infrared to 0.1 mm (3 THz). Sunlight , at an effective temperature of 5,780 K (5,510 °C, 9,940 °F), is composed of near-thermal-spectrum radiation that is slightly more than half infrared. At zenith , sunlight provides an irradiance of just over 1 kW per square meter at sea level. Of this energy, 527 W
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#17327803454101342-456: 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 markedly less sensitive to light above 700 nm wavelength, so longer wavelengths make insignificant contributions to scenes illuminated by common light sources. Particularly intense near-IR light (e.g., from lasers , LEDs or bright daylight with
1403-578: Is efficiently detected by inexpensive silicon photodiodes , which the receiver uses to convert the detected radiation to an electric current . That electrical signal is passed through a high-pass filter which retains the rapid pulsations due to the IR transmitter but 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
1464-587: Is emitted or absorbed by molecules when changing 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 absorption and transmission of photons in the infrared range. Infrared radiation is used in industrial, scientific, military, commercial, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without
1525-491: Is 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 emission from an object, due to the vibration of its molecules at a given temperature. Thermal radiation can be emitted from objects at any wavelength, and at very high temperatures such radiation
1586-552: Is especially useful since some radiation at these wavelengths can escape into space through the atmosphere's infrared window . This is how passive daytime radiative cooling (PDRC) surfaces are able to achieve sub-ambient cooling temperatures under direct solar intensity, enhancing terrestrial heat flow to outer space with zero energy consumption or pollution . PDRC surfaces maximize shortwave solar reflectance to lessen heat gain while maintaining strong longwave infrared (LWIR) thermal radiation heat transfer . When imagined on
1647-676: Is infrared radiation, 445 W is visible light, and 32 W is ultraviolet radiation. Nearly all the infrared radiation in sunlight is near infrared, shorter than 4 μm. 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. Black-body, or thermal, radiation is continuous: it radiates 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 far more infrared than visible-light energy. In general, objects emit infrared radiation across
1708-403: Is no universally accepted definition of the range of infrared radiation. Typically, it is taken to extend from the nominal red edge of the visible spectrum at 780 nm to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz. Beyond infrared is the microwave portion of the electromagnetic spectrum . Increasingly, terahertz radiation
1769-457: Is often the case, that the surrounding environment is cooler than the objects being viewed). When an object has less than perfect emissivity, it obtains properties of reflectivity and/or transparency, and so the temperature of the surrounding environment is partially reflected by and/or transmitted through the object. If the object were in a hotter environment, then a lower emissivity object at the same temperature would likely appear to be hotter than
1830-530: Is one of the primary parameters studied in research into global warming , together with solar radiation . A pyrgeometer is utilized in this field of research to perform continuous outdoor measurements. This is a broadband infrared radiometer with sensitivity for infrared radiation between approximately 4.5 μm and 50 μm. Astronomers observe objects in the infrared portion of the electromagnetic spectrum using optical components, including mirrors, lenses and solid state digital detectors. For this reason it
1891-467: Is that low clouds such as stratus or fog can have a temperature similar to the surrounding land or sea surface and do not show up. However, using the difference in brightness of the IR4 channel (10.3–11.5 μm) and the near-infrared channel (1.58–1.64 μm), low clouds can be distinguished, producing a fog satellite picture. The main advantage of infrared is that images can be produced at night, allowing
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1952-519: Is that the IR energy heats only opaque objects, such as food, rather than the air around them. Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, and print drying. In these applications, infrared heaters replace convection ovens and contact heating. A variety of technologies or proposed technologies take advantage of infrared emissions to cool buildings or other systems. The LWIR (8–15 μm) region
2013-438: Is the dominant band for long-distance telecommunications networks . The S and L bands are based on less well established technology, and are not as widely deployed. Infrared radiation is popularly known as "heat radiation", but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49% of the heating of Earth, with the rest being caused by visible light that
2074-402: Is the most common way for remote controls to command appliances. Infrared remote control protocols like RC-5 , SIRC , are used to communicate with infrared. 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, except for
2135-520: Is the spectroscopic wavenumber . It is the frequency divided by the speed of light in vacuum. In the semiconductor industry, infrared light can be used to characterize materials such as thin films and periodic trench structures. By measuring the reflectance of light from the surface of a semiconductor wafer, the index of refraction (n) and the extinction Coefficient (k) can be determined via the Forouhi–Bloomer dispersion equations . The reflectance from
2196-404: Is typically in the range 10.3–12.5 μm (IR4 and IR5 channels). Clouds with high and cold tops, such as cyclones or cumulonimbus clouds , are often displayed as red or black, lower warmer clouds such as stratus or stratocumulus are displayed as blue or grey, with intermediate clouds shaded accordingly. Hot land surfaces are shown as dark-grey or black. One disadvantage of infrared imagery
2257-560: The Middle Ages (during the 14th century), possibly through Old Spanish . It was reduced to samt ("direction") and miswritten as senit / cenit , the m being misread as ni . Through the Old French cenith , zenith first appeared in the 17th century. The term zenith sometimes means the highest point , way, or level reached by a celestial body on its daily apparent path around a given point of observation. This sense of
2318-535: The IR spectrum is thereby divided varies between different areas in which IR is employed. Infrared radiation is generally considered to begin with wavelengths longer than visible by the human eye. There is no hard wavelength limit to what is visible, as the eye's sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700 nm. Therefore wavelengths just longer than that can be seen if they are sufficiently bright, though they may still be classified as infrared according to usual definitions. Light from
2379-488: The Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation that may be concentrated by a lens into a beam that the user aims at the detector. The beam is modulated , i.e. switched on and off, according to a code which the receiver interprets. Usually very near-IR is used (below 800 nm) for practical reasons. This wavelength
2440-521: The astronomer Sir William Herschel discovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer . Slightly more than half of the energy from the Sun was eventually found, through Herschel's studies, to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth's climate . Infrared radiation
2501-428: The day or night pass culminate (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location. In celestial navigation this allows latitude to be determined, since the declination of the star equals the latitude of the observer. If the current time at Greenwich is known at the time of the observation, the observers longitude can also be determined from
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2562-539: The division of infrared radiation into the following three bands: ISO 20473 specifies the following scheme: Astronomers typically 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, and hence different environments in space. The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover
2623-473: The eye is given a moment to adjust to the extremely dim image coming through a visually opaque IR-passing photographic filter, it is possible to see the Wood effect that consists of IR-glowing foliage. In optical communications , the part of the infrared spectrum that is used is divided into seven bands based on availability of light sources, transmitting/absorbing materials (fibers), and detectors: The C-band
2684-499: The frequencies of infrared light. Typically, the technique is used to study organic compounds using light radiation from the mid-infrared, 4,000–400 cm . A spectrum of all the frequencies of absorption in a sample is recorded. This can be used to gain information about the sample composition in terms of chemical groups present and also its purity (for example, a wet sample will show a broad O-H absorption around 3200 cm ). The unit for expressing radiation in this application, cm ,
2745-537: The gray-shaded thermal images can be converted to color for easier identification of desired information. The main water vapour channel at 6.40 to 7.08 μm can be imaged by some weather satellites and shows the amount of moisture in the atmosphere. In the field of climatology, atmospheric infrared radiation is monitored to detect trends in the energy exchange between the Earth and the atmosphere. These trends provide information on long-term changes in Earth's climate. It
2806-411: The infrared light can also be used to determine the critical dimension, depth, and sidewall angle of high aspect ratio trench structures. Weather satellites equipped with scanning radiometers produce thermal or infrared images, which can then enable a trained analyst to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features. The scanning
2867-496: The infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nm or 9–14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black-body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence
2928-412: The infrared wavelengths of light compared to objects in the background. Infrared radiation can be used as a deliberate heating source. For example, it is used in infrared saunas to heat the occupants. It may also be used in other heating applications, such as to remove ice from the wings of aircraft (de-icing). Infrared radiation is used in cooking, known as broiling or grilling . One energy advantage
2989-479: The local zenith - that is, the complement of the altitude angle (or elevation angle ). The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the Tropic of Cancer and the Tropic of Capricorn . In Islamic astronomy , the passing of the Sun over the zenith of Mecca becomes the basis of the qibla observation by shadows twice a year on 27/28 May and 15/16 July. At
3050-438: The name). A hyperspectral image is a "picture" containing continuous spectrum through a wide spectral range at each pixel. Hyperspectral imaging is gaining importance in the field of applied spectroscopy particularly with NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defence, and industrial measurements. Thermal infrared hyperspectral imaging can be similarly performed using
3111-405: The near-infrared spectrum. Digital cameras often use infrared blockers . Cheaper digital cameras and camera phones have less effective filters and can view intense near-infrared, appearing as a bright purple-white color. This is especially pronounced when taking pictures of subjects near IR-bright areas (such as near a lamp), where the resulting infrared interference can wash out the image. There
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#17327803454103172-446: The near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region. These letters are commonly 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 is the region closest in wavelength to the radiation detectable by the human eye. mid- and far-infrared are progressively further from
3233-431: The observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds , to detect objects such as planets , and to view highly 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 assist firefighting, and to detect
3294-506: The overheating of electrical components. Military and civilian applications include target acquisition , surveillance , night vision , homing , 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 , spectroscopy , and weather forecasting . There
3355-426: The radiation damage. "Since the eye cannot detect IR, blinking or closing the eyes to help prevent or reduce damage may not happen." Infrared lasers are used to provide the light for optical fiber communications systems. Wavelengths around 1,330 nm (least dispersion ) or 1,550 nm (best transmission) are the best choices for standard silica fibers. IR data transmission of audio versions of printed signs
3416-412: The telescope observatory at a high altitude, or by carrying the telescope aloft with a balloon or an aircraft. Space telescopes do not suffer from this handicap, and so outer space is considered the ideal location for infrared astronomy. Zenith The zenith ( UK : / ˈ z ɛ n ɪ θ / , US : / ˈ z iː n ɪ θ / ) is the imaginary point on the celestial sphere directly "above"
3477-479: The temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry . Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to greatly reduced production costs. Thermographic cameras detect radiation in
3538-508: The use of liquid mirrors meant these telescopes could only point straight up. On the International Space Station , zenith and nadir are used instead of up and down , referring to directions within and around the station, relative to the earth. Zenith stars (also "star on top", "overhead star", "latitude star") are stars whose declination equals the latitude of the observers location, and hence at some time in
3599-476: The visible light filtered out) can be detected up to approximately 780 nm, and will be perceived as red light. Intense light sources providing wavelengths as long as 1,050 nm can be seen as a dull red glow, causing some difficulty in near-IR illumination of scenes in the dark (usually this practical problem is solved by indirect illumination). Leaves are particularly bright in the near IR, and if all visible light leaks from around an IR-filter are blocked, and
3660-472: The visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (the common silicon detectors are sensitive to about 1,050 nm, while InGaAs 's sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). No international standards for these specifications are currently available. The onset of infrared
3721-404: The word is often used to describe the position of the Sun ("The sun reached its zenith..."), but to an astronomer, the Sun does not have its own zenith and is at the zenith only if it is directly overhead. In a scientific context, the zenith is the direction of reference for measuring the zenith angle (or zenith angular distance ), the angle between a direction of interest (e.g. a star) and
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