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Infrared thermography ( IRT), thermal video or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object. It is an example of infrared imaging science. Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum (roughly 9,000–14,000 or 9–14 μm) and produce images of that radiation, called thermograms.
Since infrared radiation is emitted by all objects with a temperature above absolute zero according to the black body radiation law, thermography makes it possible to see one's environment with or without optical spectrum illumination. The amount of radiation emitted by an object increases with temperature, and thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds. For example, humans and other warm-blooded animals become easily visible against their environment in day or night. As a result, thermography is particularly useful to the military and other users of surveillance cameras.
Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics. Thermography is used in allergy detection and veterinary medicine. Some alternative medicine practitioners promote its use for breast screening, despite the FDA warning that "those who opt for this method instead of mammography may miss the chance to detect cancer at its earliest stage". Notably, government and airport personnel used thermography to detect suspected swine flu cases during the 2009 pandemic.
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past 50 years. use thermography to see through smoke, to find persons, and to locate the base of a fire. Maintenance technicians use thermography to locate overheating joints and sections of power lines, which are a sign of impending failure. Building construction technicians can see thermal signatures that indicate heat leaks in faulty thermal insulation, improving the efficiency of heating and air-conditioning units.
The appearance and operation of a modern infrared camera is often similar to a camcorder. Often the live thermogram reveals temperature variations so clearly that a photograph is not necessary for analysis. A recording module is therefore not always built-in.
Specialized thermal imaging cameras use Staring array (FPAs) that respond to longer wavelengths (mid- and long-wavelength infrared). The most common types are InSb, InGaAs, HgCdTe and QWIP FPA. The newest technologies use low-cost, uncooled as FPA sensors. Their resolution is considerably lower than that of optical cameras, mostly 160×120 or 320×240 pixels, and up to 1280 × 1024 FLIR x8500sc Thermal imaging camera specifications . Retrieved on 2019-07-10. for the most expensive models. Thermal imaging cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted due to potential military uses. Older or more sensitive models such as InSb require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or with liquid nitrogen.
This phenomenon may become clearer upon consideration of the formula:
where incident radiant power is the radiant power profile when viewed through a thermal imaging camera. Emitted radiant power is generally what is intended to be measured; transmitted radiant power is the radiant power that passes through the subject from a remote thermal source, and; reflected radiant power is the amount of radiant power that reflects off the surface of the object from a remote thermal source.
This phenomenon occurs everywhere, all the time. It is a process known as radiant heat exchange, since radiant power × time equals radiant energy. However, in the case of infrared thermography, the above equation is used to describe the radiant power within the spectral wavelength passband of the thermal imaging camera in use. The radiant heat exchange requirements described in the equation apply equally at every wavelength in the electromagnetic spectrum.
If the object is radiating at a higher temperature than its surroundings, then Energy transfer takes place radiating from warm to cold following the principle stated in the second law of thermodynamics. So if there is a cool area in the thermogram, that object will be absorbing radiation emitted by surrounding warm objects.
The ability of objects to emit is called emissivity, to absorb radiation is called absorbance. Under outdoor environments, convective cooling from wind may also need to be considered when trying to get an accurate temperature reading.
A black body is a theoretical object with an emissivity of 1 that radiates thermal radiation characteristic of its contact temperature. That is, if the contact temperature of a thermally uniform black body radiator were , it would emit the characteristic black-body radiation of . An ordinary object emits less infrared radiation than a theoretical black body. In other words, the ratio of the actual emission to the maximum theoretical emission is an object's emissivity.
Each material has a different emissivity which may vary by temperature and infrared wavelength. For example, clean metal surfaces have emissivity that decreases at longer wavelengths; many dielectric materials, such as quartz (SiO2), sapphire (Al2O3), calcium fluoride (CaF2), etc. have emissivity that increases at longer wavelength; simple oxides, such as iron oxide (Fe2O3) display relatively flat emissivity in the infrared spectrum.
The spectrum and amount of thermal radiation depend strongly on an object's Temperature. This enables thermal imaging of an object's temperature. However, other factors also influence the received radiation, which limits the accuracy of this technique: for example, the emissivity of the object.
For a non-contact temperature measurement, the emissivity setting needs to be set properly. An object of low emissivity could have its temperature underestimated by the detector, since it only detects emitted infrared rays. For a quick estimate, a thermographer may refer to an emissivity table for a given type of object, and enter that value into the imager. It would then calculate the object's contact temperature based on the entered emissivity and the infrared radiation as detected by the imager.
For a more accurate measurement, a thermographer may apply a standard material of known, high emissivity to the surface of the object. The standard material might be an industrial emissivity spray produced specifically for the purpose, or as simple as standard black insulation tape, with an emissivity of about 0.97. The object's known temperature can then be measured using the standard emissivity. If desired, the object's actual emissivity (on a part of the object not covered by the standard material) can be determined by adjusting the imager's setting to the known temperature. There are situations, however, when such an emissivity test is not possible due to dangerous or inaccessible conditions, then the thermographer must rely on tables.
Other variables can affect the measurement, including absorption and ambient temperature of the transmitting medium (usually air). Also, surrounding infrared radiation can be reflected in the object. All these settings will affect the calculated temperature of the object being viewed.
Sometimes these monochromatic images are displayed in pseudo-color, where changes in color are used rather than changes in intensity to display changes in the signal. This technique, called density slicing, is useful because although humans have much greater dynamic range in intensity detection than color overall, the ability to see fine intensity differences in bright areas is fairly limited.
In temperature measurement the brightest (warmest) parts of the image are customarily colored white, intermediate temperatures reds and yellows, and the dimmest (coolest) parts black. A scale should be shown next to a false color image to relate colors to temperatures.
Thermal cameras convert the energy in the far infrared wavelength into a visible light display. All objects above absolute zero emit thermal infrared energy, so thermal cameras can passively see all objects, regardless of ambient light. However, most thermal cameras are sensitive to objects warmer than .
Some specification parameters of an infrared camera system are number of pixels, frame rate, responsivity, noise-equivalent power, noise-equivalent temperature difference (NETD), spectral band, distance-to-spot ratio (D:S), minimum focus distance, sensor lifetime, minimum resolvable temperature difference (MRTD), field of view, dynamic range, input power, and mass and volume.
Their resolution is considerably lower than that of optical cameras, often around 160×120 or 320×240 pixels, although more expensive ones can achieve a resolution of 1280×1024 pixels. Thermographic cameras are much more expensive than their visible-spectrum counterparts, though low-performance add-on thermal cameras for became available for hundreds of US dollars in 2014. Thermal camera answers age-old question by Fraser Macdonald, 4 October 2014, Hot Stuff
Without cooling, these sensors (which detect and convert light in much the same way as common digital cameras, but are made of different materials) would be 'blinded' or flooded by their own radiation. The drawbacks of cooled infrared cameras are that they are expensive both to produce and to run. Cooling is both energy-intensive and time-consuming.
The camera may need several minutes to cool down before it can begin working. The most commonly used cooling systems are which, although inefficient and limited in cooling capacity, are relatively simple and compact. To obtain better image quality or for imaging low temperature objects Stirling cryocoolers are needed. Although the cooling apparatus may be comparatively bulky and expensive, cooled infrared cameras provide greatly superior image quality compared to uncooled ones, particularly of objects near or below room temperature. Additionally, the greater sensitivity of cooled cameras also allow the use of higher F-number lenses, making high performance long focal length lenses both smaller and cheaper for cooled detectors.
An alternative to Stirling coolers is to use gases bottled at high pressure, nitrogen being a common choice. The pressurised gas is expanded via a micro-sized orifice and passed over a miniature heat exchanger resulting in regenerative cooling via the Joule–Thomson effect. For such systems the supply of pressurized gas is a logistical concern for field use.
Materials used for cooled infrared detection include based on a wide range of narrow gap semiconductors including indium antimonide (3-5 μm), indium arsenide, mercury cadmium telluride (MCT) (1-2 μm, 3-5 μm, 8-12 μm), lead sulfide, and lead selenide. Infrared photodetectors can also be created with structures of high bandgap semiconductors such as in quantum well infrared photodetectors.
Cooled bolometer technologies can be superconducting or non-superconducting. Superconducting detectors offer extreme sensitivity, with some able to register individual photons. For example, ESA's Superconducting camera (SCAM). However, they are not in regular use outside of scientific research. In principle, superconducting tunneling junction devices could be used as infrared sensors because of their very narrow gap. Small arrays have been demonstrated, but they have not been broadly adopted for use because their high sensitivity requires careful shielding from background radiation.
In uncooled detectors the temperature differences at the sensor pixels are minute; a 1 °C difference at the scene induces just a 0.03 °C difference at the sensor. The pixel response time is also fairly slow, at the range of tens of milliseconds.
Uncooled infrared sensors can be stabilized to an operating temperature to reduce image noise, but they are not cooled to low temperatures and do not require bulky, expensive, energy consuming cryogenic coolers. This makes infrared cameras smaller and less costly. However, their resolution and image quality tend to be lower than cooled detectors. This is due to differences in their fabrication processes, limited by currently available technology. An uncooled thermal camera also needs to deal with its own heat signature.
Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. The material are used to form pixels with highly temperature-dependent properties, which are thermally insulated from the environment and read electronically.
Ferroelectric detectors operate close to phase transition temperature of the sensor material; the pixel temperature is read as the highly temperature-dependent polarization charge. The achieved NETD of ferroelectric detectors with F-number optics and 320×240 sensors is 70-80 mK. A possible sensor assembly consists of barium strontium titanate bump-bonded by polyimide thermally insulated connection.
Silicon microbolometers can reach NETD down to 20 mK. They consist of a layer of amorphous silicon, or a thin film vanadium(V) oxide sensing element suspended on silicon nitride bridge above the silicon-based scanning electronics. The electric resistance of the sensing element is measured once per frame.
Current improvements of uncooled focal plane arrays (UFPA) are focused primarily on higher sensitivity and pixel density. In 2013 DARPA announced a five-micron LWIR camera that uses a 1280 × 720 focal plane array (FPA). Some of the materials used for the are amorphous silicon (a-Si), vanadium(V) oxide (VOx), lanthanum barium manganite (LBMO), lead zirconate titanate (PZT), lanthanum dopant lead zirconate titanate (PLZT), lead scandium tantalate (PST), lead lanthanum titanate (PLT), lead titanate (PT), lead zinc niobate (PZN), lead strontium titanate (PSrT), barium strontium titanate (BST), barium titanate (BT), antimony sulfoiodide (SbSI), and polyvinylidene difluoride (PVDF).
At temperatures of 600 °C and above, inexpensive cameras with CCD and CMOS sensors have also been used for pyrometry in the visible spectrum. They have been used for soot in flames, burning coal particles, heated materials, SiC filaments, and smoldering embers. This pyrometry has been performed using external filters or only the sensor's . It has been performed using color ratios, grayscales, and/or a hybrid of both.
Some infrared cameras marketed as night vision are sensitive to near-infrared just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. However, these are not usually used for thermography due to the high equivalent black-body temperature required, but are instead used with active near-IR illumination sources.
In passive thermography, the features of interest are naturally at a higher or lower temperature than the background. Passive thermography has many applications such as surveillance of people on a scene and medical diagnosis (specifically thermology).
In active thermography, an energy source is required to produce a thermal contrast between the feature of interest and the background. The active approach is necessary in many cases given that the inspected parts are usually in equilibrium with the surroundings. Given the super-linearities of the black-body radiation, active thermography can also be used to enhance the resolution of imaging systems beyond their diffraction limit or to achieve super-resolution microscopy.
There is also a difference in refresh rate. Some cameras may only have a refreshing value of 5 –15 Hz, other (e.g. FLIR X8500sc) 180 Hz or even more in no full window mode.
There are various types of lenses available, including fixed focus, manual focus, and auto focus. Most thermal cameras only support digital zoom and lack true optical zoom capabilities. However, a few models (e.g. FOTRIC P7MiX) offer dual-view optical zoom, combining lenses with different fields of view (e.g., 25° and 12°, or 25° and 7°).
Many models do not provide the irradiance measurements used to construct the output image; the loss of this information without a correct calibration for emissivity, distance, and ambient temperature and relative humidity entails that the resultant images are inherently incorrect measurements of temperature.F. Colbert, "Looking Under the Hood: Converting Proprietary Image File Formats Created within IR Cameras for Improved Archival Use", Professional Thermographers Association
Images can be difficult to interpret accurately when based upon certain objects, specifically objects with erratic temperatures, although this problem is reduced in active thermal imaging. Infrared Temperature Theory and Application . Omega.com. Retrieved on 2013-06-18.
Thermographic cameras create thermal images based on the radiant heat energy it receives. As radiation levels are influenced by the emissivity and reflection of radiation such as sunlight from the surface being measured this causes errors in the measurements. Real Time Emissivity Measurement for Infrared Temperature Measurement . Pyrometer.com. Retrieved on 2013-06-18.
By using proper camera settings, electrical systems can be scanned and problems can be found. Faults with steam traps in steam heating systems are easy to locate.
In the energy savings area, thermal imaging cameras can see the effective radiation temperature of an object as well as what that object is radiating towards, which can help locate sources of thermal leaks and overheated regions.
Cooled infrared cameras can be found at major astronomy research , even those that are not infrared telescopes. Examples include telescopes such as UKIRT, the Spitzer Space Telescope, WISE and the James Webb Space Telescope
For automotive night vision, thermal imaging cameras are also installed in some luxury cars to aid the driver, the first being the 2000 Cadillac DeVille.
In smartphones, a thermal camera was first integrated into the Cat S60 in 2016.
In building inspection, thermography can be used in: Infrared Building Inspections — Resources for Electrical, Mechanical, Residential and Commercial Infrared/Thermal Inspections . Infrared-buildinginspections.com (2008-09-04). Retrieved on 2013-06-18.
Healthcare-related uses include:
In weapons systems, thermography can be used in military and police target detection and acquisition:
In computer hacking, a thermal attack is an approach that exploits heat traces left after interacting with interfaces, such as touchscreens or keyboards, to uncover the user's input.
The first civil sector application of IR technology may have been a device to detect the presence of icebergs and steamships using a mirror and thermopile, patented in 1913.L. Bellingham, "Means for detecting the presence at a distance of icebergs, steamships, and other cool or hot objects," US patent no. 1,158,967. This was soon outdone by the first accurate IR iceberg detector, which did not use thermopiles, patented in 1914 by R.D. Parker.Parker (R.D.)- Thermic balance or radiometer. U.S. Patent No 1,099,199 June 9, 1914 This was followed by G.A. Barker's proposal to use the IR system to detect forest fires in 1934.Barker (G.A.) – Apparatus for detecting forest fires. U.S. Patent No 1,958,702 May 22, 1934 The technique was not genuinely industrialized until it was used to analyze heating uniformity in hot steel strips in 1935.Nichols (G.T.) – Temperature measuring. U.S. Patent No 2,008,793 July 23, 1935
The first British infrared linescan system was Yellow Duckling of the mid-1950s. This used a continuously rotating mirror and detector, with Y-axis scanning by the motion of the carrier aircraft. Although unsuccessful in its intended application of submarine tracking by wake detection, it was applied to land-based surveillance and became the foundation of military IR linescan.
This work was further developed at the Royal Signals and Radar Establishment in the UK when they discovered that mercury cadmium telluride was a photoconductor that required much less cooling. Honeywell in the United States also developed arrays of detectors that could cool at a lower temperature, but they scanned mechanically. This method had several disadvantages which could be overcome using an electronic scanning system. In 1969 Michael Francis Tompsett at English Electric Valve Company in the UK patented a camera that scanned pyro-electronically and which reached a high level of performance after several other breakthroughs during the 1970s. Tompsett also proposed an idea for solid-state thermal-imaging arrays, which eventually led to modern hybridized single-crystal-slice imaging devices.
By using video camera tubes such as vidicons with a pyroelectric material such as triglycine sulfate (TGS) as their targets, a vidicon sensitive over a broad portion of the infrared spectrum is possible. This technology was a precursor to modern microbolometer technology, and mainly used in firefighting thermal cameras.
Towards the end of the 1990s, the use of infrared was moving towards civilian use. There was a dramatic lowering of costs for uncooled arrays, which along with the significant increase in developments, led to a dual-use market encompassing both civilian and military uses.A. Rogalski, "IR detectors: status trends," Progress in Quantum Electronics, vol. 27, pp. 59–210, 2003. These uses include environmental control, building/art analysis, functional medical diagnostics, and car guidance and collision avoidance systems.C. Corsi, "Rivelatori IR: stato dell’arte e trends di sviluppo futuro," Atti della Fondazione Giorgio Ronchi, vol. XLVI, no.5, pp. 801–810, 1991.L. J. Kozlowski and W. F. Kosonocky, "Infrared detector arrays," in Hand-Book of Optics, M. Bass, Ed., chapter 23, Williams, W. L.Wolfe, and McGraw-Hill, 1995.C. Corsi, "Future trends and advanced development in I.R. detectors," in Proceedings of 2nd Joint Conference IRIS-NATO, London, UK, June 1996.M. Razeghi, "Current status and future trends of infrared detectors," Opto-Electronics Review, vol. 6, no. 3, pp. 155–194, 1998.Corsi, Carlo. "Infrared: A Key Technology for Security Systems." Advances in Optical Technologies 2012 (2012): 1-15.
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