What is Industrial thermal inspection lenses and Why Do We Use Them?

Infrared imaging used to reveal temperature This article is about the infrared imaging technique. For the printing technique called thermography, see thermographic printing. For thermography in medicine, see Non-contact thermography.

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 nanometers or 9–14 μm) and produce images of that radiation, called thermograms.

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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 visible 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".[1] Notably, government and airport personnel used thermography to detect suspected swine flu cases during the pandemic.[2]

Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past 50 years. Firefighters 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 thermographic 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 focal plane arrays (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 microbolometers as FPA sensors. Their resolution is considerably lower than that of optical cameras, mostly 160×120 or 320×240 pixels, and up to × [3] 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 bolometers or more sensitive models such as InSb require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or with liquid nitrogen.

Thermal energy

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Thermal images, or thermograms, are visual displays of the total infrared energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is sometimes difficult to get an accurate temperature of an object using this method. A thermal imaging camera uses processing algorithms to reconstruct a temperature image. Note that the image shows an approximation of the temperature of an object, as the camera integrates multiple sources of data in the areas surrounding the object to estimate its temperature.[4]

This phenomenon may become clearer upon consideration of the formula:

Incident Radiant Power = Emitted Radiant Power + Transmitted Radiant Power + Reflected Radiant Power;

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 power 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 absorptivity. Under outdoor environments, convective cooling from wind may also need to be considered when trying to get an accurate temperature reading.

Emissivity

[edit] Main article: Emissivity

Emissivity (or emissivity coefficient) represents a material's ability to emit thermal radiation, which is an optical property of matter. A material's emissivity can theoretically range from 0 (completely not-emitting) to 1 (completely emitting). An example of a substance with low emissivity would be silver, with an emissivity coefficient of 0.02. An example of a substance with high emissivity would be asphalt, with an emissivity coefficient of .98.

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 50 °C (122 °F), it would emit the characteristic black-body radiation of 50 °C (122 °F). 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.[5] 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.

Measurement

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A thermal imaging camera requires a series of mathematical algorithms to build a visible image, since the camera is only able to see electromagnetic radiation invisible to the human eye. The output image can be in JPG or any other image formats.

The spectrum and amount of thermal radiation depend strongly on an object's surface 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.

Color scale

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Images from infrared cameras tend to be monochrome because the cameras generally use an image sensor that does not distinguish different wavelengths of infrared radiation. Color image sensors require a complex construction to differentiate wavelengths, and color has less meaning outside of the normal visible spectrum because the differing wavelengths do not map uniformly into the color vision system used by humans.

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.

Cameras

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A thermographic camera (also called an infrared camera or thermal imaging camera, thermal camera or thermal imager) is a device that creates an image using infrared (IR) radiation, similar to a normal camera that forms an image using visible light. Instead of the 400–700 nanometre (nm) range of the visible light camera, infrared cameras are sensitive to wavelengths from about 1,000 nm (1 micrometre or μm) to about 14,000 nm (14 μm). The practice of capturing and analyzing the data they provide is called thermography.

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 −50 °C (−58 °F).

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 × pixels. Thermographic cameras are much more expensive than their visible-spectrum counterparts, though low-performance add-on thermal cameras for smartphones became available for hundreds of US dollars in .[6]

Types

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Thermographic cameras can be broadly divided into two types: those with cooled infrared image detectors and those with uncooled detectors.

Cooled infrared detectors

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Cooled detectors are typically contained in a vacuum-sealed case or Dewar and cryogenically cooled. Cooling is necessary for the operation of the semiconductor materials used. Typical operating temperatures range from 4 K (−269 °C) to just below room temperature, depending on the detector technology. Most modern cooled detectors operate in the 60 Kelvin (K) to 100 K range (-213 to -173 °C), depending on type and performance level.[7]

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 peltier coolers 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 photodetectors 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.

Uncooled infrared detectors

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Uncooled thermal cameras use a sensor operating at ambient temperature, or a sensor stabilized at a temperature close to ambient using small temperature control elements. Modern uncooled detectors all use sensors that work by the change of resistance, voltage or current when heated by infrared radiation. These changes are then measured and compared to the values at the operating temperature of the sensor.

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.[8] 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/1 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 DARPA announced a five-micron LWIR camera that uses a × 720 focal plane array (FPA).[9] Some of the materials used for the sensor arrays are amorphous silicon (a-Si), vanadium(V) oxide (VOx),[10] lanthanum barium manganite (LBMO), lead zirconate titanate (PZT), lanthanum doped 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).

CCD and CMOS thermography

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Non-specialized charge-coupled device (CCD) and CMOS sensors have most of their spectral sensitivity in the visible light wavelength range. However, by utilizing the "trailing" area of their spectral sensitivity, namely the part of the infrared spectrum called near-infrared (NIR), and by using off-the-shelf CCTV camera it is possible under certain circumstances to obtain true thermal images of objects with temperatures at about 280 °C (536 °F) and higher.[11]

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.[12] This pyrometry has been performed using external filters or only the sensor's Bayer filters. It has been performed using color ratios, grayscales, and/or a hybrid of both.

Infrared films

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Infrared (IR) film is sensitive to black-body radiation in the 250 to 500 °C (482 to 932 °F) range, while the range of thermography is approximately −50 to 2,000 °C (−58 to 3,632 °F). So, for an IR film to work thermographically, the measured object must be over 250 °C (482 °F) or be reflecting infrared radiation from something that is at least that hot.

Comparison with night-vision devices

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Starlight-type night-vision devices generally only magnify ambient light and are not thermal imagers.

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.

Passive vs. active thermography

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All objects above the absolute zero temperature (0 K) emit infrared radiation. Hence, an excellent way to measure thermal variations is to use an infrared sensing device, usually a focal plane array (FPA) infrared camera capable of detecting radiation in the mid (3 to 5 μm) and long (7 to 14 μm) wave infrared bands, denoted as MWIR and LWIR, corresponding to two of the high transmittance infrared windows. Abnormal temperature profiles at the surface of an object are an indication of a potential problem.[13]

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.[14] 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.[15]

Advantages

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Thermography shows a visual picture so temperatures over a large area can be compared.[16][17][18] It is capable of catching moving targets in real time.[16][17][18] It is able to find deterioration, i.e., higher temperature components prior to their failure. It can be used to measure or observe in areas inaccessible or hazardous for other methods. It is a non-destructive test method. It can be used to find defects in shafts, pipes, and other metal or plastic parts.[19] It can be used to detect objects in dark areas. It has some medical application, essentially in physiotherapy.

Limitations and disadvantages

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Quality thermography cameras often have a high price (often US$3,000 or more) due to the expense of the larger pixel array (state of the art ×), although less expensive models (with pixel arrays of 40×40 up to 160×120 pixels) are also available. Fewer pixels compared to traditional cameras reduce the image quality making it more difficult to distinguish proximate targets within the same field of view.

There is also a difference in refresh rate. Some cameras may only have a refreshing value of 5 –15 Hz, other (e.g. FLIR Xsc[3]) 180 Hz or even more in no full window mode.

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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.[20]

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.[21]

Thermographic cameras create thermal images based on the radiant heat energy it receives.[22] 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.[23]

• Most cameras have ±2% accuracy or worse in measurement of temperature and are not as accurate as contact methods.[16][17][18]
• Methods and instruments are limited to directly detecting surface temperatures.

Applications

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Thermography finds many uses, and thermal imaging cameras are excellent tools for the maintenance of electrical and mechanical systems in industry and commerce. For example, firefighters use it to see through smoke, find people, and localize hotspots of fires. Power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning.

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 telescopes, even those that are not infrared telescopes. Examples include telescopes such as UKIRT, the Spitzer Space Telescope, WISE and the James Webb Space Telescope[24]

For automotive night vision, thermal imaging cameras are also installed in some luxury cars to aid the driver, the first being the Cadillac DeVille.

In smartphones, a thermal camera was first integrated into the Cat S60 in .

Industry

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In manufacturing, engineering and research, thermography can be used for:

• Process control
• Research and development of new products
• Condition monitoring
• Electrical distribution equipment diagnosis and maintenance, such as transformer yards and distribution panels
• Nondestructive testing
• Fault diagnosis and troubleshooting
• Program process monitoring
• Quality control in production environments
• Predictive maintenance (early failure warning) on mechanical and electrical equipment
• Data center monitoring
• Inspecting photovoltaic power plants[25]

In building inspection, thermography can be used in:[26]

• Roof inspection, such as for low slope and flat roofing
• Building diagnostics, including building envelope inspections, and energy losses in buildings[27]
• Locating pest infestations
• Energy auditing of building insulation and detection of refrigerant leaks[28]
• Home performance
• Moisture detection in walls and roofs (and thus in turn often part of mold remediation)
• Masonry wall structural analysis

Health

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Some physiological activities, particularly responses such as fever, in human beings and other warm-blooded animals can also be monitored with non-contact thermography. This can be compared to contact thermography such as with traditional thermometers.

Healthcare-related uses include:

• Dynamic angiothermography
• Peripheral vascular disease screening.
• Medical imaging in infrared
• Thermography (medical) - Medical testing for diagnosis
• Carotid artery stenosis (CAS) screening through skin thermal maps.[29]
• Active Dynamic Thermography (ADT) for medical applications.[30][31][32]
• Neuromusculoskeletal disorders.
• Extracranial cerebral and facial vascular disease.
• Facial emotion recognition.[33][34]
• Thyroid gland abnormalities.
• Various other neoplastic, metabolic, and inflammatory conditions.

Security and defence

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Thermography is often used in surveillance, security, firefighting, law enforcement, and anti-terrorism:[35]

• Quarantine monitoring of visitors to a country
• Technical surveillance counter-measures
• Search and rescue operations
• Firefighting operations
• UAV surveillance[36]

In weapons systems, thermography can be used in military and police target detection and acquisition:

• Forward-looking infrared
• Infrared search and track
• Night vision
• Infrared targeting
• Thermal weapon sight

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.[37]

Other applications

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Other areas in which these techniques are used:

• Thermal mapping
• Archaeological kite aerial thermography
• Thermology
• Veterinary thermal imaging[38]
• Thermal imaging in ornithology and other wildlife monitoring[39]
• Nighttime wildlife photography
• Stereo vision[40]
• Chemical imaging
• Volcanology[41]
• Agriculture, e.g., Seed-counting machine[42]
• Baby monitoring systems
• Chemical imaging
• Pollution effluent detection
• Aerial archaeology
• Flame detector
• Meteorology (thermal images from weather satellites are used to determine cloud temperature/height and water vapor concentrations, depending on the wavelength)
• Cricket Umpire Decision Review System. To detect faint contact of the ball with the bat (and hence a heat patch signature on the bat after contact).
• Autonomous navigation

Standards

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ASTM International (ASTM)

• ASTM C, Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings
• ASTM C, Standard Practice for the Location of Wet Insulation in Roofing Systems Using Infrared Imaging
• ATSM D, Standard Test Method for Detecting Delamination in Bridge Decks Using Infrared Thermography
• ASTM E, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems
• ASTM E, Standard Guide for Examining Electrical and Mechanical Equipment with Infrared Thermography

International Organization for Standardization (ISO)

• ISO , Thermal insulation – Qualitative detection of thermal irregularities in building envelopes – Infrared method
• ISO -1, Condition monitoring and diagnostics of machines – Thermography – Part 1: General procedures
• ISO -7, Condition monitoring and diagnostics of machines – Requirements for qualification and assessment of personnel – Part 7: Thermography

Regulation

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Higher-end thermographic cameras are often deemed dual-use military grade equipment, and are export-restricted, particularly if the resolution is 640×480 or greater, unless the refresh rate is 9 Hz or less. The export from the USA of specific thermal cameras is regulated by International Traffic in Arms Regulations.

In biology

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Thermography, by strict definition, is a measurement using an instrument, but some living creatures have natural organs that function as counterparts to bolometers, and thus possess a crude type of thermal imaging capability. This is called thermoception. One of the best known examples is infrared sensing in snakes.

History

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Discovery and research of infrared radiation

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Infrared was discovered in by Sir William Herschel as a form of radiation beyond red light.[43] These "infrared rays" (infra is the Latin prefix for "below") were used mainly for thermal measurement.[44] There are four basic laws of IR radiation: Kirchhoff's law of thermal radiation, Stefan–Boltzmann law, Planck's law, and Wien's displacement law. The development of detectors was mainly focused on the use of thermometers and bolometers until World War I. A significant step in the development of detectors occurred in , when Leopoldo Nobili, using the Seebeck effect, created the first known thermocouple, fabricating an improved thermometer, a crude thermopile. He described this instrument to Macedonio Melloni. Initially, they jointly developed a greatly improved instrument. Subsequently, Melloni worked alone, creating an instrument in (a multielement thermopile) that could detect a person 10 metres away.[45] The next significant step in improving detectors was the bolometer, invented in by Samuel Pierpont Langley.[46] Langley and his assistant Charles Greeley Abbot continued to make improvements in this instrument. By , it could detect radiation from a cow from 400 metres away and was sensitive to differences in temperature of one hundred thousandths (0. C) of a degree Celsius.[47][48] The first commercial thermal imaging camera was sold in for high voltage power line inspections.

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 .[49] This was soon outdone by the first accurate IR iceberg detector, which did not use thermopiles, patented in by R.D. Parker.[50] This was followed by G.A. Barker's proposal to use the IR system to detect forest fires in .[51] The technique was not genuinely industrialized until it was used to analyze heating uniformity in hot steel strips in .[52]

First thermographic camera

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In , Hungarian physicist Kálmán Tihanyi invented the infrared-sensitive (night vision) electronic television camera for anti-aircraft defense in Britain.[53] The first American thermographic camera developed was an infrared line scanner. This was created by the US military and Texas Instruments in [54][failed verification] and took one hour to produce a single image. While several approaches were investigated to improve the speed and accuracy of the technology, one of the most crucial factors dealt with scanning an image, which the AGA company was able to commercialize using a cooled photoconductor.[55]

The first British infrared linescan system was Yellow Duckling of the mid-s.[56] 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,[further explanation needed] but they scanned mechanically. This method had several disadvantages which could be overcome using an electronic scanning system. In 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 s.[57] Tompsett also proposed an idea for solid-state thermal-imaging arrays, which eventually led to modern hybridized single-crystal-slice imaging devices.[55]

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[58] is possible. This technology was a precursor to modern microbolometer technology, and mainly used in firefighting thermal cameras.[59]

Smart sensors

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One of the essential areas of development for security systems was for the ability to intelligently evaluate a signal, as well as warning of a threat's presence. Under the encouragement of the US Strategic Defense Initiative, "smart sensors" began to appear. These are sensors that could integrate sensing, signal extraction, processing, and comprehension.[60] There are two main types of smart sensors. One, similar to what is called a "vision chip" when used in the visible range, allow for preprocessing using smart sensing techniques due to the increase in growth of integrated microcircuitry.[61] The other technology is more oriented to specific use and fulfills its preprocessing goal through its design and structure.[62]

Towards the end of the s, 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.[63] These uses include environmental control, building/art analysis, functional medical diagnostics, and car guidance and collision avoidance systems.[64][65][66][67][68][69]

See also

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• ASTM Subcommittee E20.02 on Radiation Thermometry
• Chemical imaging – Simultaneous measurement of spectra and pictures
• Fluorescent microthermography
• Hyperspectral imaging – Multi-wavelength imaging method
• Infrared and thermal testing
• Infrared non-destructive testing of materials – Materials testing procedurePages displaying short descriptions of redirect targets
• Infrared camera – Infrared imaging used to reveal temperaturePages displaying short descriptions of redirect targets
• Infrared detector – Detector that reacts to infrared (IR) radiation
• Infrared photography – Near-infrared imaging
• Infrared thermometer – Thermometer which infers temperature by measuring infrared energy emission
• Night vision – Ability to see in low light conditions
• Non-contact thermography
• Ora (film)
• Passive infrared sensor – Electronic sensor that measures infrared light
• Sakuma–Hattori equation – Formula for the thermal radiation emitted by a perfect black body
• Thermal imaging camera (firefighting) – Thermal imaging camera in firefightingPages displaying short descriptions of redirect targets
• Thermographic inspection

References

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Infrared thermography is the process of using a thermal imager to detect radiation from an object. Below, we'll discuss how infrared thermography works and how to use it in preventive maintenance.

What Is Infrared Thermography?

Infrared thermography is the process of using a thermal imager to detect radiation (heat) coming from an object, converting it to temperature and displaying an image of the temperature distribution. Images of the detected temperature distribution are called thermograms, and they make it possible to see heat-producing objects invisible to the naked eye. It's widley-used in predictive maintenance and condition monitoring.

Since all objects above absolute zero (-459.67 degrees Fahrenheit) give off thermal infrared energy, thermal imagers can easily detect and display infrared wavelengths regardless of ambient light. A common example of this is using night-vision goggles to detect objects in the dark. Infrared thermography is commonly used in a variety of industries and applications including:

• Machine condition monitoring
• Building diagnostics like moisture, roof and energy-loss inspections
• Medical imaging including peripheral vascular disease, thyroid abnormalities, and metabolic and inflammatory condition monitoring
• Law enforcement and security imaging
• Chemical imaging
• Earth science imaging
• Electrical system monitoring
• Fluid system monitoring

Specific to plant maintenance and condition monitoring, infrared thermography is used in applications such as:

• Monitoring the electrical and mechanical conditions of a motor
• Bearing inspections (abnormal bearing friction)
• Monitoring refractory insulation
• Locating gas, liquids and sludge levels

The primary goal of infrared thermography is to confirm machinery is running normally and to detect abnormal heat patterns within a machine, indicating inefficiency and defects. Inspecting mechanical equipment using infrared thermography is a big advantage for asset managers tasked with condition monitoring. Even though infrared imagers are simple to use, interpreting the data they produce can be a bit more challenging to break down. It's important not only to have a working knowledge of how infrared imagers work, but also baseline knowledge of radiometry and heat transfer processes.

Types of Infrared Thermometers

An infrared thermometer in its most basic form consists of a lens that focuses the infrared thermal radiation onto a detector, which turns the radiant energy into a color-coded signal. Infrared thermometers are designed to measure temperature from a distance, preventing the need for contact with the object being measured. Today, there are a variety of infrared thermometer configurations for specific applications. Following is a look at three of the most common types of infrared thermometers.

• Spot infrared thermometers: Also known as a pyrometer, a spot infrared thermometer resembles a handheld radar gun and is used to detect and measure the temperature at a specific spot on a surface. Spot infrared thermometers are ideal for measuring thermal radiation on hard-to-reach assets or assets operating under extreme conditions.

  You may have seen heating, ventilation and air conditioning (HVAC) technicians using a spot infrared thermometer by pointing the gun toward the ceiling vents to check the temperature of a ventilation system in your office building or home. Common applications for using a spot infrared thermometer for preventive maintenance include:

  • Checking bearings and belts
  • Monitoring electrical rooms
  • Energy audits looking for heat loss
  • Fluid-handling systems
  • Water leaks
  • Panel boards
  • Rotating motor monitoring
  • Boiler operations and steam system monitoring

  Spot infrared thermometers work by using field of view (FOV) and distance-to-spot ratio (D:S). When measuring the temperature of an asset with a spot thermometer, make sure the target is completely in the thermometer's FOV. Also, consider the distance-to-spot ratio, as an error can occur if the background temperature varies from the target temperature. The distance-to-spot ratio is the ratio of the distance to the object you're measuring and the diameter of the temperature measurement area. The larger the ratio number, the better the instrument's resolution and the smaller the area that can be measured. For example, a spot thermometer with a 40-to-1 ratio more accurately measures a smaller object than one with a 10-to-1 ratio.

• Infrared scanner systems: These infrared thermometers scan larger areas and are often used in manufacturing plants with conveyors or web processes. Scanning objects on a conveyor belt or sheets of glass or metal leaving an oven are common applications for infrared scanner systems.
• Infrared thermal-imaging cameras: A thermal-imaging camera is an advanced type of radiation thermometer used for measuring temperature at multiple points across a large area and creating two-dimensional thermographic images. Thermal-imaging cameras are considerably more software- and hardware-based than a spot thermometer. Most cameras display real-time images and can be hooked up to specialized software for deeper evaluation, accuracy and report generation. Modern thermal-imaging cameras are handheld.

  Infrared thermal-imaging cameras let users toggle between multiple color palettes to help decipher various temperature differences.

  • Iron palette: The iron palette is the most common. It shows the coldest areas in black, slightly hotter areas in blue/purple, mid-range temperatures in red/orange/yellow and white for the hottest temperatures.
  • Black and white palette: Sometimes called grayscale, this color palette displays details very well by only showing black to white colors passing through multiple levels of gray. The most common application for grayscale is night vision or security cameras. It's rarely used in machinery imaging because it's more difficult to distinguish temperature variation when only two colors are used.
  • Rainbow palette: The rainbow palette shows thermal sensitivity the best by displaying temperature differences through multiple colors. Similar to the iron palette, the rainbow palette uses more color to indicate greater temperature variation.

  Other infrared camera features include a color alarm, picture-in-picture and fusion blending. The color alarm lets you select a temperature, so the camera will only show a color thermal image of an asset below or above the selected temperature. Fusion blending allows you to blend the minimum or maximum average temperature of a thermal image with a standard digital image.

  It's easy to get distracted with the features of a thermal-imaging camera, and many of those features offer valuable information. So, what should you look for in a thermal-imaging camera? The two most important features you should consider are the detector resolution and thermal sensitivity.

  • Detector resolution: Detector resolution tells you the number of pixels displayed in your images. Your camera should include the most common resolutions of 160x120, 320x240 and 640x480. A 640x480 imager displays an image made up of 307,200 pixels.
  • Thermal sensitivity: This refers to the smallest temperature difference the thermal-imaging camera can detect. For example, a camera showing a sensitivity of 0.05 degrees means it can tell the difference between two surfaces with a five-hundredth of a degree temperature difference.

  Also, consider the thermal-imaging camera's temperature range, which is the minimum and maximum temperature the camera can measure. The typical temperature range is minus 4 degrees Fahrenheit to 2,200 degrees Fahrenheit.

Obtaining the Best Image for Analyzing

Just like using a digital single-lens reflex (DSLR) camera, to get the best thermal image out of your camera, you'll need to make adjustments. Consider adjusting the focus, emissivity setting, reflective temperature setting and thermal tuning. Emissivity refers to the amount of radiation an object is giving off compared to if both objects were the same temperature.

As the name suggests, adjusting for reflective temperature lets you compensate for the temperature of surrounding objects that may be reflecting on the target object. Finally, thermal tuning your camera is the process of adjusting its temperature range while it's in manual mode. Once you've adjusted the camera to the desired range, it should always display that range. Be sure to adjust these four settings when taking a temperature measurement or when comparing the temperature of two objects.

How to Use Infrared Thermography

Infrared thermography is a valuable tool for condition monitoring and preventive maintenance. Not only does it allow you to detect thermal abnormalities of machines, but it lets you do so in a non-intrusive, hands-off way while still getting results in real-time. Thermographers usually employ one of three methods when performing thermal inspections: comparative, baseline and thermal trending. Determining which method to use comes down to the type of equipment you're testing and the type of data you want to see.

• Comparative thermography: Comparative thermography is used to measure the temperature of similar components under similar conditions. By comparing the results, useful information about the components' condition is obtained, thanks to the uncovering of hidden problems. Comparative thermography comparisons can be quantitative or qualitative. Quantitative inspections measure precise temperature and/or temperature distribution and are typically performed by a highly trained thermographer.

  Qualitative inspections focus on the differences in temperature rather than actual temperatures. Nearly all (90 percent) industrial and mechanical applications for modern thermography are qualitative, but using quantitative measurements in tandem with qualitative measurements can help determine the severity of the condition as well as the problem itself.

• Baseline thermography: Baseline thermography is used to set a precedent or establish a reference point for an asset by taking temperature readings when the asset is in good working order. It is used in comparison with other thermal images to identify potential issues early. It's recommended to take baseline measurements on all critical assets when they are new or have just been repaired.
• Thermal-trending thermography: Just as trends show changes over time, thermal-trending thermography shows how temperature is dispersed in a component or asset over time. It's a great method for looking at mechanical equipment with complex thermal signatures or when thermal signatures develop slowly. A great example of using thermal trending is monitoring high-temperature refractory insulation in a boiler over time to help determine a proper maintenance schedule that minimizes downtime.

Infrared Thermography Assessment Criteria

When using infrared thermography as a tool for condition monitoring, it's recommended you establish severity criteria. Severity criteria can be presented in two forms: general categories identifying temperature levels or specific categories of machines or components. Severity criteria develop over time with an accumulation of data. It's best practice to develop severity criteria specific to each category of equipment based on the equipment's design, operation, installation, maintenance characteristics, criticality and failure modes.

Establishing severity criteria on individual machines or components is based on a number of factors, including temperature rise vs. historical data, determining the rate of deterioration and time to failure, how critical the machine or component is to the overall process, safety, etc. Rises in temperature for critical machines, mechanical components, bearings, electrical supply and more are common applications used by thermographers to classify temperature severity or mechanical abnormalities.

• Relative temperature criteria: Relative temperature criteria are a set of safety criteria based on temperature rises divided into categories. For example, you may have advisory, intermediate, serious and critical categories. Under the advisory category, you may have a set rule stating that a machine falls under the advisory category when the temperature rises 10 degrees above a reference or baseline temperature. The critical category may state that a machine falls under the critical category when the temperature rises more than 104 degrees above a reference or baseline temperature.
• Absolute temperature criteria: A thermographer may use material or design criteria based on the absolute maximum allowable temperature derived from previously published data. Material criteria are used when the monitoring focus is on the machine's material, while design criteria are used when the monitoring focus is on the machine's design. Although the criteria are divided into these two categories, design usually encompasses the material aspect, which makes it the preferred criteria when it comes to monitoring reliability. If you're using material criteria to measure the heating of multiple adjacent components, the component material with the lowest temperature specification should be used as your "alarm criteria."
• Profile assessment criteria: When you compare temperature differences and patterns across any surface, you're practicing a process known as profile assessment. In the case of thermography, to perform a profile assessment, you must first conduct a severity assessment to determine the absolute and differential temperatures. This will tell you the condition of the machine or component based on two categories: "as new" or "failed." The key areas of a profile assessment are temperature gradients, historical changes, localized differences, absolute temperatures or location of abnormalities, according to Hitchcock.

Interpreting Infrared Data Correctly

As mentioned previously, ease of use is why infrared thermography has become such a widely used tool for preventive maintenance. However, interpreting the data and understanding imager capabilities can lead to some common mistakes. These include:

• Not understanding resolution: Infrared thermography imagers have limitations on what they see and measure. As discussed earlier, make sure you know the resolution limitations before purchasing an infrared imager.
• Discounting ambient conditions: Wind, rain, ambient air temperature and sunlight all factor into the final temperature measurement. Take note of and account for things like wind and rain in your reporting. Inspect surfaces out of direct sunlight to avoid abnormal temperatures.
• Disregarding cold spots: While it's a given that you will be looking for hot spots, sometimes the problem comes from cold spots. This is especially true with electrical or steam systems. Cold spots could indicate that no electrical current is flowing through a capacitor or that a steam trap is not functioning properly.
• Solely focusing on surface temperature: Due to ambient factors, thermal imaging isn't great at showing accurate temperatures. Because of this, focus on differences in thermal patterns rather than the apparent temperature. Put simply, when comparing multiple components with infrared thermography, the one that looks different than the others is probably the one causing the issues.

Infrared Thermography Testing Techniques

When it comes to infrared thermography testing techniques, there are several options from which you can choose. Your selection will depend on the considerations discussed above, including what set of data you're needing and what you're monitoring. Let's take a look at some of the most common infrared thermography testing techniques.

• Passive thermography: This testing technique involves taking thermal images while the machine is running or immediately after operation. This allows you to gather data without an external energy source or taking the machine offline.
• Active thermography: This technique requires an external energy source to create temperature variances in the component which are influenced by interior materials and defects. It is used to show how heat flows through a component and for locating abnormalities in components while in use.
• Flash thermography: This technique uses pulses of light to locate gaps, inclusions or other obstructions that block heat flow into a component.
• Vibrothermography: By introducing acoustic waves into a machine or component, vibrothermography can determine where cracks may have formed in the material. The disturbance caused by acoustic energy creates friction between the two rough edges on either side of the crack. This produces heat, which is detected by the infrared camera.
• Lock-in thermography: Like many of the other techniques, lock-in thermography requires that an external energy source (light, sound, heat, etc.) be applied to a component's surface to reveal abnormalities below the surface. It's important to know the depth, size and location of the abnormality, as well as the material makeup and properties for this technique to be accurate. While it takes longer than the techniques previously discussed, it can penetrate components with thicker walls.

Infrared Thermography and Preventive Maintenance

Infrared thermography is a highly recommended preventive maintenance tool in nearly all industries. You won't find another tool that gives you such accurate, real-time data without disrupting the process flow from shutting down your systems. Working infrared thermography into your regularly scheduled maintenance procedures is a great way to catch abnormalities in components and machines quickly. Using baseline thermography on new equipment or after repairing equipment will provide a set of thermal images to compare all other tests against and lets you more easily troubleshoot future issues.

Advantages vs. Disadvantages of Infrared thermography

Advantages of using infrared thermography in preventive maintenance include:

• Requires no contact with components or machines
• Real-time output
• Can obtain data on large surface areas at one time
• Easy-to-read visual images
• Data can be uploaded to software for further analysis
• Infrared cameras offer great mobility
• No downtime or production interruptions for testing

While there are many advantages to using infrared thermography, it's always a good idea to be aware of some of the disadvantages:

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