Thermography

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This article is about the infrared imaging technique. For the printing technique called thermography, see thermographic printing.
Thermogram of a traditional building in the background and a "passive house" in the foreground

Infrared thermography (IRT), thermal imaging, and thermal video are examples of infrared imaging science. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nanometers or 9–14 µm) and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects 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; therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to military and other users of surveillance cameras.

Thermogram of a cat

Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics. Thermography is often used for breast screening, allergy detection, and in veterinary use. Government and airport personnel used thermography to detect suspected swine flu cases during the 2009 pandemic.[1]

Right breast cancer
Thermal imaging camera and screen. Thermal imaging can detect elevated body temperature, one of the signs of the virus H1N1 (Swine influenza).

Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past fifty years. Firefighters use thermography to see through smoke, to find persons, and to localize 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 and can use the results to improve 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.

Non-specialized 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 and higher.[2]

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 160x120 or 320x240 pixels, up to 640x512 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 the military uses for this technology. Older bolometers or more sensitive models such as InSb require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or liquid nitrogen.

Thermal energy[edit]

Thermal images, or thermograms, are actually visual displays of the amount of infrared energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature. This phenomenon may become clearer upon consideration of the formula Incident Energy = Emitted Energy + Transmitted Energy + Reflected Energy where Incident Energy is the energy profile when viewed through a thermal imaging camera. Emitted Energy is generally what is intended to be measured. Transmitted Energy is the energy that passes through the subject from a remote thermal source. Reflected Energy is the amount of energy that reflects off the surface of the object from a remote thermal source.

If the object is radiating at a higher temperature than its surroundings, then power transfer will be taking place and power will be 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 the radiation emitted by the warm object. The ability of both objects to emit or absorb this radiation is called emissivity. Under outdoor environments, convective cooling from wind may also need to be considered when trying to get an accurate temperature reading.

This thermogram shows a fault with an industrial electrical fuse block.

The thermal imaging camera would next employ a series of mathematical algorithms. Since the camera is only able to see the electromagnetic radiation that is impossible to detect with the human eye, it will build a picture in the viewer and record a visible picture, usually in a JPG format.

In order to perform the role of noncontact temperature recorder, the camera will change the temperature of the object being viewed with its emissivity setting. Other algorithms can be used to affect the measurement, including the transmission ability of the transmitting medium (usually air) and the temperature of that transmitting medium. All these settings will affect the ultimate output for the temperature of the object being viewed. This functionality makes the thermal imaging camera an excellent tool for the maintenance of electrical and mechanical systems in industry and commerce. By using the proper camera settings and by being careful when capturing the image, 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, the thermal imaging camera can do more. Because it can see the radiating temperature of an object as well as what that object is radiating at, the product of the radiation can be calculated using the Stefan–Boltzmann constant.

Emissivity[edit]

Emissivity is a term representing a material's ability to emit thermal radiation. Each material has a different emissivity, and it can be difficult to determine the appropriate emissivity for a subject. A material's emissivity can range from a theoretical 0.00 (completely not-emitting) to an equally theoretical 1.00 (completely emitting); the emissivity often varies with temperature. An example of a substance with low emissivity would be silver, with an emissivity coefficient of .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 which will radiate infrared radiation at its contact temperature. If a thermocouple on a black body radiator reads 50 °C, the radiation the black body will give up will also be 50 °C. Therefore a true black body will have an emissivity of 1.

Thermogram of a snake held by a human

Since there is no such thing as a perfect black body, the infrared radiation of normal objects will appear to be less than the contact temperature. The rate (percentage) of emission of infrared radiation will thus be a fraction of the true contact temperature. This fraction is called emissivity.

Some objects have different emissivities in long wave as compared to mid wave emissions. Emissivities may also change as a function of temperature in some materials.[3]

To make a temperature measurement of an object, the thermographer will refer to the emissivity table to choose the emissivity value of the object, which is then entered into the camera. The camera's algorithm will correct the temperature by using the emissivity to calculate a temperature that more closely matches the actual contact temperature of the object.

If possible, the thermographer would try to test the emissivity of the object in question. This would be more accurate than attempting to determine the emissivity of the object via a table. The usual method of testing the emissivity is to place a material of known high emissivity in contact with the surface of the object. The material of known emissivity can be as complex as industrial emissivity spray which is produced specifically for this purpose, or it can be as simple as standard black insulation tape, emissivity 0.97. A temperature reading can then be taken of the object with the emissivity level on the imager set to the value of the test material. This will give an accurate value of the temperature of the object. The temperature can then be read on a part of the object not covered with the test material. If the temperature reading is different, the emissivity level on the imager can be adjusted until the object reads the same temperature. This will give the thermographer a much more accurate emissivity reading. There are times, however, when an emissivity test is not possible due to dangerous or inaccessible conditions. In these situations the thermographer must rely on tables.

Difference between infrared film and thermography[edit]

IR film is sensitive to infrared (IR) radiation in the 250°C to 500°C range, while the range of thermography is approximately −50°C to over 2,000°C. So, for an IR film to work thermographically, it must be over 250°C or be reflecting infrared radiation from something that is at least that hot. (Usually, infrared photographic film is used in conjunction with an IR illuminator, which is a filtered incandescent source or IR diode illuminator, or else with an IR flash (usually a xenon flash that is IR filtered). These correspond with "active" near-IR modes as discussed in the next section.

Night vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. However, again, these are not usually used for thermography due to the high temperature requirements, but are instead used with active near-IR sources.

Starlight-type night vision devices generally only magnify ambient light.

Passive vs. active thermography[edit]

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 vision 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.[4]

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.

Advantages of thermography[edit]

  • It shows a visual picture so temperatures over a large area can be compared
  • It is capable of catching moving targets in real time
  • It is able to find deteriorating, 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[5]
  • It can be used to detect objects in dark areas
  • It has some medical application, essentially in kinesiotherapy

Limitations and disadvantages of thermography[edit]

  • Quality cameras often have a high price range (often US$ 3,000 or more), cheaper are only 40x40 up to 120x120 pixels
  • 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[6]
  • 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[7]
  • Accurate temperature measurements are hindered by differing emissivities and reflections from other surfaces[8]
  • Most cameras have ±2% accuracy or worse in measurement of temperature and are not as accurate as contact methods[9]
  • Only able to directly detect surface temperatures
  • Condition of work, depending of the case, can be drastic: 10°C of difference between internal/external, 10km/h of wind maximum, no direct sun, no recent rain, ...

Applications[edit]

Kite aerial thermogram of the site of Ogilface Castle, Scotland.

Thermal imaging cameras convert the energy in the 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 only see objects warmer than −50°C.

The spectrum and amount of thermal radiation depend strongly on an object's surface temperature. This makes it possible for a thermal imaging camera to display an object's temperature. However, other factors also influence the radiation, which limits the accuracy of this technique. For example, the radiation depends not only on the temperature of the object, but is also a function of the emissivity of the object. Also, radiation originates from the surroundings and is reflected in the object, and the radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.

Standards[edit]

International Organization for Standardization (ISO)
  • ISO 6781, Thermal insulation – Qualitative detection of thermal irregularities in building envelopes – Infrared method
  • ISO 18434-1, Condition monitoring and diagnostics of machines – Thermography – Part 1: General procedures
  • ISO 18436-7, Condition monitoring and diagnostics of machines – Requirements for qualification and assessment of personnel – Part 7: Thermography

Biological counterpart[edit]

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

See also[edit]

References[edit]

  1. ^ FLIR Infrared Cameras Help Detect the Spreading of Swine Flu and Other Viral Diseases. Applegate.co.uk (2009-04-29). Retrieved on 2013-06-18.
  2. ^ Poryev V.A., Poryev G.V. "Experimental determination of the temperature range of a television pyrometer" in Journal of Optical Technology, Volume 71, Issue 1, pp. 70–71 (2004)
  3. ^ Table of Emissivities. monarchserver.com. Retrieved on 2013-06-18.
  4. ^ Maldague X. P. V., Jones T. S., Kaplan H., Marinetti S. and Prystay M. "Chapter 2: Fundamentals of Infrared and Thermal Testing: Part 1. Principles of Infrared and Thermal Testing," in Nondestructive Handbook, Infrared and Thermal Testing, Volume 3, X. Maldague technical ed., P. O. Moore ed., 3rd edition, Columbus, Ohio, ASNT Press, 2001, 718 p.
  5. ^ Using Thermography to Find a Class of Latent Construction Defects. Globalspec.com. Retrieved on 2013-06-18.
  6. ^ F. Colbert, "Looking Under the Hood: Converting Proprietary Image File Formats Created within IR Cameras for Improved Archival Use", Professional Thermographers Association [1]
  7. ^ Infrared Temperature Theory and Application. Omega.com. Retrieved on 2013-06-18.
  8. ^ Real Time Emissivity Measurement for Infrared Temperature Measurement. Pyrometer.com. Retrieved on 2013-06-18.
  9. ^ Snell, J.; Renowden, J. (2000). "Improving results of thermographic inspections of electrical transmission and distribution lines". 2000 IEEE ESMO – 2000 IEEE 9th International Conference on Transmission and Distribution Construction, Operation and Live-Line Maintenance Proceedings. ESMO 2000 Proceedings. Global ESMO 2000. The Power is in Your Hands (Cat. No.00CH37183). p. 135. doi:10.1109/TDCLLM.2000.882811. ISBN 0-7803-6625-5. 
  10. ^ Breckon, T.; Gaszczak, A., Han, J., (2011). "Real-time People and Vehicle Detection from UAV Imagery". In Röning, Juha; Casasent, David P; Hall, Ernest L. Proc. SPIE Conference Intelligent Robots and Computer Vision XXVIII: Algorithms and Techniques. Intelligent Robots and Computer Vision XXVIII: Algorithms and Techniques 7878: 78780B. doi:10.1117/12.876663. 
  11. ^ Pinggera, P.; Breckon, T.P.; Bischof, H.; (2012). "On Cross-Spectral Stereo Matching using Dense Gradient Features". In Proc. British Machine Vision Conference: 526.1–526.12. doi:10.5244/C.26.103. ISBN 1-901725-46-4. 
  12. ^ Thermographic images in active volcanoes surveillance system — TIIMNet project Vesuvius and Solfatara INGV Naples Italy. Ipf.ov.ingv.it. Retrieved on 2013-06-18.
  13. ^ Thermographic images in active volcanoes surveillance system — TIIMNet project equipment details. Tecnovideocctv.com. Retrieved on 2013-06-18.
  14. ^ Infrared Building Inspections — Resources for Electrical, Mechanical, Residential and Commercial Infrared/Thermal Inspections. Infrared-buildinginspections.com (2008-09-04). Retrieved on 2013-06-18.

External links[edit]