Infrared thermography, thermal imaging, and thermal video are examples of infrared imaging science. Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9000–14,000 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.
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past fifty years. Government and airport personnel used thermography to detect suspected swine flu cases during the 2009 pandemic.[1] 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 tell-tale 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. Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics.
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 image of an 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.
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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.
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 is a term representing a material's ability to emit thermal radiation. Each material has a different emissivity, and it can be quite a task 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.
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.
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 show something, it must be over 250°C or be reflecting infrared radiation from something that is at least that hot. 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. Starlight-type night vision devices generally only magnify ambient light.
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.
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.