Инфракрасная термография: принципы и применение

Infrared imaging used to reveal temperature
the infrared imaging technique

Infrared thermography (IRT), thermal video and/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 in a process, which are examples 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. 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; 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 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". 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 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. 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 1280 x 1024 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.

Emissivity is a term that is often misunderstood and misused. It represents a material's ability to emit thermal radiation and is an optical property of matter. 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. A material's emissivity can range from a theoretical 0.00 (completely not emitting) to an equally theoretical 1.00 (completely emitting). 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 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 , the black body would emit thermal radiation characteristic of
An ordinary object emits less infrared radiation than a theoretical black body. The fraction of its actual emission to the theoretical emission (of the black body) is its emissivity (or emissivity coefficient). In order to make a temperature measurement of an object using an infrared imager, it is necessary to estimate or determine the object's emissivity. For quick work, a thermographer may refer to an emissivity table for a given type of object, and enter that value into the imager. The imager would then calculate the object's contact temperature based on the value entered from the table and the object's emission of infrared radiation as detected by the imager. In order to get a more accurate temperature measurement, a thermographer may apply a standard material of known, high emissivity to the surface of the object. The standard material might be as complex as 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 that is not covered by the standard material) can then 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. In these situations, the thermographer must rely on tables.

IR film is sensitive to infrared (IR) radiation in the range, while the range of thermography is approximately So, for an IR film to work thermographically, the measured object must be over 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. 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.

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. 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.

Thermography shows a visual picture so temperatures over a large area can be compared. It is capable of catching moving targets in real time. It can be used to detect objects in dark areas. It has some medical application, essentially in physiotherapy.

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.

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 and higher. 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 Bayer filters. It has been performed using color ratios, grayscales, and/or a hybrid of both.