Thermography
Infrared thermography, also known as thermal imaging, is a measurement and imaging technique in which a thermal camera detects infrared radiation originating from the surface of objects. This radiation has two main components: thermal emission from the object's surface, which depends on its temperature and emissivity, and reflected radiation from surrounding sources. When the object is not opaque, i.e. exhibits nonzero transmissivity at the cameras operating wavelengths, transmitted radiation also contributes to the observed signal. The result is a visible image called a thermogram. Thermal cameras most commonly operate in the long-wave infrared range ; less frequently, systems designed for the mid-wave infrared range are used.
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". Notably, government and airport personnel used thermography to detect suspected swine flu cases during the 2009 pandemic.
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past 50 years. 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 that respond to longer wavelengths. 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 1280 × 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 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
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.This phenomenon may become clearer upon consideration of the formula:
where incident radiant power is the radiant power profile when viewed through a thermal imaging camera.
Emitted radiant power is generally what is intended to be measured;
transmitted radiant power is the radiant power that passes through the subject from a remote thermal source, and;
reflected radiant power is the amount of radiant power that reflects off the surface of the object from a remote thermal source.
This phenomenon occurs everywhere, all the time. It is a process known as radiant heat exchange, since radiant power × time equals radiant energy. However, in the case of infrared thermography, the above equation is used to describe the radiant power within the spectral wavelength passband of the thermal imaging camera in use. The radiant heat exchange requirements described in the equation apply equally at every wavelength in the electromagnetic spectrum.
If the object is radiating at a higher temperature than its surroundings, then 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
Emissivity 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 to 1. 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, it would emit the characteristic black-body radiation of. An ordinary object emits less infrared radiation than a theoretical black body. In other words, the ratio of the actual emission to the maximum theoretical emission is an object's emissivity.
Each material has a different emissivity which may vary by temperature and infrared wavelength. For example, clean metal surfaces have emissivity that decreases at longer wavelengths; many dielectric materials, such as quartz, sapphire, calcium fluoride, etc. have emissivity that increases at longer wavelength; simple oxides, such as iron oxide display relatively flat emissivity in the infrared spectrum.
Measurement
A thermal imaging camera performs radiometric processing to convert the detected infrared radiation into estimates of the object's surface temperature. This is achieved through the application of the thermography equation, which accounts for emitted and reflected components of radiation, as well as the influence of the atmosphere, which emits its own thermal radiation and attenuates the radiation originating from the measured surface. In the case of a radiometric thermal camera, the output images contain not only visual information but also radiometric data that represent the detected radiation and allow accurate temperature evaluation based on the computational model provided by the thermography equation.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 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. 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
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 parts of the image are customarily colored white, intermediate temperatures reds and yellows, and the dimmest parts black. A scale should be shown next to a false color image to relate colors to temperatures.