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Infrared thermography is a non-contact imaging technique for visualising infrared radiation. The IR radiation emitted from an object has different intensity depending on its surface temperature. An IR camera detector senses the IR radiation and electronically displays a visual image of the temperatures – a thermal image or thermogram.
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.
There are three types of thermography: liquid crystal thermography (LCT), infrared thermography (IRT) and microwave thermography (MWT). The non-invasive and high resolution characteristics of the thermographic systems make them valuable diagnostic as well as therapeutic aids.
Before the year 1800, the existence of the infrared portion of the electromagnetic spectrum wasn’t even suspected. The original significance of the infrared spectrum as a form of heat radiation is perhaps less obvious today than it was at the time of its discovery in 1800 by Sir William Herschel during his search for a new optical material.
The infrared ray is a kind of electromagnetic wave with a frequency higher than the radio frequencies and lower than visible light frequencies. The infrared region of the electromagnetic spectrum is usually taken as 0.77 and 100 μm for convenience it is often split into near infrared (0.77 to 1.5μm), middle infrared (1.5 to 6μm) and far infra (60-40μm) and far far infrared (40 to 100μm)
Infrared rays are radiated spontaneously by all objects having a temperature above absolute zero (-459.67 .Black body radiation law is the actual principle which works on thermography. A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body’s temperature.
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.
The total energy ‘W’ emitted by the object and its temperature are related by the Stefan Boltzmann formula, W= Where W = radiant flux density Stefan Boltzmann constant =5.64*10-2 T = Absolute temperature
Based on the intensity of the IR radiation, it determines the temperature of the object’s surface, and makes it visible for the human eye with a thermal image. A thermal image allows us to sense the temperature of an object or at least accurately tell its temperature relative to its environment.
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 pass band 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.
Thermography is mainly divided into two types, i.e.:
In passive thermography, the feature 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.
Where as in active thermography, an energy 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.
Medical thermography is the estimation of spatial distribution of temperature on the body surface. It is the only passive medical imaging modality, utilizing radiation energy produced by the body itself. The human body absorbs IR radiation almost without reflection, and at the same time, emits part of its own thermal energy in the form of infrared radiation. It often facilitates detection of pathological changes before any method of investigation.
Infrared thermography is based on analysis of skin surface temperatures as a reflection of normal or abnormal human physiology using a highly specialized IR- camera. In a fraction of second, a large area of the body can be imaged to an accuracy of less than 0.1 as well as a spatial resolution of 25-50 micrometres and, dynamic responses to stimuli are easily documented.
A Thermographic camera is a non-contact device that forms an image using infrared radiation, similar to a common camera that forms an image using visible light. Instead of the 450-750 nanometer range of the visible light camera, infrared cameras operate in wavelength as long as 14000 nm (14µm)
An infrared camera is a non-contact device that detects infrared energy (heat) and converts it into an electronic signal, which is then processed to produce a thermal image on a video monitor and perform temperature calculations. Heat sensed by an infrared camera can be very precisely quantified, or measured, allowing you to not only monitor thermal performance, but also identify and evaluate the relative severity of heat-related problems.
Images from infrared cameras tend to have a single color channel because the cameras generally use a sensor that does not distinguish different wavelengths of infrared radiation. Color cameras require a more complex construction to differentiate wavelength and color has less meaning outside of the normal visible spectrum because the different wavelengths do not map uniformly into the system of color vision 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 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. This technique is called density slicing.
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