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Physics of thermography and its types

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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.

  • Sir William Herschel (1738-1822), Royal Astronomer to King George III of England – and already famous for his discovery of the planet Uranus – was searching for an optical filter material to reduce the brightness of the Sun’s image in telescopes during solar observations. As the blackened thermometer was moved slowly along the colours of the spectrum, the temperature readings showed a steady increase from the violet end to the red end.
  • The Italian researcher Landriani, in a similar experiment in 1777, had observed much the same effect. It was Herschel, however, who was the first to recognize that there must be a point where the heating effect reaches a maximum, and that measurements confined to the visible portion of the spectrum failed to locate this point.
  • When Herschel revealed his discovery, he referred to this new portion of the electromagnetic spectrum as the “thermometric spectrum.” The radiation itself he sometimes referred to as “dark heat” or simply “the invisible rays. “However, it wasn’t Herschel who originated the term “infrared.” The word only began to appear in print around 75 years later, and it’s still unclear who originated it.
  • In the late 1950s and 1960s, Texas Instruments, Hughes Aircraft, and Honeywell developed single-element detectors that scanned scenes and produced line images. The military had a lock on the technology because it was expensive and had sensitive military applications. These basic detectors led to the development of modern thermal imaging.
  • The pyroelectric vidicon tube was developed in the 1970s by Philips and EEV and became the core of a product first used by the Royal Navy for shipboard fire fighting.
  • In 1978, Raytheon’s R&D group, then part of Texas Instruments, patented Ferro-electric infrared detectors that used barium strontium titanate, or BST, which is the material that coats the thermal imager’s sensor.
  • Raytheon first demonstrated the technology to the military in 1979.
  • In the late 1980s, the federal government awarded high-density array development or HIDAD contracts to both Raytheon and Honeywell for the development of thermal imaging technology for practical military applications. Raytheon went on to commercialize BST technology.
  • Honeywell developed vanadium oxide (VOx) micro bolometer technology. Later, federal programs such as LOCUSP (Low-Cost Uncooled Sensor Program), provided funding for both companies to develop their thermal imaging technologies into equipment systems, including rifle sights and drivers’ viewers.
  • After the 1991 Gulf War, production volumes increased and costs decreased, so the use of thermal imaging was introduced to municipal fire fighting services.
  • In late 2004, Raytheon’s Commercial Infrared Division was sold to L-3 Communications. Meanwhile, the Honeywell micro bolometer was awarded a patent in 1994. Boeing, Lockheed-Martin (who sold its infrared business to British Aerospace, or BAE), and others licensed VOx technology from Honeywell developed infrared detectors for military applications.
  • Thermal imagers based on both BST and micro bolometer technologies are now available for non-military applications. In fact, thermal imaging has expanded for used in law enforcement, commercial and industrial applications, security, transportation, and many other industries. Bullard introduced its first thermal imager specifically designed for fire fighting in 1998.
  • The American Society of Non-Destructive Testing developed and approved standards for teaching thermal imaging courses in 1992. These classes are called Level I, II and III. By the early 2000s, infrared camera prices continued to fall and the cameras were getting smaller, so new uses for the building industry began to emerge in earnest. By 2006, thermal imaging using infrared cameras by home inspectors and contractors became more common.
  • In 2008, the International Association of Certified Home Inspectors – InterNACHI – developed its Infrared-Certified® program to teach home inspectors how to use infrared cameras in the wide variety of building inspection applications. Since that time, InterNACHI has been the leading home inspector association to promote and teach its members the effective use of thermal imaging.

Physics of thermography

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.

Types of Thermography

Thermography is mainly divided into two types, i.e.:

  • Active thermography
  • Passive thermography

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

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.

  • 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 deterioration, 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.
  • It can be used to detect objects in dark areas.

Limitations

  • Quality cameras often have a high price range.
  • Accurate temperature measurements are hindered by differing emissivity and reflections from other surfaces.
  • Methods and instruments are limited to directly detecting surface temperatures.

Thermographic cameras

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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