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A thermal imager, sometimes called a thermal camera or thermograph, is an instrument which senses the distribution of infrared radiation from a target area and presents it as a visible display. It creates a picture of the thermal radiation in a scene and hence gives information about the distribution of temperature on the surfaces in view.

Thermal imagers, developed principally for military purposes, became available in the 1960s. Originally they were used to gain tactical advantage by detecting vehicles and people in darkness. Performance and reliability improved with advances in technology; accompanied by reductions in cost, size and weight, they now have a wide variety of applications. Military use expanded to include more sophisticated applications such as missile guidance.

Thermography is used in the medical and veterinary fields for various diagnostic purposes such as detecting tumours and identifying damage to soft tissue which give rise to locally raised temperatures. Industrial uses include predictive and preventative plant maintenance. Here, thermal imagers perform routine inspections to identify developing fault conditions by observing anomalies in the surface temperature of plant or equipment. Thermography is also used for quality control and process monitoring. Pulsed thermography is a technique in which the surface of an object is subjected to an impulse of heat. Observation of the rate of change of the object's surface temperature following the pulse can reveal the presence of internal defects.

Thermal imaging systems are used widely for surveillance. Modern instruments are becoming sufficiently compact and inexpensive to be used as night vision devices in vehicles, aircraft and vessels. Other applications include satellite surveys, astronomical measurements and a variety of research applications.

Most thermal imaging systems operate in wavebands extending between 3 and 5 μm, or 8 and 14 μm in which reasonable levels of target exitance are combined with good atmospheric transmission.

The performance of a thermal imaging system usually is expressed in terms of spatial and temperature resolutions. The former is expressed as the angular size of the smallest target that can be discerned by the instrument, given that the target has a large temperature difference from its background. Values of 1 to 3 milliradians are typical. Temperature resolution usually is described by the "Noise Equivalent Temperature Difference", which expresses the noise created in the instrument's detector and signal processing electronics as a temperature fluctuation, and describes the limiting value of temperature difference that can be discerned in a scene. Typical values range from 0.05 to 1.0 Kelvin. A combined parameter which may be used to describe the overall performance is "Minimum Resolvable Temperature Difference", which defines the smallest increment of temperature that can be detected in a target of a given size.

Essentially, a thermal imager consists of an optical system, an infrared detector, a signal processing system and a display. Infrared detectors are either quantum devices in which the incidence of radiation causes a change in the electronic state of the detector material, or thermal devices in which the heating of the material causes a measurable change in one of its physical properties. The detector may be a two-dimensional array, each element of which indicates the radiant intensity of a single point in the scene. Alternatively, a detector which contains a single element or a linear array of elements may be scanned across the whole scene by a system of mirrors or prisms. The choice of detector type is governed by a range of considerations which include performance, cost, weight, size, power and other logistic requirements.

The signal processing system converts the raw detector output into a format suitable for display and recording. Often this is in the form of a standard television signal which can be used to operate a CRT or LCD display and is suitable for recording on video equipment. The output may also be in the form of a stream of digital information to allow interpretation of the thermal scene by computer. Other signal processing functions may include the addition of artificial color to provide a display which allows easy identification of features of interest, and the accurate measurement of temperature at points within the scene.

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