Thermography is a thermal imaging technique using an infrared camera. Any material that is at a temperature above 0 K (−273.16°C) gives out electromagnetic radiation at a particular wavelength. The wavelength of this electromagnetic radiation is just above the wavelengths of visible light. The wavelength of the visible light is 4000 to 7000 angstrom (1 angstrom = 10 −10 m). Electromagnetic waves radiated in the range of 9000 to 14,000 angstrom are known as (IR) infrared waves.
This thermal radiation is measured by suitable infrared detectors to provide a means to know the surface temperature of the thermal energy-emitting body The energy emitted by the body is directly proportional to the fourth power of its temperature in Kelvin scale. This constant of proportionality is known as the Stefan–Boltzmann constant, which has a value of 5.67 × 10−8 W/m2K4. The fractions of the radiated energy, which describe the absorption of the incident energy, is denoted by α, the reflected energy is ρ, and the transmitted energy is τ. Each coefficient can vary anywhere from zero to one. A body where no energy is transmitted or reflected and all the incident energy is absorbed is known as a black body. This amount of energy absorbed by a black body is independent of the wavelength of thermal radiation. A black body is also a perfect emitter and the energy emitted by a black body is independent of the direction of radiation.
no real-world body is a perfect black body and thus every body that radiates thermal energy is characterized by its surface emissivity An object that has the same emissivity at all wavelengths is known as a grey body. Thus the Stefan-Boltzmann law is given as in Equation (10.3) W = εσT4 (10.3 Based on the above principles of thermal radiation, commercial infrared detectors are available for measuring the surface temperature of a radiating body. Thermal imaging devices The thermal energy emitted by a black body varies according to the wavelength of the radiation and is given, according to the Plank distribution
The Stefan–Boltzmann law in Eq. 11-3 gives the total blackbody emissive power Eb, which is the sum of the radiation emitted over all wavelengths. Sometimes we need to know the spectral blackbody emissive power, which is the amount of radiation energy emitted by a blackbody at an absolute temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength .
E = monochromatic black body emissive power [W/m2⋅μm] λ = wavelength, μm T = temperature, K C 1 = 3.743 × 108 W·μm4/m2 C 2 = 1.4387 × 104 μm·K bλ
Optical pyrometer Optical pyrometers are used to measure surface temperature of radiating bodies, where, because of the high temperatures (as in the blast furnace of a steel plant), conventional contact-type measurements at high temperatures above 500°C would not work. no direct contact between the pyrometer and the object whose temperature is to be found out. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a
lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength.
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A reference lamp, which is powered with the help of a battery.
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A rheostat to change the current and hence the brightness intensity.
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A red filter placed between the eye piece and the reference bulb helps in narrowing the band of wavelength.
The radiation from the source is emitted and the optical objective lens captures it. The lens helps in focusing the thermal radiation on to the reference bulb. The observer watches the process through the eye piece and corrects it in such a manner that the reference lamp filament has a sharp focus and the filament is super-imposed on the temperature source image. Filament disappears. Thus, there is equal brightness between the filament and temperature source. At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.
When the glow of the reference matches with the hot object through an eye piece, that electric current is measured to calibrate the temperature of the hot body. In front of the eye piece there is a coloured glass (usually red), to make lights monochromatic. Optical Pyrometers work on the basic principle of using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument. The optical system contains filters that restrict the wavelength-sensitivity of the devices to a narrow wavelength band around 0.65 to 0.66 micons
infrared cameras operate in wavelengths as long as 14,000 nm (14 µm) It works even in total darkness because ambient light level does not matter.