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Infrared Thermography

The Electromagnetic Spectrum

Wavelengths used by thermographic systems

Wavelengths used by thermographic systems

Blackbody

A hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated toa given temperature emit thermal radiation with the same spectrum, as required by arguments of classical physics involving thermal equilibrium. However, the distribution of blackbody radiation as a function of wavelength, known as the Planck law, cannot be predicted using classical physics. This fact was the first motivating force behind the development of quantum mechanics.

The Radiation Laws

Planck’s Law Planck’s Law is sometimes called the “black-body” formula works very well for celestial bodies: cox14
Where E (lambda) is the amount of radiant energy emitted at a given wavelength, lambda. T is the temperature of the object, and a and b are constants. The spectrum of wavelengths emitted by a body at a temperature, T, has a characteristic shape that is strongly dependent on the wavelength (to the inverse fifth power).

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Stefan-Boltzmann Law (“E equals sigma T to the fourth”) cox16
where E is the total energy, sigma is a constant, and T is temperature.

Wein’s Law The wavelength of the peak radiance [lambda (max)] decreases linearly as the temperature increases, where c

is a constant:cox17

Radiation Parameters of Objects

  • Emissivity(ε) : ability of an object to emit infrared radiation
  • - Absorption(α) : ability of an object to absorb infrared radiation
  • - Transmissivity(τ) : ability of an object to transmit infrared radiation
  • - Reflectivity(ρ) : ability of an object to reflect infrared radiation

Blackbody Radiation Spectrum

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Multi-step calibration process

To normalize different FPA detector gains and offsets, the first correction step is offset compensation. This brings each detector response within the dynamic range of the camera’s A/D converter electronics.

To normalize different FPA detector gains and offsets, the first correction step is offset compensation. This brings each detector response within the dynamic range of the camera’s A/D converter electronics.

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After offset compensation, slope correction is applied.

After gain factors are brought to the same value, non-uniformity correction (NUC) is applied so that all detectors have essentially the same electronic characteristics.

Infrared Thermography