# Thermography technique

Laws that link the absolute temperature of the emitting object, peak radiation, the intensity and wavelength are the Plank’s Law, the Stefan-Boltzmann Law and the Wein’s Displacement Law. It can be seen in Fig. 1 that total energy radiated by a blackbody i. area under the spectral radiant emittance increases with increasing temperature of the blackbody. Further it can be seen that maxima of the spectral radiant emittance is shifting towards lower wavelength with increasing temperature of the blackbody. If one differentiates equation (1) with respect to  and equates the differential to zero then one gets the relationship between the temperature of the blackbody and the wavelength corresponding to the maximum spectral radiance.

This relationship is known as Wein’s law and is mathematically expressed as [2]: Integrating equation (1) with respect to  between the limits  = 0 to ∞ for a given temperature T of a blackbody, one gets total radiant power emitted into a hemisphere from the blackbody. This relationship is known as Stefan’s-Boltzmann law and is mathematically expressed as: This law states that total radiant power is proportional to the fourth power of the source temperature.

One can compute the power radiated by human body using equations (3). Let us take human body temperature as 300 K and an external surface area of say 2 m2; then the total radiated power is approximately 1 kilowatt. Detailed review of the principles of infrared imaging is presented in reference [3]. A typical infrared imaging system consists of the infrared camera, control unit, image acquisition and analysis unit. The heart of the system is infrared scanner. This scanner unit converts the electromagnetic thermal energy radiated from an object into electronic video signals.

These signals are amplified and transmitted to a display monitor via an inter-connecting cable. Infrared imaging system can be classified as qualitative or quantitative systems. A qualitative system displays only an isothermal map. This isothermal map is not corrected for emissivity variations, system non-linearity or atmospheric effects. Thermal measurements are not possible from image, as it does not include the temperature. In a quantitative system, the infrared signal is temperature calibrated using an internal blackbody reference. Appropriate correction factors are also applied such that the infrared image displayed has a temperature distribution approaching the true surface

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