Thermography technique Essay Example | Topics and Well Written Essays - 1750 words

Thermography technique - Essay Example Thus a complete surface temperature map of the object can be obtained in a non-contact manner. With appropriate calibration, it is also possible to get the absolute temperature values of any point on the surface of the object. Standards are required for calibration and these standards are materials of known emissivity in the temperature range of calibration. Infrared refers to a region of spectrum between the visible and microwave. The infrared spectrum extends from 0.75 mm to 1000 ?m wavelength range. However for practical applications it is the 1 – 15 ?m wavelength band, which is used. The properties of infrared radiations are similar to those of other electromagnetic radiations like light. These radiations travel in straight lines; propagate in vacuum as well as in solid, liquid and gases. These radiations can be optically focussed and directed by mirrors and lenses. The laws of geometrical optics are valid for infrared radiations as well. The energy and intensity of infrar ed radiation emitted by an object primarily depends on its temperature and can be calculated using the analytical tools such as Wein’s law, Plank’s law and Stefan Boltzmann law. When a body is heated, there is an increase in the temperature and emitted energy. The spectrum of infrared radiation emitted by a heated object contains a continuous band of wavelength over a specific range. The fundamental equations or radiation 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. The Plank’s law describes the spectral distribution of radiation intensity from a black body and is mathematically expressed as: [Wm-2sr-1?m-1] †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦.. (1) Where, W? = Blackbody spectral radiant emittance at wavelength ? (?m) c = 3x108 m/s is the velocity of light in vacuum h = 6.634 x10-3 4 Js is Plank’s constant k = 1.4 x10-23 J/K is Boltzmann’s constant T is absolute temperature of the blackbody Spectral radiant emittance of a blackbody at different temperatures is shown in Fig.1 [1]. Fig. 1: Spectral radiant emittance of a blackbody at different temperatures It can be seen in Fig. 1 that total energy radiated by a blackbody i.e. 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]: †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦.. (2) Where, ?max is the wavelength corresponding to the maximum spectral emittance T is absolute temperature of the blackbody This equation supports left ward shift of the spectral emittance peak with increasing temperature of the blackbody as in Fig. 1. 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: Total emittance [W/cm2] †¦