The structure of such a black body is in principle very simple. A heatable hollow body has a small hole at one end through which infrared radiation can escape. If the hollow body is heated to a defined temperature, it is in temperature equilibrium, and black radiation of the set temperature reaches the outside through the small opening. It is important that the outlet opening is as small as possible with respect to the total area of the hollow body in order to minimize the deviation from the ideal state. Cavity radiators achieve emissivities of up to 99%. In practice, however, area radiators are often used for calibration. These have a heated surface with a pigmented paint coating. Area radiators achieve emission and absorption levels up to 97%.
The calibration of infrared measuring devices is done by means of black bodies of different temperature. It is important to know the exact radiation temperature of the radiators. This is determined, for example, with a transfer standard radiation thermometer. Optris uses a CTlaser PTB which can be traced back to the international temperature scale of 1990 and is calibrated at regular intervals by PTB.
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