Definition and realisation of a black body

A black body is defined as a body where all the incident radiation penetrates it and is completely absorbed within it. No radiation is reflected or allowed to pass through it. This holds for radiation of all wavelengths falling onto the body from all angles. In addition to this the black body is a diffuse radiator. Its spectral intensity LXs does not depend on direction, but is a universal function iAs(A,T) of the wavelength and the thermodynamic temperature. The hemispherical spectral emissive power MXs(X,T) is linked to Kirchhoff s function LXs(X,T) by the simple relationship 

The determination of the universal functions LXs(X,T) or MXs(X,T) is a fundamental task of physics. This was solved experimentally towards the end of the 19th century, but a theoretical basis for the measured data was first found by M. Planck4 in 1900. Through assumptions, which later formed one of the foundations of quantum theory, he was able to formulate the law for the spectral intensity 

The black body got its name because of the property of a good absorber to appear black to the eye when visible light falls on its surface. However, the eye is only an indicator of absorption capability of a body within the very small wavelength region of visible light, see Fig. 5.1. Therefore, surfaces which appear to be black, such as soot or black platinum only approximately attain the complete absorption of radiation at all wavelengths, which is the prerequisite for a black body. 

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