Radiation real surface behavior

Equation (8-8) is called Kirchhoffs identity. At this point we note that the emissivities and absorptivities which have been discussed are the total properties of the particular material i.e., they represent the integrated behavior of the material over all wavelengths. Real substances emit less radiation than ideal black surfaces as measured by the emissivity of the material. In reality, the emissivity of a material varies with temperature and the wavelength of the radiation. 

The increase of emissivity with sintering and fusion is due to increased transmission of radiation into the surface of the deposit layer. In the range of surface temperatures of interest, namely between 800 K and 1400 K, measured total emissivities on probes of sintered real furnace deposits exhibit only slight variations with surface temperature ( ), ( ) However, measurements by Becker of spectral emissivities of deposits on laboratory prepared probes showed distinctive nongray behavior. For typical flame temperatures of 1700 K and typical surface temperatures of 1100 K, up to 0.2 higher values were found for emissivities than for absorptivities (Figure 6). 

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