Blackbody, spectral radiant emittance

Figure 5 Spectral characteristics of blackbody radiation. The spectral radiant emittance is plotted as a function of the wavelength for several values of the absolute temperature. The slartted, dashed line indicates Wien s displacement law.

The emissivity, S, is the ratio of the radiant emittance of a body to that of a blackbody at the same temperature. Kirchhoff s law requires that a = e for aH bodies at thermal equHibrium. For a blackbody, a = e = 1. Near room temperature, most clean metals have emissivities below 0.1, and most nonmetals have emissivities above 0.9. This description is of the spectraHy integrated (or total) absorptivity, reflectivity, transmissivity, and emissivity. These terms can also be defined as spectral properties, functions of wavelength or wavenumber, and the relations hold for the spectral properties as weH (71,74—76). 

There is a maximum amount of radiant energy emitted by a body at a given absolute temperature T at a wavelength X. This maximum amount of radiant emission is the spectral blackbody radiation intensity Ixb(T) the emitter of such radiation is named a blackbody. This spectral blackbody radiation intensity is independent of direction. For a blackbody at an absolute temperature T and emitting radiative energy into a vacuum, I. b(T) is calculated from the relation given by Planck, 1959 [1], in the form... 

The spectral emittance is defined as the ratio of the radiant power per unit area leaving the surface of a body at some given wavelength to that leaving a blackbody at the same temperature. The spectral emittance can be determined practically by comparing the observed or apparent surface temperature of a material with that of a blackbody cavity existing in the same material. The normal spectral emittance is a special case in which the viewing direction is normal to the smooth, opaque surface of the crystalline material. Emissivity is a property of the surface of real specimens. 

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