Radiation from real surfaces

Black body is an idealized concept in radiation. A black body absorbs all incoming radiation and transmits none. Black body is also a perfect emitter, since it emits radiation of all wavelengths. Its total emissive power is theoretically the highest that can be achieved at any given temperature.

Thus, all real surfaces emit thermal radiations lower than black surface at any temperature such that

(4)

In which  is emissivity and  is total radiation from a real surface. Obviously  for a black body and   for all a real surfaces. Thus, monochromatic emissive power of a real surface is

(5)

In view of the Stefan-Boltzmann equation, the total emissive power of a real surface is

(6)

Qualitatively the spectral emissivity of metals decreases with increasing wavelength. For the bare metal surface, the emissivity decreases with increasing wavelength. Roughness increases emissivity.