Abstract: In this paper, we establish the form of the spectral degree of spatial coherence of the field thermally emitted by a planar opaque surface. Our approach is based on the fluctuation-dissipation theorem. Several new properties are derived. It is shown that for lossy media, the coherence length in a plane very close to the surface is essentially the skin depth. Thus, it can be much smaller than the wavelength. However, if the metal can support a surface wave, the coherence length can be much larger than the wavelength.!6
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