Spatial coherence of thermal near fields
  We analyze the spatial coherence of the electromagnetic field emitted by a half-space at temperature T close to the interface. An asymptotic analysis allows to identify three different contributions to the cross-spectral density tensor in the near-field regime. It is shown that the coherence length can be either much larger or much shorter than the wavelength depending on the dominant contribution. 
C. Henkel, K. Joulain, R. Carminati, J.-J. Greffet