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The emission of light by a material at temperature T has been shown recently to be coherent in the near field. These properties were attributed to the thermal excitation of surface polaritons. We review the origin of this phenomenon. We analyze the influence of the microstructure and temperature on the coherence properties and show how to engineer thermoradiative properties of surfaces. We report the design of a quasi-isotropic source and a very directional source of thermal light. We also report a measurement of the transverse coherence length of a thermal source of light.
Marquier et al. (2004) studied this question.