Statistical properties of the electromagnetic field at blackbody cavity opening and in its far zone

We consider radiation from a blackbody cavity opening and derive an explicit expression for the electric coherence matrix in the aperture and in its far zone. The aperture and far-zone coherence and polarization properties are assessed by measures valid for three-component fields. We analyse the far-zone coherence created by an incoherent, partially polarized planar source and show that paraxially it coincides with the blackbody result. Our conclusions are useful in modelling thermal sources and natural light.