Poincaré cycles and coherence of bounded thermal radiation fields

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Abstract
We study the thermal radiation field in finite cavities bounded by perfectly reflecting walls. Previous analytical results on the spatially averaged temporal correlation tensors for the cube-shaped blackbody are generalized for cuboidal cavities of arbitrary edge lengths including the infinite slab resonator. Computational results for very small cubes and sufficiently long times demonstrate the almost periodic character of the coherence function. The transition from cyclic to aperiodic relaxation is shown for the visibility, the phase, and the field autocorrelation. Improved results on the spectral mode density and total radiation energy are obtained as byproducts.