  We study the properties of thermal radiation emitted by a thin dielectric
slab, employing the framework of macroscopic fluctuational electrodynamics.
Particular emphasis is given to the analytical construction of the required
dyadic Green's functions. Based on these, general expressions are derived for
both the system's Poynting vector, describing the intensity of propagating
radiation, and its energy density, containing contributions from
non-propagating modes which dominate the near-field regime. An extensive
discussion is then given for thin metal films. It is shown that the radiative
intensity is maximized for a certain film thickness, due to Fabry-Perot-like
multiple reflections inside the film. The dependence of the near-field energy
density on the distance from the film's surface is governed by an interplay of
several length scales, and characterized by different exponents in different
regimes. In particular, this energy density remains finite even for arbitrarily
thin films. This unexpected feature is associated with the film's low-frequency
surface plasmon polariton. Our results also serve as reference for current
near-field experiments which search for deviations from the macroscopic
approach.
