  The approach based on the generalized Kirchhoff's law for calculating
photoluminescence (PL) spectra of one-dimensional (1D) multi-layered
structures, in particular, 1D photonic crystals has been developed. It is valid
in the local thermodynamic equilibrium approximation and leads to simple and
explicit expression for the photoluminescence intensity. In the framework of
the present theory the analytical expression for the spontaneous emission
intensity enhancement factor (IEF) for a 1D photonic crystal has been derived.
It takes a particularly simple form in the case of a sufficiently large number
of the layers and is well suitable for analysis; in particular, it explains the
difference in the emission intensity at frequencies near different edges of
photonic band-gaps (PBGs), where the intensity is relatively high, and
specificity of suppression of the emission in a given frequency range. Also,
the developed approach is discussed in connection with the standard method
using the Fermi's golden rule and the concept of the local density of (optical)
states (LDOS).
