  The paper is devoted to the thermodynamics of normal surface electromagnetic
fields within a nonuniform dispersive and absorptive system. This system is
formed by vacuum and lossy medium separated by a plane interface. As a medium,
we used dielectric and metal samples characterized by local and nonlocal
optical properties. Thermodynamic properties of surface eigenmodes of plane
interfaces are discussed. Various definitions of density of states and spectral
characteristics of surface polaritons in equilibrium at the interface formed by
vacuum and lossy medium are described and discussed. All formulas for
thermodynamic functions are represented in terms of density of states. The
generalized density of states is calculated based on the Barash-Ginzburg theory
and dispersion relations for the surface states in different approaches. It is
exemplified that different definitions of the density of states are identical
in the case of dissipationless materials. The spectral functions and integrated
over all frequencies thermodynamic characteristics and their temperature
dependences are demonstrated.
