  We propose in this article an unambiguous definition of the local density of
electromagnetic states (LDOS) in a vacuum near an interface in an equilibrium
situation at temperature $T$. We show that the LDOS depends only on the
electric field Green function of the system but does not reduce in general to
the trace of its imaginary part as often used in the literature. We illustrate
this result by a study of the LDOS variations with the distance to an interface
and point out deviations from the standard definition. We show nevertheless
that this definition remains correct at frequencies close to the material
resonances such as surface polaritons. We also study the feasability of
detecting such a LDOS with apetureless SNOM techniques. We first show that a
thermal near-field emission spectrum above a sample should be detectable and
that this measurement could give access to the electromagnetic LDOS. It is
further shown that the apertureless SNOM is the optical analog of the scanning
tunneling microscope which is known to detect the electronic LDOS. We also
discuss some recent SNOM experiments aimed at detecting the electromagnetic
LDOS.

Optics