  This is the first of a series of papers devoted to develop a microscopical
approach to the dipole emission process and its relation to coherent transport
in random media. In this Letter, we deduce general expressions for the decay
rate of spontaneous emitters and the power emission of induced dipoles embedded
in homogenous dielectric media. We derive formulae which apply generically to
virtual cavities and, in the continuum approximation, to small real cavities.
