  We calculate the Casimir energy for scalar fields in interaction with
finite-width mirrors, described by nonlocal interaction terms. These terms,
which include quantum effects due to the matter fields inside the mirrors, are
approximated by means of a local expansion procedure. As a result of this
expansion, an effective theory for the vacuum field emerges, which can be
written in terms of generalized $\delta$-potentials. We compute explicitly the
Casimir energy for these potentials and show that, for some particular cases,
it is possible to reinterpret them as imposing imperfect Dirichlet boundary
conditions
