  We revisit the Casimir effect perceived by two surfaces in the presence of
infrared (IR) transparency. To address this problem, we study a model, where
such a phenomenon naturally arises: the DGP model with two parallel 3-branes,
each endowed with a localized curvature term. In that model, the UV modes of
the 5-dimensional graviton are suppressed on the branes, while the IR modes can
penetrate them freely. First, we find that the DGP branes act as "effective"
(momentum-dependent) boundary conditions for the gravitational field, so that
the (gravitational) Casimir force between them emerges. Second, we discover
that the presence of an IR transparency region for the discrete modes modifies
the standard Casimir force -- as derived for ideal Dirichlet boundary
conditions -- in two competing ways: i) The exclusion of soft modes from the
discrete spectrum leads to an increase of the Casimir force. ii) The non-ideal
nature of the boundary conditions gives rise to a "leakage" of hard modes. As a
result of i) and ii), the Casimir force becomes weaker. Since the derivation of
this result involves only the localized kinetic terms of a quantum field on
parallel surfaces (with codimension one), the derived Casimir force is expected
to be present in a variety of setups in arbitrary dimensions.
