  The Casimir force can be understood as resulting from the radiation pressure
exerted by the vacuum fluctuations reflected by boundaries. We extend this
local formulation to the case of partially transmitting boundaries by
introducing reflectivity and transmittivity coefficients obeying conditions of
unitarity, causality and high frequency transparency. We show that the
divergences associated with the infiniteness of the vacuum energy do not appear
in this approach. We give explicit expressions for the Casimir force which hold
for any frequency dependent scattering and any temperature. The corresponding
expressions for the Casimir energy are interpreted in terms of phase shifts.
The known results are recovered at the limit of a perfect reflection.
