  The Bogoliubov transformation in thermofield dynamics, an operator formalism
for the finite-temperature quantum-field theory, is generalized to describe a
field in arbitrary confined regions of space and time. Starting with the scalar
field, the approach is extended to the electromagnetic field and the
energy-momentum tensor is written via the Bogoliubov transformation. In this
context, the Casimir effect is calculated for zero and non-zero temperature,
and therefore it can be considered as a vacuum condensation effect of the
electromagnetic field. This aspect opens an interesting perspective for using
this procedure as an effective scheme for calculations in the studies of
confined fields, including the interacting fields.
