  In acoustic, ultrasonic or electromagnetic propagation, crossed media are
often modelled by linear filters with complex gains in accordance with the
Beer-Lambert law. This paper addresses the problem of propagation in media
where polarization has to be taken into account. Because waves are now
bi-dimensional, an unique filter is not sufficient to represent the effects of
the medium. We propose a model which uses four linear invariant filters, which
allows to take into account exchanges between components of the field. We call
it bi-filter because it has two inputs and two outputs. Such a circuit can be
fitted to light devices like polarizers, rotators and compensators and to
propagation in free space. We give a generalization of the Beer-Lambert law
which can be reduced to the usual one in some cases and which justifies the
proposed model for propagation of electromagnic beams in continuous media.
