  We present a general radiative transfer model which allows the Zeeman
diagnostics of complex and unresolved solar magnetic fields. Present modeling
techniques still rely to a large extent on a-priori assumptions about the
geometry of the underlying magnetic field. In an effort to obtain a more
flexible and unbiased approach we pursue a rigorous statistical description of
the underlying atmosphere. Based on a Markov random field model the atmospheric
structures are characterized in terms of probability densities and spatial
correlations. This approach allows us to derive a stochastic transport equation
for polarized light valid in a regime with an arbitrary fluctuating magnetic
field on finite scales. One of the key ingredients of the derived stochastic
transfer equation is the correlation length which provides an additional degree
of freedom to the transport equation and can be used as a diagnostic parameter
to estimate the characteristic length scale of the underlying magnetic field.
  It is shown that the stochastic transfer equation represents a natural
extension of the (polarized) line formation under the micro- and macroturbulent
assumption and contains both approaches as limiting cases. In particular, we
show how in an inhomogeneous atmosphere asymmetric Stokes profiles develop and
that the correlation length directly controls the degree of asymmetry and net
circular polarization (NCP). In a number of simple numerical model calculations
we demonstrate the importance of a finite correlation length for the polarized
line formation and its impact on the resulting Stokes line profiles.
