Role of spatial coherence in polarization tomography

We analyze an experimental setup in which a quasi-monochromatic spatially coherent beam of light is used to probe a paraxial optical scatterer. We discuss the effect of the spatial coherence of the probe beam on the Mueller matrix representing the scatterer. We show that according to the degree of spatial coherence of the beam, the \emph{same} scattering system can be represented by \emph{different} Mueller matrices. This result should serve as a warning for experimentalists.