In this paper we discuss problems in an interferometer which arise when the interferometer uses a conventional, so-called 'non-cooperative' target rather than a mirror. Under these conditions, the reflected light assumes the character of a speckle field which requires that we describe its statistical properties by applying first order statistics of Stokes parameters. Next we discuss how the interference signal is influenced by a speckle field with an arbitrary degree of polarization. Finally we show how the detrimental effects arising from the speckle field can be reduced when the target surface fluctuations are sufficiently (compared to the wavelength) small.