  Most of the reflectometry methods which are used for determining the phase of
complex reflection coefficient such as reference method and Variation of
Surroundings medium are based on solving the Schr\"odinger equation using a
discontinuous and step-like scattering optical potential. However, during the
deposition process for making a real sample the two adjacent layers are mixed
together and the interface would not be discontinuous and sharp. The smearing
of adjacent layers at the interface (smoothness of interface), would affect the
reflectivity, phase of reflection coefficient and reconstruction of the
scattering length density (SLD) of the sample. In this paper, we have
investigated the stability of reference method in the presence of smooth
interfaces. The smoothness of interfaces is considered by using a continuous
function scattering potential. We have also proposed a method to achieve the
most reliable output result while retrieving the SLD of the sample.
