  Long baseline interferometry of microlensing events can resolve the
individual images of the source produced by the lens, which combined with the
modelling of the microlensing light curve, leads to the exact lens mass and
distance. Interferometric observations thus offer a unique opportunity to
constrain the mass of exoplanets detected by microlensing, and to precisely
measure the mass of distant isolated objects such as stars and brown dwarfs,
and of stellar remnants such as white dwarfs, neutron stars, and stellar black
holes. Having accurate models and reliable numerical methods is of particular
importance as the number of targets is expected to increase significantly in
the near future. In this work we discuss the different approaches to
calculating the fringe complex visibility for the important case of a single
lens. We propose a robust integration scheme to calculate the exact visibility,
and introduce a novel approximation, which we call the `thin-arcs
approximation', which can be applied over a wide range of lens--source
separations. We find that this approximation runs six to ten times faster than
the exact calculation, depending of the characteristics of the event and the
required accuracy. This approximation provides accurate results for
microlensing events of medium to high magnification observed around the peak
(i.e. a large fraction of potential observational targets).
