  We report theoretical calculations and experimental observations of
Pancharatnam's phase originating from arbitrary SU(2) transformations applied
to polarization states of light. We have implemented polarimetric and
interferometric methods which allow us to cover the full Poincar\'e sphere. As
a distinctive feature, our interferometric array is robust against mechanical
and thermal disturbances, showing that the polarimetric method is not
inherently superior to the interferometric one, as previously assumed. Our
strategy effectively amounts to feed an interferometer with two copropagating
beams that are orthogonally polarized with respect to each other. It can be
applied to different types of standard arrays, like a Michelson, a Sagnac, or a
Mach-Zehnder interferometer. We exhibit the versatility of our arrangement by
performing measurements of Pancharatnam's phases and fringe visibilities that
closely fit the theoretical predictions. Our approach can be easily extended to
deal with mixed states and to study decoherence effects.
