The differential polarization visibilities R-Q and R-U of an object are the ratios
of its visibilities corresponding to orthogonal polarizations, the interferometric
analogs to Stokes Q and U intensity images. The measurement of differential polarization
visibilitites can be used e.g. for constraining inner parts of circumstellar envelopes
of young or evolved stars at the diffraction limited resolution of the feeding telescope.
Here we demonstrate the estimation of both amplitude and phase of RQ and RU from data
obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope
using the Differential Speckle Polarimetry technique. The correction for biases arising
due to instrumental polarization effects is discussed. The accuracy of R-Q, R-U measurement
with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts
to 5 x 10(-3).
