Classical polarimetry with a twist: a compact, geometric approach.

We present an approach to classical polarimetry that requires no moving parts, is compact and robust, and that encodes the complete polarization information on a single data frame, accomplished by replacing the rotation of components such as wave plates with position along a spatial axis. We demonstrate the concept with a polarimeter having a quarter wave plate whose fast axis direction changes with location along one axis of a 2D data frame in conjunction with a fixed-direction polarization analyzer, analogous to a classical rotating quarter wave plate polarimeter. The full set of Stokes parameters is obtained, with maximal sensitivity to circular polarization Stokes V if a quarter wave retarder is used. Linear and circular polarization terms are encoded with spatial carrier frequencies that differ by a factor two, which minimizes cross-talk. Other rotating component polarimeters lend themselves to the approach. Since the polarization modulation spatial frequencies do not change greatly, if at all, with wavelength such devices are close to achromatic, simplifying instrument design. Since the polarimetric information is acquired in a single observation, rapidly varying, transient and moving targets are accessible, loss of precision due to sequential data acquisition is avoided, and moving parts are not required.