Development of an imaging polarimeter for water wave slope measurements

An imaging polarimetric slope sensing instrument for measuring water waves has been developed. From themeasurement of the intensities of three different linear polarization states, it is possible to determine the first three components of the polarization Stokes vector. The slope of the water surface is computed from the measurement of the polarization of reflected light. Unlike in common polarimeters, custom optics are not required in this simple setup consisting of three cameras aligned in parallel and each equipped with a standard polarization filter. The trade-off for the simple setup is the need for more extensive system calibration and image post-processing. The camera setup was fully calibrated (extrinsic, intrinsic, and distortion parameters) with a specialized calibration procedure using a custom built target. The analyzer matrix, which transforms the intensities of the measured polarization states into the Stokes vector components was determined and verified experimentally. A data set collected during an experiment on board the research vessel Meteor is analyzed. In an experiment at the Hamburgische Schiffsbau-Versuchsanstalt, the polarimeter was successfully operated under laboratory conditions. It is shown to be capable of measuring the slope distribution of mechanically generated waves. Elevation power spectra, determined by integration of the slope measurements, show good agreement with reference measurements with a wave wire. Deviations at low wave frequencies due the small size of the polarimeter footprint can be compensated with a transfer function that is derived from the measurements