Determination of Nearshore Surface Slope Field and Wave Heights Using Optical Polarimetry

In order to gain an accurate, predictive modeling capability for nearshore dynamics, it is necessary to be able to measure the heights of waves. We are developing a technique to do this remotely, by exploiting the polarization properties of light reflected from the ocean surface. A polarization camera was built for the purposes of laboratory testing, and was calibrated to determine filter orientations and to correct for gain differences between pixels, lens distortion, and slightly varying fields of view of the four component cameras before being used in laboratory tests. The camera’s ability to determine degree and azimuth of linear polarization (DOLP and AOLP), from which sea surface slope and, in turn, wave height can be determined, was then tested by comparing calculated polarization parameters to known conditions created by an external polarizer. While error in DOLP could not be precisely quantified without further tests, we found that we could determine AOLP to within one degree. Following this, the camera was used to make measurements of known wave conditions at the Hinsdale Wave Research Laboratory at Oregon State University. Incident band waves were isolated from noise and high-frequency chop using spectral processing of time series of AOLP values at individual pixels. The processed signals were then used to make estimates of surface slope and wave amplitude, which were compared to ground-truth data from electric and acoustic wave height sensors installed at the wave tank. While further analysis is necessary to fully demonstrate and quantify the accuracy of this technique, we have confidence in our ability to measure wave height based on the preliminary results obtained.