Using polarization to estimate surface normals at air-water interfaces for correction of refraction in seafloor imaging

Peters, Enno; Atkinson, Gary; Stephan, Maurice

Abstract

The retrieval of sea surface normal vectors using shape-from-polarization is investigated for the purpose of correcting for refraction at the water–air interface when imaging from above the water. In shallow clear water and overcast conditions, spectral longpass filtering (using a hard-coated 850 nm cut-on wavelength filter) is demonstrated to 1) avoid artifacts from the ground in the measured polarization state, and 2) reduce polarization from water-leaving radiance sufficiently to derive shape information exclusively from the polarization produced by specular reflection. The dependence of the method on meteorological conditions is studied. Measurements are performed with a commercial polarization filter array (PFA) camera. Due to the decreasing PFA efficiency towards the near-infrared, rigorous characterization and calibration measurements were performed and recommendations (e.g., on the f-number) elaborated. Overcoming the paraxial approximation, normal vectors are then retrieved with systematic errors of 0.1∘ (image center) to 0.5∘−0.8∘ (edges/corners) for a flat water surface. An image of the sea floor corrected for surface refraction shows maximum displacements of 10–20 pixels only (corresponding to 0.25∘) with respect to a validation image without water.

• Journal Article Type: Article
• Acceptance Date: Nov 24, 2024
• Online Publication Date: Dec 16, 2024
• Publication Date: Jan 1, 2025
• Deposit Date: Dec 16, 2024
• Publicly Available Date: Dec 19, 2024
• Journal: Applied Optics
• Print ISSN: 1559-128X
• Electronic ISSN: 2155-3165
• Publisher: Optical Society of America
• Peer Reviewed: Peer Reviewed
• Volume: 64
• Issue: 1
• Pages: 9-22
• DOI: https://doi.org/10.1364/ao.544305
• Public URL: https://uwe-repository.worktribe.com/output/13532060
• Additional Information: Received: 7 October 2024; Accepted: 24 November 2024