Pronounced turbidity in coastal waters such as those found in the North Sea and Baltic Sea make optical inspection tasks difficult. In particular, the identification and status assessment of wrecks, offshore structures, quay walls and dumped munitions pose a great challenge. When using robots (ROVs) or divers, high visibility is desirable when strong currents are present, allowing status assessment from a safe distance (collision avoidance). However, as soon as little ambient light is available (e.g. night operations, great depths or strong turbidity) and artificial light sources are used for illumination, image degradation occurs due to backscattering of particles in the water column. Furthermore, forward scattering effects cause blurring in the image.
Visibility is reduced due to backscattering of particles in water (DLR / NSW-Project)
In addition to acoustic sonar systems, active optical camera systems offer solutions for improving visibility through turbid waters. The aim of this project is to develop a gated-viewing sensor, that effectively suppresses backscatter by time-synchronising nanosecond light pulses with a delayed exposure of the camera. However, the targeted system is still susceptible to blurring due to its area detector. For this purpose, methods for estimating the point spread function (PSF) will be developed, which can be used in image processing to reconstruct a sharpened image by deconvolution. To support this development, optical water parameters such as spectral transmission and light availability in coastal regions will be collected and evaluated.
Inherent eye-safety, as well as improved robustness against large particles directly in front of the illumination unit, are addressed by the development of an area LED illumination. The reduction image blurring due to forward scattering is targeted by a multispectral approach, which primarily offers the advantage of performing PSF estimations based on viewing conditions along the entire optical path.
J. Schmidt, E. Peters, M. Stephan and O. Zielinski, "Feasibility study on LED-based underwater gated-viewing for inspection tasks," OCEANS 2022, Hampton Roads, Hampton Roads, VA, USA, 2022, pp. 1-8, doi: 10.1109/OCEANS47191.2022.9977303
