DZK 33UX250: Overcoming Imaging Challenges to Improve the Safety and Security of Maritime Infrastructures

Overview

Over the last century, the complexity of maritime infrastructures has increased dramatically. At its Institute for the Protection of Maritime Infrastructures, the German Aerospace Center (DLR) researches methods and technologies for improving the safety and security of maritime infrastructures such as offshore installations, ports, and ships. Specifically, this includes vision-based systems for improved monitoring and reconnaissance of undersea infrastructures. Dr. Enno Peters, a scientist at the institute, has been working with The Imaging Source's industrial camera, DZK 33UX250, featuring Sony's Polarsens sensor. The sensor's on-chip polarization technology captures multi-directional polarized images allowing for a number of image processing benefits such as improved contrast, and reduced glare.

The Challenge

The view through the water's surface for recording the seabed, for example from drones, or the detection and identification of submerged objects is made more difficult by the refraction of light. This can lead to distorted images and localization errors especially when using conventional camera systems. Polarization measurements can be used to obtain information about the orientation of object surfaces. Traditionally, diffusely reflected radiation is used in the so-called "shape-from-polarization" method. For the reconstruction of the water surface, however, specularly reflected sky light is used. As the Fresnel coefficients for the s- and p- components of the reflected light are different, the reflected radiation is partially polarized depending on the viewing geometry. The challenge lies in the fact that with each measurement, the light reflected at the surface is superimposed with light rising from the body of water. The latter is refracted when it re-emerges at the surface and is therefore also partially polarized, meaning that both components act against one another.

How It Was Done

In a research study, the DZK 33UX250 was used to investigate a method working with spectral long-pass filters which suppresses the contribution of light from the water. The effect is based on the fact that the absorption of liquid water increases strongly in the infrared range. In addition, an algorithm was developed to calculate the local inclination of the water surface with pixel accuracy from the polarization measurements and compensates for the projection (paraxial approximation), making it also suitable for short focal lengths. Building on this, it was successfully demonstrated that the data obtained from the water surface can be used to compensate for the distortion caused by refraction in images of the seabed. These results were published as a scientific article in the journal Applied Optics.

Further Development

Research is now focusing on reducing the camera's exposure time to capture accurate data even on moving water surfaces. Studies are underway to adapt the technology to different weather conditions and water types, as the current method is dependent on unpolarized irradiation (cloud cover). In a 2-channel system, the Polarsens could then be combined with a conventional camera in recording the seabed.