Contrast-enhanced optical imaging of submersible targets

摘要

Abstract: Sunken objects at sea such as lost ship cargo are often not detectable by conventional video cameras because of their low contrast due to the high turbidity of the water column. A well-known contrast enhancing imaging technique in turbid media is range gated video recording. Synchronization of laser emission and camera gate time allows to suppress backscattered light from the water column and to record only the light backscattered by the object. This results in a contrast enhanced video image that increases the visibility range in turbid water up to eight attenuation lengths. Furthermore, image reconstruction algorithms can be applied if the modulation transfer function of the water column is known. However, these approaches only lead to an increase in visibility of a target if the intensity of light scattered by the seafloor differs from signals that originate from the object under investigation. Conventional monochrome video cameras as well as rang gated systems record the intensity of light, whereas its spectral composition and polarization characteristics are not considered. A linear polarized monochromatic illumination source and suitable cut-off and polarization filters in front of the camera allow to record the fluorescence images and to detect the degree of depolarization which is a function of the roughness of the scattering surface. This can result in a better contrast between target and seafloor. A prototype of a submarine imaging instrument has been realized for operation in the German Bight that makes use of techniques described above. Results of tank trials are presented. !11