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Ambient light is strongly attenuated in turbid media. Moreover, natural light is often more highly attenuated in some spectral bands, relative to others. Hence, imaging in turbid media often relies heavily on artificial sources for illumination. Scenes irradiated by an off-axis single point source have enhanced local object shadow edges, which may increase object visibility. However, the images may suffer from severe nonuniformity, regions of low signal (being distant from the source), and regions of strong backscatter. On the other hand, simultaneously illuminating the scene from multiple directions increases the backscatter and fills-in shadows, both of which degrade local contrast. Some previous methods tackle backscatter by scanning the scene, either temporally or spatially, requiring a large number of frames. We suggest using a few frames, in each of which wide field scene irradiance originates from a different direction. This way, shadow contrast can be maintained and backscatter can be minimized in each frame, while the sequence at large has a wider, more spatially uniform illumination. The frames are then fused by post processing to a single, clearer image. We demonstrate significant visibility enhancement underwater using as little as two frames.
Date of Publication: Nov. 2012