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12 June 2012 Scattered photons as useable signal for underwater imaging
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Abstract
The orthodox approach to designing an underwater imaging system with artificial illumination has been to consider only the unscattered target photons as useable signal while looking at scattered photons as a nuisance to be mitigated. Photons scattered from the target towards the receiver cause blurring of fine target details in the collected imagery, while photons backscattered by the water column as the artificial source illuminates the target act as a veiling luminance that reduces overall image contrast. Typical performance for the Laser Line Scanner and Pulsed Range-Gated imagers can reach up to 6 attenuation lengths, which can still represent very short ranges in the turbid waters of coastal regions. In the early 1970's, with the goal of extending these performance ranges, the Visibility Laboratory explored an unconventional concept that was called imagery by means of Time Varying Intensity (TVI). TVI uses both scattered and unscattered photons from the laser-scanned target as useable signal. This novel approach enabled high-quality imagery to be collected over 20 attenuation lengths between the target and receiver. Although this system was eventually shelved, it has been resurrected by using a modulated laser illuminator to communicate critical information about the laser scan to a distant receiver via both the scattered and unscattered photons. With this knowledge, a high-fidelity image of target detail can then be recreated. In this paper, a real-time interactive simulation of TVI's expected imaging performance is presented and model predictions are compared with experimental imagery acquired when laser and receiver are both located underwater.
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Walton McBride, Alan Weidemann, Linda Mullen, Alan Laux, and Derek Alley "Scattered photons as useable signal for underwater imaging", Proc. SPIE 8372, Ocean Sensing and Monitoring IV, 837204 (12 June 2012); https://doi.org/10.1117/12.920469
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