Improved LLS imaging performance in scattering-dominant waters

摘要

Experimental results from two alternate approaches to underwater imaging based around the well known Laser Line Scan (LLS) serial imaging technique are presented. Traditionally employing Continuous Wave (CW) laser excitation, LLS is known to improve achievable distance and image contrast in scattering-dominant waters by reducing both the backscatter and forward scatter levels reaching the optical receiver. This study involved designing and building prototype benchtop CW-LLS and pulsed-gated LLS imagers to perform a series of experiments in the Harbor Branch Oceanographic Institute (HBOI) full-scale laser imaging tank, under controlled scattering conditions using known particle suspensions. Employing fixed laser-receiver separation (24.3cm) in a bi-static optical geometry, the CW-LLS was capable of producing crisp, high contrast images at beyond 4 beam attenuation lengths at 7 meters stand-off distance. Beyond this stand-off distance or at greater turbidity, the imaging performance began to be limited mainly by multiple backscatter and shot noise generated in the receiver, eventually reaching a complete contrast limit at around 6 beam attenuation lengths. Using identical optical geometry as the CW-LLS, a pulsed-gated laser line scan (PG-LLS) system was configured and tested, demonstrating a significant reduction in the backscatter reaching the receiver. When compared with the CW-LLS at 7 meters stand-off distance, the PG-LLS did not become limited due to multiple backscatter, instead reaching a limit (believed to be primarily due to forward-scattered light overcoming the attenuated direct target signal) beyond 7 beam attenuation lengths. This result demonstrates the potential for a greater operational limit as compared to previous CW-LLS configuration.