Optical and Acoustic Imaging Comparison in a Controlled Underwater Environment

Accurate sensing of objects in the marine environment under varying environmental conditions can be performed using a variety of sensing modalities; acoustic methods (sonar) dominate due to the inherent high transmissivity of sound in water, but visual sensing (cameras) are also valuable tools, that are, however, more affected by turbidity and lighting. In this work, the operational limits with respect to turbidity, lightning, and target object distance, of these two main sensing modalities are compared in a controlled underwater environment, embodied by two off-the-shelf commercial products, chosen for high-measurement resolution and ease of integration, in the form of a high-frequency forward-looking imaging sonar and an infrared and color stereo camera, which is additionally assisted by a laser line reference. It is shown that the stereo camera yields high fidelity measurements up to a turbidity of 1 FTU at distances up to 1.7 m, whereafter clouding of the image renders the sensor unable to calculate object distances and dimensions. The sonar is unaffected by the additional turbidity up to the upper test limit of 6 FTU.