Real-time super-resolved 3D in turbid water using a fast range-gated CMOS camera

Abstract

We present a range-gated camera system designed for real-time (10 Hz) 3D estimation underwater. The system uses a fast-shutter CMOS sensor (1280×1024) customized to facilitate gating with 1.67 ns (18.8 cm in water) delay steps relative to the triggering of a solid-state actively Q-switched 532 nm laser. A depth estimation algorithm has been carefully designed to handle the effects of light scattering in water, i.e., forward and backward scattering. The raw range-gated signal is carefully filtered to reduce noise while preserving the signal even in the presence of unwanted backscatter. The resulting signal is proportional to the number of photons that are reflected during a small time unit (range), and objects will show up as peaks in the filtered signal. We present a peak-finding algorithm that is robust to unwanted forward scatter peaks and at the same time can pick out distant peaks that are barely higher than peaks caused by sensor and intensity noise. Super-resolution is achieved by fitting a parabola around the peak, which we show can provide depth precision below 1 cm at high signal levels. We show depth estimation results when scanning a range of 8 m (typically 1-9 m) at 10 Hz. The results are dependent on the water quality. We are capable of estimating depth at distances of over 4.5 attenuation lengths when imaging high albedo targets at low attenuation lengths, and we achieve a depth resolution (σ) ranging from 0.8 to 9 cm, depending on signal level.

State of the Art of Underwater Active Optical 3D Scanners

“…More specifically, gated intensifiers coupled to charge-coupled device (CCD) image sensors allow for precise range gating. However, complementary metal-oxide-semiconductor (CMOS) chips achieve high frame rates and high spatial and depth resolution with a lower system complexity [76].…”

“…On the one hand, the whole scene can be illuminated at once using a homogeneizer diffractive optical element (DOE) (see Figure 12). This technique needs a 2D ToF sensor in order to resolve the 3D position of the scanned scene [75,76]. This is considered an active method because, even though the light direction is not actively controlled, its time structure provides information for the 3D reconstruction.…”

Behavioural and welfare implications of a new slipping methodology for purse seine fisheries in Norwegian waters

“…Therefore, our observations can be thought of as random snapshots of escape behaviour throughout the slipping event, with no systematic biases. Recent advances in underwater video technology [84] may, however, allow future observations to be conducted at distance from the discharge channel, avoiding the need to attach cameras to moving gear and thereby increasing the amount of observed time.…”

Range-Gated Imaging System for Underwater Monitoring in Ocean Environment

“…To access the dual benefits of high-contrast images and 3D information, an adaptive 3D reconstruction algorithm has been developed to identify both the object distance as well as the range of optimal contrast on a per-pixel basis [22]. A final image is then reconstructed consisting of the high-contrast grey scale overlaid with the 3D information.…”