Abstract
High-precision underwater 3D cameras are required to automate many of the traditional subsea inspection, maintenance and repair (IMR) operations. In this paper we introduce a novel multi-frequency phase stepping (structured light) method for high-precision 3D estimation even in turbid water. We introduce an adaptive phase-unwrapping procedure which uses the phase-uncertainty to determine the highest frequency that can be reliably unwrapped. Light scattering adversely affects the phase estimate. We propose to remove the effect of forward scatter with an unsharp filter and a model-based method to remove the backscatter effect. Tests in varying turbidity show that the scatter correction removes the adverse effect of scatter on the phase estimates. The adaptive frequency unwrapping with scatter correction results in images with higher accuracy and precision and less phase unwrap errors than the Gray-Code Phase Stepping (GCPS) approach.
Highlights
Traditional subsea inspection, maintenance and repair (IMR) operations are costly because they require manual control using Remotely Operated Vehicles (ROVs)
In this paper we propose an underwater structured light system which uses an multi-frequency phase-shifted (MFPS) method adapted for underwater use
Notice that while turbidity increases the errors along the Gray code boundaries for the Gray-Code Phase Stepping (GCPS) method, increasing turbidity increases the errors on the boundary of the plate for the MFPS which is not scatter corrected
Summary
Traditional subsea inspection, maintenance and repair (IMR) operations are costly because they require manual control using Remotely Operated Vehicles (ROVs). In practice the light-water interaction causes light attenuation and scattering which affects the performance of optical 3D systems. Light attenuation limits the potential range of the system, while the scattering has primarily two effects. 532 nm lasers are frequently used as a light source underwater to increase the imaging range by diminishing the effect of light attenuation and scattering. An alternative faster approach is to illuminate the whole field-of-view with short powerful light pulses and use a fast range-gated camera to estimate the time-of-flight [14,15,16]. The advantage over laser-based scanning approaches for subsea intervention and inspection is the potential for real-time frame-rates and high-resolution both laterally and depth-wise in the 0.5 m-2 m range. Gray-codes are often used to phase-unwrap the phase-maps from high-frequency phase-shifted sinusoidal patterns [21].
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