Adaptive Structured Light with Scatter Correction for High-Precision Underwater 3D Measurements

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.

State of the Art of Underwater Active Optical 3D Scanners

“…Risholm et al [76,134] and Chua et al [75] used a doe to flash diffuse light in every direction with no spatial structure. Other authors employ spatially-structured light: Massot-Campos and Oliver-Codina [60] used a special doe to project 25 lines onto the surface of the scanned object, whereas Sarafraz and Haus [135] and Risholm et al [112,136] used commercial projectors to illuminate the scene with specifically-designed, more complex patterns. Bleier and Nüchter [137] simply used two laser lines forming a cross.…”

“…Risholm et al [112] projected a gray code phase stepping (GCPS) pattern using a LED with a DLP projector. In a newer version [136], they developed a multi-frequency phase stepping (MFPS) pattern, which was reportedly more accurate in-depth and more robust to turbidity than GCPS.…”

“…Most of the analyzed triangulation sensors achieve depth accuracies better than 5 mm, whereas the accuracies of ToF devices are in the range of a few centimeters (see Figure 21). In fact, the best absolute and relative depth accuracies are achieved by Lopes et al [140] and Risholm et al [136]. On the other hand, ToF sensors can work at longer ranges of up to 20 m, which makes their relative depth error at longer distances comparable to triangulation scanners working at short ranges (usually less than 2 m), as can be seen in Figure 22.…”

“…The baseline of triangulation sensors is a very important design decision, as discussed in Section 3.2. The reported baselines are 100 mm (Lopes et al [140]), 100 to 200 mm (Risholm et al [112,136]), 265 mm (Matos et al [143]) and 150 to 400 mm (Palomer et al [48]). Bodenmann et al [61] use a larger baseline between 800 and 1500 mm.…”

“…Scanners that are able to provide more frames per second are more suitable for scenes with faster dynamics. The reported refresh rates are 0.1 Hz (Risholm et al [136]), 0.1 Hz to 6 Hz, depending on lateral resolution (Palomer et al [48]), 0.2 Hz (Imaki et al [82]), 1 Hz (Digumarti et al [97] with Intel RealSense) and 10 Hz (Risholm et al [76,134] and Anwer et al [147] with Kinect v2). Typical underwater manipulation tasks require a refresh rate higher than a few hertz.…”