This work studies the application of laser structured light scanning to measure the small scale roughness of seabed. We use a CCD camera to capture the dislocation of laser light. The location of the laser light in pixel coordinates can be converted into world coordinates if the CCD camera is calibrated. We propose an algorithm which is analogous to the idea of longitudes/latitudes in map projection. The idea is to place a calibration board to be aligned with the laser scanning sheet. On the calibration board, grid points of 50mm are laid to represent the intersection of the longitudes and latitudes. The position of a point in pixel coordinates can be obtained by referring to its neighboring graticule. We designed three experiments to verify the accuracy of the system: The first experiment consists of measuring the distance between feature points on the calibration board, then check and correct the optic distortion effects of the lenses. The second experiment is to measure the slice of laser scanning image of a known object, and check the accuracy of our laser scanning system by measuring the object's height and width. In the third experiment, we measure an object which has a small height variety of its surface, to test the resolution of the system. The results indicate that the error is under 1%, only then that we proceed with the design, analysis, and measurement of artificial seabed. The artificial seabed model is made by using a 210mm * 210mm * 30mm acrylic board with sand ripples forms in the 150mm * 150mm square. The amplitude of the ripples is no higher/larger than ¡Ó 8mm, and no lower/smaller than ¡Ó 1.5mm. Contour map of the sand ripples would be plotted to analyze the results obtained from the measurements. The analysis is carried out by obtaining slice data from a reconstructed surface of the sand ripples, then compare it with the theoretical values. From the result we know that the error between sand dune ideal wave index and measured index is in the range of ¡Ó 2mm. To further test the system's tolerance with turbidity, we incorporate conditions which would alter environmental turbidity into the seabed experiments before running the experiments for analysis. The results show that the system is able to maintain a stable performance in an environment below 2.3 NTU (Nephelometric Turbidity Unit), and the error between ideal sand dune ideal wave index and measured index is still in the range of ¡Ó 2mm.

This paper reports the design and development of an underwater laser scanning system to measure the geometry of underwater objects. The application of structure light scanning method requires a calibrated CCD camera as the input device. Because the underwater environment is by far different from that in the air. Conventional calibration methods adopted in the air can not be applied for the underwater cases. In this paper we propose an algorithm which is analogous to the idea of longitudes/latitude in map projection to calibration the CCD. The calibration board pattern is fabricated by laying vertical and horizontal grid dots of 5cm span with an NC milling machine. To obtain the higher accuracy, we redesign the laser source holder to make the board and laser scan line coplane. We use a new laser that is focus adjustable. So we can capture clearer image of the edge on the target. Then, we calibrate the CCD camera with the calibration board. For testing our new system, two test pieces are used. One is sine waves ripples with varying amplitudes from 8mm to 3 mm. The other one is a rough surface with know spatial power spectrum. Scanning results show that: Scanning from rough 1 meter away, the absolute error for the sine wave ripples is less than 1mm along vertical direction. The power spectrum for the rough surface is accurate to the order of 3 to 5mm wave number. In order to survive in the harsh underwater environment, we design and make a rotational scanning system. The system was designed as an automatic image-capturing system, utilizing single board computer as control plane to work in conjunction with PLC(Programmable Logic Controller) for System power management. When using two 12V batteries as main power source, obtaining samples once per hour, capturing 360 images per operation, the system may run for approximately 39 hours.