Duo-Min He

Underwater lidar imaging (UWLI) system in short turbid water tank

We are first to demonstrate the underwater lidar imaging (UWLI) in such a 3 m short water tank with highly turbid water successfully, and show the range-gated phenomenon in water much more clearly based on our newly designed serial targets. The target set comprises a series of three bar test targets (five in total), which are set at intervals of 22.5 cm roughly along the laser illumination direction since the light speed in water is 22.5 cm/ns. We synchronize precisely (approximately 0.5 ns) the UWLI system to range-gate on the targets which we want to capture their images, and to range- gate out the targets which we do not want their images. The attenuation coefficients in water are 1.0/m and 2.3/m (extremely turbid). Compared with non-gated case, the most distinct difference between the gated images and non-gated images in turbid water is that the nearer the target is located, the clearer its image is on the non-gated photos, but for the gated case the situation will be inverted completely when the delay time is adjusted suitably; that is, the image of the farther target could be much clearer than the image of the nearer target even in very turbid water.

The effect of illumination volume in underwater camera image

Video cameras are standard equipment on, practically, all of todays underwater robotics vehicles. As a part of a camera system setting, the arrangement of active illumination affects the quality of acquisition, particularly in turbid water environment. Large illumination volume helps to increase image brightness but depresses modulated contrast. This paper elaborates on the effects of illumination volume due to the forward and backward scattering progresses. An intensity model is proposed to include the geometrical parameters of the illumination setup. This model is based on the volume scattering function (VSF) and Lambertian reflection effects. The distribution of scattering noise in the image is predicted for different size of light beam. Experiments were conducted in a short water tank using a continuous wave, 532nm laser light source with an adjustable parallel beam output. In application, as laser beam scanning is a well-known method to improve underwater visibility, the theoretical model helps in determining the optimum illumination setting in different water conditions, and accordingly reducing the time of photomosaicing.

Underwater vision enhancement in turbid water by range-gated imaging system

Summary form only given. The importance of visual formation in the operation of robots cannot be undervalued always, since about 80% of human sensed information is due to vision. Although developments of underwater robotics vehicles (URV) have been rapid in recent years, the underwater vision is still regarded as a major tough undertaking, particularly in turbid water condition. Sonar imaging offers a large range for detection but often falls short in achieving the required resolution for identification. The normal optical image method such as underwater camera offers inherently high resolution capability, but the absorption and scattering characteristics of the marine medium severely limits the range performance, e.g. the visible range in the Singapore coastline area is only 2-3 meters. Currently, an experimental range-gated imaging system has been finished and precisely demonstrated ns (10E-9 sec) level gated images in a short turbid water tank (<3m). Our system uses very short (5 ns), high energy (160mj/p), 532 nm laser pulse as an active illuminating, and a fast gated intensified CCD camera for capturing image. The camera gated width i.e. optical gated duration (OGD) and illumination laser pulse width should match as good as possible, and the delay between them should correspond to the light round trip time to a target.

Underwater lidar imaging (UWLI) in 3 m tank with highly turbid waters

Based on our newly-designed series of targets at intervals, the range-gated phenomenon of underwater lidar imaging is much clarified. In extremely turbid water with attenuation 2.3/m, targets 1.5 m apart in distance cannot be observed by the non-gated camera at all, but its image could be recorded by our UWLI system.