High Spatial Resolution Survey Using Frequency-Shifted Feedback Laser for Transport Infrastructure Maintenance

We propose a remote surface measurement system that uses a laser to inspect tunnel walls. To prevent accidents caused by aging parts of the transportation infrastructure, such as tunnels and bridges, the maintenance of such structures has grown in importance. Although these structures are checked by human inspectors, it is hoped that the process can be further automated through the development of remote sensing technologies. In this article, we focus on the detection of cracks on tunnel surfaces. As the concrete surfaces of tunnels can have many discolored areas, the precision of conventional remote inspection methods based on digital cameras is limited. Employing a frequency-shifted feedback (FSF) laser to overcome this difficulty, we adopt three measurement principles: reflectance imaging, 3D measurement, and spectroscopy. We have realized high spatial resolution, which is essential to our purpose. Using reflectance imaging, we have detected cracks of more than 200 μ m in width on a concrete surface. Using 3D measurement with an FSF laser, we have detected as 3D shape a 0.35 mm crack on an actual concrete surface. We also have detected the presence of water on a concrete surface using 2.95 μ m mid-infrared light in the laboratory. We discuss the use of our system to reliably detect 0.2-mm-wide cracks on the basis of experimental results. The measurement results for the reference targets and real concrete are described.