Extending the Exposure Time in High-Resolution Mobile Tunnel LIDAR

We are developing a high-speed laser measurement system of tunnel concrete wall with the purpose of automating the maintenance of civil infrastructures. For the automated diagnosis in a road tunnel, it is essential to realize remote measurement from the traveling vehicle so that measurement can implement without closing the tunnel. Although standards vary depending on the country, specifically, in Japan, the running speed of the vehicle is 30 km/h or more, and resolution capable of reliably detecting cracks with a width of 200 μm is required. A vehicle-mounted type tunnel measurement system with sufficient resolution has not been established. Various remote measurements based on LIDAR technology are expected to be useful for the tunnel measurement. However, it is difficult to achieve high resolution of 200 μm, due to the short exposure time associated with high-speed traveling. The spatial resolution of the existing vehicle-mounted measurement system is 1 mm in imaging, 5 cm in ranging [1]. The problem is an insufficient SN ratio caused by the insufficient number of photons. We estimated signal intensities under various restrictions such as the safety standard on vehicular lights, the minimum speed of vehicles that can avoid traffic restrictions, and the allowable exposure time for imaging with a pixel resolution 150 μm. We finally showed that the light energy corresponding a pixel is only a few photons. With this signal intensity, the obtained images become a black and white random pattern by shot noise.