Detecting and Mapping Transparent or Mirror-like Surfaces with Lidar

The presence of specular surfaces is often revealed by intense multibounce returns. For instance, when one directly illuminates a diffusely reflecting surface, nearby specular surfaces may produce mirror images of the true laser spot (also referred to as highlights) that appear just as bright as the original. Alternatively, when a specular surface is illuminated directly, the beam is often deflected such that it lands on a diffusely reflecting surface nearby—thus producing a clearly visible spot. These multibounce signals are easy to observe when one wave's a laser pointer around any room containing mirrors and windows, and yet they have been largely overlooked by the lidar sensing community as useful sources of information.

In this project we demonstrate that multibounce returns are both an important cue that reveals the presence of specular surfaces, as well as an information source that can be used to estimate a specular reflector's shape. We review the geometry of multibounce returns that are encountered in scenes that contain both diffuse and specular reflectors. Using our knowledge of this geometry, we propose criteria for unambiguously detecting the presence of specular multibounce signals, as well as a set of equations that relates the time- and angle-of-arrival of multibounce impulses to the position and orientation of points on the specular surface.