Combination of dot-matrix lighting and floodlighting for multipath interference suppression in ToF imaging

Abstract

Time-of-flight (ToF) cameras obtain the depth information of the whole scene simultaneously by floodlighting it. In the presence of mutual reflection between the targets, the measurement would suffer from the multipath interference (MPI), because the signal received by the sensor is a mixture of direct component and multipath (or global) component. MPI would lead to significant measurement errors. In this paper is introduced a method of separating the direct and global components by once dot-matrix lighting and twice floodlighting with different frequencies. With dot-matrix lighting, the depth information at the dot matrix position is almost only determined by the direct component. The phase value of the direct component is readily calculated. The global component at the dot position with floodlighting is separated by minimizing the separation error through solving the nonlinear least square problem. Then the global component of the whole scene can be obtained by two-dimensional interpolation from data at dot matrix position. As a result, the direct component can be calculated by subtracting the interpolation result from the floodlighting signal, and the depth were calculated only from direct component. Semi-physical experiments were made for three kinds of scenes, blank corner with uniform reflectivity, the corner with different reflectivity areas and the corner with an object placed in front of it. The results demonstrate that the MPI has been significantly suppressed in all scenes. Especially in the area with strong MPI in the first two kinds of scenes, the measurement errors can decrease to about 10%~20%.