Simultaneous Multifrequency Demodulation for Single-Shot Multiple-Path ToF Imaging

Abstract

Indirect Time-of-Flight (iToF) sensors measure the received signal's phase shift or time delay to calculate depth. In realistic conditions, however, recovering depth is challenging as reflections from secondary scattering areas or translucent objects may interfere with the direct reflection, resulting in inaccurate 3D estimates. We propose a new measurement concept including a single-shot on-chip multifrequency demodulation method with periodically-repeated ultrashort-pulsed illumination using a novel pixel array architecture to address a main limitation of conventional iToF, the Multi-Path Interference (MPI). Due to the careful hardware/software codesign, the proposed single-shot architecture provides close-to-optimal Fourier measurements to a spectral estimation algorithm that retrieves the unknown parameters of the interfering return paths in a closed form. Electrical simulations of the on-chip multifrequency demodulation circuit demonstrate the feasibility of distance retrieval in double and triple bounce conditions in a single shot with high accuracy. Furthermore, we propose a set of methods for processing the resulting sensor measurements that exploit valuable a priori information and structural constraints of the data and observe that they yield a substantial increase in accuracy.