SNLOS: Non-line-of-sight Scanning through Temporal Focusing

Over the last decade, several techniques have been developed for looking around the corner by exploiting the round-trip travel time of photons. Typically, these techniques necessitate the collection of a large number of measurements with varying virtual source and virtual detector locations. This data is then processed by a reconstruction algorithm to estimate the hidden scene. As a consequence, even when the region of interest in the hidden volume is small and limited, the acquisition time needed is large as the entire dataset has to be acquired and then processed.In this paper, we present the first example of scanning based non-line-of-sight imaging technique. The key idea is that if the virtual sources (pulsed sources) on the wall are delayed using a quadratic delay profile (much like the quadratic phase of a focusing lens), then these pulses arrive at the same instant at a single point in the hidden volume – the point being scanned. On the imaging side, applying quadratic delays to the virtual detectors before integration on a single gated detector allows us to ‘focus’ and scan each point in the hidden volume. By changing the quadratic delay profiles, we can focus light at different points in the hidden volume. This provides the first example of scanning based non-line-of-sight imaging, allowing us to focus our measurements only in the region of interest. We derive the theoretical underpinnings of ‘temporal focusing’, show compelling simulations of performance analysis, build a hardware prototype system and demonstrate real results.

I. INTRODUCTION

NON-line-of-sight imaging (NLOS) refers to the reconstruction of texture and geometry of objects hidden by occluding surfaces from both the source and the detector. Over the last decade, several NLOS imaging techniques have been developed that exploit either phase [18], [29], time-of-flight [19], [34], motion [5], known occluders in the hidden volume [31] or just intensity information [20]. Among these, time-of-flight (or transients) based techniques have been the most popular and have resulted in some of the best available results so far.