Non–line-of-sight imaging over 1.43 km

Abstract

Non–line-of-sight (NLOS) imaging has the ability to reconstruct hidden objects from indirect light paths that scatter multiple times in the surrounding environment, which is of considerable interest in a wide range of applications. Whereas conventional imaging involves direct line-of-sight light transport to recover the visible objects, NLOS imaging aims to reconstruct the hidden objects from the indirect light paths that scatter multiple times, typically using the information encoded in the time-of-flight of scattered photons. Despite recent advances, NLOS imaging has remained at short-range realizations, limited by the heavy loss and the spatial mixing due to the multiple diffuse reflections. Here, both experimental and conceptual innovations yield hardware and software solutions to increase the standoff distance of NLOS imaging from meter to kilometer range, which is about three orders of magnitude longer than previous experiments. In hardware, we develop a high-efficiency, low-noise NLOS imaging system at near-infrared wavelength based on a dual-telescope confocal optical design. In software, we adopt a convex optimizer, equipped with a tailored spatial–temporal kernel expressed using three-dimensional matrix, to mitigate the effect of the spatial–temporal broadening over long standoffs. Together, these enable our demonstration of NLOS imaging and real-time tracking of hidden objects over a distance of 1.43 km. The results will open venues for the development of NLOS imaging techniques and relevant applications to real-world conditions.

Onsite Non-Line-of-Sight Imaging via Online Calibrations

“…Several main factors, including sensitivity, dark count rate, temporal jitter, and hold-off time, define the signal-to-noise ratio and the temporal resolution and are related to the physical configuration and the material properties of the SPAD. For instance, silicon-based SPADs cover the visible spectrum, e.g., 400 −800 nm, with full width at half maximum (FWHM) of tens of ps [5][6][7], whereas InGaAs/InP-based SPADs cover the infrared spectrum with FWHM of approximately 200 ps [10,11,16]. Wu et al [10] constructed a long-range NLOS capture system, over 1.43 km, exploiting InGaAs/InP SPADs at 1550 nm and a fiber laser with 500 ps pulses and 300 mW average power.…”

“…The first bounce corresponds to direct reflection and enables us to recover the shape and albedo of the relay surface. By removing or gating the photons from the first bounce, photons from the third bounce are employed to reconstruct [3][4][5][6][7][8][9][10] and localize [5,6,[10][11][12] hidden objects in the NLOS scene. Potential applications are numerous, including autonomous driving, remote sensing, and biomedical imaging.…”

“…Substantial efforts have been made to both improve the hardware of the capture system [4,5,10,13,14] and provide better NLOS reconstruction [6,7,[15][16][17]. For the former, the efforts have been focused on using more accurate and affordable detectors and lasers.…”

Observing quantum coherence from photons scattered in free-space

“…For instance, quantum communication capable of operating over a scattering channel could accommodate free space communication with non-line-of-sight between multiple users such as indoors around corners, or with short range links with moving systems. Furthermore, the photon coherence recovered from scattered light could be utilized to improve noise performance in low-light and 3D imaging, non-line-ofsight imaging 10,11 , velocity measurement 12 , light detection and ranging (LIDAR), surface characterization, or biomedical sample identification.…”

Non-line-of-sight imaging of moving objects obscured by a random corridor

“…On the other hand, speckle correlations 11,12 , wavefront-shaping 13 , computational periscope 14 , and spatial coherence analysis 15,16 is capable of imaging the hidden object with only the spatial information of scattered light from the relay surface. The novel principle and technology greatly improve the efficiency, economy, minimum resolution and maximum working distance of NLOS imaging 8,9,14,17,18 . In spite of recent advances, challenging but important NLOS imaging in another type of complex environment 'random corridor' has received little attention.…”