Optical imaging through diffusive, visually-opaque barriers, and around
corners is an important challenge in many fields, ranging from defense to
medical applications. Recently, novel techniques that combine time-of-flight
(TOF) measurements with computational reconstruction, have allowed breakthrough
imaging and tracking of objects hidden from view. These light detection and
ranging (LiDAR)-based approaches, however, require active short-pulsed
illumination and ultrafast time-resolved detection. Here, bringing notions from
passive RADAR and passive geophysical mapping approaches, we present an optical
TOF technique that allows to passively localize light sources and reflective
objects through diffusive barriers and around corners. Our approach retrieves
TOF information from temporal cross-correlations of scattered light, providing
temporal resolution that surpasses the state-of-the-art ultrafast detectors by
three orders of magnitude. We demonstrate passive localization of multiple
white-light sources and reflective objects hidden from view, using a simple
setup, with interesting potential for covert imaging.Comment: Article: 20 pages, 5 figures. Supplementary materials: 14 pages, 8
figure