Keyhole Imaging: Non-Line-of-Sight Imaging and Tracking of Moving Objects Along a Single Optical Path

Non-line-of-sight (NLOS) imaging and tracking is an emerging technology that\nallows the shape or position of objects around corners or behind diffusers to\nbe recovered from transient, time-of-flight measurements. However, existing\nNLOS approaches require the imaging system to scan a large area on a visible\nsurface, where the indirect light paths of hidden objects are sampled. In many\napplications, such as robotic vision or autonomous driving, optical access to a\nlarge scanning area may not be available, which severely limits the\npracticality of existing NLOS techniques. Here, we propose a new approach,\ndubbed keyhole imaging, that captures a sequence of transient measurements\nalong a single optical path, for example, through a keyhole. Assuming that the\nhidden object of interest moves during the acquisition time, we effectively\ncapture a series of time-resolved projections of the object's shape from\nunknown viewpoints. We derive inverse methods based on expectation-maximization\nto recover the object's shape and location using these measurements. Then, with\nthe help of long exposure times and retroreflective tape, we demonstrate\nsuccessful experimental results with a prototype keyhole imaging system.\n