Seeing Around Corners Using Lasers and Imaging

A direct line of sight between an object and a camera or detector is typically needed for every imaging application except for extreme cases such as light bending due to gravitational lensing in astronomy. But for the most part, imaging applications are limited to light propagating in a straight line. However, that is starting to change as some cutting edge research is opening up possibilities to image around corners and around obstacles. A combination of lasers, sensitive cameras, and computational reconstruction methods can be used to detect objects hidden by obstacles by scattering light off of surrounding objects.

The Future is Just Around the Corner

The process for non-line-of-sight imaging is similar to that of LiDAR (light detection and ranging), where a laser pulse is sent towards an object and the time-of-flight of the light scattering back off of the object is used to measure the distance between the object and a detector. However, non-line-of-sight imaging images objects obscured by obstacles by adding another scattering event to this process.¹

Figure 1: To image a hidden target obscured by some obstacle, ultrafast laser pulses are sent towards an object, such as a wall, near the target

Figure 2: The laser pulse hits the wall, or other object near the target, and scatters off of it. This scattered light propagates towards the hidden target

Figure 3: This light then scatters a second time off of the hidden target and propagates back towards the wall. The highly sensitive camera detects this secondary scatter as it hits the wall. A 3D model of the hidden target is formed by repeating this process for many different laser positions on the wall

Reconstructing a Model of the Hidden Target

Highly sensitive cameras such as single-photon avalanche photodiode array cameras are needed to measure the propagation of picosecond and femtosecond pulses of light in real time. The detector receives two different return signals: an initial signal of light scattered directly off of the wall and a secondary signal of light scattered off of the target, which is the signal used for non-line-of-sight imaging. This time-of-flight information is then used to reconstruct a series of ellipsoids that all overlap at a given point on the hidden target, allowing computational software to calculate the distance between the camera and the hidden target and recreate a 3D model of the target.

A 3D object can be broken down into a collection of many individual points that scatter light. The summation of all of these points can reconstruct a model of the original object. If the detector can distinguish return pulses with a temporal resolution of 100ps, this corresponds to a spatial resolution of points on the hidden target of approximately 1.5cm. ¹

Figure 4: Illustration of how non-line-of-sight imaging generates a 3D model of an obscured object by summing up a series of points determined through the process described in Figures 1-3

Real World Applications

Autonomous Vehicles

Allow cars to sense approaching vehicles or pedestrians around corners before they are in the car’s direct line of sight²

Public Safety

Let law enforcement, firefighters, and emergency medical services detect presence of people in dangerous situations from a safe distance²

Medical Imaging / Microscopy

Investigate small 3D structures hidden from the system’s direct line of sight²

The Future of Non-Line-of-Sight Imaging

Taking this emerging technology and creating a practical solution for real-world use that is portable and not dangerous to observer’s eyes is extremely challenging. One of the main issues with non-line-of-sight imaging is the limited amount of light that makes its way back to the detector, which must be able to pick up this very small amount of light and differentiate it for any ambient light sources. The return signal to the detector is the result of two consecutive scattering events, leading to an extremely high loss. Return signals can be as low as one photon per laser pulse.¹

However, the Stanford Computational Imaging Lab has developed a non-line-of-sight imaging system that works outdoors under indirect sunlight.² They successfully imaged an object made out of retroreflective tape that was obscured by a wall, which bodes well for the future of this technology.

The lab of Aristide Dogariu of University of Central Florida is investigating non-line-of-sight imaging utilizing the spatial coherence of light hitting a wall instead of laser light scattered off of that wall and the target behind it.³ This could lead to modeling of the hidden target without requiring ultrafast laser illumination, making real-world applications of the technology more portable and easy to use.

More development is still needed before non-line-of-sight imaging technology becomes available in practical commercial systems, but it is a promising solution for the next generation of imaging applications.

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