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A new foliage penetrating LiDAR system

3 July 2017

Shortly after lasers were first developed in the 1960s, LiDAR—whose name originated as a combination of “light” and “radar”—capitalized on the newly unique precision they offered for measuring both time and distance.

LiDAR quickly became the standard method for (3-D) land surveys and is now used in a multitude of sensing applications, such as self-driving cars. By scanning areas of land with lasers, often from airplanes, LiDAR’s travel-time measurements for light reflected back from the scanned area provide the distances that make up a resulting high-resolution topography.

As laser and electronic technology evolved, LiDAR’s abilities adapted to overcome several limitations and obscuring effects unavoidably produced by real environments, like dynamic weather patterns.

With a specially designed laser system and a new methodology based on gated digital holography, research from the Naval Research Laboratory, in Washington, D.C., now provides a method to give LiDAR an enhanced ability to see through otherwise obscuring elements of terrain like foliage or netting.

Paul Lebow, from the Naval Research Laboratory, will present this work at The Optical Society’s Imaging and Applied Optics Congress, held 26 -29 June, 2017 in San Francisco, California.

“This was an attempt to address one of the problems with something called foliage-penetrating LiDAR,” Lebow said. “You can actually use it to detect three-dimensional images behind an obscuration such as a tree canopy, for instance, in a disaster relief situation where you wanted to find people in trouble. You can illuminate using LiDAR through the leaves and get enough light coming back through to be able to recreate a three-dimensional, topographic view of what’s going on beneath.”

Until now, LiDAR measurements of surfaces hidden behind foliage have been difficult to acquire. A majority of the original light in these cases gets thrown away, as far as the camera detecting light from the ground is concerned, since the light hitting the leaves never reaches the ground in the first place. Moreover, the light blocked, and therefore reflected, before getting to the ground often overpowers the signal hitting the camera and hides the fainter signal that does make it to the ground and back.

“We have been working with a process called optical phase conjugation for quite some time and it dawned on us that we might be able to use that process to essentially project a laser beam through the openings of the leaves and be able to see through a partial obscuration,” Lebow said. “It was something that until maybe the last five years was not viable just because the technology wasn’t really there.

The stuff we had done about 20 years ago involved using a nonlinear optical material and was a difficult process. Now everything can be done using digital holography and computer generated holograms, which is what we do.” This new system uses a specially designed laser that alone took a year and a half to develop, but was a necessary component according to Lebow and his colleague, Abbie Watnik, who is also at the Naval Research Laboratory and another of the work’s authors. Read more.

Photo: Specially designed laser system and a new methodology based on gated digital holography enable LiDAR to see through obscuring elements like foliage and netting. Credit: US Naval Research Laboratory

Source: pddnet.com

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