Shadows aren’t usually credited with bringing things to light. They’re more often associated with clandestine meetings and dark corners, at least in spy movies. In contrast, new research published today in the journal Nature describes a technique developed by researchers at Boston University that uses shadows to reveal things otherwise hidden from view.
|From left to right: A hidden image, the data used to reconstruct the image, the reconstructed image.
Image Credit: Nature and Charles Saunders.
One of the main drivers of optical technology has been the desire to see things we can’t otherwise see—telescopes to see the heavens, microscopes to see the tiny, periscopes to see the enemy, and glasses to see the chalkboard. Since 2012, researchers have been especially interested in optical systems that let us see around corners, also known as non-line-of-sight techniques. From seeing into burning buildings to avoiding car crashes, the applications have enormous potential.
Non-line-of-sight techniques have come a long way in the last few years, but they generally require expensive pulsed lasers and light detectors, or have other constraints that make them impractical. In this new research, Charles Saunders, John Murray-Bruce, and Vivek Goyal from Boston University demonstrate a non-line-of-sight technique that uses an ordinary, 4-megapixel digital camera—no lasers or extra detectors required.
The basic idea behind this new non-line-of-sight technique is to reconstruct what you can’t see based on its reflection. Take the simple case of the periscopes used in the trenches of World War I. For a solider in a deep trench, the enemy is not within the line-of-sight. However, if the solider holds up a periscope, the reflection of the hidden enemy becomes visible to the solider.
|Left: Diagram of a simple periscope with mirrors shown in blue. Image credit: Arz (CC BY-SA 3.0) from Wikimedia Commons. Right: A British trench at Cape Helles during the Battle of Gallipoli in World War I. Image credit: Photo from The War Illustrated, 1915. Caption reads: The unseen watcher. British soldier, with the aid of a periscope, keeps an eye on the Turkish trenches while his comrades enjoy a well-earned repose. Public domain.|
Now imagine that instead of wanting to see what’s happening on the other side of a trench, you want to see what’s happening behind a wall or around the corner of a building. In some situations you could use a simple periscope, but what if that’s not possible or preferable? The new technique aims to “see” what’s hidden via its reflection on a wall.
The problem is that interpreting the reflection off of a wall is a lot trickier than interpreting the reflection off a mirror: mirrors keep reflected images intact, walls don’t. This means that you have to be able to decode the light spectrum of the wall using complicated computer algorithms.
In other modern non-line-of-sight approaches, short laser pulses illuminate one spot at a time on the wall and a detector records the time it takes scattered photons to return. With this information, an algorithm can use a kind of triangulation approach to spatially reconstruct the hidden object. A regular digital camera doesn’t have this capacity.
The researchers overcame this challenge by introducing a penumbra—a partial shadow—on the wall. Their “hidden object” was an image displayed on an LCD screen. They positioned an opaque object, smaller than the LCD screen, between the screen and the wall. The opaque object cast a shadow onto the wall that blocked some of the image.
|The experimental setup: Controlled by a laptop PC, a standard digital camera (upper right) takes a snapshot of the distribution of light on a wall (left). The light captured by the camera includes the light from a hidden image (lower right) and the penumbra from an opaque object (center) sitting between the hidden image and the wall. A computer algorithm reconstructs the hidden image from the snapshot.
Image Credit: Charles Saunders.
How does introducing a shadow help reveal a hidden image? Although it blocks the light behind it, the penumbra doesn’t block all of the information about that light; the brightness of the penumbra depends directly on the image it is blocking. The penumbra introduces an element of contrast to the distribution of light on the wall that is essential to the algorithm’s ability to reconstruct the image.
Using this set up, the researchers were able to snap a photo of the wall, feed it into a computer algorithm, and reconstruct hidden images in color. The reconstruction isn’t perfect, but it’s definitely recognizable. That’s pretty good for a 4-megapixel camera and a computer! The approach isn’t ready for prime time yet, but it definitely brings us closer to an accessible, affordable technology for peaking around corners and exploring areas too dangerous or inaccessible to inspect in other ways.
Now, who’s ready for a spy movie?