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Science: Ghost imaging in 3D – minus the camera

Snapping a picture with a camera that doesn’t exist is pretty spooky, but doing it in stereo is doubly so. A strange kind of photography known as ghost imaging has been extended to work in three dimensions.

A 3D picture created with no camera

Although the time-consuming technique will not replace your digital camera any time soon, it could make it cheaper to take 3D pictures in wavelengths beyond the visible spectrum, so it could find uses in arenas as diverse as airport security and natural gas prospecting.

A digital camera focuses incoming light using lenses and then records the position and intensity of that light with a multi-pixel sensor. To capture a 3D image, two cameras are placed side by side, like eyes in a human head, and their slightly different views are combined on a computer.

Ghost imaging was developed because it can take pictures of objects by recording light that doesn't actually hit them, making it promising for surveillance. The method was originally thought to rely on quantum effects linking pairs of photons, which allow someone to see what happens to photons hitting an object by recording the photons that do not.

Researchers later realised that any paired light, such as a laser beam split in two, would also work. In this set-up, one part of the beam is aimed at a target with a single-pixel detector nearby. This device picks up reflected photons but can only measure the intensity of incoming light and not its position. The other part of the beam is recorded by a traditional camera, which builds an image based on the behaviours of the photon pairs.

For their version of ghost imaging, Matthew Edgar of the University of Glasgow, UK, and his colleagues got rid of the camera altogether.

Instead, a projector shines hundreds of random computer-generated, black-and-white patterns on an object, while four single-pixel detectors record the amount of light reflected back. Patterns that happen to match the shape of the object reflect more light than those that don't. The computer weights each black-and-white pattern according to the intensity recorded by the detectors and overlays the results, so that a picture of the object gradually emerges.

The 3D system's four detectors are placed above, below and on either side of the projector. These detectors measure slightly varying intensities of reflected light and create pictures with different shading.

It turns out these images are exactly the same as ones taken if an ordinary camera is placed at the position of the projector and light bulbs take the places of the detectors.

"We're actually turning a projector into a camera and a detector into a light source," says Edgar. Using this method, his team created a 3D image of a polystyrene dummy head (pictured, above right).
Clean ghost

"I think that the results are beautiful," says Robert Boyd of the University of Rochester in New York. The images are much more detailed than previous ghost images, and are good enough to capture 3D imagesMovie Camera for the first time, he says. "I have never seen any other sort of ghost imaging that gave results that are this clean."

Real-world applications are a bit limited for now, though, because it currently takes about half an hour to take a picture. The capture speed is dependent on the refresh rate of the projector. Edgar says the team is now testing a much faster system.

If it can be made a bit snappier, the method could prove useful for taking pictures in multiple wavelengths. Digital camera sensors for non-visible wavelengths are expensive or non-existent, but single-pixel detectors are available. Used as ghost imagers, infrared detectors could identify potential gas fields from warm gas leaking out of the ground, while detectors of terahertz-wavelength light, which can pass through non-conducting materials such as clothes, wood and plastic, might be used in medical imaging or for airport security scanners.

Journal reference: Science, DOI: 10.1126/science.1234454

Syndicated content: Jacob Aron, New Scientist

(Image: University of Glasgow. A 3D picture, created with no camera)


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