Tuesday, January 29, 2013

Ghost Images in 3D

Physicists are snapping photos of buildings using light that never left the lab. Using a technique called ghost imaging, scientists at the Chinese Academy of Sciences have proposed a method of three-dimensional remote sensing and used it to take images of buildings almost a mile away.

3D ghost imaging of a building located about 1200 meters away.
(a) the field-of-view taken with a telescope, (b) the field-of-view obtained by 3D ghost imaging.
W. Gong, et. al.  arXiv:1301.5767v1
When you snap a photo with a digital camera, light reflected from your object enters the camera. Inside the camera, a tiny CCD chip made of an array of sensors collects these incoming photons, each sensor "bin" records the light from a specific location in the field-of-view and converts them to electric signals that compose your digital image.

In ghost imaging, light is emitted from a light source and split into two beams. One beam (beam 1) goes directly to the CCD camera. The other half of the beam (beam 2) is sent towards the objects you're trying to take a picture of. When the second beam hits the object– in this case some industrial-looking buildings– some of the light is reflected. The reflected light is collected by one tiny "single bin" detector.

Think about this. The CCD camera simply has a wash of light, straight from the source after splitting. There's no image there. The single-pixel detector is literally little bucket that collects photons and doesn't record any spatial information.

The key is in the comparison. As the researchers explain in their paper, by comparing the light intensity registered by the CCD camera to the photons collected by the single pixel detector, they can construct an image that correlates the spatial grid on the CCD camera with the intensity information from the reflected photons collected by the single detector.

The name 'ghost imaging' comes from the fact that the final image is made, in part, with the light hitting the CCD camera– light that never touched the object.

To expand two-dimensional ghost imaging to three dimensions, the researchers made the light pulse, like a strobe light, in 10 ns bursts. Then, they timed when the single pixel detector turned on so that it only captured the light that was reflected from a specific distance away.

Using the pulsed light beam and detector, the team could compile the time-stamped measurements to form a spatial map in three dimensions. For ghost photos over one kilometer away (0.62 miles or ~3200 feet), like the image above, the researchers achieved spatial resolution of about twenty centimeters (~7 inches), and depth resolution of about sixty centimeters (~2 feet).

The researchers say that ghost imaging is an improvement over current methods of laser-based radar-type systems (laser/light detection and ranging, LADAR/LIDAR). These current methods rely on scanning the field-of-view region point-by-point. Ghost imaging collects data from the whole field-of-view at once, using only a fraction of the time and data required for scanning imaging. Unlike current methods of scanning imaging, the ghost method is even compatible when the object is moving relative to the imaging camera.

The research was published January 24, 2013 on the arXiv.

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