ENSCHEDE, Netherlands, and FLORENCE, Italy, Nov. 7, 2012 — A noninvasive method that unscrambles scattered light to retrieve sharp images of objects hidden behind an opaque screen could have important applications in biological tissue imaging.
Materials such as paper, skin, ground glass and fog scatter light in complicated and unpredictable ways, which is why it is impossible to get a clear view of objects hidden behind them. Many methods have been developed to retrieve images through materials in which only a small fraction of light follows a straight path, but it was not possible to resolve an image from light that had been completely scattered.
Now, scientists from the University of Twente’s MESA+ Institute for Nanotechnology and from the University of Florence have developed a technique that can sort randomly scattered light from light that has not changed direction to reconstruct an image — and it doesn’t require a detector to be placed behind the layer.
A cartoon by Clive Goddard, inspired by the research. Courtesy of Clive Goddard ©email@example.com.
The work was published this week in Nature (doi: 10.1038/nature11578
). In describing the challenge the team faced in an accompanying article, Demetri Psaltis and Ioannis N. Papadopoulos used the analogy of a golfer trying to get her ball out of the woods by aiming for the trees, whacking the ball hard and hoping that it will hit wood, ricochet and emerge back onto the green.
“[Jacopo] Bertolotti and colleagues demonstrate that it is possible to form an image of an object hiding behind a scattering screen without the need to put a detector or a light beacon behind the screen,” wrote Psaltis and Papadopoulos of the Optics Laboratory in the School of Engineering at Ecole Polytechnique Federal de Lausanne, Switzerland, who were not involved in the research.
The new method involved scanning the angle of a laser beam that illuminated an opaque diffuser. At the same time, a computer recorded the amount of fluorescent light that was returned by a tiny object hidden behind the diffuser.
The team successfully unscrambled light signals to retrieve detailed images of human-cell-sized fluorescent objects hidden 6 mm behind scattering layers and a complex biological sample enclosed between two opaque screens.
A graphical illustration of the method described in the paper. (a) The test object used was the Greek letter π, written in fluorescent ink and 100× smaller than the one printed here. The test object was covered by a strongly scattering ground-glass diffuser that hid it from view. (b) A laser beam scanned the ground glass. The test object yielded only a diffuse glow of fluorescent light. (c) The intensity of the fluorescence was measured versus the angle of the laser beam and recorded by a computer. The seemingly random pattern bears no resemblance to the test object. (d) The computer searched for similarities in the measured pattern, which it used to calculate the true shape of the object. Courtesy of Dr. Elbert van Putten.
“While the measured intensity of the light cannot be used to form an image of the object directly, the information needed to do so is in there, but in a scrambled form,” said Dr. Allard P. Mosk of the Complex Photonic Systems research group at MESA+. Two of the first authors of the paper set out to determine whether the scrambled information was sufficient to reconstruct the image, he said.
Their method involved a computer program that initially guesses the missing information, then tests and refines the guess. The researchers succeeded in making an image of a hidden fluorescent object just 50 µm across — the size of a typical cell.
The work could lead to new microscopy methods capable of forming razor-sharp images in a strongly scattering environment.
“This will be very useful in nanotechnology,” Mosk said. “We would like to bring structures to light that are hidden inside a complex environment like computer chips.” The team also would like to examine objects under the human skin, but the method is not yet fast enough, he said.
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