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Imaging Array Captures Minute Details From a Distance

Photonics Spectra
Jul 2017
HOUSTON — A unique camera that can capture a detailed micron-resolution image from a distance uses a laser and techniques that borrow from holography, microscopy and "Matrix"-style bullet time.

Rice University graduate student Yicheng Wu demonstrates the SAVI prototype.
Rice University graduate student Yicheng Wu demonstrates the SAVI prototype, which is able to capture fine details of an object from a distance, effectively replacing a large telephoto lens. The prototype camera is on a motorized track in the foreground at left, while a laser at right creates a speckle pattern on the target, a fingerprint. Courtesy of Jeff Fitlow/Rice University.

Engineers from Rice and Northwestern Universities built the prototype device that compares interference patterns between speckled images by reading a spot illuminated by a laser, which captures the pattern with a camera sensor. The raw data from the camera is fed to a computer program that interprets it and constructs a high-resolution image.

The Synthetic Apertures for long-range, subdiffraction-limited Visible Imaging (SAVI) system works with coherent illumination sources such as lasers, but researchers hope to one day use it in visible light.

"The problem we're solving is that no matter what wavelength of light you use, the resolution of the image — the smallest feature you can resolve in a scene — depends upon this fundamental quantity called the diffraction limit, which scales linearly with the size of your aperture," said Ashok Veeraraghavan , an assistant professor of electrical and computer engineering at Rice. "With a traditional camera, the larger the physical size of the aperture, the better the resolution.”

A schematic shows the single-beam SAVI system developed at Rice and Northwestern universities.
A schematic shows the single-beam SAVI system developed at Rice and Northwestern universities. The system employs a single beam, multiple images and sophisticated software to capture detailed images from a distance. Courtesy of Jason Holloway/Rice University.

SAVI's "synthetic aperture" sidesteps the problem by replacing a long lens with a computer program that resolves the speckle data into an image.

"You can capture interference patterns from a fair distance," Veeraraghavan said. "How far depends on how strong the laser is and how far away you can illuminate."

With SAVI, the images are taken from slightly different angles with one camera that is moved between shots instead of many fired in sequence.

Details from a fingerprint image taken from a distance of 1 meter by the SAVI prototype.
Details from a fingerprint image taken from a distance of 1 meter by the SAVI prototype developed at Rice and Northwestern universities. At top is one of many speckle patterns captured from a laser reflecting off the original image. At bottom, a clear print is the result of combining dozens of images of the fingerprint taken from slightly different angles and processed by a "synthetic aperture" program. Courtesy of Jason Holloway/Rice University.

The speckles serve as reference beams and essentially replace one of the two beams used to create holograms. When a laser illuminates a rough surface, the viewer sees grain-like speckles in the dot. That's because some of the returning light scattered from points on the surface has farther to go and throws the collective wave out of phase. The texture of a piece of paper - or even a fingerprint - is enough to cause the effect.

Jason Holloway, a Rice alumnus who is now a postdoctoral researcher at Columbia University, suggested that an array of inexpensive sensors and plastic lenses that cost a few dollars each may someday replace traditional telephoto lenses that cost more than $100,000.

Such an array would eliminate the need for a moving camera and capture all the data at once, potentially creating an avenue toward real-time, high-resolution capture using this synthetic aperture approach.

Veeraraghavan said SAVI leans on work by the California Institute of Technology and the University of California, Berkeley, which developed the Fourier ptychography technique that allows microscopes to resolve images beyond the physical limitations of their optics.

The SAVI team's breakthrough was the discovery that it could put the light source on the same side as the camera rather than behind the target.

The research has been published in the journal Science Advances (doi: 10.1126/sciadv.1602564).

Research & TechnologyeducationRice UniversityNorthwestern UniversitycamerasimagingsuperresolutionopticslasersAshok VeeraraghavanJason HollowaySynthetic Apertures for long-rangesubdiffraction-limited Visible ImagingSAVITech Pulse

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