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Origami lens slims high resolution cameras

Engineers at the Jacobs School have folded up a telephoto lens in order to build a powerful yet ultra thin digital camera that may find its way into cell phones, unmanned surveillance aircraft or infrared night vision applications.

Origami lens slims high resolution cameras

"Our imager is about seven times more powerful than a conventional lens of the same depth," says Eric Tremblay, a UCSD electrical and computer engineering graduate student and the first author of a recent Applied Optics paper describing the new technology. Tremblay works with professor Joseph Ford in the Photonic Systems Integration Lab at the Jacobs School. The research is funded by the U.S. Defense Advanced Research Projects Agency (DARPA) as part of the "MONTAGE" imager program.

"This type of miniature camera is very promising for applications where you want high resolution images and a short exposure time. This describes what cell phone cameras want to be when they grow up," says Ford. "Today's cell phone cameras are pretty good for wide angle shots, but because space constraints require short focal length lenses, when you zoom them in, they're terrible. They're blurry, dark, and low contrast."

To reduce camera thickness but retain good light collection and high-resolution capabilities, Tremblay and colleagues replaced the traditional lens with a "folded" optical system that bends and focuses light within a single mirror-coated, 5-millimeter thick calcium fluoride optical crystal. In contrast, traditional zoom lenses generally rely on a series of separate mirrors and lenses to bend and focus light.

The term "folded" refers to the fact that the light bending and focusing elements— the optic surfaces—are all etched onto the same side of the crystal, thanks to recent advances in the mechanical machining process of diamond turning.

Cameras with the new slim folded lenses take digital photographs, such as the one on the laptop screen, that are comparable—in terms of resolution, color and quality—to photographs taken by cameras with traditional lenses.
Cameras with the new slim folded lenses take digital photographs, such as the one on the laptop screen, that are comparable—in terms of resolution, color and quality—to photographs taken by cameras with traditional lenses.

This design forces light entering the ring-shaped aperture to bounce back-and forth between the two mirrored surfaces. The light follows a predetermined zigzag path as it moves from the largest of the four concentric optic surfaces to the smallest and then to the digital camera's CMOS light sensor.

Folding the optic addresses performance issues facing today's cell phone cameras by increasing the "effective focal length"—the zooming power of the camera— without increasing the distance from the front of the optic to the light sensor.

In the laboratory, the engineers compared a 5 millimeter thick, 8-fold imager optimized to focus on objects 2.5 meters away with a conventional high-resolution, compact camera lens with a 38 millimeter focal length.

At best focus, the resolution, color and quality of digital photographs taken with the two cameras are comparable, the authors report.

Cameras with the new slim folded lenses take digital photographs, such as the one on the laptop screen, that are comparable—in terms of resolution, color and quality—to photographs taken by cameras with traditional lenses. Traditional lens vs Folded lens: The engineers reduced camera thickness but retained good light collection and high-resolution capabilities by replacing the separate mirrors and lenses used in traditional lens designs (bottom) with a "folded" optical system (left) in which all light bending and focusing elements are etched onto one surface.

One initial drawback with the new folded camera was its limited depth of focus. Digital post-processing techniques and design changes implemented in the latest generations of the camera are addressing these issues.

The team is also designing variable focus folded optical systems that have gel or air between the reflective surfaces of the imager. Such imagers may be especially useful for lightweight, inexpensive infrared vision applications. The all-reflective systems also enable ultra-broad-spectrum imaging and thus may be useful for ultraviolet lithography—an emerging technique for squeezing more transistors onto silicon in order to create more powerful chips.

This work is part of a larger Multi- Domain Optimization research project led by Mark Neifeld at the University of Arizona, which includes a related UCSD research effort on nanophotonics led by Jacobs School electrical engineering professor Shaya Fainman, and researchers from CDM Optics and MIT. To create fully functional prototype imagers, the UCSD engineers partnered with Illinois-based optics company Distant Focus.