Imaging the Eight-Fold Way
An annular aperture and concentric zone reflectors shrink imager by 80 percent.
Although she was not talking about cameras, the Duchess of Windsor nonetheless had it right: You can never be too thin — at least for certain applications. For example, surveillance cameras for miniaturized unmanned aerial vehicles cannot be big and bulky. The same is true for cameras in portable infrared telescopes. However, shortening the length of conventional imaging optics degrades resolution and decreases light collection.
By using multiple concentric zone reflectors and an annular aperture, researchers folded a standard refractive imaging system (a) into an imaging system that was at least five times thinner (b). Images used with permission from Applied Optics.
Now a team from the University of California, San Diego, and Distant Focus Corp. of Champaign, Ill., has demonstrated a reflective multiple-fold approach that overcomes these problems. Its prototype camera had an effective focal length of 38 mm, a numerical aperture of 0.7 and an effective aperture of 27 mm. However, it was only 5 mm thick — more than five times thinner than a conventional optical setup with similar capabilities.
Joseph E. Ford, an associate professor of electrical and computer engineering at the university, noted that the technique could show up in consumer products. “One potential application is cell phone cameras, where space is small but the user’s expectations are large.”
Ford, along with graduate student Eric J. Tremblay, designed, assembled and tested the optical prototype. Their colleagues from Distant Focus designed circuit boards and packaging, developed software and helped with testing and demonstration.
Here conventional and eight-fold imagers are compared. Using a dual-camera setup (a), researchers imaged staggered resolution charts (b). On the bottom are the results, with the conventional camera capturing the image on the left (c) and the eight-fold camera acquiring the image on the right (d).
In their prototype, the investigators used an annular aperture and concentric zone reflectors to achieve an eight-fold optic in which light enters through an annulus that runs around the outermost edge of the disk-shaped optic. It then bounces off a rear reflector that is arranged in a concentric ring and off the front edge of the imager, which directs it to the next concentric rear reflector. In this back-to-front-to-back manner, the light travels from the edge of the optic to the center, where a sensor sits.
The folds enable the imaging optics to be reduced in depth, with the decrease tracking the number of folds. The trade-off for a thinner lens, however, is a decrease in the field of view and in the depth of focus, along with some loss of light collection resulting from the number of reflecting surfaces.
Annular folded optics also are not easy to manufacture. The prototype required eight concatenated mirrors with tight surface tolerances. After considering a more complex design, the researchers settled on a layout that consisted of a planar surface on one side, four concentric aspheric reflectors on the other and a patterned dielectric mirror coating. This approach benefited from recent advances in diamond machining of optical surfaces, which helped in fabricating the reflectors.
The group constructed the prototype folded optic out of a solid calcium fluoride substrate, using a megapixel CMOS imager from Omnivision Technologies Inc. of Sunnyvale, Calif., for the sensor. Resolution and image quality of the folded optic were comparable to those of conventional optics. “The imager worked well and matched simulations well,” Tremblay said.
With the prototype proven, the group is now working with partners on both defense and consumer applications. It also is looking into ways to increase efficiency and to reduce light losses, with one goal being an ultrathin multisensor imager that collects 14-megapixel images.
Applied Optics, Feb. 1, 2007, pp. 463-471.
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