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Single-Lens System Offers Simultaneous Fields of View

Photonics Spectra
Feb 2006
Richard Gaughan

Optical zoom lenses can provide variable magnification, so a single system can generate images of low magnification and large field as well as images with high magnification and a narrow field. It had not been possible to provide high magnification while retaining a large field of view. Researchers at Bar-Ilan University in Ramat Gan, Israel, however, have applied superresolution techniques to make a double-field, single-snapshot camera.

An imaging system essentially is a lens in front of a multielement detector. The lens creates a one-to-one correspondence between a detector element and a section of a scene. The magnification is a measure of how much of the scene fits onto one detector element. For a given detector, there is a trade-off between field of view and magnification — that is, the higher the magnification, the smaller the field.

The new technique does not rely on any moving parts; a single lens replaces the multielement lens of a traditional optical zoom system. The system is designed to be diffraction-limited in the center of the field where the lens resolution is highest, and detector-limited for the remainder of the field.

At the heart of the system is a Fourier transform grating filter that decreases the effective pixel size in the center of the field. The filter is a combination of three cosine filters, each representing one-third of the spatial frequency distribution of the object. It has the effect of “folding in” all the spatial frequencies of the object into the central image region. Digital postprocessing then deconvolves the image, mathematically removing the aliasing created by the filter.

Increased resolution

The different regions of spatial frequency are separated in the grating mask, which increases the resolution of the central region of the final image by a factor equal to the number of regions. In the researchers’ proof-of-principle system with three cosine filters, the zoom portion of the image has three times the resolution of the original image. The image contains more pixels than the detector because of the increased number of effective pixels in the superresolved central region.

According to Zeev Zalevsky, the lead researcher on the project, the trick involves trading resolution outside the center of the field for resolution within the center. There is a penalty in terms of a reduced dynamic range, but it is roughly equivalent to jpeg compression.

Because the method does not require a longer focal length lens, like conventional zoom systems, the mass and volume are reduced while the reliability and the speed increase. Zalevsky said this makes the technique especially suited for cell phone cameras or unmanned aircraft.

Xceed Imaging Ltd. of Rosh Ha’Ayin, Israel, which holds the patent on the method, is designing a compact form factor polychromatic zoom camera using the single-lens technique.


GLOSSARY
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
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