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Novel Camera Employs Microlenses

Photonics Spectra
Jan 2006
Hank Hogan

Researchers at Stanford University in California have found that one lens may be good but that thousands of lenses are better for taking pictures.

The group at Pat Hanrahan’s lab has developed and field-tested under a variety of conditions a device that looks and functions like a typical digital camera except that it lets the user choose what is in focus after the fact. This can extend the depth of focus and so minimize the common effect wherein some objects in a picture look sharp while others are blurry. The improved depth could be a boon for camera users everywhere, from amateur shutterbugs to serious scientific microscopists.

Novel Camera Employs Microlenses
These three images — with foci on the far statue, near statue, and both statues and background wall simultaneously — were computed from a single photograph taken with a prototype camera. Although the first two images could have been taken with a conventional camera, the latter could not. The camera changes focus after the fact and offers such focal depth bysubstituting a microlens and photosensor array for the standard detectorin a typical digital camera. Courtesy of Ren Ng, Stanford University.

The scheme involves a twist on standard optics. In a typical digital camera, a lens focuses the incoming light onto a detector. In the new camera, the engineers have replaced the detector with a microlens array at the focal plane of the main lens. A photosensor array behind the microlenses sits at such a distance that the image from each microlens spans many photosensors.

This is unlike other methods that employ one microlens atop each pixel to concentrate and intensify light, noted Ren Ng, a computer science graduate student and developer of the technique. “We collect how much light arrives at each pixel along rays from each direction, rather than a single value for the pixel,” he said.

The extra data are used in a process of synthetic photography to extend the depth of focus. The number of pixels imaging each microlens determines the maximum improvement. The sharpest results, according to calculations and experiments, are obtained when the effective f numbers of the microlens and the main lens apertures match.

Devices based on the design could be used in any application in which traditional cameras fail to produce images of adequate clarity. The researchers hope to build a microscopy system that will exploit the higher signal-to-noise ratio and greater depth of focus.

At present, setting the critical distance from microlens to photosensor array is performed manually using an attachment assembly designed with the aid of the engineering firm Duval Design. Ng said that this approach would not present a problem for potential applications.

“The calibration is quite manageable,” he said. “It is very mechanically stable.”


GLOSSARY
digital camera
A camera that converts a collected image into pixels that are black or white digital or shades of gray. The digital data may then be manipulated to enhance or otherwise modify the resulting viewed image.
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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