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Cell-Phone Cameras That Zoom

A new design could put the power of a telephoto lens into thin cameras.
February 6, 2007

While each generation of mobile-phone camera captures more megapixels, the images still can’t match the quality of those taken with stand-alone cameras. The major reason: the lens. In a mobile-phone camera, the embedded lenses are frozen in place, without the ability to physically zoom in on a subject.

This novel lens design has the zooming capabilities of a telephoto lens 40 millimeters long, even though it’s only 5 millimeters thick. The trick is to collect light from the outer edge (the dark ring); reflect it within the lens eight times, using mirrors on the front and in the back; and focus it onto a camera sensor.

But now researchers at the University of California, San Diego (UCSD), working with Illinois-based optics company Distant Focus, have developed a new type of lens that could let mobile-phone cameras take close-up shots. Joseph Ford, professor of electrical and computer engineering at UCSD, and his group have developed a five-millimeter-thick lens that has the power of an optical system that is usually 40 millimeters long. The group’s novel design collects light and reflects it within the lens to obtain the full 40-millimeter optical path, and then it focuses the light onto the camera’s sensor. Ford says the lenses could be used in, in addition to mobile-phone cameras, any situation in which a small and lightweight but powerful camera, from a telescope to a military imaging system, is needed.The research is funded by the U.S. Defense Advanced Research Projects Agency as part of the “MONTAGE” imager program. 

The research is based on technology called a folded optical system, which can be found in some telescopes today. In these telescopes, a series of separate lenses and mirrors are used to increase the distance that light travels before it reaches the imaging sensor, a distance known as the focal length. Light is collected using a lens at one end, reflected between mirrors, and then focused onto a sensor. The longer the focal length of a system, the larger the final image will appear. Ford’s group compressed this idea into a novel thin lens and designed it in such a way that light reflects inside the lens eight times before hitting the sensor.

To do this, the researchers made extreme modifications to a traditional lens. First, they used diamond machining to carve mirror surfaces out of a lens material called calcium fluoride. The mirrors steer the light, altering its path so that all the light converges onto the camera’s sensor. Second, they coated both the front and the back of the calcium fluoride with mirrors so that the light reflects inside the lens.The key to making this lens work is precise allignment between the mirrors, which is accomplished by machining them all from a single piece.

The mirror on the front of the lens blocks roughly 90 percent of the light from entering, says Ford, which can reduce the contrast in an image. However, even with so much light blocked, he says, the group’s camera was able to perform almost as well as a conventional lens almost ten times as long, producing images that are only slightly less crisp than those created using a traditional camera, in which 100 percent of the light from an image passes through the lens.

Blocking the lens, as the group has done, makes a tiny, fuzzy ring appear around an image. However, this ring is about a micrometer in diameter, and since most of the light sensors in cameras are only sensitive to a resolution of two micrometers, the ring is undetectable. “If you had a perfect detector with infinite resolution this would be a disaster,” says Ford. “But it works well for the kind of sensors that we’d find for [digital cameras].”

The UCSD research takes advantage of a well-known optical system, says José Saisián, professor of optical sciences and astronomy at the University of Arizona, in Tucson, but he admits that the design is unique. “I think it has some merit,” he says. “They took this idea, analyzed it well, and it can have some interesting applications.”

Ford acknowledges that there were some drawbacks to the group’s initial prototype. The first prototype had a limited depth of focus, for example, which meant that anything approximately two inches in front of or behind the focus point of the lens will appear blurry. However, he says, his team has explored different-shaped lenses that increase the depth of focus, and it has built successful camera prototypes. A second, smaller prototype, using a slice cut from a round lens, matched the depth of field of the conventional camera. Ford claims that the third generation imager, which is now being tested, will be even smaller. 

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