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The Eyes have it

New York Times, Ian Austen, 18 Mar 04

MINIATURIZING the electronic parts of digital cameras is relatively easy. Witness the proliferation of cellphones and hand-held computers that have built-in cameras.

But in trying to make ever-smaller lens assemblies that can focus and zoom, camera designers encounter difficulties. These lenses require moving parts, and as moving parts become smaller, overcoming friction between them is increasingly difficult. As a result, many miniature cameras make do with fixed-focus lenses and simply forget about zooming.

But scientists at Philips' research center in Eindhoven, the Netherlands, say the answer may lie with lenses based on liquids rather than glass or plastic. Fluid lenses, that is, that can change their shape, and thus their focus, through electronic commands.

"With this you have a very simple concept in which you don't move the lens, you only change the lens," said Benno Hendriks, a physicist specializing in optics at Philips.

The FluidFocus lens, as Philips has named it, is based on an effect called electrowetting, using an electrical charge to alter the surface tension of a liquid.

Electrowetting is not new. But Dr. Stein Kuiper, the lens project leader at Philips, said that until 1993, the effect was mostly a laboratory curiosity. To alter a liquid significantly, researchers had to use so much voltage that the experiments short-circuited.

The breakthrough that opened the way to potentially practical applications seems, in retrospect, rather simple. The short circuits can be prevented by putting an insulator between the electrodes holding the electrical charge.

Philips began working on several technologies based on electrowetting in 1994, including electronic displays. When presented with an early prototype electrowetting lens, Dr. Kuiper had his doubts. Like most people, he had visions of a lens that could spill or slosh around when the camera was jiggled.

"I was skeptical," Dr. Kuiper recalled. "I had to shake it and turn it around many times."

The current prototype lens is a sealed transparent tube just 3 millimeters (about one-eighth of an inch) in diameter and 2.2 millimeters long. Its inside surfaces are covered with water-repelling chemicals. The tube is filled with two liquids: one that conducts electricity readily, for example, water, and another that does not, such as oil.

Outside of their electrical properties, the type of liquid does not matter. "You could make a lens with vinegar and olive oil," Dr. Hendriks said.

And indeed, at least one lens at the Philips lab was made using soup. (It rendered color very poorly.)

Without an electrical charge, the surface of the conducting liquid forms a curve. But when a charge is applied through the electrodes, the surface tension of the liquid changes, altering its curvature and thus the focal point of light passing through it. Different voltages produce different curvature changes. For zooming, changes in the shape of liquid lens elements replace the mechanical parts that shift the relative positions of glass or plastic lenses.

Surface tension also keeps the small amount of liquid in the lens stable. By way of explanation, Dr. Hendriks noted that while it is easy to spill a full glass of wine, it is difficult to dislodge the few droplets that remain at the bottom, even with vigorous shaking, because of surface tension.

Like the wine, the liquid lens only maintains its stability at a small scale. The Philips researchers estimate that the technology will only work for lenses up to 4 millimeters in diameter.

The researchers have made a prototype cellphone camera that they anticipate will be the model for future products. The prototype has a field of view roughly comparable to a 35-millimeter wide-angle lens on a film camera. In tests, Philips found that the liquid lens was able to change focus rapidly; the lens would be ready for another shot long before the digital imaging chip was.

For cameras, the researchers predict that manufacturers will use fluid elements only to provide focus or zoom effects. Other optical corrections will be left to traditional glass or plastic lenses.

As for the liquids, Dr. Hendriks said, the best conducting fluid is water mixed with lithium chloride as an antifreeze and, if necessary, other chemicals to fine-tune its optical properties. Silicon-based oil is the most likely insulator.

With the addition of extra electrodes, liquid lens elements can take on tasks other than just focusing. For example, tiny medical cameras might contain liquid lens elements that could peek around corners by swiveling on their optical axes.

Similarly, home video cameras could use the system to replace electronic image stabilization systems, which generally degrade picture quality. A liquid lens element would take signals from a motion detector and jiggle around to optically offset the user's unsteady hand.

The simplicity and reliability of the liquid lens set it apart from other research efforts, Dr. Kuiper said. "Normally you start with a good idea then you try to find an application," he said. "This can be used in virtually any field where lenses are needed. We didn't have to find applications. Even before this was finished, it was sold."

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