Low-Power Liquid Lens

An adjustable-focus lens made of two drops of water could be used in small cameras.

Lenses made of liquid have interested researchers and engineers for decades because of the technology’s ability to quickly change shape and focal length. But traditional approaches, which use an electric current to change the surface shape of a liquid, require a lot of power. Now, researchers at Rensselaer Polytechnic Institute, in Troy, NY, are proposing a type of liquid lens–made of only two drops of water–that changes shape when bombarded with sound waves. Using sound requires much less power than previous methods and could, with improvements in resolution, make the lens attractive for use in small surveillance cameras and cell phones.

Droplet optics: This series of pictures shows droplets of water in a hollow Teflon cylinder every four milliseconds. The curved shape of the water forms a lens, which changes shape, and therefore focal length, when sound waves are applied to one side of the cylinder.

With glass, plastic, and other hard materials, it’s impossible to quickly change the shape of the lens, and therefore to focus: to adjust the focal length, you need to physically move the lens. Extremely small cameras and many cell phones simply don’t have enough room to allow users to move a rigid lens the distance required for a range of focal lengths. An adaptive liquid lens, however, enables small cameras to focus without needing any extra room. “Liquids are a favorite material to work with when you want to change the shape of a lens,” says Amir Hirsa, a professor of mechanical and aerospace engineering at Rensselaer and lead researcher on the project.

The researchers’ lens system, described in October’s Nature Photonics, is composed of a Teflon cylinder less than two millimeters in diameter. The cylinder is overfilled with water so that droplets bulge out on either side. A speaker is hooked up to one side of the cylinder, which is in a pressure-sensitive chamber. The researchers pumped sound at between 50 and 160 hertz into the chamber, changing the shape of the droplet’s surface.

Two companies, Philips and Varioptic, which is based in France, have developed products that use an alternative liquid-lens system. Both use two different liquids that are in contact with each other, which creates an imaging lens at the interface. In a process called electrowetting, the shape of the interface is altered when an electric current is applied to it, changing the surface tension of both liquids.

The advantage of his new approach, says Hirsa, is that it requires much less power. He says that electrowetting requires an electric potential of tens to one hundred volts is required to adjust the conventional liquid lens. In contrast, only a small potential of a couple of millivolts is required to drive his two-droplet design.

Finding focus: These three pictures were taken, from left to right, after 0.007, 0.020, and 0.033 seconds. The focal point of the lens changes as the sound waves alter the shape of the droplet. The researchers suggest that a camera using this liquid lens could take tens of pictures at the press of a button and let software sort out the one that’s most in focus.

In tests, Hirsa’s liquid-lens camera was able to take 250 images per second, at varying focal lengths. He envisions a camera that could instantly capture tens of images with different focal lengths, and then use simple image-analysis software to determine the most in-focus image. “Say you capture 60 frames per second,” he says. “Just take the one that’s the sharpest.”

Stein Kuiper, the Philips researcher who developed the electrowetting technique, notes that the researchers’ ideas seem original, but he sees drawbacks to the approach. Since the lens is continuously moving, this means that “a significant amount of light is lost, as most of the time the object is out of focus.”

Additionally, these early results are not high resolution, notes Yuhwa Lo, a professor of electrical engineering at the University of California, San Diego. “Even the low-end cameras have a pretty strict resolution requirement,” he says. However, he says that at this stage, the lens could be good for other optical applications, such as simply focusing beams of light instead of taking high-quality pictures.

Hirsa says that his team is looking to improve the resolution, possibly with different types of fluid, such as those that change shape in response to magnetic fields. He’s also exploring the possibility of working with electronics manufacturers and says that Samsung has expressed interest in the lens.

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