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  • Liquid lens: water + sound = fast focusing

Photonics Spectra
Dec 2008
Melinda Rose,

TROY, N.Y. – An adaptive liquid lens that is powered by water, sound and surface tension can capture 250 pictures per second using very little energy, and its ability to focus extremely quickly (in 0.01 seconds) makes it promising for use in cell phone cameras, as well as autonomous robots and tiny unmanned spy planes.

The millimeter-scale lens, essentially two water droplets that change focus when high-frequency sounds cause them to vibrate back and forth, was designed by researchers at Rensselaer Polytechnic Institute (RPI) with funding from the National Science Foundation. The device becomes a miniature camera lens when light is passed through the droplets.

By using imaging software to automatically capture in-focus frames and discard any out-of-focus frames, the researchers can create streaming images from lightweight, low-cost, high-fidelity miniature cameras.

“The lens is easy to manipulate, with very little energy, and it’s almost always in focus – no matter how close or far away it is from an object,” said project leader Amir H. Hirsa, professor and associate department head for graduate studies in the department of mechanical, aerospace and nuclear engineering at RPI. “There is no need for high voltages or other exotic activation mechanisms, which means this new lens may be used [in] and integrated into any number of different applications and devices.”


A Rensselaer Polytechnic Institute technique for creating liquid lenses with water and sound could enable a new generation of low-cost, lightweight, energy-efficient cameras. This series of time-lapse photos shows how the lens, made up of two droplets of water vibrating at a high speed, changes shape and, in turn, moves in and out of focus. (Photo courtesy of Rensselaer/Carlos A. Lopez.)

Most current liquid lenses are focused by changing the size and shape of the area where the liquid contacts a surface, which takes time and consumes valuable energy. A key energy-saving feature of his technique, Hirsa said, is that the water stays in constant, unchanging contact with the surface as it is manipulated.

His method involves two droplets of distilled water trapped in a cylindrical hole drilled into a Teflon plate. When exposed to certain frequencies of sound, the device exploits inertia and water’s natural surface tension and becomes an oscillator: The water droplets resonate back and forth with great speed and a springlike force. The rate of these oscillations can be controlled by exposing the droplets to different sound frequencies.

As the water droplets move back and forth through the cylinder, the lens moves in and out of focus, depending upon how close it is to the object. The images are captured electronically, and software can be used to automatically edit out any unfocused frames, leaving the user with a stream of clear, focused video.

“The great benefit of this new device is that you can create a new optical system from a liquid lens and a small speaker,” Hirsa said. “No one has done this before.”

The size of the droplets is the key to how fast they oscillate. The researchers have demonstrated high-fidelity imaging at 100 Hz (100 times per second). Hirsa said that, with small enough apertures and properly selected liquid volumes, he should be able to create a lens that oscillates as fast as 100,000 times per second – and still be able to effectively capture those images.

“We have gone up to about 150 Hz, but it’s with relatively large droplets,” he said.

Hirsa and his colleague Carlos A. Lopez, who earned his doctorate at RPI and now works in Mexico at Intel Corp.’s Platform Technologies Lab, Guadalajara Design Center, have filed a provisionary patent on the technology.

The researchers can see the lens being used in cell phones, camcorders and other small consumer products that record video, and it also could lead to new types of 3-D imaging devices.

“I have been approached by all the major cell phone makers and am in discussions with them,” Hirsa said.

The next step will be to make a robust, commercial-grade device, he said. A challenge will be to create packaging that addresses issues such as evaporation.

Hirsa said he also envisions small, lightweight, liquid lens cameras being integrated into a new generation of unmanned and micro air vehicles used for defense and homeland security applications for imaging at all angles simultaneously.

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