Lenses of Liquid

We don’t expect much from a cell-phone camera. For one thing, only a handful of camera phones have a lens system capable of automatically focusing on objects at different distances – causing many fuzzy snapshots.

But there may be a solution to the problem of camera phone focus – and one that could find uses in other devices as well. Saman Dharmatilleke, Isabel Rodriguez, and colleagues at the Institute of Materials Research and Engineering in Singapore have proposed replacing the stationary plastic lens in most camera phones with a drop of liquid, such as water, that could be auto-focused by varying the amount of pressure applied to the drop. The team’s lens has no moving parts, making it rugged, and it uses only minimal electricity, so it would not drain a cell-phone battery.

[Click here to view images of the liquid lens.]

Additionally, the optical properties of liquids can be better than standard lens material. “Water is more transparent to light than glass or plastic,” Rodriguez says. “Water cannot be scratched and, in principle, is defect free.”

The technology, which appeared online in the January 26 issue of Applied Physics Letters, is based on the fact that a drop of a liquid with a high surface tension has a natural curvature similar to that of a conventional lens. When the drop is placed in a small well, and pressure is applied to it, the curvature of the drop alters; more pressure increases the curvature, and less flattens out the drop. As the curvature changes, so does the lens’s focal length, allowing a clear image to be captured from various distances. In most cameras, the auto-focus feature mechanically moves the solid lens forward or back in order to adjust focal length. But in a liquid lens camera, the droplet stays put and only its curvature changes.

The researchers tested varying sizes of drops, from 100 microns to 3 millimeters: all responded to pressure changes within milliseconds. The bigger the lens, of course, the more light it collects, and more light produces better pictures. But when a droplet becomes too large, it is more difficult to keep stable. “Up to two millimeters the lens stays perfectly in the aperture by surface tension,” Rodriguez says. “You need to shake it very hard for it to move out.” She suspects that lenses one to two millimeters in diameter are ideal for most miniaturized imaging systems.

Although this team is not the first to use liquid for lenses, they’re the first to adjust the focal length by simply applying pressure. In 2004, Philips announced a liquid lens system, using a technique called “electrowetting,” which relies on the intrinsic electrical conductivity of water-like liquids. With this technique, an electrical current is applied to a liquid lens, altering the bonds of the liquid’s molecules, thereby changing the curvature of the drop. In January, the French company Varioptic introduced a cell phone containing an electrically variable liquid lens, which uses a version of electrowetting that, Rodriguez says, pre-dates the Philips design.

Stein Kuiper, the Philips researcher who developed the electrowetting technique for his company’s liquid lenses, sees advantages in using pressure instead. “The electrical properties of the liquid are not relevant, which allows for a wider range of liquids, and thus optical and mechanical properties of the lens.” Additionally, Kuiper says, the voltage required to change the pressure within a liquid lens system may be less than is required in a system using electrowetting. For these reasons, he says, Philips has “built up” intellectual property rights on both types of lenses.

Currently, Dharmatilleke and his team have partnered with a local company to fine tune and manufacture their liquid lens system, and they’re seeking commercialization of the technology. As Rodriguez notes, applications for these lenses extend beyond camera phones, into webcams and portable medical devices.

Images courtesy of Isabel Rodriguez at the Institute of Materials Research and Engineering.