'Letric Specs.
Carefully tuned voltages can change the focusing power of these prototype glasses (above) and bring a blurry image into focus.


Bye Bye Bifocals

Here's the long and short of it: A lens that electronically switches its focus from far to near may someday provide an alternative to traditional bifocal lenses.

The natural lens in your eye is supposed to bend light rays and focus them on the retina at the back of the eye. If the eye is misshapen or not strong enough to do the job itself, glasses help bend the light rays through a process called refraction--the same one that makes a stick appear to kink when one end is submerged in water. Just how much a given lens bends light depends on its precise shape and curvature. Bifocals are essentially two lenses ground into a single piece of glass or plastic. In contrast, the new electronic lens is flat and focuses light through a phenomenon known as diffraction, in which light waves overlap either peak-to-trough to cancel one another out, or peak-to-peak to reinforce one another.

The new lens consists of a layer of material known as a liquid crystal, which is sandwiched between two thin sheets of glass, report optical scientists Guoqiang Li and Nasser Peyghambarian of the University of Arizona in Tucson and colleagues. The inner surfaces of the glass sheets are covered with circular transparent electrodes arranged in a bulls eye pattern. Applying a voltage to one of these electrodes affects the speed with which light can pass through that part of device. So light waves passing through different rings get out of synch with one another.

That may sound like a bad thing, but by adjusting the voltages on the rings, the researchers exploited the canceling and reinforcing effects to make the flat plates focus the light. So simply by applying the voltages, they could switch the device from a transparent plate to a lens, they report online this week in Proceedings of the National Academy of Sciences. That makes the lenses suitable for reading glasses that can switch on and off electronically, as the researchers demonstrated with a prototype. As for bifocals, the new technology could be combined with a standard lens to make one whose focusing power increases when the electricity is switched on, Li says.

Compared to bifocals, "the advantage to this is that you get a full field of view correction," says vision scientist Mark Bullimore of Ohio State University in Columbus. Such electronic lenses would have to compete against not only traditional glasses but also bifocal contact lenses and lenses surgically implanted in the eye. Still, Bullimore says, "if the technology were to mature, I can see where this might be an attractive option for some patients."