- Liquid Crystal Lenses May Replace Bifocals
Lenses switch focusing power with applied voltage.
By age 50, some people notice that their arms have become too short to hold a newspaper or book where they can read it. That’s a consequence of presbyopia, an age-related inability of the eye to focus on nearby objects. Common solutions include the use of multifocal prescription eyeglasses such as bifocals, trifocals or progressive lenses.
Multifocal glasses, however, present their own problems, explained Guoqiang Li, an assistant research professor of optical sciences at the University of Arizona in Tucson. The wearer must look up or down through the lenses, depending on the vision task, which can result in dizziness or other discomfort.
Now Li, Nasser Peyghambarian, and a team from Arizona and from Georgia Institute of Technology in Atlanta have demonstrated a liquid-crystal-based lens that quickly switches its focusing power with the application of an electric signal to provide a distortion-free image of the entire field of view. The lens reverts to being a passive device with the power off, ensuring no interference with such everyday tasks as driving in the event of power loss.
The key is the use of a 5-μm-thick layer of E7 liquid crystal between two flat pieces of glass. The researchers constructed the lenses with eight concentric electrodes spanning an aperture larger than 10 mm. With the electrodes, they control the state of the liquid crystal material by applying 2 V or less. Switching on the voltage for a particular electrode alters the effective refractive index of that part of the lens. By turning the eight subzones on or off, they change the focusing power of the lens from nothing to a few diopters, an ophthalmic measure of focusing power.
Besides requiring only a low voltage to switch states, the thinness of the liquid crystal material results in a fast response time and high transmission, Li said. Specifically, switching from one state to another takes less than 1 s, and the transmissivity is 85 percent with the voltage off and nearly as high when the device is turned on. This should increase with the use of higher-quality substrates and antireflection coatings.
Results from initial tests involving a model eye and clinical investigations have been promising, according to Li and Peyghambarian. More work is needed, with the possibility of developing glasses that automatically switch among near-, intermediate- and far-vision tasks without wearer intervention.
The work has attracted commercial interest, but it is unclear at this point how expensive mass-produced glasses made with the new lenses would be, Li said.
PNAS, April 18, 2006, pp. 6100-6104.
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