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Adjustable lenses may mean no more bifocals

May 2006
Hank Hogan,

Along with thinning hair, a few more wrinkles and some added pounds, by age 50 nearly everyone notices that print seems harder to read. It’s the result of presbyopia, an age-related inability of the eye to focus on near objects.

Solutions to the problem have included reading glasses as well as bifocals, trifocals and progressive lenses. However, all multifocal lenses have a limited field of view, which makes some reading tasks difficult, and they sometimes cause dizziness and discomfort.

Now a team from the University of Arizona in Tucson and Georgia Institute of Technology in Atlanta has demonstrated a lens that changes focusing power on demand while providing power-failure-safe operation and avoiding the problems of multifocals. “Using the switchable lens, the entire lens aperture is used for one vision task,” said Guoqiang Li, an assistant research professor of optical sciences at Arizona. Nasser Peyghambarian, a professor of optical sciences and materials science at Arizona, was the principal investigator.

In building switchable lenses with adjustable focus, researchers constructed optics comparable to high-quality reading glasses. The unwrapped phase map of the 10-mm aperture lens is on the top, and the bottom map shows the difference between the measured wavefront and a best-fit spherical wave and tilt. This shows there are few higher-order aberrations. Reprinted with permission of PNAS.

The lenses consist of a 5-μm-thick layer of liquid crystal sandwiched between two pieces of glass. In building these, the researchers first fabricated eight concentric conducting rings on a glass substrate. They covered everything with polyvinyl alcohol to act as a liquid crystal alignment layer and attached the other substrate, leaving a small gap that they filled with liquid crystal material. They sealed the two together and connected the drive electronics.

Switching states

When the investigators applied 2 V, the liquid crystal material switched on, changing the effective refractive index in the eight subzones of the 10-mm-diameter lenses. This changed the lens from simply being a passive device to one adding focusing power.

Because the material was so thin, transmissivity was high — about 85 percent with the voltage off and nearly as much with the voltage on. Switching from one state to another took less than a second. With no voltage applied, the lenses appeared to be plain glass, which Li noted is a valuable characteristic for a variety of common tasks. “Power-failure-safe operation is very important for driving,” he said.

After characterizing the lenses for various optical parameters, the researchers tested them with a model of a human eye constructed from a set of lenses, a filter and a sensor. They placed an object at a distance of 30 cm in front of the model eye and captured the resulting blurry image.

They then turned on their lens and brought the object into sharp focus. These findings have been confirmed by some clinical studies, which have had promising results. According to Li and Peyghambarian, a company is working to commercialize the prototype used in those studies.

As for the future, the technology could create lenses that allow multiple focal lengths to handle near, intermediate and far vision tasks. The result could be glasses that adjust through the use of a distance-determining near-infrared sensor and without requiring intervention by the wearer.

“The electronics control system decides the needed focusing power and the corresponding voltages to apply,” Li said.

PNAS, published online April 5.

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