Thursday, September 24, 2009

Adaptive optics: not high-tech, just humanitarian

Physicist Josh Silver's specs may look retro, but they can change lives.

Back in July, I wrote about the 2009 TED Global Conference, held at Oxford. The Global Conference is a sort of carnival of ideas, with talks and presentations by by great thinkers of every stripe, from storytellers to designers, anthropologists to physicists, and videos of this years talks have started to trickle onto TED's online archive.

Physicists, of course, were well-represented among the ranks of TED speakers, but when Oxford prof Joshua Silver took the stage, the audiences weren't in for the usual science lecture. Silver is an atomic physicist, but lately he's been obsessed with optics; not because he wants to design an invisibility cloak or improve high-speed communication, but because he wants to address a very important problem for the world: bad vision.

As Silver points out in his talk, glasses, contact lenses, and even laser eye surgery are facts of life for about half the people in the developed world. You can imagine that the same should be true for people in the developing world. But while there is equal need for vision correction among people, say, in Sub-Saharan Africa, there is a serious dearth of optometrists, about one to every eight million people. In the UK, by contrast, the ratio of optometrists to people is about 1 to 10,000.

Believe it or not, this is a problem for physicists to solve. Or, at least, that's the way Joshua Silver saw it. Glasses and lenses are fairly cheap and plentiful; optometrists are not. The answer: a pair of glasses that a patient could adjust on her own to fit her needs.

At a basic level, correcting bad vision boils down to high school optics. In the eye, muscles squeeze or release a small lens at the front of the eye, that focuses an image on the retina, the brain's outboard receiver for visual information. Squeezing the lens makes it fatter, which focuses nearby objects; relaxing the lens flattens it, focusing far away objects.

The eye's lens, from the Centre for Vision in the Developing World

But if the geometry of the eye is slightly off—if the lens is deformed, or the retina too far away from the lens—the lens's focal point will hit just in front of the retina (nearsightedness) or behind the retina (farsightedness.) Glasses are additional lenses, shaped to bend the light in a way that compensates for a refracting system that doesn't work. Usually these lenses are made from plastic and are shaped according to the patients' needs. Once made, they can't be adjusted.

So Silver came up with an ingenious alternative to prescription glasses. He invented a pair of glasses that had a space encased by two flexible plastic walls in place of each lens. By pumping in or removing liquid from the space between the walls, the patient can put the glasses on and change the shape of the lens until he can see clearly.

Fluid-filled lenses, from the Centre for Vision in the Developing World

The glasses cost only about $19—though Silver wants to drive the price even further down, since many of his prospective patients live on a dollar a day— and the whole adjustment procedure takes less than ten seconds, as Silver demonstrates in the talk below, shot at the TED Global Conference. Not only that, Silver set up the Centre for Vision in the Developing World to study and remedy the problem of access to vision correction and cook up other new ways of making adjustable lenses. Now that's physics in action.

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