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Eyeglasses On the Cheap

Graduate student inventor sets his sights on correcting the world's vision problems with tabletop lens molding

4 min read

For an astonishingly large number of people in poor countries, uncorrected vision prevents them from doing things some of us take for granted, like reading street signs or comparing ads to decide which market has the best prices.

According to the World Health Organization, as many as a billion people need vision correction but will never get it. Eyeglasses are scarce in developing nations because they cost too much for the average person--sometimes more than the average monthly income--and there are few people qualified to diagnose eye problems and then provide the proper corrective lenses. But now, something is being done about the world's vision problems.

A chance encounter with Kenya's education minister, who reported that a lack of eyeglasses was one of the country's biggest problems, motivated a young inventor to develop a system that can provide a vision-impaired person with low-cost lenses in a matter of minutes. In the scheme devised by Saul Griffith while a graduate student at the Massachusetts Institute of Technology, in Cambridge, a desktop molding machine whips out a pair of lenses, right after the needed prescription is determined by his electronic goggles. Griffith [see photo, " Budget Glasses"] came up with the idea for the machine and goggles and built a prototype system in 2002. Having earned his Ph.D. in media arts and sciences last September, he was the winner earlier this year of the US $30 000 Lemelson-MIT Student Prize for Inventiveness in recognition of his achievement.

Much of the cost of vision care comes from opticians and suppliers' having to maintain an inventory of 500 to 1000 lenses, to fit every possible eyewear prescription. Griffith's technique eliminates this cost by shrinking the inventory to a single programmable mold that can be precisely shaped to accommodate a person's eyes one at a time, used to make a lens, and then reshaped to make lenses for someone else. Each mold, which is made of the mylar polyester film used for tinting car windows, makes 20 to 30 lenses before it needs replacing. As a result, says the inventor, "it should be possible to offer an eye exam and a new pair of glasses for $10 to $15." A good bit of that cost, he says, pays the salary of the technician operating the examination and molding devices.

The more complicated part of providing corrective glasses is the diagnosis--assessing the extent of refractive errors that cause objects close up or far away to appear fuzzy. Heretofore, obtaining an accurate lens prescription was impossible in some areas of the world. The equipment is expensive and fragile, and there are few people with the requisite training. But Griffith has come up with a device that is portable, easy to use after a short training period, and at less than $250, relatively cheap.

The goggles eliminate guesswork by automatically doing what an optician would do to arrive at the right prescription. Infrared beams are shined into each of the patient's eyes. As the beams are moved slightly, discrepancies introduced by a misshapen cornea cause a noticeable difference in the movement of the light reflected off the back of the eye from what would be expected in reflections from a normal eye. An array comprising 12 infrared photosensors picks up these errors and routes the data to a rudimentary microprocessor, which determines how the adjustable lenses in the goggles should be manipulated by the technician in order to cancel the refractive error in the eye. The adjustable lenses in the goggles are calibrated so that the lenses' final shape will tell the technician what the prescription is.

The device is cheap because the microprocessor uses the same simple decision tree that an optician would use when doing a manual retinoscopy. Does this lens setting cancel the refractive error of a myopic (nearsighted), hypermetropic (farsighted), or astigmatic eye? If not, then try another lens setting. The chip "has about the same processing power as a television remote control," Griffith told IEEE Spectrum .

Griffith says that the goggles will run easily on rechargeable AA batteries. Exactly how many will depend on whether a light-emitting diode (LED) display is needed. The display could be used in a manually assisted system as an indicator telling the technician to add or remove optical power from a lens. "You would have a red and a green LED," said Griffith, "with the red LED saying 'give me more' and the green saying 'give me less.'" That is the cheapest possible model. More costly but less power hungry is a fully automatic version being developed by Griffith and his collaborators that doesn't need a display. Little servomotors will control the focusing of the lenses. "So you could think of it as an autofocusing pair of goggles."

Once the patient's vision problems have been diagnosed, the programmable mylar film is placed in the palm-size lens-making device on top of a ring whose shape is adjustable. The shape of this ring is important because it determines what type of eye problem (nearsightedness, farsightedness, or astigmatism) the resulting lens will correct for. Griffith notes that more complex ring shapes can yield progressive or bifocal lenses.

The lens gets its shape--which determines the degree of correction--from the curvature of the film's surface. To produce a concave surface, baby oil that fills the cavity under the ring is drawn out, pulling the film against the ring. For convex lenses, the ring sits on top of the mylar membrane and baby oil is injected into the cavity. When the proper curvature is reached, a containment ring that represents the proper shape of the lens's outer rim is placed on the mold. The procedure saves on time spent shaping the edges of the lenses and on wasted materials.

To form the actual lens, the same type of UV-curable acrylic/monomer lens that is used in 95 percent of the world's eyeglasses is poured into the ring. Then the technician shines a device comprising four ultraviolet LED arrays on the monomer, providing enough energy to polymerize (and solidify) it. The UV cures the polymer in 3 to 5 minutes. When the polymer is pulled away from the mylar membrane and popped out of its containment ring, it is a finished lens that is ready to be fitted into a frame and worn right away.

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