Engineers from the University of Rochester and seven corporate partners, working with the U.S. Department of Defense's Advanced Research Projects Agency (DARPA), have developed the first system to automate the manufacture of unusually shaped lenses known as aspheres. Officials at the University's Center for Optics Manufacturing (COM) say the machine is capable of producing these aspheric lenses in minutes, not days, at a fraction of the current cost.
Its developers say the savings in time and money should trickle down to products ranging from 35-millimeter cameras and medical endoscopes to military-grade night-vision goggles. High costs -- up to $4,000 for a meticulously formed, doorknob-sized piece of glass -- have limited the use of aspheres, despite their ability to deliver much better optical performance and image quality than traditional spherical lenses. The new asphere machining system greatly streamlines production; engineers believe the machine will push costs down to as little as $25 to $100 per lens.
"There isn't an optical device around that wouldn't benefit from aspheres," says Harvey Pollicove, director of COM, which is now testing the new machine. "Aspheres are used in nearly every application for which they're affordable, and every engineer who designs optical devices wants to use them."
Aspheres are well suited for a wide range of consumer goods, including compact disk players, photocopiers, and projection televisions; top-rated cameras and video camcorders now rely on three or four aspheres to achieve lightweight, compact designs with improved image quality. Beyond the consumer realm, aspheres are also used in virtual reality helmets, professional-quality movie cameras, surgical lasers, bar-code laser scanners, and in endoscopes to see inside the body. The new machine might also provide a cheaper way to produce aspheric elements that are found in the $500,000 lenses now used to make computer chips by tracing out integrated circuits on wafers of silicon.
"Aspheres are better than spherical lenses in these applications because they bend light rays more precisely," says President Donald Golini of Rochester optics firm QED Technologies, which was not part of the consortium that developed the asphere grinder.
An aspherical lens or mirror focuses incoming rays to a single point, while spherical lenses cause blurring. "For example," says Golini, "if you were to build a reflective telescope with a spherical primary mirror, and use it to look at a star far away, the starlight striking the edge of the mirror would focus at a different spot than the light striking the mirror's center. You'd actually need additional corrective optics to reduce this aberration, which is normally avoided altogether in telescopes through the use of aspheric mirrors."
Since it focuses light more precisely, a single asphere can take the place of two or more spherical elements in many optical devices, such as night-vision goggles worn by soldiers. Replacing bulky groupings of three or four spherical lenses with an asphere or two would make the goggles 30 percent smaller and lighter, Pollicove says, while also boosting image quality and resolution.
But aspheres' subtly irregular curves make them a real chore to produce. Most of today's manufacturers use a process Golini describes as "home-grown," rigging up expensive machines like high-precision lathes for double-duty as asphere grinders. The final painstaking round of hand-smoothing, known as "lapping," done by specialized artisans, can take hours or even days. "There's now no efficient way, and certainly no single machine, for making high-precision ground aspheres from start to finish," Golini says.
Many companies sidestep these difficulties by using mass- produced molded plastic or small glass aspheres in their optical products. But plastic aspheres are susceptible to heat and humidity, and molding limits their glass counterparts to a diameter of about an inch. Even before reaching the mass-production stage, companies often spend hundreds of thousands of dollars creating asphere prototypes the old-fashioned way; Golini expects that much of the demand for the new grinder will actually come from large companies that spend heavily on hand-crafted templates while developing new products.
"Since this new system will reduce initial tooling costs, it will make all plastic and glass molded aspheres more affordable, and it's much better suited to the task of producing high-precision aspheres than any of the previous techniques," Pollicove says.
A technician can just plunk a piece of glass into the new machine, plug the desired curvature of the final lens into the machine's computer, and let the of diamonds embedded in its two bronze grinding wheels go to work. Using a first-of-its-kind measuring system to ensure that precision shape requirements are met, the computer alternates between measuring the lens and manipulating the grinding wheels that sculpt it. A regular ball-bearing spindle spins the rough grinding wheel to quickly shape the lens, and then an advanced spindle that floats a fine grinding wheel on a cushion of air produces an almost polished finish. The machine's built-in measuring system can then automatically feed relevant data to a magnetorheological finishing device for a final round of ultra-smooth polishing. Total time required: roughly 15 to 30 minutes.
"We've incorporated several new technologies into this machine," says Kevin Uhlig, vice president of machinery systems at Bridgeport, Conn.-based Moore Tool Company, the firm that actually built the new asphere grinder. "It's the first machine to fully automate asphere production; earlier machines couldn't muster the precision needed to shape aspherical lenses. It's also the first to successfully tackle the final round of smoothing, typically done by hand."
The machining system, designed by a consortium consisting of COM, Moore, Byelocorp Scientific, Raytheon TI Systems, Eastman Kodak, Opkor, Lockheed Martin, and OptiPro Systems, and funded partly by DARPA, is expected to sell for about $250,000 -- roughly $100,000 less than the cost of the techniques now used to produce aspheres. COM is currently testing the wardrobe-sized, computer-controlled grinder, and Moore will begin selling the machine later this year.
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