"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 counter
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Contact: Steve Bradt
sbradt@admin.rochester.edu
716-273-4726
University of Rochester
7-May-1998