SPECTACLE LENSES
Good-bye Thick Lenses
An in-depth look at how aspheric technology is changing the face of optical
By Joseph L. Bruneni

The use of aspheric curves for ophthalmic lenses is expanding in both innovative and interesting ways. With aspheric lenses in a state of evolution, this is a good time to review the entire concept of aspheric lenses--from their inception as post-cataract lenses to the newest ways these sophisticated lenses are impacting today's and tomorrow's ophthalmic eyewear.

The initial use of aspheric curves was in the 1950s--and they were introduced to a market that badly needed the enhanced acuity provided by these lenses. Back then, patients had to wear strong plus lenses, usually over +10.00D, following cat-aract surgery. The lenses were thick and extremely heavy (most lenses were glass), post-cataract patients had severely compromised vision, particularly when looking away from the optical center of the lens.

Post-cataract lenses that featured aspheric curves made a major impact on this specialty market. Zeiss produced them early in the century, but they were never sold in this country. In the early 1950s, Dr. David Volk introduced aspheric lenses he called Conoids, primarily for low vision uses. Then in 1958, American Optical introduced the Aolite Aspheric Cataract Lens. Other major lens producers followed with their own aspheric cataract lenses, including the famous Welsh 4-Drop.

As a result, cataract patients gained vastly improved vision. In addition, these lenses didn't bulge as much from the frame, were thinner, and best of all, provided greatly improved vision with a much wider field of view.

Aspheric cataract lenses proliferated in both conventional and lenticular form until that market was wiped out a few years later as surgeons began to implant
interocular lenses (IOL) during cataract surgery. Aspheric cataract lenses are still available, however, and are now used primarily for older patients operated on prior to IOLs.

A CONTINUING EVOLUTION

Asphericity for progressives. All modern progressive addition lenses (PALs) incorporate aspheric curves in the bottom half of the lens--to change power from one point to another. The earliest patent for a progressive lens was in 1907, but it was never produced, most likely because of manufacturing difficulties.

When the first Varilux progressive was designed in France by Bernard Maitenaz, he created changing powers through use of a series of spherical surfaces. That Varilux I lens was introduced to the United States in 1969, and when Maitenaz improved his design through use of aspheric curves, the Varilux II lens was brought to the United States in 1974 by Multi-Optics (an Essilor subsidiary). Since then, all progressives have used aspheric curves.

Dress and premium lenses. The year 1991 saw the first use of aspheric curves for dress lenses when Rodenstock introduced the Cosmolit^� lens to the United States. It was available only in plus power, differing from cataract lenses in that the lenses were available in all plus powers, not just over +10.00D. The success of Cosmolit prompted other manufacturers to develop aspheric lenses, including minus powers.

Material applications. The next major step for aspherics was a decision by Sola USA to incorporate aspheric curves in its high index line called Spectralite, which is available in both plus and minus. And when Seiko launched its new Changers^� photochromic lens a few years ago, the entire line was aspheric.

More recently, Vision-Ease brought out a new line of polycarbonate lenses, which are also aspheric, called Tegra. Adding an aspheric design enhances acuity and cosmetics, but it also elevates Tegra lenses into the premium class, justifying higher fees and better profits. Since premium lenses are increasingly important to the professions in today's market, aspheric lenses are gaining even more attention than before from both the dispensing population and the vendor community.

Semi aspherics. One company, Bristol, produces only aspheric lenses, in both CR 39^� and polycarbonate, in single vision, bifocals, and trifocals. Bristol's design differs from other aspherics in that the lenses are spherical in the central portion and aspheric toward the edge. This feature allows Bristolite lenses to be decentered for prism (aspheric lenses cannot be decentered for prism but must be surfaced by the laboratory).

The newest aspherics. A new manufacturing facility was recently formed to manufacture ophthalmic lenses in a totally different way. The company, 2C Optics, utilizes aspheric designs in its entire line of single vision, bifocal (CurveTop 28), and progressive lenses.

Two things distinguish this new company. First, lenses are produced with a casting process called APT (Automated Prescrip-tion Technology) in which each lens is individually cast to prescription, using a proprietary monomer. Unlike previous lens casting processes, 2C lenses are individually cast by the manufacturer, not in a doctor's office.

Second, several major world-class companies have invested in this new technology company, including PPG, Dow Chemical, and Rodenstock. In addition, Rodenstock has granted 2C an exclusive license to use Rodenstock lens designs. In the 2C progressive, the final prescription is modified with an "as worn" feature (similar to the "position of wear" concept), and all 2C bifocals have a curve top. The company is currently selling its lenses in limited markets and plans to broaden distribution in the near future.

Aspheric multifocals. A number of manufacturers produce bifocals and trifocals in aspheric form (see chart on page 122). These lenses provide the same advantages as single vision aspherics. (Note: segments in some aspheric bifocals/trifocals have a curved top rather than a conventional flat top design.)

THE ATORIC ASPHERIC STORY

Can aspherics be improved? In a word, yes. Today's aspheric lenses offer positive benefits to the wearer that include improved acuity, cosmetically more attractive lenses, less bulging in plus corrections, and thinner edges with minus lenses. However, aspheric lenses all share an inherent problem when the prescription includes a cylinder correction.

The cylinder challenge. Ophthalmic lenses are produced in a series of front base curves, ranging from flatter (for high minus) to steeper (for high plus). Depend-ing on the sophistication of the lens design, the front curves change for every one to two diopters of Rx power. Each spherical base curve is designed to be used with a range of inside curves.

Aspheric lenses use a similar system of base curves. Each aspheric base curve is used with a one to two diopter range of prescription powers. In an ideal world, aspheric base curves would change with every change in lens power. That, however, would require laboratories to carry an inventory of thousands of semi-finished blanks. For practical reasons, manufacturers design each aspheric base curve to work with a range of lens powers.

This becomes more complicated when the lens has cylinder. Cylinder lenses have two back curves, at crossed meridians, while the aspheric front curve is the same in all meridians. As a result, the aspheric front curve can maximize optics for one meridian of the cylinder, but not both.

The back-surface solution. There is, however, a way around this problem. Mov-ing aspheric curves to the back-surface eliminates any need for compromise. When a back surface cylinder surface is aspherized, it becomes an atoric surface. Atorics permit curves in each meridian to be optimized specifically for the spherical front surface.

Such back-surface aspheric lenses have been available for more than a year. Two major manufacturers, Sola and Optima, produce high index single vision stock lenses with atoric back curves. Sola also produces its design in polycarbonate. Both base curve and cross curve are aspherized and optimized specifically to the spherical front. Customizing of aspheric curves provides the ultimate in sharp vision, with an additional cosmetic benefit. Placing aspheric curves on the backside of a minus lens produces considerably thinner edges, particularly in strong corrections. For this reason, atoric stock lenses are only produced in minus powers.

Atoric lenses produce another benefit--the patient ends up with a much wider field of vision (see comparison graphic on page 120). This enhancement is particularly useful with atoric progressives.

United States laboratories, however, do not have equipment capable of fining and polishing atoric surfaces. As a consequence, atoric single vision lenses are only available in stock lens form.

When the patient's single vision prescription falls outside the stock range, atorics cannot be ordered. Fortun-ately, however, atoric stock lenses are available up to -16.00D.

Atoric PALs. What about progressives? As stated earlier, all modern progressives utilize aspheric curves in the lower half to accomplish their magic. Some recent designs feature aspheric curves in the top portion as well. And, one progressive (American Optical's AO55�) features an aspheric top portion only in higher powers.

These represent among the newest and most sophisticated lenses in the aspheric family. The most immediately noticed benefit is an extraordinarily wide field of view. This wider field manifests itself throughout the lens, benefiting distance, intermediate, and near vision.

Two manufacturers, Rodenstock and Zeiss, currently produce atoric progressives. Until recently, only Rodenstock's lens (Multigressiv) has been available in the United States. The Zeiss lens (Gradal^� Top) is just starting to show up in the United States. Rodenstock ads in Europe show a man wearing the Multigressiv. Seen behind the wheel of a convertible, the man is looking over his shoulder as he pulls out from the curb, a severe test for any progressive. It graphically demonstrates one of the benefits of a wider field of view.

Atoric progressives differ from conventional progressives in another major way. Because American laboratories cannot fine or polish atoric surfaces, both the Roden-stock and Zeiss atoric progressives are fabricated in Europe.

A comparison of the error-free fields of view of a flattened lens with a spherical base curve, an aspheric and an atoric lens, each with -2.00D of cylinder power (based on data supplied by Sola USA). 

THE FUTURE OF ASPHERICS

All precision camera lenses have aspheric curves. Camera makers learned long ago that aspheric optics enhance the precise imaging required for photography. The increasing number of aspheric ophthalmic lenses suggests that, ultimately, all quality prescription lenses will be aspheric.

This new trend echoes the industry's gradual transition from six base lenses to corrected curve--that is, best form--lenses. That transition took more than ten years, but eventually six base lenses were phased out.

Today, all non-aspheric lenses are best form and have been for some 30 years. It seems likely that another transition will take place in which ophthalmic lenses will be seen gradually converting from best form to the more exacting aspheric design. That possibility makes knowing what's available in aspherics even more important for eyecare professionals.

The chart on page 122 provides an easy way to keep track of what's available in this modern lens design. Keep it for reference so that you can keep up-to-date and be able to communicate the availability of aspherics to patients. EB