ASPHERIC LENSES

 

Ever since aspheric lenses were introduced for conventional prescriptions in 1989, Eyecare Business has been covering this rapidly growing category. For 40 years prior to that time, aspheric lenses had represented the most complex lens design for eyeglasses a design that was limited to the post-cataract field.

The cosmetic and visual advantages for high plus corrections made aspherics the preferred lens following cataract surgery. Unfortunately, just about the time aspheric cataract lenses hit their stride, interocular lenses (IOLs) were invented, and in time, virtually eliminated the use of cataract spectacles (IOLs can also be ordered in aspheric form). That seemed to relegate those complex lenses to history..

All this changed in 1989, however, when Rodenstock introduced a new lens series called Cosmolit These were CR 39 single vision lenses with aspheric curves on the front surface, similar to the earlier post-cataract lenses, but available for corrections down to plano. Initially, the lenses were made in plus powers only.

Eyecare professionals soon discovered how aspheric lenses improved both optics and cosmetics. Adding to their appeal was the fact that these new lenses fell into the premium category, producing improved profits for everyone involved.

The use of aspherics grew rapidly. Two years later, minus power lenses became available, and other manufacturers began producing aspheric lenses. The race was on.

An explanation of aspheric lenses will help in understanding this special lens category.

All modern lenses utilize minus cylinders with the cylinder curves placed on the backside of the lens. The front surface is considered to be the base curve, and all lab surfacing takes place on the backside of the lens.

– Superior optics. The word aspheric is defined as non-spherical The aspheric curves on the front surface, in combination with the back curves, produce optics superior to those of conventional lenses especially when looking away from the optical center of the lens.

– Cosmetic advantages. Much of their popularity comes from the fact that they also improve the appearance of eyewear. In plus powers, aspheric lenses are flatter with an immediately noticeable decrease in bulging.

Flattening is less obvious with minus lenses, but the aspheric curves reduce edge thickness, always an advantage for myopes. Lens designers have long known that flatter minus lenses would have thinner edges, but simply flattening the front curves introduced severe optical errors. When the front surface is made aspheric, minus lenses can be considerably flatter with no optical distortions.

– Atoric technology. A later development involved moving the aspheric curves to the backside. Aspherizing both cylinder curves on the back surface produces even better optics for wearers. This new type of lens required a new term atoric.. Atoric lenses provide the ultimate in visual acuity for ophthalmic lenses.

By 1992, aspheric lenses had become an accepted premium lens category, and 17 lens manufacturers were producing these lenses.

That extraordinary growth prompted the author to make the following prediction in an article appearing in the June, 1992 issue of Eyecare Business: The ultimate lens of the future may very well end up being an aspheric lens made in a higher index material.

The rest of this article will review what is currently available in aspheric form so readers can decide for themselves whether that statement was clairvoyant or mere hyperbole.

One good example of how the use of aspheric curves has increased is progressive addition lenses. PALs have always used aspheric curves in the lower front surface of the lens. Many PALs introduced in the last several years now feature aspheric curves in the top portion of the lens as well as the bottom. Descriptive literature usually describes them as all aspheric.

Aspheric curves in the top half of new progressives are less aspherized than in single vision lenses. It now seems likely that new progressive designs in the future will feature aspheric (or atoric) curves throughout the lens.

Aspheric lens designs can be produced in any material. The major influence on aspheric growth has been the tendency for manufacturers to produce their newer lenses totally in aspheric or atoric form. If this trend continues as expected, all modern lenses will eventually be either aspheric or atoric. Here are some examples of lenses that have been produced totally in aspheric or atoric form: n Spectralite: In 1990, Sola Optical produced a proprietary 1.54 index plastic with some unique features. All Spectralite lenses use an aspheric design that Sola calls ASL. Even with a relatively low index, the aspheric curves provide cosmetic benefits similar to 1.60 lenses.

– Tegra: This all-aspheric polycarbonate line was launched by Vision-Ease.

– Seiko Changers: Though now discontinued, these plastic photochromic lenses were only available in aspheric form.

– Vizio: Introduced in 1998 by Sola, these 1.66 index minus stock lenses (also made in polycarbonate) are only made in aspheric/atoric form.

– Resolution: Introduced this year by Optima as its first polycarbonate lenses, the entire series is made in aspheric/atoric form.

– Hyper Index 1.74: Optima recently introduced these super high-index stock lenses in aspheric form only.

– Phoenix: Hoya is bringing out Phoenix, a line of lenses made of PPGs new Trivex material that are totally aspheric.

– Trilogy: Younger Optics is also producing an all-aspheric line of Trivex lenses that it calls Trilogy.

– Gradal Top: This Zeiss progressive also features an atoric design.

– 2C Optics: This entire line of cast-to-prescription lenses (now part of Rodenstock North America) is aspheric.

– J&J Visioncare: This entire line of cast to prescription lenses is aspheric. The line is temporarily on hold, but is still expected to be re-released.

– Optical Dynamics: Next month, this in-office casting company is releasing a totally new all-aspheric progressive design called Continual Focus Lens that also features atoric curves on the back surface.

– High index. Some manufacturers now produce their high-index lenses in two forms, standard and aspheric.

These events seem to indicate that aspheric or atoric optics will probably continue to show up in most, if not all, new lens introductions. The future for ophthalmic lenses is clearly revealed. These facts also validate the prediction in EB that eventually all lenses would be made in aspheric form.

Readers who have been in the optical business for more than 30 years will remember when lens manufacturers produced lenses in two forms: Standard 6 base and corrected curve. Suppliers spent fortunes trying to convert the professions over to corrected curve lenses, now called best form.

It took 30 years for corrected curve lenses with their superior performance to totally eliminate the less sophisticated 6 base lenses. With the current accelerated use of aspheric/atoric lenses, we can expect the next transition from corrected curve to aspheric/atoric lenses to take place over a much shorter period of time.

With these new designs as the foundation, it will surely be interesting to watch modern lenses develop and evolve.06/15/09

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