LENS TECHNICAL REPORT
The Fine Art of Aspherics
This technical report suggests that aspherics may well be the ultimate lens design
By Joseph L. Bruneni
illustration by David Merrell

Asphericity is everywhere. But, like some forms of art, it is often misunderstood. Nonetheless, most authorities acknowledge that aspherics are the most advanced ophthalmic lenses available today. The best way to understand why, and to put aspherics in the proper context, is to begin by reviewing the evolution of eyeglass lens design.

This review serves two purposes. First, it illuminates how far ophthalmic lenses have advanced in the past 800 years. And, two, it's impossible to understand where technology is going in the future unless you look at where advances came from in the past.

LENTIL BEAN
1200 AD: The word "lens" comes from the Latin word "lentil," a bean that resembles a lens. Early lenses were convex (plus) for magnification and looked somewhat like a bean. They were flat and slightly convex on both sides. This basic flat lens form would be used for the next 600 years.

Flat lenses helped people see, but they had a major flaw. The human eye moves. Looking straight forward (through the optical center) produced acceptable vision, but move the eye a fraction of a millimeter and the image rapidly degraded.

PERISCOPIC LENSES
1800: Distortion to the sides was minimized by grinding a slight concave curve on the backside (-1.25D). This improved vision slightly and produced a somewhat wider field of vision. Called periscopic, these slightly curved lenses became available throughout the 1800s.

SIX-BASE LENSES
1890: The lens curve was increased to six diopters, and this advanced form became the standard by the time of World War II. A six-diopter curve was chosen because it approximated the curve of the eye and positioned lenses away from it. Increased vertex distance widened the field of view considerably, while degradation of image as the eye rotated away from the optical center was greatly reduced.

CORRECTED CURVE LENSES
1913: The next step in lens evolution was more subtle. During the 1900s, frames and lenses grew larger and lens aberrations increased. The term coined for these distortions was "marginal astigmatism," and the first company to work on the problem was Germany's Carl Zeiss with its Punktal lenses. Carefully altering the front curve of each lens power reduced marginal astigmatism and almost eliminated it. Developed in 1908 and introduced to America in 1913, these lenses were state-of-the-art for that time.

Punktal lenses did, however, have a major drawback. Each correction required a different front base curve. This meant that labs had to stock hundreds of various semi-finished blanks to process Punktal orders. Never commercially successful, World War II ended their availability.

Punktal lenses did establish one thing: They proved that six-base lenses could be improved. This led directly to development of corrected curve lenses.

1919: American Optical tried to find a compromise for the Punktal approach. AO's corrected curve (the term used to indicate lenses corrected for marginal astigmatism) lenses changed base curves for every one to two diopters of lens power. This reduced the required number of semi-finished blanks to less than 10 variations. Other manufacturers soon designed their own corrected curve lenses and began offering both standard six-base lenses (called toric) and corrected curve lenses.
It took more than 30 years for the industry to convert from six base to corrected curve. The difference in price was only 30 cents, but that made a difference in those days before the advent of premium lenses. Eventually, six-base lenses were discontinued, and all modern lenses are considered to be corrected curve. Today's term is "best form."

1960: Then came a struggle to convert single vision lenses to minus cylinder form. Until then, single vision lenses had cylinder curves on the front side. Most segments were on the front surface, so labs had to grind cylinder on the backside of multifocals. As patients became presbyopic and switched from single vision to bifocals, they often experienced difficulty adjusting from plus to minus cylinders. Though manufacturers were reluctant to convert to minus cylinders for cost reasons-it meant new inventories and substantial equipment investments-refractionists wanted minus cylinders and eventually they prevailed.

New Lens Standard

The point of this exercise in lens history is to illustrate a confirmed evolutionary trend for ophthalmic lenses. Once improved optical lens technology is accepted in the marketplace, the improved lens form eventually replaces earlier forms and becomes the standard.

Understanding this evolution makes it understandable to predict that as aspheric lenses become accepted and more widely used, they will eventually become the lens standard. Expect to see this happen before the year 2010. Already, a number of all-aspheric lens lines seem to confirm this prediction.

That's especially likely when you consider that most improvements in modern lenses during the past 15 years have been devised to answer one or more of three basic consumer concerns:

1. Cosmetic (thinner, more attractive lenses)
2. Comfort (lighter eyewear)
3. Visual (better vision)

In today's marketplace, how do we answer those basic consumer needs and desires? High index use has exploded during the past 10 years, and it now represents at least 50 percent of all lenses dispensed. High index lenses certainly address concern #1. With strong corrections there is some weight savings, answering concern #2. High index lenses do not claim to provide concern #3, better vision. In other words, high index lenses answer two of the three basic consumer desires.

What propels aspheric lenses to the head of the class is the fact that in most cases, they address all three consumer concerns. They are flatter and thinner, definitely a cosmetic asset. The lenses have considerably less bulk, and this contributes considerably to comfort and lighter weight. Lastly, the unquestioned superior optics of aspherics provides enhanced vision for wearers. The visual advantages of an aspheric design benefits all corrections, although plus corrections gain the most in visual acuity. Once these facts are recognized, it's logical to forecast that all lenses will eventually be aspheric.

In terms of cost, there is usually not much cost difference to the patient between high index lenses and aspheric lenses. Why is it, then, that far more high index lenses are dispensed than aspheric lenses?

One consideration that can influence whether high index or aspheric lenses are recommended to the patient involves the experience or skill of the dispenser. Dispensing high index lenses requires no special skills. If you can spell h-i-g-h i-n-d-e-x, you can explain and dispense these lenses. Aspherics are somewhat different. They require more explaining (demonstrating) and require greater care and proficiency in dispensing. It's unfortunate that dispensers who don't fully understand aspheric lenses tend to avoid recommending or using them. This is unfair to the patient and a disservice to the practice, as well.

So, what makes dispensing aspherics more difficult? Let's review how dispensing aspheric lenses differs from dispensing conventional lens designs.

Aspheric Fitting Guidelines

It's important to note that many of these guidelines do, in fact, apply to all lenses and are not exclusive to aspherics. They are simply hallmarks of good dispensing.

Decentration: Avoid large frames requiring more than 3mm decentration. When lenses are decentered, the nasal or temporal edge becomes thicker (depending on power), detracting from the overall cosmetic appearance. The best-looking eyewear is produced when the frame PD approximates the patient's PD. Though this rule applies to any lens, it's particularly important with aspheric lenses.

Center vertically. Select frames in which the patient's eyes are centered vertically, thus minimizing lens weight and thickness.

Pre-adjust frame. Pre-adjust the frame to the patient's face, and take monocular PDs. A digital pupilometer is recommended for doing this.

Prism. Aspheric lenses cannot be decentered to induce prism. Rather, prescribed prism must be ground in the lens by the lab.

Fitting height. Determine the optical center height for each eye. Position yourself at the same height as the patient. Have that patient gaze into the distance, and then place a mark on the demonstration lens at each pupil center. Measure the distance from dot to bottom of the frame groove.

Lower centers. Determine the frame's pantoscopic tilt, and lower O.C. 1mm for every 2� of tilt-but no more than 5mm.

Check cut-out. Aspheric manufacturers produce a "cut-out chart" for their lenses. Use it by positioning the selected frame with the pupil markings over the center of the chart, and then make sure the lens will cut out. If it won't, select a smaller frame with the patient.

Vertex. Adjust the frame for a vertex distance of 13mm or less.

Dispeners shouldn't view all of this as extra work, because only one of the above rules is actually exclusive to aspherics-the one about lowering optical centers, depending on pantoscopic tilt.

The steps are neither difficult nor time-consuming, and fitting aspherics simply requires a little more care than most other lenses. As dispensers who recommend them to their patients have found, the superior profits of these lenses more than justify any small degree of extra time and effort.

In fact, patients are inevitably impressed with the care demonstrated by these steps. Accompany that with a simple show-and-tell demonstration, and patients are even more impressed. Fortunately, there are a number of ingenius demo kits available that will eliminate the need for detailed verbal explanations of exactly how aspheric lenses work.

Clues to the Future

Manufacturers who produce aspheric lenses also produce high index lenses. In fact, most aspheric lenses are made of mid- to high-index materials.

Looking at the future of manufacturing and what the future holds for aspheric lenses, consider these clues:

Mid-index. When Sola introduced a proprietary mid-index line (called Spectralite�) a few years ago, it chose to produce the entire mid-index line in aspheric design. The same held true for Seiko when it introduced a mid-index photochromic several years ago. And more recently, when 2C Optics (now part of Rodenstock North America) created a totally new mid-index "molded to prescription" lens in 1998, they produced the entire line in aspheric form.

PALs. Most progressive addition lenses introduced in the last several years are aspheric throughout the lens, top and bottom.

Atorics. Now produced by several companies in various high index materials, atoric lenses are simply aspheric lenses taken to the extreme by moving the aspheric curves to the backside of the lens. When there is cylinder, both meridians of the cylindrical surface are aspherized.

Top-of-the-line progressives from Rodenstock and Zeiss feature atoric surfaces. Currently produced only by their labs abroad, this technology is eventually expected to be available to selected U.S. labs.

Given all this activity in aspherics, it doesn't take a rocket scientist to forecast that standard lenses in the future will most likely be either aspheric or atoric. These state-of-the-art lenses will address all three consumer concerns: Improved cosmetics, superior wearing comfort, and improved visual optics.

Do aspheric lenses represent the ultimate and final lens improvement?

Many people say yes, but though it's difficult to visualize what could possibly replace them, never say never. In fact, you can be sure that, as you read this article, a lens designer somewhere out there is working hard to design an even better lens. EB