Dispensing Aspherics
A hands-on guide to solving this dispensing quandary
By Joseph L. Bruneni
Photography by Peter Baker

"Form follows function" is a common concept, affecting the design of everything from furniture to automobiles to buildings. It has also impacted ophthalmic lens design over the years. Lens form changed considerably as the function of eyewear changed.

Improvements in lens design continue, spurred on mostly by vastly improved equipment for producing lenses. Another important contributing factor to lens design has come from advancements in computers--in speed and computing power.

Eyecare professionals are familiar with the evolution of ophthalmic lenses--how they started out shaped in a primitive flat shape (bi-concave or bi-convex). This was the only practical way to make lenses in the days of hand- or foot-powered lens making equipment, even though it was known that flat lenses produced severe distortions when the wearer looked anywhere away from the lens center.

Fortunately, eyeglasses were small in those early days, and wearers tended to look directly through the lens center. As lens grinding equipment evolved, it became easier to produce curved lenses that more closely matched the curve of the eye. Curving the lens also minimized distortions when looking away from the lens center. As lenses grew larger in the mid-1900s, however, it became obvious there were still inherent optical distortions even in curved lenses.

The term for these peripheral distortions is "marginal astigmatism." The best optics would occur if front curves could change with every power change. Logistically, however, this was totally impractical, so lens designers compromised by changing base curves for approximately each two-diopter range of lens powers. Called "corrected curve," these lenses eventually became the standard. All modern lenses are considered to be corrected curve, though the modern term is "best form." Front curves are carefully selected to provide the best optics possible for a two-diopter range of power.

A NEW CONCEPT

For 40 years, corrected curve lenses represented the best in lenses. That changed in 1989 when a lens category called aspheric was introduced. Though this was a new concept for conventional lenses, older dispensers were familiar with them. The first ones were all strong plus powers (+10.00D and higher) used following cataract surgery.

Removing the lens of the eye during cataract surgery required replacing the plus power normally provided by the removed lens. For years, it was easy to identify post-cataract patients since they all wore heavy plus lenses. Lens designers found that replacing spherical front curves with aspheric curves produced remarkable improvements in acuity and appearance. Over the following years, aspheric post-cataract lenses became an important, profitable market, representing the ultimate in the lens maker's art.

That entire market was eventually wiped out by intraocular lens implants. Aspheric cataract lenses are still manufactured but seldom used except as replacement lenses.

In 1989, a new lens from Rodenstock was introduced to the United States. Made of conventional CR 39, these single vision lenses used aspheric front curves, similar to earlier cataract lenses but now available for plus corrections down to plano. This new lens provided two major benefits to wearers: They enhanced acuity and improved the cosmetic appearance as well. Within a year, minus aspheric lenses became available and soon most major lens producers added aspheric lenses to their lines.

RULE OF TILT

Figure 1 Vertical placement of optical centers should be in accord with the eye's center of rotation. If a lens had no pantoscopic tilt (Fig. 1), the center would be placed directly in the center of the pupil. As lens tilt increases (seen in Fig. 2), the O.C. must be moved downwards 1mm to compensate for each 2 degrees of tilt (never more than 5mm). For example, if tilt is 8 degrees, order the O.C. 4mm below the pupil center marking (8 divided by 2 times 1mm). If tilt is 12 degrees, order the O.C. 5mm lower than the pupil center marking. An alternative way of computing aspheric center placement is to have patients tilt their heads backwards until lenses are vertical to the floor. Mark pupil center in that position. That marking becomes the correct height for the aspheric O.C. Figure 2

LENS DESIGNS

Lens designers are mostly concerned about what happens as wearers look away from the optical center (O.C.). As eyes rotate away from the O.C., distortions begin to show up. These distortions or optical errors are called marginal astigmatism. Lens manufacturers use a minimal number of base curves, each designed to serve a specific range of powers. As long as labs use the base curve recommended by the manufacturer, marginal astigmatism falls within an acceptable range.

Another design concern is cosmetic. Flattening front curves will produce a thinner, better looking lens, but flatter curves increase marginal astigmatism and degrade optical performance. This has been the lens designer's dilemma for the past 100 years.

Now designers have a new tool for designing lenses: Aspheric curves. Aspheric means "non-spherical," so the word could apply to any cylinder lens. A better definition of aspheric is a lens with a rotationally symmetrical front surface that gradually changes power from the center out to the edge of the lens. These minor curve changes minimize or eliminate marginal errors created as wearers look away from the optical center. Aspheric curves are, therefore, the ideal way to reduce that age-old bugaboo--marginal astigmatism.

Aspherizing the front surface offers another primary benefit: It allows flattening the lens without compromising optics. From a cosmetic standpoint, this is a major benefit of aspheric lenses, second only to improving acuity.

Aspheric lenses are generally flatter and thinner than conventional lenses, and because of this, they bulge less in the frame and, overall, produce more attractive eyewear. Flatter curves position lenses closer to the eyes, reducing magnification or minification of eyes in stronger powers. Aspheric base curves are optimized to a smaller range of powers so aspherics typically have more base curves than conventional lenses.

DISPENSING ASPHERICS

There is one peculiarity that sets them apart from most high-tech lenses--they are often dispensed without the eyecare professional realizing the lenses are aspheric. How does this happen?

It's generally accepted that aspheric lenses represent the ultimate in optics. Using aspheric (or atoric) curves in a lens produces two benefits: Better vision and improved cosmetics. As a result, lens producers are now using aspheric designs for most new lens introductions. This immediately places the lens into a premium category, providing better profits for everyone--manufacturers, labs, and eyecare professionals. The result is that aspherics are now the fastest growing lens category.

This is imposing new responsibilities on dispensers because aspherics can be less forgiving than conventional lenses and require more precise positioning by the dispenser. A poorly fit aspheric lens can provide poorer, not better vision.

Think of the front surface of an aspheric lens as a cone with the front curve tapering to a point directly over the optical center of the lens. The front curves change as the eye moves away from the optical center or point of the cone. This is how aspheric lenses accomplish their magic. When the eye looks 10 degrees away from the O.C., the front curve has changed just enough to counteract marginal astigmatism normally experienced at that distance from the O.C. If the apex or point of the cone isn't positioned precisely in front of the pupil, the designer's precise calculations are off and vision is reduced rather than enhanced.

Figure 3 - In marking the vertical O.C. height, the conventional method is to have the patient face the dispenser with head in normal position. Mark pupil center and apply "Rule of Tilt" formula to lower O.C.

DISPENSING DIFFERENCES

Fitting aspheric lenses is similar to the way progressive lenses are fit.

PD. Precise monocular PD measurements position the point of the cone in front of the pupil.

Vertical height. The vertical height of each pupil center is marked on the dummy lens of the selected frame.

Prism. Decentering aspheric lenses to create prism is never done. When prism is required, labs must grind it into the lens.

Rotation. Rotation of the eye must be considered. This requires lowering the optical center based on the pantoscopic angle of the frame. The rule is to lower the O.C. 1mm for every 2 degrees of pantoscopic tilt. The maximum drop of the O.C. is 5mm (see sidebar, p. 88).

Lab order. Vertical pupil height (after subtracting for pantoscopic angle) is specified on the order going to the lab.

Cut-out chart. Like progressives, the manufacturer's cut-out chart is used to determine if the lens will cut out of the factory blank. Labs can not grind prism to shift the optical center for P.D. purposes.

Determining pupil height is a new dispensing responsibility with aspheric lenses. It can be as important as centering the lens horizontally to the patient's PD. When aspheric lenses are dispensed without recognizing they are aspheric, vertical measurements are usually overlooked and this can adversely affect the fitting.

COMPLICATING THE ISSUE

Fitting aspheric lenses requires additional dispensing skills, but should not be an undue burden. What can complicate the dispensing task is the fact that manufacturers are taking varying approaches in designing aspheric lenses.

Flatness. The difference between brands of aspherics can be substantial. One difference is the degree of overall flatness. The amount of flattening from the normal six-base curve is determined by the designer. Some designers flatten their lens less as a way of keeping the dispensing process more simple. Others flatten their lenses less to minimize back surface reflections coming from nearly flat back curves.

Central asphericity. Another variance is how the designer treats the central portion of their aspheric design. In a true aspheric lens, asphericity begins in the center of the lens, the O.C. This is why aspheric lenses can't be shifted to create prism. To eliminate this restriction, one manufacturer currently features a 45mm spherical central zone. All aspherizing takes place outside this spherical cap. For similar reasons, another manufacturer uses a 10mm spherical cap design.

Modified Asphericity. One modern approach to ease the critical aspects of dispensing aspheric lenses is to lessen asphericity slightly in the central area. Unfortunately, modifying aspheric designs in this way is usually not identified in the company's literature.

	Figure 4 - An alternative method for determining vertical O.C. height is to have the patient raise his head until lenses are vertical to the floor. Mark pupil center and request that height for the O.C. "Rule of Tilt" formula is not required.

Alternative materials. More often than not, new aspheric lenses are produced in higher index materials than CR 39. In general, higher refractive index imposes no additional requirements to the dispensing routine.

Looking forward, eyecare professionals can expect to dispense more aspheric lenses in the future, not fewer. Properly fit aspheric lenses currently offer the maximum in visual acuity and provide substantial cosmetic benefits as well. Notice the key words are "properly fit."

Maximizing the benefits of aspheric lenses requires some relatively simple but important procedures be followed during the dispensing process. Conventional non-aspheric lenses are relatively forgiving when improperly fit. Poorly fit aspheric lenses can create visual problems patients may not be able to accept. Here are suggestions on coping with this increasingly complex subject.

The key is to know what the manufacturer recommends for that brand. Only the lens manufacturer knows the exact details used in its aspheric design. The fitting guidelines published should take those details into consideration. For this reason, it's important to have the fitting guidelines for each brand of aspheric lens dispensed in the office. Dispensing aspheric brands that do not provide dispensing information for its lens should be approached cautiously.

RULES OF THUMB

Observing the following rules of thumb should ensure a successful fitting for even inexperienced dispensers.

• Take monocular PDs (pupilometer recommended)
• Select frames where the eye is centered as much as possible, horizontally and vertically
• Adjust frame with proper face wrap and pantoscopic angle
• Spot pupil center for each eye (exactly like fitting PALs)
• Lower height using Rule of Tilt (see sidebar)
• Check markings against cut-out chart published for the aspheric brand being dispensed
• Verify and readjust frame when dispensing finished eyewear

Recommending anti-reflective coatings for aspheric lenses has become routine in many offices for three important reasons. First, many aspherics have a higher index than CR 39 or glass, and AR brings light transmission up to 99.5 percent, no matter what the index. Second, aspheric lenses are flatter and more apt to cast backside reflections into the wearer's eyes.

The third reason is more philosophical in nature. Many consumers now equate AR coatings with high-tech eyewear. As a result, combining an aspheric design with an AR coating firmly establishes that the patient is ordering state-of-the-art eyeglasses. Who can argue with that concept?

In preparing this article, the author consulted with a number of knowledgeable sources for technical and dispensing information on aspheric lenses. In particular, we thank Clifford Brooks, OD, Renato Cappuccitti, Michael Kurze, James Sheedy, OD, Darryl Meister, Bill Cooper, Valerie Manso, and Ignacio Cytrynowicz for providing valuable aspheric information. The conclusions in the story, however, are those of the author and do not necessarily reflect individual opinions of the advisors.