Aug
2004

L&T 101

by Timothy Herrick


Wraparound sunwear is hot and getting hotter. An ever-expanding range of brands and styles is attracting sports- and fashion-minded patients looking for high-performance eye protection and comfort.

Dispensers must take special care when ordering the sunlenses because of the technical demands of fabricating an Rx wrap. That means knowing the parameters of the product and providing precise information to the wholesale lab so the sunglass can be customized.

There are two interrelated issues dispensers must be aware of when ordering wraparound sunlenses. The first concerns the front base curve of the lens, which must conform to the wraparound frame design instead of the patient’s prescription. This means that prescriptions, such as -2.50D, normally on a four base curve, must be ground on an eight base lens for wraparounds. These steeper base curves are not the optimum for the patient’s Rx and can cause peripheral distortions.

The second ordering issue concerns the wrap angle of the frame, generally 13 to 23 degrees. This angle rotates the optical axis of the lens toward the temporal area and creates power errors and unwanted prism.

The goal of wraparound sunlens processing is to compensate for the optical problems caused by the steeper base curve and wrap angle. In order to minimize unwanted prism and power error in the patient’s straight-ahead vision and reduce peripheral distortions, optical labs must take into account prescription power, the patient’s monocular PD and the measurements of the frame when selecting and processing the wraparound sunlens.

The optical effects of steeper base curves and wrap lenses can be minimized if the dispenser conveys exact prescription and frame information to the optical lab, then has an in-depth discussion with the optical lab about lens options for the individual patients. Dispensers have the responsibility to advise patients that their eyes may not accommodate to this minimal peripheral blur immediately. “Even moderate myopes can have a problem,” says Richard Homeier, owner/optician of Optical Perspectives in Estes Park, Colo. “I tell patients that it may take a couple of days to get used to the wraparound lens, that it may seem different at first. The vast majority of patients love the lens and knowing this up front eases the process.”

“As I became more confident in fitting Rx wrap sunwear, the patients became more confident in adapting to the wrap lenses,” explains Maria Robbins, owner/optician of Optik Eyewear in Langhorn, Pa. “There are always high-maintenance patients and sometimes I steer them away from this product. But if you advise most patients, especially those in a higher power, that it can take them a couple of days to get used to the lenses, the vast majority of the patients are fine.”

Changing Curves
Hyperopes have fewer issues than myopes because plus powers typically use front base curves of six and higher, so the base curve switch is not as severe. Most dispensers are hesitant to fit moderate-to-high hyperopes with wrap sunlenses—edges can be too thin to mount—but Rxs of +2.00D and below have few problems. The point to remember is to order a spherical, not aspherical product. “Asphericity flattens curves,” says Thomis Buell, owner/optician of Tom’s Sportique Eyewear in Boulder, Colo. “Aspherics are contraindicated, but you should stress spherical or at least inform the lab it is for a wrap, because there are so many aspheric lenses now on the market.”

For myopes the base curve change is dramatic. In general, base curve is determined by a prescription. In the case of wraps, however, base curve is determined by the cosmetics of the frame. “There is a range of prescriptions optimum for each base curve,” says Mark Mattison- Shupnick, CE coordinator for 20/20 magazine. “The philosophy is different with wrap sunwear because you are ordering a base curve based on the curvature of the frame. The idea is to try and get a lens curve that is close to the one normally used for that prescription.”

Lens manufacturers offer front base curves in one to two diopter steps. The higher the value, the steeper the curve. Manufacturers differ in how they specify their base curves. Dispensers should review the seven to nine base availability of the lenses they prefer for the patient. In general, curves within 0.5 diopters of the wraparound frame curve will work fine.

Most wrap sunwear requires an eight base curve. But there are six base curve wraps and even some 10 base frames. These are typically plano although more ambitious dispensers and labs have had success with mounting prescriptions in these “severe wrap” frames. Use a lens clock to measure the front base curve of the frames’s demo lens and, with your lab, find a lens that will best match that curve and still be able to contain the patient’s prescription.

Due to the need for steeper curves, there are general prescription limitations for wrap sunwear: +2.00D to a -4.00D sphere and cylinder powers up to -2.00D. But these are more rules of thumb than written in stone. Other factors, such as eye size, frame design and patient PD play significant roles in determining if a wrap lens can be processed. It requires discussing the prescription—and frame selection—with the lab.

In lens processing, the lab grinds a back curve on the lens and the prescription is essentially a combination of the front and back curves. Some eight base blanks will not be thick enough for higher minus prescriptions or the lens edges will be too thick for mounting.

“In some cases, we can order an extra thick factory blank,” says Forrest Reukauf, president of Optics Plus, a wholesale lab in Amherst, N.Y. “It depends on the other components of the job. It helps to be familiar with a lot of the frames to know if edge thickness will be a problem.”

Sometimes it’s necessary to choose a different type of lens. For example, polarized lenses, which are typically in an eight base curve for wraps, may not be the best lens choice for patients with prescriptions above a -3.00D with a cylinder power of -2.00D. For these patients, Reukauf often suggests going to polycarbonate or high-index material, generally 1.60. “There are some high-index manufacturers that produce a 7.25 base curve, which will fit nicely in most wraps and still accommodate the Rx,” he explains. “The thinner material will mean less steep curves, edges are thinner and the lens won’t pop out of the frame.”

Ordering Prism
Choosing a base curve to match the frame curve is only part one of the ordering process. In wrap styles, the optical axis of the lens rotates more temporally than in other frame styles because of the frame’s wrap angle. This causes unwanted prism and shifts decentration. In a wrap design, the eye is looking through the lens differently than in other lenses and prescriptions have to be adapted to these changes. For most myopic prescriptions, a compensating prism—base in prism—must be ground into the lens to enable the patient to see through his or her wraparound sunwear just like they do through flatter more typical styles.

Base in prism, which is ordered in 1/4 diopter steps, reduces the prismatic effect that comes from rotating lenses in wrap sunwear. “Prism is added so that the optical axis of the lens matches the straight ahead visual axis of the eye,” notes Mattison-Shupnick. “The rule of thumb is, for powers less than 2.50 diopters, use a 0.25 prism diopters base in. For powers 2.50 and greater, order 0.50 prism diopters base in.”

Many labs prefer to calculate prism. “The first thing the dispenser tells us is the base curve, which is almost always an eight,” says Reukauf. “Dispensers don’t tell us how much prism to grind, but we discuss how much will be needed.”

“If the patient requires base in prism, we make that adjustment. We have developed a sophisticated computer program to make those calculations,” says Becky Wilkinson, Rx director for sunglass supplier Oakley. “Unless the doctor orders prism, we prefer not to have the prism on the prescription. We grind to what the doctor orders, but the prescription is adjusted when we make the calculations. So in a way it is modified, but it is essentially the prescription for that wraparound.”

Buell places the patient’s current prescription lens in a lensmeter, then “tilts” the lens to the same degree of the wraparound frame. This technique mimics what happens to the prescription in the lens in a wrap. “What you are reading is the perceived error. Often, the prescription will change slightly and you will determine the degree of prism you will need to order. You adjust the prescription for the optical error in the lens and order the prism amount but in the opposite direction.”

Buell will either order the lens with the prescription adaptation, including base in prism, or at least discuss the lensmeter readings with his lab. “You want the prescription to work as best it can, within the tilt of the frame and the steeper base curve. The idea is the same as when you adjust for pantoscopic tilt, but here you are adjusting for wrap angle.”

Consider Measurements
Even powers on the high end or just outside recommended prescription range can be used for wraparounds, as long as such things as patient PD, frame measurements and the direction of lens power is taken into consideration. “If the cylinder power is too high in the 90 degree meridian, then we may start looking at using a lower curve than an eight base,” says Reukauf. “But we have to discuss the frame with the dispenser to see if it can handle a less steep lens and still have the same cosmetics.”

In addition to the prescription power, the patient’s PD is a factor in determining wraparound sunwear. In general, a 58 PD or narrower can pose problems, but those problems can be mitigated by prescription power and/or wraparound frame dimensions. “A wider PD won’t be as much of a problem with a lower prescription,” says Mattison-Shupnick “Opticians would be well to suggest a smaller eye size, which can also compensate for the narrow PD as well as the Rx. Too narrow a PD and a large frame may cause problems such as too thick edges, not enough lens diameter or not enough blank thickness. Too large a PD and a large frame could mean not enough blank diameter or thickness, and the inside bevel of the frame may be unable to secure the lens. Certain edge thicknesses in minus Rxs may not allow the temples to close.”

Dispensers should be aware that a narrow PD can be an issue, but through discussions with the lab, a wraparound solution is possible. “You have to look at ED measurement of the frame, how narrow the patient’s PD is and how strong the prescription is,” says Al Gleek, director of business development for SOLA Sunlens. Gleek is based at the SOLA Technologies lab in Hebron, Ky., which produces Spazio, a sunlens used in exclusive brand programs with wraparound manufacturers. “We have proprietary software,” he explains. “We use an atoric front surface and surface the lens differently to compensate for too narrow PDs and adapt the prescription to the wrap angle and base curve. It is dependent on the information we get from the eyecare professional.”


Tech Tips for Wrap Artists
1 Take monocular PDs.
2 Inform patients there will be an adaptation process. 
3 Sell Rx wrap sunglasses only when there is already a working prescription. It is not recommended to have patients adapt to both wrap sunwear and a new Rx at the same time. 
4 Avoid dispensing lenses outside of the range +2.00D to -4.00D sphere, with cylinders to -2.00D.
5 Wraps with smaller eye sizes or a less severe tilt work best for stronger powers or narrow PDs.

 

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