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Selecting, Filling and Fitting Wrap Prescription Eyewear

By Dave Carr

Release Date: April, 2006
Expiration Date: April 30, 2008
 
This course is supported by an unrestricted educational grant from Luxottica Group.
 

Learning Objectives:
Upon completion of this program, the participant should be able to:

  1. Describe the lab processing procedure.
  2. Explain the details of frame selection and fitting.
  3. List the benefits of an Rx clip

Faculty/Editorial Board: 
Mark SachsDave L. Carr, BSME

Dave Carr, BSME, spent nine years as a manufacturing engineer and product developer for SOLA Optical and SOLA Technologies. Carre earned a Bachelor of Science in mechanical engineering at Yale University.


Credit Statement:
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). Course #: STJP011-1
Please check with your state licensing board to see if this approval counts toward your CE requirement for relicensure.

 

Everywhere you look these days, you will find consumers wearing sports wrap eyewear, from cyclists on the road to golfers on the links to even businesspeople on the street. From their unremarkable beginnings as a niche product in the 1980s, the wrap category has grown dramatically. Adoption of wraps by high-profile professional athletes has raised their profile. And specialty eyewear manufacturers are investing in advanced design and manufacturing technology that has left most old-line eyewear makers behind. Overall the sunwear category has grown to more than $2 billion of the $15 billion plus in annual optical sales.

It�s easy to understand the popularity of wrap sunglasses. The curved, contoured eight-base polycarbonate lenses in most wraps block UV, shield the wind, reduce glare and protect from impact. They provide excellent optics tailored to all sorts of outdoor conditions, whether cycling, golfing, hiking or fishing. They are designed to be lightweight, comfortable and versatile.

Your customers are already buying wrap eyewear�and maybe not from you, either. But they still need vision correction when they are outdoors. Maybe some of your customers have already begun to ask if you can provide Rx capability in their sports eyewear. This course provides a key to prescription wrap sunwear.

WRAP EFFECTS AND OPTIMIZATION

The curvature and wrap that makes the lens so functional in terms of lens coverage also requires care in setting up the Rx. Compared to a standard �flat� lens in the same Rx, a wrap lens changes the base curve and tilts the lens in the vertical plane. These changes will affect the wearer�s vision, not usually for the better, unless the optics is optimized for the effects of wrap. Fortunately, the optimization is well understood.

TILT (WRAP) OPTIMIZATION

The primary optical effect of wrapping a powered Rx lens is an offset in the prism and power of the lens as perceived by the wearer. The fact that the wearer looks through a lens differently when it is wrapped is key. To consider it another way, if you take an ordinary �flat� lens of a certain Rx power and introduce wrap (faceform tilt), the lens will still read the original Rx only when measured along the original OC and visual axis of the lens. However, in the new wrap configuration the wearer is now looking through the lens on a different optical axis corresponding to the wrapped positioning.

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The resulting prism and blur can be objectionable, especially in higher powers. Even a plano lens introduces prism when wrapped, causing discomfort for the wearer; this is why so many ophthalmic manufacturers of better sunwear provide decentered (i.e. prism corrected) lenses in their plano sports eyewear. In an Rx lens, the lens power makes those effects even stronger.

Fortunately, the corrections required to optimize the wrap are fairly straightforward and formulas can be found in many ophthalmic optical texts. The calculation takes into account the Rx power and the wrap dimensions of the frame, and the resulting optimization usually adds some base-in prism at the eye point and small adjustments of power, cylinder and axis compared to the original �flat� Rx. These adjustments are ground into the Rx during surfacing.

In the example opposite, to create a -4.00 sphere for the wearer to satisfy a -4.00 sphere Rx, the lab would need to supply a -3.75 -0.25 x 180 with 0.35� Base In.

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LAB PROCESSING

How these calculations are applied depends on who is doing it. In fact, many labs still fill a wrap Rx as if it were a conventional flat frame, grinding the �dumb� Rx onto an eight-base lens and mounting in the frame without regard to the wrap angle or optical correction. The resulting optics is bound to disappoint your patient.

Labs that understand the need for wrap optimization, however, have procedures for making the required adjustments. For example, there are proprietary labs for processing wrap prescriptions. These companies use proprietary formulas to calculate surfacing optimization in Rx lenses or offer specialty wrap lens designs in SV and progressive with additional atoric optimization of the front surface for improved peripheral optics.

Specialty labs also may have other tools and tricks for delivering wrap eyewear, such as surfacing processes that are optimized for the steep back curves and large apertures of a wrap Rx and edging and tracing solutions that can handle the sinuous frame grooves. These labs can also assist with selecting which wrap frames are most suitable for Rx.

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Some manufacturers have built their own dedicated labs to service their own products with factory-direct Rx processing and OEM lens colors. Other brands have partnered with specific labs to provide factory-approved optics and processes specifically tailored for their wrap frames.

All this doesn�t mean that a wrap Rx can�t be filled in your own office. The formulas and technology are out there and a specialty wrap Rx service could be an attractive niche for your practice. Talk to your lab to understand their capability and then decide for yourself. (�Wrap Technologies� below includes additional discussion of wrap processing.)

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ORDERING WRAP LENSES

If it is decided to outsource wrap jobs to a lab, there are a few steps to follow. First, talk to your lab about their capability and procedures, and make sure you understand the product you are getting. Also ask if the lab has a list of frames they consider Rx capable (and not capable) to assist in the frame selection process. Finally, discuss their Rx range limitations and turnaround time capability.

When ready to place an order, generally the process should be quite straightforward. Call or fax the order, the job is started immediately, then supply the frame from inventory or have it drop shipped from the manufacturer; the lab does the rest. Do not bother working out the wrap corrections because the lab will do that and it could be confusing if you try to second-guess their calculation by sending pre-corrected Rx figures.

MEASURING WRAPS � SURFACED VS. �AS-WORN� MEASUREMENTS

When the job comes in and it is time to check the Rx accuracy, remember the wrap-corrected Rx differs from the original Rx. If the lens is simply clamped into your lens meter, the value will be the �corrected� power that was surfaced into the lens. The lab that compensates the Rx will supply a checking ticket that lists this corrected value so you know that your measurement matches their �adjusted Rx.�

Alternatively, try measuring the eyewear in the �as-worn� position� that is, get your lens meter to look through the lens along the same visual axis that the wearer will experience�which then should report the exact power called for in the original Rx refraction. The feasibility of �as-worn� measurements depends on the configuration of your lens meter; manually position the frame over the read head and eyeball its position to get the right alignment. It may help to open the temples and use them as a guide; align the temples so they are parallel to the optical axis of the lens meter and then read the Rx along the same optical axis.

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BASE CURVE CONSIDERATIONS

As with any base curve change in Rx eyewear, it is important to consider the effects of eight-base curves on patients. The 8D base curve is selected for wrap coverage and to equal the curve of the frame, not optics. Wearers may perceive a change in magnification compared to their regular specs, especially in higher minus powers where their regular lenses may be a two- or four-base. Increased vertex distance and front base curves will increase magnification of the patient�s world. These changes may also be more significant if the new sunwear does not have wrap-optimized optics. Make sure to explain the differences to patient and inform them there may be a short adaptation period required. Most patients love the fact they are free from having to wear ordinary flat or uncorrected sunglasses outdoors.

RX RANGE

Generally, wrap lenses are recommended for a narrower Rx range than conventional frames, from perhaps +200 to -600 depending on the manufacturer. There are several good reasons for this. For example, high minus Rxs can be very thick in the larger eye sizes typical of wraps; wrap prism and magnification errors are more pronounced in stronger Rxs; and higher minus Rxs can result in steep back curves that are difficult to surface. Discuss Rx ranges with your lab and consider steering higher Rx patients to other products (for example contact lenses combined with regular plano wraps).

MULTIFOCAL LENSES

For the most part vendors have offered Rx wraps in single-vision lenses only, leaving out many wearers such as the presbyopic golfer who wants to read her scorecard as well as her lie. In expert hands, however, a variety of multifocal solutions can be delivered in wrap frames. The same wrap corrections used with spherical single-vision lenses can also be applied to spherical flat-top or round-seg lenses.

Progressives are tougher since most PAL designs are optimized for specific Rx ranges, which in eight-base designs means high-plus powers only; therefore the design may not perform as well when ground to a more typical emmetropic or myopic correction. Alternative solutions are softer design or spherical design progressives like AO b�Active, Varilux Panamic or Younger Image where wrap optimized surfacing with a wide-periphery PAL design can provide an effective wrap PAL solution. Talk to your lab about their capability.

GETTING INTO THE DETAILS: FRAME SELECTION AND FITTING ADJUSTABILITY

With many wraps, fitting is either a �does� or �doesn�t� proposition. A large proportion of wrap sunglasses are injection-molded plastic with limited adjustability. The shape of the patient�s head and nose are often the primary factors in fit. However, fit-friendly features do exist:

  • Some models feature adjustable nosepads and/or temples.
  • Many wraps are engineered with compliant parts that form to a wear-er�s face (for example, ribbed rubber nosepads and flexible temples).
  • Certain brands offer two sizes in popular styles to help find the best fit.
  • Most metal-frame wraps are made with the same components as regular ophthalmic frames and thus are easily adjusted.

Overall, the process of finding the best fit is still going to be trial-and-error, and over time the models that work well for customers� facial features will be learned.

SELECTING AND FITTING WRAP FRAMES

With limited adjustability, fitting needs to be optimized on the first visit before the Rx is ordered. Have a patient try on several styles and check the fit. To get the best sport performance consider the following factors:

  • Nose: Requires comfort and grip. The nose and temples must securely hold the frame in place during active sports despite shocks and sweat. The wearer can�t take the time to push the eyewear back in place while stretching for a point. Extreme conditions will also expose the comfort limitations of a frame.
  • Temples: : Ensure secure fit and comfort. Flexible, conforming temples may fit perfectly for one customer yet create uncomfortable �hot spots� for another. Ask whether the customer will be wearing the glasses with a hat or helmet as this will affect the fit.
  • Lash crash: Many wraps fit very close to the wearer�s face and their eyelashes could contact the wrap lens. Keep in mind the Rx solution that will be employed; most lens-in-frame styles place the front surface flush with the frame front, bringing the back surface closer to the lashes. Clip-on styles add an additional lens element between the sun lens and the eye, further reducing the available space. In many cases the shape of the person�s nose will determine whether a particular frame will fit or not. Keep an Rx lens or an Rx clip on hand to try on with the frame.
  • Vertex distance: : In stronger prescriptions, a large change in vertex distance compared to the patient�s optical frame could affect the power and magnification experienced in the new wrap pair. Measuring vertex is difficult with wraps, but try to select a frame that keeps the lens about the same distance as the optical frame.
  • Coverage: Since the patient expects good sun and wind protection from a wrap, watch for gaps at the cheekbones, eyebrows and temples. Most wraps are not panto adjustable, they either work or they don�t. In shield or rimless models, manufacturers often offer multiple lens shapes of different depths that may help with coverage. Remember that deeper B sizes also curve in toward the face more due to the eight-base curvature, giving the effect of greater panto. Have the patient wear the frame outdoors to check out sun protection.
  • Field of View: For Rx wraps, particularly clip-on models, consider the wearer�s peripheral vision needs in the intended application. Place the frame on the wearer and have them strike a sport-specific pose. Does the frame provide sufficient view through the periphery of the Rx lens or clip to see the pitch or putt?
  • Weight: : Consider the weight of the eyewear with the Rx and/or clip, which in higher powers can double the weight of a lightweight plastic frame. The movement and bouncing in sports puts a premium on fit. Don�t be afraid to have the patient jump up and down to see if the frame stays put.
  • Style: Considering the radical look of many sport sunglasses, some wearers could be self-conscious about the �attitude� that comes with the performance. Additionally, wearing a pair of dark glasses in the store under artificial lighting, without correction, can make it hard for patients to judge the look. Consider using a virtual mirror or a digital camera to capture an image of the wearer.

CLIPS

For many wrap sunwear styles, the only Rx solution available for many years has been an Rx clip. A clip mounts behind the frame front and accepts clear lens inserts in a more or less traditional plastic or wire rim. The benefits of this system are:

  • Availability: Makes Rx available in a wider range of frame styles.
  • Flexibility: Many styles give the wearer the option to insert different colored interchangeable plano lenses for different light situations, which is a popular option in the plano sun market. Also, contact lens wearers can remove the Rx clip altogether when they choose to wear contacts with their sunwear.
  • Simplicity: Since clips are built more like traditional frames, they are straightforward to Rx without specialized equipment, often even in-office. Also, clip sales mean extra revenue from the sale of the clip itself since it is usually offered separately from the frame.

However, clips have a few drawbacks to consider:

  • View: Most clips have smaller apertures that restrict the wide field of view wearers expect from wrap sunwear. Some wearers may compare this to looking through a keyhole.
  • Fit: Clips add weight and volume behind the lens, affecting fit and lash-crash. Some types require a stacked nose-bridge that pushes the frame away from the face, compromising the �speedy� look.
  • Optics: Clips introduce multiple surfaces that reflect light and attract fingerprints. Additionally, clips may not offer advanced wrap-optimized lenses that many suppliers offer in full-frame wrap eyewear, so prism and blur may be an issue. In general, though, clips tend to have less wrap than the parent frame and therefore present fewer optimization issues.

FULL-RIM AND THREE-PIECE WRAPS

When it comes time to Rx these frames, it is generally easy going since three-piece and conventional metal frames share the same materials and mounting features as conventional optical frames. Many suppliers are also starting to design injection-molded plastic wraps with Rx capability. Still, there are a few practical issues which must be considered:

For a full aperture, true wrap Rx solution, many manufacturers offer eight-base wraps in full-rim and three-piece styles. These offer advantages to the wearer such as:

  • Full field of view: The lens fills the entire aperture of the frame providing the widest possible view.
  • Seamless integration: Instead of clipping to the back, the Rx lens takes the place of the plano lens for an integrated, �factory original� appearance.
  • Reduced volume: Eliminating the clip eliminates much of the concern over weight and lash crash.
  • Wrap optimization: Even though the detailing of these frames may look conventional, the wrap configuration and eight-base lenses make these frames the prime candidate for wrap Rx optimization. Filling these frames with a �dumb� Rx could result in compromised vision for your patients. Also remember to send the actual frame in with the order so the lab can measure the correct wrap parameters before surfacing.
  • Cutout: Since wraps generally have larger eye sizes than most ophthalmic frames, lens cutout could be a concern with certain lens products in smaller blank sizes. Discuss with your lab eight-base SF/SV or progressive lens availability to ensure good cut-out. Specialty wrap lenses like SOLA�s Spazio feature larger diameters and decentered optics to provide adequate cutout in nearly all applications.
  • Edging and mounting: While the wire or channel on a full-rim frame may look conventional, the edging challenge is not. Tracers designed for flat frames may have trouble following the steeply curving bevels of a wrap frame. In edging, the sharp corners and curving can cause trouble for the surface probes and large diamond wheels of a conventional wet edger, leading to poor bevel shape and placement. Equipment which works well with wraps, such as the National Optronics� 4T tracer and 6E edger uses a 3D trace and router cutting blades to produce custom bevels.
  • Plastic frame compatibility: Inspect injection-molded plastic wraps carefully for Rx edging compatibility. Unless they are designed to accommodate an Rx bevel, the grooves are usually shaped to grip square-edged plano lenses and may have a wide rear flange that prevents Rx mounting. Additionally, thick plastic structures at the nose or hinge area could interfere with thick Rx lenses. Your lab may be able to help you select frames that are Rx capable.
  • Drilling: Three-piece frames require additional attention. First, make sure the studs or screws are long enough for thick Rx lenses. Some screws are countersunk into the lens and will not require cutting. Second, make sure the base curve of the Rx lens matches the base curve of the original plano; sometimes three-piece wraps use tilted six-base lenses instead of true eight-base wraps. Using the wrong Rx base will change the fit, wrap and apparent temple length. Finally, drill holes at a perpendicular angle to the front surface, otherwise the frame pieces will not mount flush and the wrap and fit will be compromised. Fortunately, newer drillers are starting to pay more attention to this requirement.

SPECIALTY WRAPS

Beyond clips and lens-in-frame models, a few manufacturers have developed unique Rx solutions that fall into neither category. One example uses a specially designed Rx element implanted into a plano shield, giving full coverage with a minimal lens profile. On the horizon, companies are working on advanced machine technologies that in the future could offer full-frame Rx lenses even in the more challenging rimless styles, opening up frame selection and eliminating the need for clips. These advanced technologies require working with a specialty lab.

PULLING IT ALL TOGETHER

To put this knowledge into practice, use the optics, selection and fitting of frames, and ordering and QC of wraps discussed here. Wrap eyewear can be a successful and popular specialty for your practice and a great way to satisfy patients with optically advanced, functional eyewear and a terrific second pair opportunity. As you develop your understanding of how to match patients to the right eyewear and deliver a great Rx, they will keep coming back for more.