The Rap On Wraps
The Wrap-Around
Prescription Eyewear Challenge

Many Patients want wrap-around prescription eyewear.
Many Optical Retailers are unaware of the challenges.

                        

                        

Wrap Effects and Optimization
From an article appearing in the April,
2005 issue of 20/20 Magazine.

The curvature and wrap (face-form tilt) that makes a 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 are 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 (face-form 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.

          

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, i.e., non-prescription, 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.

For example, to create a -4.00D sphere for the wearer to satisfy a -4.00D sphere Rx, the lab would need to supply a -3.75 -0.25 x 180 with 0.5D Base In.

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 it 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, two labs have proprietary labs for processing wrap prescriptions. Oakley has a proprietary formula to calculate surfacing optimization in Rx lenses comparable to the companies XYZ optical technology. The AO-SOLA SOLA Technologies lab uses special software to calculate surfacing optimization, plus the lab offers a "Spazio" specialty wrap lens design 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.

Some manufacturers such as Oakley and Maui Jim have built their own dedicated labs to service their own products with factory-direct Rx processing and OEM lens colors. Other brands such as Nike, Bolle and Serengeti 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.

Add prism to wrap Rxs to adjust for the "tilted" visual axis. A rule of thumb is:

For Rxs less than ±2.50D in the horizontal meridian, order lenses with 0.25D Base In.

For Rxs less than >2.50D in the horizontal meridian, order lenses with 0.50D Base In.

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 that 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.

For example, with Oakley, call or fax the order, the job is started immediately and the frame is supplied from the manufacturer's inventory; for SOLA Technologies select the "Spazio" wrap product on the order form and send in the frame and regular Rx script. 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 VERSUS "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. Oakley, SOLA and other labs 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 lensometer and then read the Rx along the same optical axis.

Conventional coordinates of an optical system

X-axis the horizontal, the Y-axis the vertical
and the Z-axis the optical axis of the system.

Frame Tilt

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 that 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 +200D to -600D 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 for example, Rudy Project.

Many wraps are engineered with compliant parts that form to a wearer'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 sunlens 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. --

Now you know!

  

Back To Basics Part I
Serving vs Selling Eyewear