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 business people 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 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 light weight,
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 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 (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.
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 0.5s 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 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: |
Rxs
less than ±2.50D in
the horizontal meridian |
Order
lenses with
0.25
Base In |
Rxs
less than >2.50D in
the
horizontal meridian |
Order
lenses with
0.50
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 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. 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 lens meter and then
read the Rx along the same optical axis.
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 +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: