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. The wrap optimization 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 an example, 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.
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.
Independent Lens Specialists understand the need for wrap optimization and have procedures
for making the required adjustments.
Independent Lens Specialists have other tools and tricks for delivering wrap eyewear,
and edging and tracing solutions that can handle the sinuous frame grooves. Independent
Lens Specialists can also assist with selecting which wrap frames are most suitable
for Rx. All this doesn't mean that a wrap Rx can't be filled. ILS have the knowledge
and technology and are able to assist your practice with a specialty wrap Rx service
could be an attractive niche for your practice.
ORDERING WRAP LENSES
ILS will do the rest. Do not bother working out the wrap corrections because ILS
will do that and it could be confusing if you try to second-guess their calculation
by sending pre-corrected Rx figures.
Phone 03 348-8821
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. ILS 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 from Shamir, have a wrap optimized surfacing with a wide-periphery
PAL design can provide an effective wrap PAL solution. Talk to ILS 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.
The following compensated lab ticket will be sent back with every script, to enable
you to check the job against the compensated values
