Who hasn’t heard a client exclaim: “I’d love to have my prescription
ground into my windshield!” People’s desire to obtain a wide, sharp
and unrestricted view from their eyewear has been around for a long
time. Today we think of prescription wrap-around eyewear as a new
technology. But the road leading up to improving the peripheral
optics of corrective
lenses actually began a
few million years ago.
EVOLUTION
AND PERIPHERAL
VISION
The importance we
humans place on
peripheral vision is
eternal. The process
of natural selection
endowed our mammalian ancestor’s eyes
with a high sensitivity
to changes that occur
in the peripheral field.
The evolutionary
importance to survival
from a prompt reaction to the simple
flicker of a blade of
grass in our periphery
cannot be understated.
Inattention here meant
missing your next meal
or worse, misjudgment
and wham; you’re someone else’s meal. Because of this, human
peripheral vision has been honed to be exquisitely sensitive to
change. This sensitivity is the foundation of our desire for no
compromise peripheral vision.
THE ROAD TO WRAP-AROUND EYEWEAR
 For the first 600 years of eyewear’s evolution, all lenses, whether
they were used for reducing glare (sunglasses), cosmetic reasons (privacy, such as hiding the
thoughts expressed by a
judge’s eyes within the
ancient Chinese courts) or
for correction, were essentially flat in shape and form.
Starting in the late 1700s,
advances in glass making,
lens performance and production techniques, along
with a more developed
understanding of the optics
of the human eye, combined to enable better quality, meniscus
(crescent-shaped) lenses to become available.
THE TASKS OF THE EARLY LENS DESIGNERS
Beginning in the early 1800s, a physicist named William Wollaston
recognized that contemporary corrective lenses permitted good
vision only through the lens center, i.e., they had a small “sweet spot”
of clear vision. He invented lenses that expanded peripheral clarity
and called them ‘periscopic’ (which means “to see around”). Unlike
the other lenses of his day, his lenses were very deeply curved, using
what we now refer to as base curves in the range of +10 diopters and more. Around the
same time, an Englishman named
Thomas Young discovers the eye aberration known as
astigmatism.
Because of all the
excitement, interest
and vision improvement realized by
correcting this axial
optical error, along
with the difficulty,
expense and the sphere-power restriction of his periscopic lenses,
Wollaston’s efforts to improve the peripheral clarity of corrective
lenses were overlooked. His research and papers languished and
were not again pursued for almost 100 years.
Around 1899, a French ophthalmologist named Ostwalt redis-covered the published investigations of Wollaston and tried his
own hand toward improving the peripheral performance of ophthalmic lenses. He attempted to improve clarity by reducing off-axis (aka marginal) astigmatism. Like Wollaston, Ostwalt’s lenses
corrected the marginal astigmatism of spherical powers only and
therefore could not be combined in a compound, toric-form lens
that would also include Young’s axial astigmatism correction.
A few years later, a German scientist by the name of von Rohr,
under the direction of the firm of Carl Zeiss, began to investigate
other possible improvements to eyeglass lenses. Along with Dr.
Gullstrand, an ophthalmologist well-versed in the optics of the
human eye and using his new scientific calculations, the first
lenses developed by von Rohr were called Katral. These eyeglass
lenses were intended for aphakic individuals, who had their natural crystalline lens removed through cataract surgery.
In 1912, von Rohr then turned his attention and renewed the
work pioneered by Wollaston and Ostwalt in improving peripheral clarity of corrective lenses. His latest lenses, called Punktal,
provided comprehensive power, axial and off-axis astigmatism
correction for all viewing gazes. Now, compound form, sphero-cylindrical ophthalmic lenses could finally provide outstanding,
point-like clarity across wide fields-of-view. The Punktal lenses
had one disadvantage: each and every lens prescription required
a unique set of front and rear curves to fully realize their optical
promise. This approach made it difficult for wide-view lenses to
enjoy the cost reductions benefits of mass production. They
were, however, tremendously sought after by ammetropic individuals of the day.
THE BIRTH OF CORRECTED CURVE LENSES
 During the first decade of the 20th century, Bausch & Lomb, an
optical firm based in Rochester, N.Y., began to manufacture and
distribute Zeiss Punktal lenses. With the requisite tooling and fabrication costs rapidly becoming unmanageable, the head of B&L’s
Scientific Bureau, Dr. W. B. Rayton, began to investigate alternatives.
The result of his research efforts was a lens series that promised most
of the performance of the Punktal lenses, but with a more favorable
fit with mass-production requirements. Dr. Rayton’s Orthogon series
of ophthalmic lenses helped to firmly establish Bausch & Lomb as a
world-wide leader in ophthlamic lenses. Similar events took place
at another firm, American Optical, where Dr. Tyllier presented a
similar, multi-base curve lens series design, which was based on an
acceptable balance to correcting most of a lens’ off-axis errors. These
two corrective curve series lenses are responsible for the exponential
growth of ophthalmic lenses in the first half of the 20th century. With
frames less dependant upon expensive, natural materials, everyone
could now afford the benefit of wide fields of sharp vision.
FRAME FIT AND PERIPHERAL VISION
Early ophthalmic lenses were made of glass and were kept small to
deliver the best vision, reduced weight as well as thickness. Frames,
based on natural materials, were made of bone and horn. This made
frames bulky and, along with the small lens diameters, restricted
fields of view. In the first half of the 20th century, dispensing opticians tried to optimize eyewear for wide fields-of-view using the few
tools and techniques available to them. Firstly, by choosing a lens
shape close to round or oval, lens thickness and weight could be
reduced, enabling an increase in lens size. Secondly, by fitting lenses
as close to the eye as possible, the field-of-view was effectively
increased. This phenomenon is known as the “key-hole” effect,
which describes the improved angular view realized by placing your
eye a close as possible to a door’s keyhole. Taking this a step further,
the lenses that can provide the ultimate field-of-view are contact
lenses. Even with all the advancements in materials and surface
chemistry contacts have enjoyed since the 1930s, they are still but an
alternative vision modality for most of the ammetropic public.
A COMPENSATED WRAP RX
DELIVERS PERFECT VISION, RIGHT?
Wrong. Keep in mind that any prescription or POW compensation
performed for wrap eyewear only impacts central or straight-ahead
vision. None of these Rx compensators purport to correct vision-degrading peripheral astigmatism and other extended vertex errors.
For example, an Rx of +2.00 diopters properly compensated and
fabricated with an 8D base curve and a face-form angle of 20 degrees,
is corrected for good acuity when only gazing directly ahead. Offaxis, peripheral errors are left
uncorrected when using simple
spherical lenses. Although we
can never completely match
the sharp central acuity in the
periphery of a wrap, we can
reduce the amount of distortion
to an acceptable level. (The term
distortion, as used here, is a term
enveloping the sum of all the
optical errors in the periphery
of wrap around eyewear.)
How is reduce peripheral wrap
distortion reduced? There are
currently two approaches:
1. A selection is made from a
series of 8D base curve blanks,
each with a differently tailored,
distortion-reducing design, which
is chosen for the intended prescription power. (For example,
lenses like SOLA Spazio, Shamir
Attitude, KbCo Wrap Solutions)
2. A digital, lens design algorithm integrates all the supplied
fitting parameters, including
monocular PD, pupil height,
pantoscopic tilt, face-form angle
and vertex distance, as well as the
frame dimensions and lens shape
geometry. This program then
optimizes an individual pair of
lenses, whose fabrication is completed using free form lens manufacturing techniques.
Both approaches offer markedly
improved peripheral clarity
compared to using Rx wrap
compensations with simple,
spherical lenses.
WHAT IS POSITION OF WEAR?
Zeiss Katral lenses were amongst the first ophthalmic lenses to use computational analysis in their
design. It is interesting to note that aphakic lenses
are also the first lenses that used position of wear
calculations to ensure that they focused “as
intended” by the doctor.
An optical error known as spherical aberration in
aphakia reduces peripheral vision and delivers
dynamic blind spots as the eye rotates to look at
objects in the periphery. One of the first adjustments opticians performed to help reduce these
negative effects was to place these lenses as close
to the eye as possible, i.e., reduce the lens-to-eye
vertex distance. But modifying the vertex distance
for strong lens powers can also result in a change
to the lens’ effective power. This new effective
power represents a change from the original
power found in the refracting lane at the conventional refracted vertex distance (usually assumed
to be between 13.5 to 14.0mm). For example,
the refraction for an aphakic individual is found
to be +15.00 diopters at a 14mm refractive
vertex distance. However, the eyewear was fitted
at a closer vertex distance of 11mm, in order to
improve field- of-view and reduce image magnification. If this same +15.00 diopter lens is placed
at 11mm, its effective power would decrease
0.75 diopters, to a total of +14.25 diopters. To
compensate for this new position of wear, the
dispenser increases the lens power by +0.75D
and orders a lens of +15.75 diopters.
Q: Have we changed the original prescription?
A: Yes, but only to ensure that we have delivered the prescription as the doctor intended. The effective power at the newly fitted, back vertex
position matches the intended power of the original prescription at the refracted vertex distance.
The change to the +15.75 power is referred to as a
position of wear (POW) compensation, or simply
an “adjusted value.”
Q: When performing a POW adjustment for
changes in back vertex distance, do I adjust only
the sphere power or should I include the cylinder
power when present as well? |
|
A: You must include the cylinder power as well,
but not in the typical way we think of cylinder
power. Using a lens power cross, map the total
power in each of the prescription’s main meridians. Calculate the back vertex distance power
adjustment for each meridian, and record these
powers on a new lens cross. From the cross, re-derive the new adjusted values that will be
ordered. In this way the adjusted Rx is compensated and ordered correctly, and will perform as
the doctor “intended.”
WHAT ARE THE PARAMETERS THAT
AFFECT POSITION OF WEAR?
Technically, any differences in parameters of the
newly fitted eyewear that deviate from the standards of the refracting lane must be addressed
and compensated. However, depending on the
strength of the original prescription, such differences are often negligible and may be ignored.
Below is a short reference list:
Wrap around eyewear makes significant changes
in three basic fitting parameters: Pantoscopic Tilt, Vertex Distance and Face Form angle (also know as panoramic angle). One or more of these almost always
requires a POW calculation in wrap eyewear. Because
of the interactive effects of these changes, manual
calculations are tediously long and complicated.
Employing any one of the various “Rx compensator”
programs, available on the Internet, is essential for
doing wrap work. Not only do these sophisticated
programs facilitate the quick calculation of POW values, they are excellent tools for helping you understand the relationship between parameter changes
and prescription values. Keep in mind, however, that
these POW compensators cannot improve a poor
performing original prescription.
Remember Wrap Eyewear’s Golden Rule:
Rx garbage in = Rx garbage out.
Most client vision complaints with their properly
made, wrap eyewear are generally traceable to
problems within the original Rx. Copying or duplicating a client’s present eyewear as a basis for new
wrap eyewear is to be discouraged, unless basic
refracting skills are available and at your disposal. |
WHAT GETS WARPED
WHEN FRAMES GET
WRAPPED?
 Besides the optical errors discussed
above, wrap eyewear introduces
other problems when crafted with traditional edging equipment.
1. Centration Errors — The increased face form angle of wrap
eyewear will effectively narrow the requested PD. Fabricating and
verifying the PD of wrap eyewear requires compensation here as
well, and depends upon the degree of wrap angle involved.
2. Lens Retention — Most traditional edgers are not designed for
placing the bevel for an eight base lens in the best manner for wrap
eyewear. Additionally, conventional lens tracers either won’t trace a
wrap frame properly or sometimes not at all. Tracing a wrap lens by
itself may help, but dimensional differences are still introduced
that will adversely affect optimal lens retention. The distortion of
lens shape that results requires the use of dimensionally stable
lens materials. Polycarbonate, Trivex, 1.6 and 1.67 substrates are
superior in this regard to CR-39.
The type and profile of lens bevel required for wrap eyewear is
usually not obtainable with a conventional edger. In view of the
above problems, dispensers are encouraged to:
- Either order their wrap eyewear complete from a lab that has the
experience and equipment to do wrap eyewear properly or
- Purchase one of the new, high-curve tracers and edgers that
enable optimal in-house fabrication of the bevel profiles required for
wrap eyewear.
THE CHALLENGES OF WRAP-AROUND EYEWEAR
Many of us often find ourselves wincing when clients return seeking
remedial attention for their eyewear. It’s natural to view the time and
attention required to evaluate, correct and/or solve problems as time
unrewarded. Yet, in the service-based industry we call home, nothing
could be further from the truth. How we tackle these problems,
whether real or perceived by your client, can define the essential
difference between success and failure.
| WRAP PEARLS |
1. Order lenses from a lab that specializes in sports
eyewear. They will compensate the Rx for prism
and power, bevel it correctly and help you to trouble
shoot any patient adaptation issues that arise.
2. For an office that does in-house edging:
a. Download one of the calculators for wrap eyewear
at www.optiboard.com, www.opticampus.
com or www.kbco.com.
b. For minus Rxs, try using a base curve one step
flatter than the lens’ curve received with the frame.
Example: For a frame received with eight or nine
base lens, try calculations with a six base lens. In
some case, the lab might tell you that the order
you’re placing is better off being fabricated with a
six base wrap lens.
c. For plus Rxs, using an eight base curve works
well. However, plus prescriptions will require more
decentration to obtain the required base-in prism
and lens thickness issues must be considered. |
|
d. Minus lenses may yield the desired base in
prism with modest, outward adjustments of the
given PD.
e. If you are using a conventional edger, note that
the most optimal bevel placement is to set the bevel
apex as close to the front of the lens curve as possible.
3. If at all possible, try to confirm if the given Rx is
corrected for full infinity viewing. Wrap sunwear
is almost never used indoors, so any lack in full
infinity focus may significantly compromise
patient satisfaction.
4. Note that almost all the wrap Rx work done by
major sun brands, such as Rudy Project, Nike, Oakley
and others, require the ECP to be aware of both
the Rx range listed for a particular frame style and
the PD range that is allowed for that style. You
must be sure both lens power and the patient’s PD
are within the allowed values before the vendor
will process your order. |
From your client’s viewpoint, the awareness or lack of comfort that
can accompany a new pair of wrap around eyewear is indistinguishable between a situation that requires your attention and one that
they simply have to “get used to.” Long term trust is built upon a
patient’s willingness to believe
your skills and expertise will satisfy their eyecare and eyewear
needs. Therefore, wrap dispensing challenges not only represent
chances to learn and extend your
personal skill set, they are indeed
the foundation of your chosen
livelihood.
GO FLAT OUT
Wrap eyewear represents a
complex, multi-faceted challenge to the dispenser and lab.
In wraps, the disciplines of optical design, technical craftsman-ship and ophthalmic dispensing
come together to satisfy the
evolutionary need of the
human eye to see sharply, both
far and wide. It’s time for all
ECPs to get out of flatland. |