Freeform Lenses 1&2

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opticianonline.net20 |Optician | 09.07.10

CET Continuing education

In the first of a three-part series looking at freeform lenses, Paul Bullock describes the evolutionof progressive power lenses as a backdrop to the introduction of freeform. Module C14211, onegeneral CET point for optometrists and dispensing opticians

Freeform lensesPart 1

T

here is no better exampleof the lack of awarenesson the part of the publicabout modern progressivelens options than an event

which occurred only a fewweeks ago.

My wife and I were in Budapestcelebrating our seventh weddinganniversary. We took an open-topbus tour around this beautiful city;however, my attention was not onthe architectural and natural wonderssurrounding me, rather than on a fellowpassenger and his curious spectaclearrangement. I was determined to takea picture of this gentleman which I amdelighted to share with you (Figure 1).

From my subsequent discussionswith him, his spectacle apparatus is

thus: Pair 1 single-vision distance, CR39,clear, uncoatedPair 2 plano aviator style sunglasses,Glass, B15 tinted Pair 3 single-vision near, CR39,clear, uncoated.

I explained that there were optionsto aid his visual experience and toimprove his cosmetics and comfort. Idemonstrated this by showing him thepair I was wearing at the time withthe following specifications; -5.00DS,prism compensated, high index, highbase, concave HMAR, flash mirror,

polarising sunglasses.Even though there was a significant

language barrier the advice was warmlyreceived and we may have anotherconvert to progressive power lenses.

I am still surprised by the number ofpresbyopes who could benefit from themodern day technological marvels thatare todays progressive power lenses(PPLs). They either decline to wearthem, preferring to wear various opticalapparatus with sometimes reducedsuccess, or have not been offered thebest fit solution to suit their lifestyleand visual requirements. I pondered

this particular gentlemans plight asI researched this series. He, like mostof our patients, would be surprised

by the development, advancementsand complexities of PPLs throughouthistory.

Te prgressve addt les(PAL) eptThe ideal spectacle lens for presbyopiawould be completely distortionfree while providing a continuousprogression in power for clear visionfrom distance to near, with wide fieldsof view for all distances. This has beenthe goal of the PAL designer since itsinception.

Current PALs provide the wearerwith zones for distance and near visionthat are wide, free of distortions andstable in power. These areas are joinedby a corridor of increasing plus power.

This corridor of transitional power isgenerated by a gradual increase in thecurvature of the lens surface.

A typical design PAL progressivesurface can be represented by a series ofconic sections stacked on one another.By joining these sections with theirapices coinciding, an astigmatism-freecorridor of increasing plus power isachieved.

This use of non-circular, asphericcross sections, the conic sections,reduces the surface astigmatism in thelens periphery without compromisingwidth of the viewing field.

From the distance to near zones,the conic sections vary from ellipses,to circles, to parabolas, to hyperbolas.

Where these sections coincide, theresultant surface is free of astigmatism.Where the conic sections do notcoincide, the front surface displaysastigmatism.

The intensity and orientation of theastigmatism varies with the PAL designand the corridor length.

These concepts which we all takefor granted today have taken muchexpertise, innovation and developmentsince Benjamin Franklin first designedthe bifocal lens in 1784.

Early PPL desgThe optical principles of PPLs havelong been understood. The earliestprogressive lens patent was granted toBritish optometrist Owen Aves in 1907,co-founder of what is now known

as the Institute of Optometry. Thelens Aves designed used two asphericcylindrical surfaces.

On one surface there was a sectionof an elliptic cylinder and the othera section of an inverted cone. Eachsurface contributed an equal cylindricaleffect with the axes mutuallyperpendicular to produce a sphere ofpower from top to bottom. This wasin essence a dual-surface progressivedesign; unfortunately the lens structuremade it unable to correct for astigmaticprescriptions and Aves design neverwent into production and instead was

limited only to prototypes.In the following years, many other

PPL lens designs followed, includingones which used a parabolic concaveback surface with an increasing radiusof curvature from the geometric lenscentre, decreasing the back surfacepower and increasing the total lenspower. This lens orientation resulted inthe central portion for distance visionwith the periphery for near.

In 1914 Henry Orford Gowllandinvented one of the first commerciallyavailable PPLs with the familiarumbilic design structure we recognise

as a PPL today. The design was a singlesurface progressive using a section ofa paraboloid on the rear surface. This

Fgre 1

A novel

approach to

multi-focals

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Continuing education CET

design was produced commercially, butwithout success. Limitations with themachining of the necessary surfaces,the optically poor performance of these

designs and the resulting unacceptableaberrations are attributed to theclassification of these early PPLs as afailure.

Frst-geerat PPLsIn 1959 the first commercially successfullens was introduced by Essel, one ofthe founding members of Essilor. Theprogressive design, named Varilux 1,gave a large spherical distance and nearzone linked along the main meridian, orumbilical line with orthogonal circularsections of decreasing radii. This designstructure resulted in strong surface

astigmatism in the periphery and to thenasal and temporal sides of the umbilical.The Varilux 1 was a symmetrical designwith no inset. The lens could be used foreither the right or left eye; the lens wouldbe twisted nasally into the frame to createthe inset required for the reading zone.

The design had no power variationin the upper half of the lens, a relativelyshort corridor, and a near zone of constantpower approximately 22mm in width.

In the next few years several othermanufacturers developed and releasedfirst-generation PPLs including SilorSuper No Line and American Optical

AO7.

Sed-geerat PPLsThe development of commerciallyviable and successful PPLs representeda technological breakthrough in surfacegeneration; the ability to controlthe angle of contact to produce thenecessary surfaces was heralded bysome as more of an innovation than

the product produced.With this new technology the lens

designers had the ability to manufacturemore complex designs to enhance thedesign performance.

First-generation designs had strongsurface astigmatism; the next stepin PPL development was to reducethis and to produce a softer design.Mathematic surface modelling wasused to research ways to reduce this;however, the limitation was still inthe capabilities of the manufacturingtechnology rather than the imaginationand expertise of the lens designers.

The Varilux 2 design was introducedin 1972 with a change of design

structure from orthogonal circularsections of decreasing radii to evolutiveconic sections of changing eccentricities.This new design, termed horizontaloptical moderation, coupled with anasymmetrical design with separate lensesfor right and left eyes with inset nearzones, produced a design optimised forbinocular vision.

The Varilux 2 design possessed an

aspheric progressive surface. The resultof this was a reduction in the intensity of

the surface astigmatism. Many similardesigns were subsequently releasedusing similar concepts, including:Silor New Super No Line, RodenstockProgressiv R and Sola Graduate.

As more manufacturers entered thePPL market, great care was takento differentiate their designs fromtheir competitors. This was done bydesigning the lens around optimumperformance for certain criteria. Somedesigns were focused around reducingthe severity of the surface astigmatismby spreading it wider over the surfaceof the lens. Some designs purposely

increased the strength of the surfaceastigmatism, thereby increasing the sizeof the distance and near zones.

Rodenstock in its Multigressiv designadded an aspheric flatter base curve toits already aspheric progressive surface.This yielded better visual performancefor higher astigmatic lenses.

Carl Zeiss in its Gradal HS designfocused on binocular vision utilising

Fgre 2 Hard design PPL Fgre 3 Soft design PPL

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an asymmetrical design equalisingacuities and prismatic effect across thehorizontal zone.

Trd-geerat PPLs

This new generation, circa 1980,introduced the multi-design PPL. First-generation PPLs displayed strong surfaceastigmatism, now classified hard designPPLs (Figure 2). Second-generationPPLs displayed weaker surfaceastigmatism, now classified softer-designPPLs (Figure 3). Both these generationsutilised the same basic lens design acrossall base curves and addition powers; this

design is known as a mono-design. A multi-design PPL adjusts the

lens design for each addition power.The design for an early presbyopewould be a soft which became harderas the addition power increased. Themulti-design PPL was aimed at visioncomfort and ease of adaptation for eachstage of presbyopia (Figure 4).

Several lenses of this era with thisdesign profile include: AmericanOptical M3, BBGR Selective and HoyaHoyalux GP.

Frter prgressve pwergeeratsFurther generations of PPLs havebeen made possible due to significantadvancements in lens design and surfacemanufacture. Advanced computer-numerically controlled cutting of theceramic moulds used for forming theglass moulds for semi-finished PPLshas reduced manufacturing limitations

and mathematic surface modellinghas become much more complexand efficient due to advancements incomputer technology.

Fourth-generation PPLs, such asVarilux Comfort and Hoyalux Wide,were designed to offer more naturalvision than previous generations. Thedesign profile was thus that the wearercould obtain the near power in a fast,effective way while adopting a natural

head posture.Fifth-generation PPLs, such as

Hoyalux Summit Pro, changed theorientation of the asphericity of the lens,matching the transmitted vertical andhorizontal powers in a transmission-based design. Other key designcharacteristics include an ergonomicinset and horizontal asymmetric design.Other fifth-generation PPLs include

Fgre 4

A multi-

design PPL,

changing

from soft to

hard designwith

increasing

add

Fgre 5 Shorter corridor PPLs were

introduced in 1999

Fgre 6 Conventional PPLs are only

optimised for one target Rx per base curve

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MuLTiPLE-choicE quESTionS take part at opticianonline.net

1Which of the following is not a second-

generation PPL?A Sola Graduate

B Rodenstock Progressiv R

c Silor New Super No Line

D American Optical AO7

2The Varilux I used a near zone of constantpower of what approximate width?A 18mm

B 20mm

c 22mm

D 24mm

3Which of the following is not a third-generation PPL?A Zeiss Gradal HSB BBGR Selective

c Hoya Hoyalux GP

D AO M3

4Which of the following best describes the

Varilux Comfort?A Second-generation PPL

B Third-generation PPL

c Fourth-generation PPL

D Fifth-generation PPL

5Which of the following best describes theHoyalux Summit Pro?A Second-generation PPL

B Third-generation PPL

c Fourth-generation PPL

D Fifth-generation PPL

6What front face form angle is suggestedas a universal standard measurement?A 2 degreesB 4 degrees

c 6 degrees

D 8 degrees

Successful participation in this module counts as one credit towards the GOC CET scheme

administered by Vantage and one towards the Association of Optometrists Irelands scheme.

The deadline for responses is August 5 2010

the Varilux Panamic.In 1999 American Optical released the

first short-corridor PPL, the AO Compactin response to market demands. Eye sizes

were becoming smaller throughout the1990s and PPLs were not suitable for theshallow frames becoming fashionable atthis time.

The near zone was positioned higherand the umbilical shortened, leadingto a harder design profile. The wearerexperienced a faster transition fromdistance to near and reduced intermediatefield of view. The fitting height wasreduced by 4mm compared to PPLs ofthe same era, more than adequate for theframe requirements (Figure 5).

cvetal desg PPLs

Up to this point, all generations of PPLscould be classified as conventionallydesigned and manufactured.Conventional PPLs are privy to twomajor limitations, base curve restrictionsand universal standard measurements.

Optional PPLs are produced fromsemi-finished blanks with a mouldedprogressive front surface and base curve.Each base curve is used for a range ofpowers but the design is only optimisedfor one target power per base curve,usually a simple spherical prescription.The design when a strong cylindricalpower is required performs very

differently to the optimised target power(Figure 6).

Due to the semi-finished nature ofconventional PPLs with a mouldedprogressive surface, the progressivedesign was based upon assumptionsregarding the target wearer. Theseuniversal standard measurements being: Pupillary distance of 63mm Back vertex distance of 15mm

Pantoscopic tilt of 8 Front face form angle of 4.

Any patients Rx that lies outside ofthe optimised target power or frame andfacial measurements that are outside of

the universal standard measurementsexperiences a PPL considered as anacceptable compromise.

The ideal PPL would incorporateframe and facial measurements uniqueto the patient as well as lifestyle andmethod of use. The design wouldbe calculated for every individualprescription rather than optimisedfor one target power per base curve,

resulting in a personal design.These new types of lenses will be

discussed in the next article in this series,as will the improvements in the toolsto design, manufacture and compute

the complex surfaces required for thenew generations of progressive powerlenses, including the introduction ofnumerically controlled cutting andpolishing machinery. The changingvisual requirements of the presbyopewill also be discussed.

Paul Bullock is professional services

manger at Hoya Lens UK

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Paul Bullock describes the manufacturing process behind modern freeform lenses.Module C14387, one general CET point for optometrists and dispensing opticians

Freeform lensesPart 2

What is freeform? Thisis a question I haveput to many people,whether they beseasoned opticalprofessionals in

CET presentations or first-year optics

students. The answers volunteered havebeen insightful, intriguing, derogatoryand many other adjectives, but alldifferent.

It is quite apparent that the termfreeform has become an industrybuzz-word that has led to muchconfusion. It is being embraced by theoptical industry, but the term is stillmisunderstood in some instances.

Freeform surfacing is a wonder ofmodern technology; we now have theability to manufacture lens designsthat were inconceivable a decade ago.

This technology is another milestonefor the optical industry and only byhaving a clear understanding of thepractical applications of this technologycan we move forward as an industryto introduce it to the audience itdeserves.

Freeform simply is a manufacturingprocess that allows the production ofcomplex surfaces and designs withextreme precision.

T vtal mtdThe conventional lens surfacing process

cannot produce the complex surfacerequired for a progressive power lens

(PPL). The process was designedto create simple spherical and toricsurfaces which can be polished by rigidlap tools. A progressive surface does nothave a consistent curvature and requiresa flexible lap tool for polishing.

To overcome this restriction the

complex progressive surface is castinto a semi-finished lens blank usinga glass mould. A metal or ceramicdye is cut with the progressive lensdesign using a computer numericallycontrolled cutting tool. Heated glassis then slumped over the dye toreproduce the complex progressivedesign cut into the surface of thedye. There are two moulds for eachlens blank, one for the front surfacecontaining the progressive lens designand one for the back, a simple sphericalcurve. The moulds are mounted in a

gasket, liquid lens monomer injectedinto the apparatus and the monomerheat polymerised. Once cured theresulting form takes the shape of thetwo moulds, a traditional semi-finishedPPL blank.

This is known as the open and shutproduction method. Importantly,the accuracy of the lens is only equalto the accuracy of the moulds. Thisprocess is a highly repeatable methodof production which yields consistentquality. The only drawback is shrinkageas the lens monomer goes through heat

polymerisation. This process producesa power accuracy of approximately

0.06D. The semi-finished blanks wouldthen have the distance prescriptionapplied to the rear surface usingtraditional surfacing equipment. Thisequipment can only produce relativelysimple surfaces because the surfacingtools they use can only produce a

spherical or toric surface. For sphericalsurfaces, there is only one radius ofcurvature used in all meridians, for thecylinder surfaces (toric) there are tworadii of curvature crossed at 90 degrees.These tools cannot create asphericor complex progressive lens designs,hence the need to use pre-mouldedsemi-finished lens blanks.

As previously discussed in Part 1(Optician, 09.07.10), the semi-finishedlens blanks are available in a set numberof base curves, each base curve coveringa range of prescriptions.

Advt f frfrm tlgyFreeform technology has beenutilised in optical lens manufacturefor approximately two decades. Itfirst came into use manufacturing themoulds for PPLs.

Freeform was introduced to helpimprove the mould making technologywhich inherently improved theaccuracy of the PPL. A freeformgenerator was used to directly cutthe surface of the glass mould withthe progressive design, rather than

using the glass slumping processas previously described. This directsurfacing improved the power accuracyto approximately 0.01D.

This fact lends itself to a question.Because the mould is manufacturedusing freeform technology and thesemi-finished lens blank cast from thismould, does this mean it is a freeformPPL? Does the use of a freeformgenerator on the mould equate to afreeform PPL?

Some manufacturers refer to productsmade in this manner as digitally

moulded, digitally cast, freeform frontsurfaced and many other names andmarket them as such. The fact is thatmost semi-finished lenses are madefrom freeform generated moulds andhave been for some time.

Fgr 1

Mathematical

modelling of

cutting tool

coordinates

4th axis angle

Csz

zy

x

z

Optical axis

Machine axis

Vertical axis

Horizontal axis

x y z

0.000 0.000 0.000 0.000

35.000 0.000 6.201 24.835

0.000 -35.000 6.201 0.000


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T frfrm gratrFreeform generators are multi-axis,computer numerically controlled (CNC)machines which rapidly translate theindividual progressive design in theform of a point file to three-dimensionaldiamond cutting tools which removematerial to create the surface. Thecombination of the point file, software

control and pinpoint CNC surfacingaccuracy means the freeform generatorcan precisely cut away material downto microns, thereby producing virtuallyany complex multi-curve surface.

Freeform surfacing begins bymathematically modelling theindividual lens surface. The design isthen translated into point file, a seriesof co-ordinates for the cutting toolsalong the x, y and z axis (Figure 1).

The lens blank then undergoes athree-stage cutting process (Figure 2): A multi-blade rough cutting tool

A polycrystalline diamond smoothcutting toolA natural diamond high speed finishcutting tool.

After surface generation, the lens isthen polished in a freeform polisher

using a CNC dynamic, flexible lap tool.(Figure 3).

Frfrm PPLApplying this technology to PPLs allows

the lens manufacturer to overcome therestrictions of conventionally surfacedand cast PPLs.

The original design of the lens is stillparamount. If a poorly designed lensis freeform manufactured, it remains apoor design; however, a well designedlens can be improved by utilising thistechnology.

Best form theory dictates a uniquebase curve for each specific prescriptionto reduce optical aberrations, but thisis not economically viable due to theeconomics of mass production. So a best

available fit is used. Conventional PPLdesigns are constrained by surfacingtechnology and parameters specific tothe individual wearer. Frame and facialcharacteristics cannot be accounted forbecause the PPL has to be designedconforming to universal standardmeasurements. These constraints meanthat the basic PPL design can only bevaried slightly for each base curve andadd power.

Freeform technology allows excitingpossibilities for lens designers as itremoves the constraints of traditional

semi-finished lens manufactureand allows the direct surfacing ofthe extremely complex progressivelens surface directly onto the lensblank. This allows the opportunity todesign a lens for every order, freeingus from the constraints of universalstandard measurement and basecurve restrictions. The lenses can nowincorporate an infinite number ofparameters specific to the individualprescription and frame fitting. It allowsa unique PPL to be custom designed foreach individual order and manufactured

with the highest of precision.

Dffrt typ f frfrmPPLThere are many different freeformPPLs on the optical market, frommany different manufacturers, each

with their own interpretation of whatthe term means and what constitutes afreeform lens. Some lens manufacturersnow have multiple freeform PPLs intheir portfolios, and even within their

own portfolio different manufacturingprocesses are employed, but all underthe banner of freeform.

Some examples of these differenttypes of freeform manufactureinclude: A semi-finished lens blank with amoulded progressive surface, cast froma freeform mould, and then utilisingtraditional surfacing for power (rear)surface A semi-finished lens blank with amoulded progressive surface, cast froma freeform mould, and then utilising

freeform surfacing for the power (rear)surface Using a spherical lens blank, andthen directly surfacing the progressiveand power surface onto the rear surfaceusing freeform surfacing Using a complete blank and thendirectly surfacing both lens surfaceswith different elements of theprogressive design and power surfaceusing freeform manufacture.

As can be seen by this very generic andnon-exhaustive list, the term freeformis used extensively to cover a wide

range of manufacturing techniques,some with more credence to the termfreeform and yielding superior opticalperformance than others.

Freeform allows the lens designerto directly surface the progressivedesign onto the lens surface. However,a large proportion of freeformPPLs are still manufactured frommoulded semi-finished blanks withthe progressive element moulded incasting. The true benefits of freeformprogressives can only be fully reachedonce the starting point of the lens

is a true blank with no restrictionsand the lens is designed for freeformmanufacture. There are factors whichmake this method of manufacturedifficult, which is why the majority ofmanufacturers do not produce PPLsusing this method, but to use freeformtechnology to its full potential and tostrive for the best optical performance,this method of manufacture must beadopted.

To manufacture a PPL from scratch,freeforming both sides of the lens,incorporating different elements of the

progressive design onto both surfacesand integrating them together toachieve the best characteristics of bothfront surface PPLs with a fast, dynamicprogression corridor and the widefields of view enjoyed by the wearers

Fgr 2 The three-stage freeform cutting cycle

Fgr 3 The CNC controlled flexiable lap tool


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MuLTiPLe-choice quesTions take part at opticianonline.net

1What is the power accuracy of the openand shut production method?A 0.01D

B 0.03D

c 0.06D

D 0.6D

2What is the power accuracy of direct

surfacing?A 0.01D

B 0.03D

c 0.06D

D 0.6D

3In freeform surfacing, how many cuttingstages are there?A 1

B 2

c 3

D 4

4The final cutting stage relies on a toolmade of which of the following?A Quartz

B Diamond

c Multicrystal

D Iron

5For an ordered power of sphere of

+2.50DS for a freeform lens, which ofthe following best represents the measuring

power from a traditional focimeter?

A +2.44DS

B -2.64DS

c +2.64DS

D +2.60DS

6What is the thickness of embeddedphotochromic material resulting from theimbibation photochromic technology process?

A 100-150 microns

B 150-200 microns

c 250-500 microns

D 150-200mm

Successful participation in this module counts as one credit towards the GOC CET scheme

administered by Vantage and one towards the Association of Optometrists Irelands scheme.

The deadline for responses is September 9 2010

of rear surface PPLs is not an easyundertaking. It requires a very complexmathematical modelling to generate thelens surfaces,2 for each lens integratedtogether for best performance to

create a dual surface PPL. In order tomanufacture a dual surface PPL, thelens must go through two freeformcycles, one for the concave side and onefor the convex side. This increases themachining costs and requires specialistequipment for concave and convexfreeform generation and polishing.

Another difficulty is in offeringa photochromatic plastic lens. Thepreferred type of photochromaticproduct by patients and opticalprofessionals is an imbibed offering.Here the semi-finished lens blank,

whether it be a semi-finished PPL tobe freeformed on the rear surface, ora semi-finished spherical lens blankto be freeformed on the rear surfacewith the power and progression tobecome a PPL, has absorbed a layerof photosensitive dye into the plasticsubstrate. This becomes permanentlyembedded into the semi-finished lenssurface at a uniform depth of between150 and 200 microns. Because thisprocess requires the lens to start froma semi-finished spherical or progressiveblank, it is not possible to create a

dual surface freeformed PPL withthis imbibing technology as a fullyfreeformed, dual surface PPL does notstart from a semi-finished lens blank ofany type, rather a large puck of lensmaterial. The imbibition photochromictechnology described is a proprietary,patented process developed andcommercially introduced by TransitionsOptical.

To overcome this issue a different typeof photochromatic technology must beused. A spin-coating process can be usedafter the lens has been manufactured

to apply a photochromatic layer.The photochromic technology is aproprietary, patented process developedand commercially introduced byHoya.

Frfrm l pratFreeform PPLs from somemanufacturers often requireadditional fitting parameters, backvertex distance, face form angle, nearworking distance, pantoscopic tiltfor example. Others use an averagevalue for these measurements. By

individually designing the lenses beforemanufacture we now have the abilityto optimise the lens performance,compensating for the effects that theseparameters have on the performanceof the lens. This technology not only

allows us the facility to optimise forthe prescription and facial parametersbut also for lifestyle and previouslens experience of the wearer. The

ability to tailor make the lenses tothe patients requirements offers thedispenser the option the deliver thehighest level of patient care with anindividual solution to the wearersvisual requirements.

The personalised or individualisedfreeform lenses will often arrivein practice with a compensatedprescription, usually described asa measured prescription (Table 1).This generally will differ slightlyfrom the prescribed prescription. Thecompensated prescription is what you

would read using a traditional manualor digital focimeter. The lens will have

the correct power in position of wearfor the patient, taking into accounthow the patient would wear it, withcompensation for back vertex distance,

pantoscopic tilt, frame form angleetc. As the measured position on thefocimeter is the unnatural wearingposition, you would not measure theprescribed power. A digital focimeterconforming to the infinity on axisprinciple will accurately read thevery latest generations of freeformmanufactured lenses.

The latest individualised lensesalong with the design base theory willbe discussed in the next article of thisseries.

Paul Bullock is professional servicesmanager for Hoya Lens UK

TABLe 1

Sph Cyl Axis Add

Ordered power +2.75 0.00 0 +2.50

Reference powerIncluding wear parameters, lens thickness,design and material properties

+2.60 +0.12 89 +2.45

Measuring powerThe actual lens power that you may measureyourself

+2.64 +0.16 86 +2.48

ISO tolerance compliantReference power minus calculated power

+0.04 +0.04 -3 +0.03

Freeform Lenses 1&2

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