Notes on Progressive Lenses

Progressive lenses Part 1How progressive power is obtained

31 | May 20 | 2005 OT

Continuing Education and Training

progression zone is governed by the powerlaw for the design. The power law may belinear, or may be more complex to provide agreater or lesser increase in power at the startof the progression.

Compared with bifocal and trifocaldesigns, the progressive power lens offers:• Vision at all distances – since the addition

increases over the progression zone• More natural use of accommodation –

accommodation does not need tofluctuate when vision is transferred fromone zone to another

• Absence of image jump – there is noabrupt change in power

• The appearance of a single vision lens –there are no dividing lines on the lens

The second of these benefits is immediatelyapparent when the accommodative demandsand ranges of vision through a trifocal lensand a progressive power lens are compared.Consider a subject who has an amplitude ofaccommodation of +1.00D and who isprescribed a correction of +3.00, Add +2.25for near. The accommodative demands andranges of vision, which are obtained whenthis specification is dispensed in trifocal formand progressive power form, are illustrated inFigure 1.

Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI

Module 2 Part 5Lens Dispensing Today

The drawbacks of trifocals are much thesame as with bifocal lenses. They havezones of fixed power, which provide thewearer with a limited focusing rangethrough each zone, there is jump at thedividing lines unless visible, no-jumpdesigns are used, and the presence of thesegment dividing lines detracts from theappearance of the lenses. These drawbacksare eliminated by dispensing progressivepower lenses, the use of which hasexpanded rapidly in the last few years.Without doubt, progressive lenses havebecome the first choice of multifocal designfor the correction of presbyopia.

Progressive power lens designA progressive power lens is designed toprovide continuous vision at all distancesinstead of the predetermined workingdistances of bifocal and trifocal lenses. Thelens can be considered to have three distinctzones just like a trifocal design – a distancezone, a progression zone, or simply, theprogression, and a near zone. Unlike atrifocal lens, the progression provides anincrease in reading addition from thedistance portion to the near portion. Therate at which the power increases in the

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Thirty years ago, if a subject required a correction for distance,intermediate and near vision, it might have been provided in theform of trifocal lenses.

About the authorMo Jalie is Visiting Professor at

the University of Ulster.

Sponsored by Rodenstock

Progress through partnership

www.rodenstock.co.uk

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Figure 1 Comparison of accommodative demands and ranges of vision through trifocals and progressive power lenses

Acc

om

modat

ion

in D

Acc

om

modat

ion

in D

Accommodation in D

Working distance in cm

0 20 40 60 80 100 200

0 20 40 60 80 100 200

Clearvision

Blurredvision

Acc in D

+2.0

+1.5

+1.0

+0.5

+2.0

+1.5

+1.0

+0.5

RP range

IP range

DP range

Figure 1a

Figure 1b


32 | May 20 | 2005 OT


Sponsored by Rodenstock - Progress through partnership

Figure 1a assumes that a trifocalcorrection with an intermediate addition of+1.25 has been provided. This intermediateaddition would provide continuous visionfrom 80cm, down to the near point at31cm through the intermediate and nearportions of the lens. Figure 1b assumesthat the correction has been dispensed inprogressive power form and it is seen thatthe accommodative demand is of the sameform as that enjoyed by the subject beforethe onset of presbyopia. Naturally, theactual position of the curve depends uponwhich zone of the lens the subject happensto be using.

In order to obtain an idea of thegeometry of a progressive surface consider,first, an E-style bifocal whose bifocalsurface is made from two different spherical

surfaces placed together so that their polesshare a common tangent at point D (Figure2). Needless to say, the two surfaces arecontinuous only at point D. At all otherpoints, there is a step between the twosurfaces whose depth increases as thedistance from D increases. If we wished toproduce a truly invisible bifocal design,that is one whose dividing line cannot bedetected, the two surfaces need to be madecontinuous by blending the two surfacestogether making the DP surface and NPsurfaces continuous at all points.

In principle, a progressive lens may beconsidered to have spherical DP and NPsurfaces connected by a surface whosetangential and sagittal radii of curvaturedecrease according to a specific power lawbetween the distance and near zones of thelens.

In theory, to make a surface where thecurvature increases at the correct rate tosatisfy whatever power law it is required toadopt, we need to be able to combinesmall segments of spheres of ever-decreasing radii, all tangential to oneanother in a continuous curve. It will beunderstood that these sections will only becontinuous along a single, so-called,meridian or umbilical line and that at allother points on the surface the sectionsmust be blended to form a smooth surface.

The simplest concept of this lattersurface is a section taken from an oblateellipsoid, as illustrated in Figure 3 wherethe radii of curvature of the sphericalsurfaces which represent the distance andnear portions are shown as rD and rNrespectively. It can be seen that the solidovoid, which is obtained by inserting theellipsoidal section between the twohemispheres shown in the figure, will resultin a surface which has no discontinuities.Along the meridian line, DD’, a crosssection through the surface would becircular and the radii of the circles in aplane parallel with either the distance or

near portion circles do indeed decreasefrom rD to rN continuously.

The ability to mass-produce surfaces ofsuch complex nature was made possible bythe introduction of computer numericallycontrolled (CNC) grinding machines. CNCmachining methods have enabled thedesign and production of both progressiveand aspheric lens designs in the last 30years. Basically, the CNC cutter, which isoften a single point diamond tool, cuts asurface under computer control, the cuttersweeping in an arc over the workpiece withthe program positioning the cutter inexactly the right place as the cutter traversesthe workpiece.

The drawback to the direct machiningmethod is that no matter how accuratelythe surface is generated, it must still besmoothed and polished. These final stagesused to be accomplished by means of afloating pad system and it was essential toensure that the pads did not remove anymore glass than intended therebymaintaining the correct surface geometry.In recent years, CNC polishing has beendeveloped where the blocked lens may betransferred directly from the generator tothe polishing cycle without de-blocking toensure that the path of the polishing toolfollows that of the generator precisely(Figure 4).

Slumping consists of using ceramicslumping moulds which are themselvesproduced by CNC cutting, upon which theglass blanks with carefully polished convexspherical surfaces are placed (Figure 5).The assembly is then heated to a hightemperature at which the glass starts toflow. The back surface of the blank thenconforms in shape with that of the ceramicmould and the convex surface of the blank,which is the progressive surface, slumps tothe required geometry. The initial shape ofthe mould must be very carefully calculatedand highly sophisticated temperaturecontrol is necessary to ensure that the glass

Figure 4Schneider Opticmachines ALG 100-4 CNC(by courtesy of Schneider Opticmachines)

Figure 2E-style bifocal made by placing together two

spherical surfaces with a common tangent at D

Spherical DP surface

Spherical NP surface

D

Figure 3Concept of a progressive surface.

Section of an oblate ellipsoid is inserted between two hemispheresof radius of curvature rD for the distance portion and rN for the near position

D

D’rN

rDrD

rN

D

D’


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flows correctly. Usually, the slumping is assisted by

vacuum forming, where the glass is heatedto just beyond its softening point at whichtemperature it is incapable of flowing bygravity alone, when a vacuum is applied tothe interface between the forming blockand the concave surface of the blank,effectively, sucking the surface into shape.

The actual geometry of a givenprogressive surface, naturally, is regarded asproprietary information by lensmanufacturers but some insight into howthe design of a surface might proceed canbe obtained by developing the conceptillustrated in Figure 3.

The CNC cutter can be programmed tocut a series of arcs, each one of which isindependently controlled, but which willresult in a continuous surface of anygeometrical configuration. For example, itis possible to cut a conicoidal surface suchas the oblate ellipsoid which, in fact, lookspromising at first sight for a progressivepower surface since both tangential andsagittal surface powers increase quiterapidly as the distance from the pole of thesurface increases. The rate of increase incurvature can be controlled by altering theasphericity of this conicoid, and the changecan be determined as follows. If an obliquepencil of rays is traced through a sphericallens there is likely to be an error, δT, in thetangential oblique vertex sphere power ofthe lens. Of course, in the case of aminimum tangential error lens form, theerror, δT, may be zero. The ray-trace alsoyields the incidence height, y1, on the frontsurface of the lens, the radius of curvatureof the front surface, F1, being denoted byr1.

It is possible to show1 that for any giventangential error, there must be a conicoidwhich will eliminate the error. If the error ispositive, a conicoid whose p-value is lessthan one must be employed, whereas, if theerror is negative, a conicoid whose p-valueis greater than one would be employed.

If the tangential error is known for agiven incidence height on the front surface,then the asphericity can be found from theequation:

p = 1 + (r1/y1)2.(1 - (F1 / (F1 + δδT))2/3)

This relationship provides some of thepreliminary information that is needed todesign a progressive power surface. Inprogressive power surface design, we want atangential error to occur, the error being themagnitude of the required reading addition.If we substitute the addition for thetangential error in the above formula, it willgive us the required asphericity to reach thevalue of the addition in the tangentialmeridian at a given distance below theoptical axis of the surface.

For example, in the case of a +6.00Dsurface worked on a material of refractiveindex 1.50, to obtain a near addition of+1.00D at 14mm below the pole of thesurface would require an asphericity of+4.5. For an addition of +2.00D, we wouldneed an asphericity of +7.2 and for an addof +3.00D, an asphericity of +9.4. Thesep-values describe oblate ellipsoidal surfaces.

Lenses which employ oblate ellipsoidalsurfaces are relatively easy to analyse usingordinary trigonometric ray-tracingtechniques and in the case of a planodistance lens which employs a convexoblate ellipsoid whose p-value is +7.2(which is designed to produce an Add of+2.00D), we obtain the field diagramdepicted in Figure 6. It can be seen that forabout a 25˚ rotation of the eye, whichcorresponds with a point about 14mmfrom the pole of the surface, the tangentialoblique vertex sphere power is indeed+2.00D, the value which it was set out toachieve.

The ellipsoid alone, however, will notproduce a good progressive power surface.The conicoidal surface is astigmatic, thesurface astigmatism is normally chosen toeliminate the aberrational astigmatism of

Figure 5Slumping a glass blank to the required geometry

b) Slumping a glass blank (forming)

a) Ceramic mould


35 | May 20 | 2005 OT


oblique incidence.The field diagram depicted in Figure 6

indicates the oblique astigmatism in therefracted pencil and it is seen that theastigmatism almost matches the increase inmean oblique power of the surface. Sincethe field diagram indicates the amount ofoblique astigmatic error in the refractedpencil, it actually gives us the amount bywhich the sagittal curvature must beincreased in order to eliminate theastigmatism. For example, at 25˚ where theastigmatism is -1.50D, if the sagittal powerof the surface is increased by +1.50D, it willeliminate the astigmatism for this zone.The field diagram gives a first indication ofthe path that the CNC cutter must take, asit traverses the progression zone toeliminate the increasing surfaceastigmatism of the simple ellipsoid.

Figure 7 illustrates the power variationalong the meridian line, AD, for progressivelenses which are assumed to have sphericaldistance and near portions. The separateplots shown in Figure 7a indicate that thetangential and sagittal powers differ, as isshown in the field diagram in Figure 6.Whereas the tangential power along themeridian line has reached the near portionpower of +4.00D, the sagittal power onlyreaches +2.62D at the bottom of theprogression zone. When the sagittalcurvatures are increased point by pointalong the meridian line, however, thesagittal powers can be increased to matchthe tangential powers in the progressionzone resulting in the power variationillustrated in Figure 7b.

Note that this description of thecorrection for astigmatism only applies topoints, which lie along the meridian lineitself. As the cutter moves away from themeridian line, the form of the surfacebecomes more complicated depending

upon the performance, which the designerwants to achieve in the periphery of theprogression zone.

It will be understood that the imageformation within the progression zonedoes not depend only upon theastigmatism at single points on the surface,for the power of the progressive surfacechanges across the refracted pencil, whichis admitted by the eye.

If the power law through theprogression zone is linear then the changein power depends upon the near addition,A, and the length of the progression zone,h. For each 1mm of progression zone theincrease in power, δF, through the zone isgiven by:

δδF = A/h

Thus, if the reading addition is +2.00D andthe length of the progression zone is10mm, the surface power through theprogression zone must be changing at the

rate of 0.20D per millimetre.When the eye uses the progression zone,

the refracted pencil which fills the pupilmust exhibit a skewed form of astigmatism,even along the meridian line. Theastigmatic nature of the pencil is illustratedin Figure 8, which depicts the tangentialand sagittal fans of a narrow circular pencilof light, which is limited by the eye’s pupil,emanating from a region of the progressionzone, which is bisected by the meridianline.

Assuming that the diameter of the beamof light traversing the progression zone is4mm, with a surface that provides a nearaddition of +2.00D over a progressionlength of 10mm, there will be a differencein the tangential and sagittal surface powersacross the zone of 0.40D. If theastigmatism across the pupil is defined asthe difference in vergence between thetangential and sagittal fans across the pupil,then the astigmatism is -2A/h D. Thisapproximate, but important, rule tells us

FFiigguurree 77Power variation for progressive power lenses with ellipsoidal progression surface of depth 14mm Rx +2.00 Add +2.00, before and after adjustment tosagittal curvatures in progression zone

FFiigguurree 66Field diagram for plano lens made with a convex oblate ellipsoidal surface

Field diagrams for spectacle lensesTangential and sagittal oblique vertex sphere powers

Ocular rotation in degrees

Oblique

ver

tex

sphe

re p

ower

s

10.00

8.00

6.00

4.00

2.00

0.00

F’T

F’S

40 35 30 25 20 15 10 5 0

rD

rN

Sagittal meridians

Tangential meridian

A

D

A

D

a) Progression zone if pureoblate ellipsoid

b) Progression zone withmodified sagittal radiito reduce astigmatism

S

T ST T

+2.62 +4.00

+1 2 3 4 5 +1 2 3 4 5

30

15

0

-15

-30


36 | May 20 | 2005 OT



that the astigmatism across the pupil isproportional to the addition, but inverselyproportional to the length of theprogression zone. In other words, thesmaller the addition and the longer theprogression zone, the smaller theastigmatism becomes.

As the eye moves away from themeridian line, the total astigmatismincreases, approximately linearly but, ofcourse, dependent upon the exact nature ofthe cross-section of the lens.

The corridor of clear vision through theprogression zone is often described as the

region in the zone where the astigmatismdoes not exceed +1.00D. It is conventionalfor manufacturers of progressive designs tocompare the width of the corridor of clearvision, either as a stated number ofmillimetres or by an isocylinder diagramwhere the contours illustrate the change insurface astigmatism in different zones ofthe lens. Such a diagram is illustrated inFigure 9, together with an iso-mean powerdiagram, which shows how the powervaries across the lens. On the basis of a1.00D limit on surface astigmatism, thelens illustrated would be described as

having a corridor width of nearly 20mm at10mm below the geometric centre of thelens.

The iso-mean power diagram illustratesthat the surface increases in power slightlyas the eye rotates upwards from thegeometric centre of the lens, which is due tothe method chosen to blend the surfacebetween the distance and near portions.Also the full reading addition of +2.00D isseen to have been reached at a point 20mmbelow the geometric centre of the lens.

A second consequence of the progressivesurface is depicted in Figure 10, whichcompares the appearance of three verticallines viewed through a bifocal lens and aprogressive power lens. Since an increase inpower is inevitably accompanied by anincrease in magnification, and it isinevitable that if there is one then theremust also be the other, vertical lines viewedthrough the progression zone exhibit skewdistortion.

The directions of the lines can beillustrated by means of a vector plot, whichshows how their orientation can beexpected to vary in a real image of the lines.The skew effect can be minimised bydecreasing the surface curvature as the eyemoves away from the meridian line. Thelower down the surface, the greater thereduction in curvature needs to become.

The development of progressive lensesover the last 40 years can be discussed interms of the way in which the CNC cutterhas been employed to blend the DP surfacewith the NP surface. In the firstcommercially successful lens, the Varilux 1design from Essel Optical Company (nowpart of Essilor International), the DP andNP surfaces were virtually spherical and theCNC followed a path, which describedcircles of ever-decreasing radius as ittraversed from the spherical distanceportion to the spherical near portion. Thisnecessitated very severe blending of thedistance and near portions with largeamounts of surface astigmatism at theperipheries of the progression. However, thedistance portion was almost completely freefrom surface astigmatism. The secondgeneration Varilux 2 design used a series ofconic sections of varying asphericity toreduce the astigmatic nature of the earlierdesign and, at the same time, usedaspherical DP and NP surfaces to furtherreduce the severity of the blend. Withoutdoubt, the Varilux 2 design pointed the wayfor later generations of progressive powerdesigns.

Third generation designs, such as theTruvision OMNI design, combinedaspherical DP and NP surfaces, whichspread the blending further into thedistance portion, softening the definition inthe distance portion but considerablyreducing the astigmatism in the lateralportions of the lens. The power profile oflenses, such as the OMNI, can be comparedwith that of an up and down curve trifocaldesign, with the full distance prescription

FFiigguurree 88Astigmatic nature of the progression zone

F

F+δF

F+2δF

Isocylinder lines Iso-mean power lines

FFiigguurree 99Isocylinder and iso-mean power lines for progressive power lens, plano add +2.00D

0.25

0.50

0.75

1.00

0.25

0.25

0.50

0.75

in 0.25D intervals

FFiigguurree 1100Skew distortion in a progressive power lens

Flat-top bifocal Progressive power lens

25 15 5 5 15 25 25 15 5 5 15 25

20

10

0

10

20

20

10

0

10

20


37 | May 20 | 2005 OT


obtained near the top of the lens and thenear prescription at the bottom. Such along progression, of course, enabled thelens to exhibit remarkably low levels ofsurface astigmatism in the ‘intermediate’peripheral zones.

The latest generation of designs hascombined the features of low-poweraspheric lenses with a progressive surfacedesign, and also adopted different powerlaws for different near additions.

This feature of progressive lens design,the ability to control in which areas of thelens the blending between the DP surfaceand the NP surface occurs, has enabledmanufacturers to decide which areas theyshould prioritise for optimum vision. If thedesigner requires a large distance portionwith the surface astigmatism confined to thelower portion of the lens, rather like theearliest progressive lens designs, the result isa hard progressive design. The arrangementof the surface astigmatism in a hard designis shown in Figure 11a. It is seen that thisdesign enables a large DP area and arelatively large NP area to be obtained, butthere are rapid discontinuities in theastigmatism in the lower portion of thelens.

If the designer wishes to reduce theamount of astigmatism which occurs in thelower portion of the lens, to speed thepatient’s adaptation to progressive lenswear, it can be spread into the distanceportion as indicated in Figure 11b. Thisarrangement results in a soft progressivedesign, and there can be no doubt thatwhen the addition is low and, hence, thesurface astigmatism is low, the softprogressive design has proved to be themost successful in enabling rapid weareracceptance of progressive design.

Some manufacturers produce progressivelens series that are deliberately soft indesign for the low addition lenses in theseries, the design tending to become harderas the additions increase. These are knownas multi-design series. Figure 12 illustrateshow the power law differs with a multi-design series for the additions, +1.00D,+2.00D and +3.00D. It can be seen how thelength of the progression zone reduces asthe addition increases for these series.

Another important feature of progressivepower lens design relates to the symmetryof the power distribution across the lens. InFigure 13, the eyes are supposed to beviewing an object at B, the visual axes

intersecting the lenses at P in the right eyeand P’ in the left eye. If the surface powerat point P were to differ much inmagnitude or orientation from the surfacepower at P’, there will be different prismaticeffects at the two points. For ease of fusionin all directions of gaze, the verticalprismatic effects at corresponding pointsmust be approximately equal. This is morelikely to be the case if each lens isindividually designed for the right and lefteyes, rather than producing a single designthat is rotated inwards, in oppositedirections for the two eyes. Progressive lensdesigns that exhibit approximately equalvertical prismatic effects at correspondingpoints, are said to possess horizontalsymmetry.

Although isocylinder and vectordiagrams are informative, it is foolhardy tosuppose that they can be used to predictwearer adaptation and acceptance of thelens.

Despite the consequences of theprogressive surface, the brain quickly adaptsto the effects of surface astigmatism andskew distortion. The adaptation required ofthe visual system is probably no greaterwhen patients wear their first pair ofprogressive lenses than that required whengiven their first pair of bifocals. Indeed, ifprogressive power lenses are introduced atthe onset of presbyopia, when the readingaddition is low, the adaptation period isprobably less than it would be for the firstpair of bifocal lenses. In an attempt toevaluate the performance of newprogressive designs, most manufacturerssubmit the lenses to clinical trials, theinformation that the wearers report beingfed back into the design loop.

Patients who, typically, find it difficultto adapt to progressive lenses are thosewith high reading additions who havesuccessfully worn bifocal lenses in the pastand have become accustomed to a widereading field of view through a largesegment. Problems with adaptation toprogressive lenses are almost always due toeither an incorrect prescription for distance,too high or low a near addition or poorlyfitted lenses2,3.

25 15 5 5 15 25

20

10

0

10

20

20

10

0

10

20

25 15 5 5 15 25

0.25

1.00

2.00

1.00

2.00

0.25

0.50

0.75

1.00

a) Hard progressive lens b) Soft progressive lens

FFiigguurree 1111Comparison of isocylinder lines for hard and soft progressive lens designs of power, plano add +2.00D

FFiigguurree 1122Power laws for +1.00, +2.00 and +3.00 additions for a multi-design series of progressive powerlenses. Note the designs tend to become harder as the reading addition increases

DRP

NRP

Add +1.00D

Add +2.00D

Add +3.00D

FFiigguurree 1133Horizontal symmetry atcorresponding point on the lens

B

P P’

DRP = distance reference point NRP = near reference point0.00 +1.00 +2.00 +3.00


38 | May 20 | 2005 OT



Thinning prismWhen a progressive lens is worked toindividual prescription by surfacing theconcave surface of the blank, it is usual toincorporate a thinning prism in thespecification of the back surface. Thepurpose of the thinning prism is to equalisethe edge thickness of the finished lens,usually at the top and bottom edges of thelens, and its magnitude depends upon thepower of the lens, the cylinder axisdirection if the lens is astigmatic, and theposition of the distance reference pointwith respect to the box centre of the lens(Figure 14). Typically for plus sphericallenses, the magnitude of the thinning prismis about two-thirds of the reading additionand its base usually lies at 270˚. As with allordered prisms, the magnitude anddirection of the prism incorporated in thelens must be measured at the prismreference point, which is normally thegeometric centre of the original blank.

It is sensible to apply an anti-reflectioncoating to any lens that incorporates prismin order to eliminate the possibility of theghost image, which is formed by total

internal reflection at the lens surfacesannoying the wearer.

Fitting progressivepower lensesA suggested fitting routine for progressivepower lenses is as follows:1. Select the final frame that the patient is

to wear and adjust it to fit properly. As ageneral guide, the frame should beclosely fitting with the smallest possiblevertex distance and the correctpantoscopic angle, and should provideadequate depth beneath the centre ofthe pupil to accommodate the readingzone of the lens. A minimum depth of22mm is usually suggested for designswith corridor lengths of 18mm, or aminimum depth of 14mm for compactdesigns. Manufacturers usually give arecommendation upon both therequired minimum depth of lens belowthe pupil centre, and the required heightof lens above the pupil centre in orderto ensure that the subject has anadequate field with the design(Figure 15).

2. If the frame is empty, attach verticalstrips of transparent adhesive tape toeach eye to enable reference points to bemarked. Otherwise, the fitting crosspositions can be marked on the existinglenses.

3. With the correctly adjusted frame inposition, ask the patient to look straightinto your eyes. If necessary, adjust theheight of your stool to ensure that youreyes are on exactly the same level as thatof the patient. Some manufacturers nowrecommend that measurements for theheight of the fitting cross should betaken with the patient standing when itis easier to judge whether their normalhead position is more or less uprightthan when they are seated.

4. Direct the patient to look straight intoyour open left eye and using a fine-tipmarking pen and, preferably, a lightcoloured ink, place a dot in front of thecentre of the patient’s right pupil.

5. Direct the patient, without moving theirhead, to look straight into your right eyeand place a second dot in front of thecentre of their left pupil.

6. Remove and replace the frame on thepatient’s face and repeat the aboveprocedure, this time without makingany marks, to ensure that the dots whichyou have marked do lie in front of thecentres of the pupils.

Record the fitting point positions andcheck by means of an appropriate blanksizing chart that the lenses can be obtainedfrom the available blank diameters. Whenordering the lenses, it is necessary to givethe fitting point positions (the vertical andhorizontal fitting distances and directionsof the fitting points from the boxed centresof the lens shapes). The horizontal fittingpoint positions are deduced from thedifference between the monocular CDsmeasured from the centre of the bridge ofthe frame, and half the horizontal boxcentre distance of the frame. It is verycommon for the monocular CDs to differbetween the right and left eyes. Thespecification should be written, forexample, as 34/32, which indicates that theright eye monocular CD is 34 and the lefteye monocular CD is 32. The heights of thepupil centres may also differ for the rightand left eyes.

It is sensible, whenever possible, todispense a frame that allows some verticaladjustment of the height of the distancereference point at, or subsequent to, thefinal fitting. This enables the lenses to beraised or lowered, if this is found to benecessary, to aid adaptation.

Of equal importance to the success ofdispensing progressive power lenses to newwearers is the confidence shown by thepractitioner when prescribing this designfor the correction of presbyopia, andespecially what advice is given in thewearing and use of the lenses at finalfitting. Needless to say, the advantages of

FFiigguurree 1144Thinning prism

FFiigguurree 1155Minimum requirements for frame depth

a) Semi-finished blankwithout thinningprism

b) Base-up prism to beremoved to equalisethickness at the topand bottom of blank

c) Base-up prism removedprovides equalthickness at top andbottom of blank

Mean power plot add +2.00Dfor a progressive design with acorridor length of 18mm does notprovide an adequate near portionin a shallow frame

Mean power plot add +2.00D for a progressivedesign with a corridor length of 14mm showsthat the lens may be mounted in a shallowframe. Note the recommended frame depthbelow the pupil is 17mm, with a minimum overallframe depth of 29mm

12

17


39 | May 20 | 2005 OT


progressive designs over other forms ofmultifocal correction should be spelt out insimple terms at the time of dispensing:• “These lenses will enable you to focus at

all distances”• To first time young presbyopes:

“They will be easier to get used to thanbifocal lenses”

• “In wear, the lenses will restore thevision of youth”

• “There are no tell-tale dividing lines onthe lenses”

At final collection, it is important tore-emphasise these advantages anddemonstrate the use of the lenses. Asuggested routine for the final delivery ofprogressive lenses to patients when theycome to collect their new spectacles is asfollows. 1. Firstly, make sure when checking the

lenses that not only the powers at theirrecommended measuring points and theprism at the prism reference point are allas ordered, but also that the horizontalpainted lines are parallel to the linejoining the centres of the engravedcircles. Also ensure that the position ofthe fitting cross on each lens has beenrestored so that it is clearly visible.

2. The frame into which the lenses aremounted should have been set up to fitthe patient properly at the time ofdispensing. However, the lens mountingprocess may have disturbed the initialset-up of the frame, so it is important toensure that the frame is returned to theadjustment that was made when the lensmeasurements were taken. Any smallfinal adjustments that may be needed atthe time of final fitting should beundertaken first.

3. Verify that the positions of the fittingcrosses do coincide with the centres ofthe patient’s pupils. The same procedurewhich was used for dotting the desiredfitting point positions should beadopted.

4. Clean off the painted marks and ensurethat the lenses and frame are completelyclean and look like new.

5. Ask the patient to look at a distantobject directly in front and verify thatthe anticipated visual acuity is obtainedfor distance vision.

6. Ask them to look at a laterally placedobject at the same height as the straight-ahead distance target, and point out ifnecessary that it might be necessary toturn the head to obtain optimum acuityfor this object.

7. Ask them to look at a test chart at near,which you should guide into their handsso that it is held directly in front of thesubject in the usual reading position,and verify that the anticipated acuity fornear vision is obtained.

8. Ask them to move the near chart to oneside and, again, ensure that the patientunderstands that they may need to turntheir head and point their nose in the

direction of view in order to see the neartarget clearly.

9. Move the near test target to a positionwhere it is about 40cm to 50cm directlyin front of the patient, and just belowthe original object chosen to verify thedistance acuity. Point out that they canalso see clearly at the intermediatedistance, if necessary by a slightadjustment of the visual axes in thevertical meridian. In particular, allow thepatient to confirm that the gaze can betransferred from the distant target to theintermediate target without moving thehead, simply by adjusting the directionof gaze of the eyes.

10.Point out the visual tasks which are noteasy with progressive power lenses, suchas reading a notice with small printwhich is above head height, readingwhen lying down or trying to watchtelevision when lying down in bed, andlooking over their shoulder as theymight when reversing a car.

Most important of all is to reassure thepatient that these visual tasks are wellunderstood, as is the optical performanceof their progressive lenses, and that mostpeople need no more than a short periodto get used to this state-of-the-art correctionfor presbyopia.

Insetting progressivepower lensesWhen single vision lenses are dispensed fornear vision, they are decentred inwards sothat the visual axes pass through the opticalcentres of the lenses. This enables thecentres of the near fields of view tocoincide and avoids the introduction ofunwanted horizontal prismatic effect fornear. In the case of multifocal lenses whosedistance portions have been properlycentred for distance vision, the main lensexerts prismatic effect at the near visualpoints, which alters the convergencerequirements for objects viewed throughthe near portion. The purpose of insettingthe near portion is to bring the near fields

into coincidence4 and in order to achievethe desired effect, it is necessary to take theprismatic effect of the distance portion intoaccount (Figure 16).

It can be seen from Figure16 that, inorder to place the centre of the nearaperture in the correct position (i.e. thecentre of the segment in the case of bifocallenses, or the centre of the near zone in thecase of progressive power lenses), theinsetting must be greater in the case of plus,and less in the case of minus distanceportions when compared with the inwarddecentration of single vision lensesdesigned for near vision. Thus, for eachcombination of base curve and readingaddition, the designer must calculate adifferent inset for the progression corridoron the lens to ensure that the wearer enjoysthe widest possible widths for theprogressive corridor and the near visionzone.

With most modern progressive designs,the inset is individually calculated takinginto account both the working distance andthe distance and near prescriptions, inaddition to the monocular centrationdistances.

Part 2 of Progressive powerlenses on June 17 will look at

the latest generation ofprogressive lens designs.

References

1. Davis JK and Fernald HG (1976) USPatent 3960442. Ophthalmic Lens Series

2. Essilor International (1997) Personalcommunication. In-house analysis ofwearer rejection of progressive lenses.

3. Sullivan CM and Fowler CW (1990)Investigation of progressive additionlens patient tolerance to dispensinganomalies. Ophthal. Physiol. Opt. 10: 16.

4. Jalie M (2003) Ophthalmic Lenses &Dispensing. Second edition. ButterworthHeinemann, Oxford.

FFiigguurree 1166Insetting progressive power lenses

a) Plus lenses b) Minus lenses

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40 | May 20 | 2005 OT



1. What jump is exerted by aprogressive lens made to theprescription, +1.00D add +2.50D?

a. 1.00Db. 2.50Dc. 3.50Dd. None

2. A progressive power lens with anaddition of +3.00D has aprogression zone 15mm in length.Assuming a linear power lawthrough the progression, whataddition would you expect thewearer to obtain 10mm below thestart of the progression?

a. +1.00Db. +1.50Dc. +2.00Dd. +2.50D

3. Why does a section taken from asimple oblate ellipsoid not form auseful progressive surface?

a. The sagittal curvature is insufficient toprovide no astigmatism along themeridian line

b. It does not join the distance and nearareas without a discontinuity in thesurface

c. It does not provide sufficient nearaddition in the tangential meridian ofthe meridian line

d. It provides too great an addition alongthe sagittal meridians of the meridianline

4. Why does skew distortion occur in aprogressive power lens?

a. Because all lenses exhibit distortionb. Because the power becomes more plus

as the eye moves down through theprogression zone

c. Because the power becomes less plusas the eye moves down through theprogression zone

d. Because the magnification becomesless as the eye moves down throughthe progression zone

MCQs

Progressive lenses Part 1 – How progressive power is obtainedPlease note there is only ONE correct answer

Module 2 Part 5 of Lens Dispensing Today

5. What are the characteristics of a softprogressive design?

a. No astigmatism in the distance area ofthe lens

b. No astigmatism in the lower half of thelens

c. Astigmatism is allowed to spread intothe distance portion of the lens

d. Astigmatism is confined to theperipheral portions of the lens

6. What is the main characteristic of amulti-design series of progressivelenses?

a. The power law varies with the readingaddition

b. The design becomes softer as theaddition increases

c. The design varies with the distanceprescription

d. The length of the progression zoneincreases with the near addition

7. What is meant by the term ‘horizontalsymmetry’ when applied to aprogressive power lens?

a. The horizontal power of the lens is thesame at corresponding points in a pairof lenses

b. The horizontal prismatic effect is thesame at corresponding points in a pairof lenses

c. The vertical prismatic effect is the sameat corresponding points in a pair oflenses

d. The thinning prism which hasbeen incorporated is the same for eacheye

8. Which of the following does notusually hinder adaptation to a pair ofprogressive lenses?

a. Too high a reading additionb. A low reading additionc. An incorrect distance prescriptiond. Poorly fitted lenses

9. If a standard prism thinning isapplied to a progressive power lenswhose power is plano / +1.00 x 90add +1.50, how much prism wouldyou expect to find at the distancereference point of the lens assumingthis to lie 5mm above the prismreference point?

a. 1.00D base downb. 1.50D base upc. 1.50D base downd. None

10. If no prism thinning is applied to aprogressive power lens whose poweris -5.00 add +2.50D, how much prismwould you expect to find at thedistance reference point of the lensassuming this to lie 4mm above theprism reference point?

a. Noneb. 1.00D base upc. 2.00D base downd. 2.00D base up

11. On verification of a progressive poweruncut lens the power at the distancereference point, 4mm above thegeometrical centre of the uncut, isfound to be +4.00D and it is notedthat the prismatic effect at this pointis 2.6D base down. What thinningprism has been incorporated in thelens?

a. Noneb. 1.00D base downc. 1.60D base downd. 1.60D base up

12. Minus power progressive lensesshould be inset in which one of thefollowing ways?

a. The same as plus lensesb. More than plus lensesc. Less than plus lensesd. The same for all minus powers

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Notes on Progressive Lenses