progressive addition lenses— matching the specific lens to ... addition lenses... · to bifocal...

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ISSUE HIGHLIGHT S ince introduction to the U.S. market in the 1960’s, 1 pro- gressive addition lenses (PALs) have steadily increased in share of the multifocal market. Studies have shown that a large percentage of patients prefer PALs, as compared to bifocal alternatives. 2-4 PALs supply a continuous change of power from distance through intermediate to near that provides the wearer with a seamless visual space and elim- inates the unusable area of visual space surrounding the top line of a bifocal segment. The seamless lens is also cos- metically more pleasing than a bifocal lens. A detracting fea- ture of PALs is that the design necessarily results in unwanted astigmatism in the periphery of the lens—usually located in the lower diagonals relative to lens center. 1 There are an infinite number of possible PAL designs because they are designed with surfaces (usually the front surface) across which the curvatures change. Some designs result in wider and larger distance, intermediate, or near viewing areas. 5,6 The magnitude of unwanted astigmatism also depends on design. More recently, PALs with a short corridor or higher near zone have been developed to accommodate the shorter fitting heights required by small frame sizes. There is considerable interdependence of the sizes and locations of the viewing zones and the magnitude of unwanted astigmatism that make it currently impossible to design a lens that is optimized for all optical attributes. There are also differences in the occupational and/or recre- ational visual needs of presbyopic patients. Some patients, such as professional drivers or many outdoor employees, have a greater demand for distance vision than near. Many indoor workers have a much greater demand for near and intermediate vision than distance. Lenses Progressive addition lenses— matching the specific lens to patient needs James E. Sheedy, O.D., Ph.D. The Ohio State University, College of Optometry, Columbus, Ohio Background: The objective of this study was to use state-of- the-art methods to measure the optical characteristics of commonly available progressive addition lenses (PALs) and to develop derivatives of the optical measurements that can be used as guidelines in selection of lenses based on patients’ visual needs. Methods: The optics of 28 PALs currently on the market were measured with a Rotlex Class Plus lens analyzer. PALs were specified with plano distance power and a near add of +2.00 D. Data were normalized to plano at the location specified by each manufacturer and acquired from each data file in 1-mm vertical steps with respect to the fitting cross. Results: The variance across lenses was greater than 2:1 for most measurements. Ratings were calculated based on equal weighting of zone width and area for distance, inter- mediate, and near zones, and also for magnitude of unwanted astigmatism. Conclusions: The results demonstrate wide ranges of optical characteristics across the PALs tested in this study. Ratings of the distance, intermediate, and near zones, as well as rat- ing of unwanted astigmatism can be used in selection of appropriate lenses to match patient visual needs. Key Words: Bifocal, multifocal, ophthalmic optics, optics, pres- byopia, progressive addition lenses 1 VOLUME 75/NUMBER 2/FEBRUARY 2004 OPTOMETRY Sheedy JE. Progressive addition lenses—matching the spe- cific lens to patient needs. Optometry 2004;75:xx-xxx.

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Page 1: Progressive addition lenses— matching the specific lens to ... addition lenses... · to bifocal alternatives. 2-4 PALs supply a continuous change ... matching the specific lens

ISSUE HIGHLIGHT

Since introduction to the U.S. market in the 1960’s,1 pro-gressive addition lenses (PALs) have steadily increasedin share of the multifocal market. Studies have shown

that a large percentage of patients prefer PALs, as comparedto bifocal alternatives.2-4 PALs supply a continuous changeof power from distance through intermediate to near thatprovides the wearer with a seamless visual space and elim-inates the unusable area of visual space surrounding the topline of a bifocal segment. The seamless lens is also cos-metically more pleasing than a bifocal lens. A detracting fea-ture of PALs is that the design necessarily results inunwanted astigmatism in the periphery of the lens—usuallylocated in the lower diagonals relative to lens center.1

There are an infinite number of possible PAL designsbecause they are designed with surfaces (usually the frontsurface) across which the curvatures change. Some designsresult in wider and larger distance, intermediate, or nearviewing areas.5,6 The magnitude of unwanted astigmatismalso depends on design. More recently, PALs with a shortcorridor or higher near zone have been developed toaccommodate the shorter fitting heights required by smallframe sizes. There is considerable interdependence of thesizes and locations of the viewing zones and the magnitudeof unwanted astigmatism that make it currently impossibleto design a lens that is optimized for all optical attributes.

There are also differences in the occupational and/or recre-ational visual needs of presbyopic patients. Somepatients, such as professional drivers or many outdooremployees, have a greater demand for distance vision thannear. Many indoor workers have a much greater demandfor near and intermediate vision than distance. Lenses

Progressive addition lenses—matching the specific lens to patient needs

James E. Sheedy, O.D., Ph.D.

The Ohio State University, College of Optometry, Columbus, Ohio

Background: The objective of this study was to use state-of-the-art methods to measure the optical characteristics ofcommonly available progressive addition lenses (PALs) andto develop derivatives of the optical measurements that canbe used as guidelines in selection of lenses based onpatients’ visual needs.

Methods: The optics of 28 PALs currently on the market weremeasured with a Rotlex Class Plus lens analyzer. PALs werespecified with plano distance power and a near add of +2.00D. Data were normalized to plano at the location specifiedby each manufacturer and acquired from each data file in1-mm vertical steps with respect to the fitting cross.

Results: The variance across lenses was greater than 2:1 formost measurements. Ratings were calculated based onequal weighting of zone width and area for distance, inter-mediate, and near zones, and also for magnitude ofunwanted astigmatism.

Conclusions: The results demonstrate wide ranges of opticalcharacteristics across the PALs tested in this study. Ratingsof the distance, intermediate, and near zones, as well as rat-ing of unwanted astigmatism can be used in selection ofappropriate lenses to match patient visual needs.

Key Words: Bifocal, multifocal, ophthalmic optics, optics, pres-byopia, progressive addition lenses

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Sheedy JE. Progressive addition lenses—matching the spe-cific lens to patient needs. Optometry 2004;75:xx-xxx.

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Table 1. Progressive addition lenses measured in this study and key data; fitting cross and distance power locations are given in millimeters above the line that connects the lens markings

wrt, With respect to.

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with wider and larger distance, intermediate, ornear optical areas would probably better meetthe needs of wearers with visual needs at thoseviewing distances. Clinically, it would be use-ful to match the patient needs with a PAL designoptimized to meet those needs. However, a sys-tematic measurement and reporting of the dis-tance, intermediate, near, and astigmatismcharacteristics of PALs has not been per-formed previously. The most-recent systematicreport of PAL characteristics was in 1987.1 Theavailable PALs in the marketplace have almostcompletely turned over since then; that reportincluded only contour plots of the lenses, whichdid not include quantification or analysis of theviewing zones.

The objective of this study is touse state-of-the art methods tomeasure the optical characteris-tics of commonly available PALsand to develop derivatives of theoptical measurements that can beused as guidelines in selection oflenses, based on patients’ visualneeds.

MethodsTwenty-eight PALs, listed inTable 1, were selected for inclu-sion in this study. Lenses wereselected in an attempt to includethe most-common currentlyavailable lenses. However,because of the large number ofdesigns available in the market-place, the list of lenses includedin this study is not exhaustive.All lenses were obtained fromtwo optical laboratories (AdvanceOptical, Cleveland, Ohio andInterstate Optical, Berea, Ohio),except for the Johnson & JohnsonDefinity lens, which is not avail-able through optical laboratoriesand was obtained from the man-ufacturer. Lenses were orderedto the following specifications:plano distance power, right lens,+2.00 add, manufacturer mark-ings to be left on the lens.

All lenses were measured with aRotlex Class Plus lens analyzer

(see Figure 1). This instrument determines lenscontour plots with a single measurement. Theinstrument is essentially a moire deflectometer,which uses a point source rather than a colli-mated beam. Diverging light from a laser pointsource is incident directly on the tested lens. Therays refracted by the lens under measure passthrough two gratings and form a moire patternon a diffusive screen. Proprietary image-pro-cessing algorithms convert the fringe data toarrays of local wavefront properties—in partic-ular, the two principal curvatures and axis direc-tions. These arrays are used to calculatetwo-dimensional maps of local power, cylinder,and axis of progressive lenses, and other phaseobjects with variable powers.

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The Rotlex Class Plus lens analyzer used in this study, with lens located on measurementstage.Figure 1

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All lenses were measured with the prism referenceline markings (the lens markings that are 34 mmapart and represent the 0–180 line on the lens)appropriately aligned in the instrument, and thedata file was saved after measurement. Foranalysis, the locations of the fitting cross, distancepower, and near power—as specified by the man-ufacturer (see Table 1) with respect to the 0–180

line—were identified in the data file. Although alllenses were ordered to have plano distance power,power errors within manufacturing toleranceexisted. All measurements taken from the data filewere determined with the “DST” mode of theinstrument—i.e., all the measures on each lenswere normalized to an assigned power of plano at the manufacturer-specified distance location.

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Schematic diagram of selected measurements taken from the measurement data file. Measurements were taken at 1-mm vertical steps (Y value). Diagram shows typical limiting values of +0.25, +0.50, +1.75, and 0.50 DC.Figure 2

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This eliminated the effects of laboratory surfacingvariances at the distance center and normalized all lenses to plano power at the manufacturer-specified distance location of each lens.

Data were acquired from eachfile in a step-wise manner byexamination of the data files in 1-mm vertical increments, begin-ning at 10 mm above the fittingcross and extending to 25 mmbelow the fitting cross. Althoughthe Rotlex instrument specifiesvertical location with respect tothe 0–180 line, for the purposesof this study, all vertical locationswere converted so that the man-ufacturer-specified location ofthe fitting cross was the referencepoint. In this manner, measure-ments across lenses are refer-enced to the location that isintended by the manufacturer tobe placed before the pupil of theeye; thus, the visual effects of thelenses can be compared to oneanother with a common visualreference point. Vertical location(Y coordinate) is specified as neg-ative for locations above the fit-ting cross and positive for below.At each Y value from –10 to +25,the following data points wererecorded, moving outward fromthe center of the corridor: the leftand right X coordinates of thelimits of 0.50 cylinder, +0.25sphere (distance area only),+0.50 sphere (distance areaonly), +1.75 (near area only), and+2.00 (near area only); value ofthe greatest amount of unwantedcylinder; and spherical power(maximum plus power) in thecenter of the corridor. An illus-tration of some of these meas-urements is provided in Figure 2.Separately, the data files wereanalyzed to provide the followingadditional data for each 0.25 Dincrement of power along thecenter of the corridor: Y location,left and right X values of 0.50cylinder limits, and maximumunwanted cylinder at that level.

In data analysis, unwanted cylinder of 0.50 DCwas used as a limit of zone width. This value rep-resents a spherical equivalent of 0.25 D, and moreassuredly creates blur than a limit of 0.25 DC.

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Width of the distance power (within ± 0.25 DS and less than 0.50 DC) at fitting cross (0),above (–1 mm), and below (1 and 2 mm) the fitting cross. Not all lenses had distance areaat 1 and 2 mm below fitting cross. Lenses sorted by width at fitting cross.

Figure 3

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Area of lens with distance power (with less than 0.25 DS and less than 0.50 DC) from 1.5 mm above the fitting cross to lowest level of distancepower.Figure 4

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ResultsDistance power zoneThe width of the distance zone at the level of thefitting cross (located at the pupil of the eye) is par-ticularly meaningful to vision because it repre-sents the width of clear distance vision with theeyes in the straight-ahead position—i.e., the hori-zon. For purposes of this study, unwantedrefractive error of 0.25 DS or 0.50 DC—whichever is most limiting—constitutes the edgeof the distance viewing zone. Although 0.25 D isgreater than the power tolerance for lower pow-ered lenses (±0.13 D) as specified in the ANSIZ80.1 standard,7 it has been chosen as the limit,because refractions and prescriptions are typicallyin 0.25 D steps and patients are generally sensi-tive to +0.25 D blur at distance. Zone widthswere not necessarily symmetrical about the fit-ting cross, and the zone width measures derivedfrom the data do not retain information aboutasymmetry.

The distance zone widths of the tested lensesdefined to the first +0.25 DS or 0.50 DC power—whichever was most restrictive on each side—aredisplayed in Figure 3. Zone widths at the level ofthe fitting cross, 1 mm above and 1 and 2 mmbelow the fitting cross, are shown. Lenses aresorted by zone width at the level of the fittingcross and presented in decreasing order, so thatthe widest zones are at the top. All lenses had adistance zone level with the fitting cross, threelenses did not have a distance area 1 mm belowthe fitting cross, and 15 of the 28 lenses did nothave a distance area 2 mm below the fitting cross.Only two lenses had a distance area 3 mm belowthe fitting cross, and none extended to 4 mmbelow the fitting cross.

Because of the normal downward gaze positionof the eyes, the distance zone near the fitting crossis the most important for visual use. All of thePALs had limitations on distance zone width atthe level of the fitting cross and also 1 mm abovethe fitting cross. For this study, the distance areaof the lens was calculated by summing the zonewidths, from 1 mm above the fitting cross downto the lowest level of the distance zone, for eachlens. This effectively integrates the area in stepsof 1 mm. The width at each 1-mm step, therefore,represents the area extending 0.5 mm above andbelow it. Thus, the area calculation above rep-

resents the area of the lens from 1.5 mm abovethe fitting cross to the lowest area with the dis-tance power. Because 1 mm on the lens surfacerepresents 2 degrees of eye movement (assuming14-mm vertex distance and 15 mm from thecorneal apex to the center of ocular rotation), theupper extent of the calculated area represents 3degrees of visual gaze angle above the fittingcross, or 3 degrees above the horizon for anupright head posture. Data for the calculated dis-tance zone area are presented in Figure 4, withthe lenses sorted according to decreasing zonearea.

Intermediate power zoneThe widths of the intermediate zone for add pow-ers of +0.75, +1.00, +1.25, and +1.50 are shownin Figure 5. The intermediate range of powers(+0.75 to +1.50 D) includes the 50% power(+1.00 D) and is slightly biased to higher inter-mediate adds (inclusion of +1.50) because anextremely common intermediate task is viewinga computer display that is typically at a distancethat requires 50% to 75% of the near add.8 Dataare sorted on the basis of zone width at +1.25 forthe same reason.

The areas of the lenses with clear intermediatepowers were calculated by summing the areafrom +0.75 to +1.50 D add in three increments(0.75 to 1.00 D, 1.00 to 1.25 D, 1.25 to 1.50 D),as limited laterally by 0.50 DC. The area in eachincrement was calculated by determination of thezone width at the upper and lower end of thezone increment (e.g., the width at +0.75 D and+1.00 D for the 0.75 to 1.00 increment), aver-aging the two widths, and then multiplying theaverage zone width by the Y difference of thelocations of the upper and lower powers. Inter-mediate zone data are presented in Figure 6.

Near power zoneThe level of first appearance (descending from thefitting cross) of +1.75 and +2.00 adds are displayedin Figure 7. Although all lenses had a nominal nearadd power of +2.00 D, 12 of the 28 designs did notprogress entirely to +2.00 D, similar to the findingin a previous study.1 However, it should be noted thatadd data were collated in 0.25 D steps, and manyof the lenses that did not reach a +2.00 add cameclose nonetheless. The maximum add powerattained across the lens population was +1.996 D

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±0.109 (mean ± standard devia-tion). All further analyses of thenear power zone are based onzones with +1.75 D add or greater.Lenses in Figure 7 are sorted by thelowest Y value of the first +1.75 Dadd, resulting in the lenses withhigher appearance of the +1.75add sorted to the top. Lenses witha higher appearance of the +1.75add are better suited for theshorter fitting heights usually asso-ciated with smaller frames. Thenear zone widths (constrained byboth an add power of +1.75 orgreater and 0.50 DC limits) at 14,16, 18, and 20 mm below the fit-ting cross are displayed in Figure8. Lenses are sorted in decreasingorder of the zone width at 18 mmin order that wider zones are at thetop. Many lenses do not have anear zone at 14 mm below the fit-ting cross; some do not at 16. Itshould also be noted that, althoughthe lenses are sorted by near zonewidth at 18 mm, zone widthordering for other levels below fit-ting cross would be quite different.This is because the rate at whichzone width increases is designdependent.

The area of the near zone wascalculated in 1-mm intervals sim-ilar to the distance calculation;the zone limits were constrainedby both an add amount of +1.75DS or greater and 0.50 DC widthlimits. The cumulative areas ofthe near zone down to 16.5, 18.5,and 20.5 mm below the fittingcross are shown in Figure 9.

Unwanted astigmatismThe highest magnitude ofunwanted astigmatism for eachlens is displayed in Figure 10.

DiscussionThe results demonstrate wide ranges of opticalcharacteristics across the PALs tested in this study.

For most of the parameters shown in Figures 3through 10, the variance is greater than 2:1; forsome it is greater than 3:1. Some lenses provide

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Width of the intermediate zone (0.50 DC limits) at add powers of +0.75, +1.00, +1.25, and+1.50 D, lenses sorted by width at +1.25 D.Figure 5

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what appear to be better distance, intermediate,or near characteristics than others. The intent ofthis study is to provide this optical informationabout PALs in a form that can be clinically use-ful in matching lens characteristics to patient

visual needs. The PAL parame-ters reported in Figures 3through 10 were selectedbecause they have prima facie rel-evance to vision. However, thisassumption warrants furtherinvestigation. If these parametersare to be used to evaluate lensperformance, then the range ofvalues for each parameter shouldbe reasonably related to visualperformance. For example, ifthe range of values for a partic-ular parameter exceeds valuesthat are meaningful for vision,then the parameter may not be avalid discriminator.

All the following angle analysesassume a vertex distance of 14mm. A 14-mm vertex distanceresults in 1 mm on the lens sur-face equating to 2 degrees ofvisual space. The visual angles ofclear vision through a PAL canbe increased with a shorter ver-tex distance. An 11-mm vertexdistance results in 1 mm equat-ing to 2.2 degrees, whereas a 16-mm vertex distance results in 1mm equating to 1.85 degrees.

Distance zoneThe utility of distance zonewidth depends on the extent towhich the eye rotates to useperipheral portions of the lens.Uemera et al.9 studied eye andhead movement in response tothe appearance of lateral fixationstimuli. This task is similar toviewing a peripheral object whiledriving. Eye movement occursbefore head movement; there-fore, there is a large initial eyemovement followed by a headmovement accompanied by aneye movement in the returndirection. For lateral stimuli at 10

and 20 degrees, initial target acquisition wasentirely with eye movement. However, the finalresting eye rotations were 2 and 5 degrees, respec-tively. For lateral stimuli of 30, 40, and 50 degrees,initial eye rotations were 28, 33, and 41 degrees,

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Area of intermediate power (+0.75 to +1.50 add and less than 0.50 DC) in mm2.Figure 6

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respectively; final eye rotationswere 11, 15, and 19 degrees,respectively. In summary, thefinal resting position of the eyescan be up to 19 degrees to oneside (38 degrees total, consid-ering both lateral directions)and the initial eye movementcan be up to 40 degrees to oneside (80 degrees total). Because1 mm on the lens surfaceequates to 2 degrees of eye rota-tion, the final and initial eyemovements require distancezone widths of 19 and 40 mm,respectively. The data in Figure3 show that no lenses meet the19-mm width requirement atthe level of the fitting cross(straight-ahead gaze position),and only four lenses meet orexceed it at 1 mm above the fit-ting cross (2 degrees superiorgaze angle). None of the lensescome close to meeting the 40-mm width requirement. Thisleads to the conclusion thateven the upper end of the dis-tance zone widths in Figure 3limits normal visual functionfor lateral gaze changes on thehorizon (or 3 degrees above it)for wearers with their heads ina normal upright posture—thus, larger values within therange should improve ability toclearly see peripherally fixatedobjects.

The zone width level with thepupil seems particularly relatedto distance visual performance,because it represents vision onthe horizon with normal headposition. However, the totalarea of the distance zone closeto and below the fitting cross(we typically gaze downward)as displayed in Figure 4 alsoseems important. The lensorderings in Figures 3 and 4 aresimilar, but contain some differences. In order torepresent distance visual performance, a metriccomprised of equal parts of both parameters was

developed. Area is calculated as height timeswidth (integrated). Therefore, a metric with equalrepresentation of area and width effectively is

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First distance along corridor (descending from fitting cross) at which +1.75 and +2.00additions occur—sorted by +1.75 data. Not all lenses achieved +2.00 add.Figure 7

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weighted twice as much for horizontal dimensionas for vertical dimension. Greater weighting of the

horizontal than vertical compo-nent seems appropriate, giventhe greater use of horizontal thanvertical eye movements in theperformance of most criticaltasks, such as driving, spectatorevents, computer viewing, andreading. For each lens, a scalarvalue from 0 to 100 was deter-mined for zone width at the fit-ting cross (based on theproportional location between 5at the low end and 20 at the highend) and also for total distancearea, as shown in Figure 4(based on proportional locationbetween 15 and 60 mm2). Theupper and lower limits of theranges were selected to closelyencompass the range of val-ues—a strategy continued in fur-ther analyses (discussed later).The two scalar values were aver-aged to develop a final ratingvalue for utility of the distancezone—the resulting ratings areshown in Table 2.

Intermediate zone and astigmatismThe width and area of the inter-mediate zone are presented inFigures 5 and 6. The validity ofthese measures, or their related-ness to how we use our eyes, canbe investigated by analyzing thevisual needs of viewing a com-puter display. A 19-inch com-puter display, tilted 10 degreesaway at the top and at a viewingdistance of 60 cm, subtends ahorizontal angle of 35.7 degreesand a vertical angle of 26.8degrees. The entire display sub-tends a solid visual angle of956.8 degrees2. A quarter of thescreen therefore subtends 239.2degrees2. These convert to a needfor 17.85-mm zone width to fix-ate both edges of the display and59.8 mm2 of lens surface to fix-ate 25% of the screen. The val-

ues in Figures 5 and 6 do not approach theserequirements; thus, larger numbers within the

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Width of the near area (+1.75 D add or greater and less than 0.50 DC) at 14, 16, 18, and20 mm below the fitting cross—sorted by width at 18 mm. Not all lenses had near zonewidths at all distances below fitting cross.

Figure 8

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measured ranges represent greater abilities to fix-ate the task without head movement.

Similar to the approach for distance vision, a scalarvalue—equally weighted for zone width and zonearea—was determined. A scalar value from 0 to 100was determined based on zone width for +1.25 D(based on proportional value between 2 and 5 mm)and on zone area (as represented in Figure 6) basedon location between 10 and 30 mm2. The two scalarvalues were averaged to develop a rating value thatrepresents utility of the intermediate zone—theresultant ratings are shown in Table 2.

Likewise, scalar values of 0 to 100 were determinedfor the unwanted astigmatism based on the mag-nitude within the range of 1.25 D to 2.75 D—withhigher ratings assigned to lower amounts of astig-matism. Those ratings are also provided in Table 2.

Near zoneThe width and area of the near zone are pre-sented in Figures 8 and 9. The validity of thesemeasures can be investigated by analyzing thevisual needs of reading. A standard 8.5” × 11”piece of paper tilted 20 degrees away at the top

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Table 2. Calculated ratings* for distance, intermediate, and unwanted astigmatismSpecialty usage—calculated ratings

* Higher ratings indicate larger and wider areas of vision and lower astigmatism magnitude.

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and viewed at 40 cm subtends 30 degrees hori-zontally and 37 degrees vertically. This representsa solid angle of 1,110 degrees2—or 555 degrees2

to fixate half of the page. Fixating either side ofthe page requires a near zone width of 15 mm,and being able to fixate half of the page requires139 mm2 of lens surface. The zone widths andareas of the lenses shown in Figures 8 and 9 areconsiderably smaller than these requirements;thus, larger numbers within the measuredranges represent greater abilities to fixate the taskwithout head movement.

Another meaningful comparison is to an FT 28bifocal. Allowing for 1.5-mm pupil clearanceunder the top of the segment, 1.5-mm clearanceon each side, and extending to 3.5 mm below theoptical center of the segment (i.e., the calculatedarea extends 3.5 mm above and 3.5 mm below theoptical center of the segment), an FT 28 has ausable width of 25 mm and contains a total areaof 175 mm2, with a full add of +2.00. This widthand area is considerably larger than provided by

any of the PALs. The near zone of an FT 28 bifo-cal is also considerably higher than provided byany of the PALs. When a bifocal is fitted at thelower limbal margin, the top of the bifocal is only5 to 6 mm below pupil center. Even if an addi-tional 2 mm for pupil clearance are considered forbelow the line, the full bifocal add occurs at 7 to8 mm below the pupil—much higher than thehighest level of add appearance in PALs (as shownin Figure 7).

The utility of the near zone is dependent on theamount of the lower part of the lens that remainsafter edging (i.e., it depends on the fitting heightin the frame). The near zone should, therefore,be evaluated down to the lowest usable portionof the lens for different fitting heights. The fittingheight is the distance from the fitting cross to thelowest portion of the lens after edging. In orderto relate the near zone measurements to usablenear vision for a given fitting height, 2 mm areadded to the Y values for each width measure-ment (because of the integration effects, this

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Table 3. Calculated ratings for near zoneNear specialty usage—calculated ratings

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results in 1.5 mm added to thearea measurements) to relatethem to fitting height. The valueof 2 mm was selected because itallows 0.5 mm extension of thelens into the frame bevel andanother 1.5 mm to represent themid-pupil location for a personwith a 3-mm pupil. Therefore,this includes the entire lens,down to the lowest portion atwhich the eye can possibly usethe lens.

Similar to the approach for dis-tance and intermediate vision, arating value equally weighted forzone width and zone area wasdetermined. In reporting thenear zone results, the Y dimen-sion values were converted to fit-ting height values (as describedearlier). Scalar values from 0 to100 were determined for widthof the +1.75 D near zone width(range, 0 to 15 mm) and for thenear zone area (range, 0 to 100mm2). Final rating was a meanof the two. Ratings were estab-lished on the basis of the sameranges for all fitting heights inorder that the rating wouldalways represent the sameamount of lens devoted to nearvision, regardless of the fittingheight. In this manner, near rat-ings are comparable across fit-ting heights. The ratings for nearvision at different fitting heightsare shown in Table 3.

Specialty usageThe ratings in Tables 2 and 3 arelabeled “specialty usage” becausethe ratings are based on a singleparameter. For wearers whohave an overriding need for dis-tance vision, intermediate vision,near vision, or reduced astig-matism, the rating value reportsthe magnitude of that particularattribute for a given lens (calculated as describedearlier) in proportion to the others. The ratings in

Tables 2 and 3 probably would be best utilized forwearers with such an overriding need for a par-

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Cumulative area of the +1.75 D add (mm2) to 16.5, 18.5, and 20.5 mm from the fittingcross—sorted by cumulative area to 18.5 m from fitting cross. Not all lenses had cumula-tive areas corresponding to the measured distances below the fitting cross.

Figure 9

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ticular usage that the PAL is almost an alternativeto a single-vision lens for distance, intermediate,or near—or they have an overriding need toreduce unwanted astigmatism. For example, a pro-fessional driver may have an overriding interestin wide and large distance vision. An emmetropic

presbyope who intendsto use the glasses asreading glasses mightprimarily be interestedin a wide and largereading area, to theexclusion of other con-siderations.

Ratings for near vision(see Table 3) are depend-ent on the fitting height.Because the near zoneratings are calculatedon the basis of the sameranges for width andarea for all fittingheights, the ratings arecomparable across fit-ting heights—i.e., thesame rating number forone fitting height repre-sents the same magni-tude of near zone as itwould for another fittingheight. The ratings forthe shorter fittingheights are not as greatas those for higher fit-ting heights. Fitting anyPAL with a short fittingheight compromises theamount of near zone.Nonetheless, the valuesin Table 3 for shorter fit-ting heights indicate thelenses that provide thegreatest width and areaof near zone for thoseheights. Because thenear zone width changesat different rates withheight across lenses, therating order of thelenses changes for dif-ferent fitting heights.

Specialty usage combinationsThe ratings in Table 4 are composites of selectedratings from Tables 1 and 2; the rating value reportsthe magnitude of that combination of attributes fora given lens (calculated as described earlier) in pro-portion to the other lenses. The distance/inter-

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Highest magnitude of unwanted astigmatism measured on the lens.Figure 10

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mediate rating is comprised (in equal parts) of thoseindividual ratings from Table 2. Similarly, the dis-tance/near rating in Table 4 is a composite of thosetwo individual values. The near ratings for a fit-ting height of 22—which is a higher fitting—areused in this composite because anyone who hasspecial intermediate/near visual needs probablywill benefit from the near zone advantages of ahigher fit.

The two composite ratings in Table 4 are shownwith and without inclusion of 25% weighting ofthe unwanted astigmatism for each lens. Thevalue of 25% was selected because it places equalemphasis on the astigmatism value with the dis-tance, intermediate, and near zone ratings. Thesame weighting value for astigmatism is used con-sistently throughout this analysis, because theeffects of unwanted astigmatism probably are thesame, regardless of the lens usage category.

The distance/intermediate ratings in Table 4 indi-cate lenses that probably would be best used forwearers whose tasks are primarily at distance andintermediate and whose needs for near vision arelimited. This could include many wearers who areprofessional drivers or are engaged in physical out-door activities. The intermediate/near rating indi-cates lenses that have larger and wider intermediateand near zones, with no consideration of the dis-tance zone. Wearers who would benefit most fromthese lenses would be those with extended view-ing needs in indoor environments with minimal dis-tance needs. These could also work well foremmetropic presbyopic wearers who intend to usethe lenses primarily for near/intermediate activitiesand would remove the lenses for distance activities.

General usage combinationsThe ratings in Tables 5 and 6 are distance/inter-mediate/near and distance/near composites.

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Table 4. Combination ratings for distance/intermediate, and for intermediate/near for a fitting height (FH) of 22 mm; ratings are presented with and without weighting for astigmatism

Specialty usage combinations

Without astigmatism weighting With 25% astigmatism weighting

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The composite ratings represent equal weightingsof the components from Tables 1 and 2. The com-posite ratings are calculated for component nearfitting heights of 18 and 22—representative ofshorter and higher fitting heights respectively. Theratings in Table 5 do not include weighting forastigmatism, whereas those in Table 6 include a25% weighting of the astigmatism component.

The general usage ratings in Tables 5 and 6 indi-cate lenses that probably would be best used forwearers who perform tasks at a variety of work-ing distances. The particular ratings that bestapply are dependent on whether intermediate isimportant, the fitting height of the lens, andwhether unwanted astigmatism is a factor in lensacceptance or performance for the particular user.

General discussionFixation of an object can be accomplished by eyemovement, head movement, or a combination of

the two. The previously discussed task analyses,based solely on eye movement, indicate that PALsprovide a narrower and smaller field of fixationthan that required by common tasks. Previousresearch,9 however, shows that the normalextent of eye movements is greater than the eyemovement requirements of the tasks that havebeen analyzed. Therefore, the analyses indicatethat PALs limit the extent of eye fixations thatwould normally be used for these tasks. This con-clusion is also supported by research that showsthat PAL wearers increase the amount of headmovement and decrease the amount of eye move-ment used to view a task.10 Selenow et al.11 testedvisual performance with PALs compared to sin-gle-vision lenses on four computer-based tasks.They found statistically significant better per-formance with single-vision lenses on one task,but not the others, and concluded that PALsshowed “marginally diminished” performancecompared to single vision lenses. It appears that

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Table 5. General usage combination ratings—no weighting for unwanted astigmatism; ratings calculated for fitting height (FH) of 18 and 22—representative of low and high fitting heights, respectively

General usage combinations—no astigmatism weighting

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wearers probably adapt quite well to the limitedvision zones of PALs by using more head move-ments, but small performance decrementsremain.

The optical measurements show wide variationsin PAL design. For most of the distance, inter-mediate, and near variables measured in thisstudy, there was more than a 2:1 range of valuesacross lenses. Selecting a lens that providesgreater width and area for a particular viewingdistance will enable the wearer to clearly view thetask with more eye movement and less headmovement. This seems desirable because it is acloser match to normal eye fixation magnitudesand would require less of a shift to head move-ments.

Trade-offs in lens design are apparent—designsthat rate high in one or two zones often are rated

lower in others. The rating magnitudes of the gen-eral usage categories (see Tables 5 and 6) are lowerthan those in the other Tables, because there isa leveling effect when all categories are includedin a composite rating. No single design can excelin all areas: distance, intermediate, near, andreduced astigmatism.

As a direct result of the trade-offs in design andthe fact that the various designs use differenttrade-offs, some designs can be expected to pro-vide better vision at distance, intermediate, near,or various combinations of those distances.Unwanted astigmatism can also be factored intothe rating. Concomitantly, all patient visual needsare not the same. The categorical ratings providedin Tables 2 through 6 list the lenses that providethe widest and largest areas of clear vision for thevarious individual or composite zones. TheseTables can be used to identify those lenses that

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Table 6. General usage combination ratings—25% weighting for unwanted astigmatism; ratings calculated for fitting height (FH) of 18 and 22—representative of low and hight fittings heights, respectively

General usage combinations—25% astigmatism weighting

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can best meet the specific visual needs of par-ticular patients. Just as there is a range of opti-cal characteristics among PALs, there is also arange of visual needs among patients. The clini-cal task is to match the two.

The rating scales developed in this study arebased on viewing zone width and area meas-urements. They have been validated insofar asthe widths and areas provided by the lensesare less than normal eye fixation movementsand also less than the calculated eye fixationrequirements of common tasks. Therefore, itcan be expected that lenses with greater widthsand areas will provide better vision. The rat-ing scales were primarily developed in thisstudy as a means to integrate the sizes of morethan one viewing zone and to integrate theeffects of unwanted astigmatism with viewingzone sizes. Several assumptions have beenmade leading to the development of the rat-ings: refractive errors of 0.25 DS or 0.50 DC(with respect to prescription of) have been usedas viewing zone limits; the area and width ofa viewing zone have been weighted equally incalculating a rating, resulting in the horizon-tal dimension having twice the weight of thevertical; unwanted astigmatism has beenweighted 25% compared to 75% for viewingzone width/area; the upper (100) and lower (0)limits of the scales have generally beenselected on the basis of the range of meas-urements obtained in this study. Most of theseassumptions have not been previouslyaddressed by research. The rating scales arelinear insofar as the rating value is directlyrelated to the measure of which it is com-posed—i.e., doubling the area/width doublesthe rating value. The relationships between rat-ing zone width/area and performance orpatient satisfaction, however, are not known;thus, the relationships between the rating fac-tor and performance/satisfaction are notknown. Data are not available to relate theseratings to task performance, patient accept-ance, or patient satisfaction. Further researchis required for such validation.

The ratings and the data from which they havebeen derived are based on state-of-the-art meas-urements, and the assumptions used are consid-ered the most-reasonable ones, given ourknowledge of PALs and the visual system. It is

likely that future improvements can be made,however, and the potential limitations must beconsidered. First of all, only one lens of eachdesign was measured. Ideally, lens manufacturewould be highly consistent, so that all lenses ofthe same design are identical. However, incon-sistencies are very possible, and better design rep-resentation might be attained by measuring andaveraging multiple lenses. It is also possible thatother derivatives of the optical measurements—such as prism magnitude or axis, measures ofoptical distortion, or higher order aberrations—could meaningfully represent performance.Binocular aspects of wearing PALs, such as cor-ridor angle or prismatic difference between theeyes, have not been evaluated, but these factorsalso can be expected to affect vision performance.In addition, several assumptions have been madeconcerning the values selected to represent theborders of the distance, intermediate, and nearzones; the relative importance of vertical vs. hor-izontal dimensions; and the relative importanceof unwanted astigmatism. Each of these issues hasbeen discussed earlier in this article. Futureresearch, advances in measurement technology,and other future findings probably will providebetter approaches and may indicate changes inthe assumptions.

ConclusionsLarge variations exist in the optical properties ofPAL designs. Measurements and analyses clearlyindicate that some designs, based on their opti-cal properties, provide better distance zones, inter-mediate zones, near zones, or reducedastigmatism. The magnitudes of PAL zone widthsand areas have also been shown to be smallerthan the eye fixation demands of those tasks—assuming no adaptive head movements. Thelenses with better optical characteristics in the dis-tance, intermediate, or near zone will enable awearer to have a wider and larger area of the taskthat can be fixated at that viewing distance with-out head movement. The results and analyses pre-sented in this study can be used to selectparticular lens designs that will optimally meetthe specific visual needs of the individualpatient. It is also hoped that the findings pre-sented in this study will serve as a stimulus forfurther research and for the ophthalmic industryto develop and market PAL designs specific tovisual needs.

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AcknowledgmentI would like to acknowledge Matthias Helfrich, for his many hours of work col-lating data from electronic files, and The Ohio State University College of Optom-etry, for providing the Rotlex Class Plus lens analyzer.This study was performed in the Vision Ergonomics Laboratory, which is supportedby Microsoft Corporation.

References1. Sheedy JE, Buri M, Bailey IL, et al. Optics of progressive

addition lenses. Am J Optom Physiol Opt 1987;64:90-9.2. Cho MH, Barnette CB, Aiken B, et al. A clinical study

of patient acceptance and satisfaction of Varilux Plus andVarilux Infinity lenses. J AM OPTOM ASSOC 1991;62:449-53.

3. Sullivan CM and Fowler CW. Analysis of a progressiveaddition lens population. Ophthalmic Physiol Opt 1989;9:163-70.

4. Gresset J. Subjective evaluation of a new multi-design pro-gressive lens. J AM OPTOM ASSOC 1991;62:691-8.

5. Simonet P, Papineau Y, Lapointe R. Peripheral power vari-ations in progressive addition lenses. Am J Optom Phys-iol Opt 1986;63:873-80.

6. Sullivan CM and Fowler CW. Grating visual acuity test-ing as a means of psychophysical assessment of pro-gressive addition lenses. Optom Vis Sci 1989;66:565-72.

7. American National Standard for Ophthalmics: PrescriptionOphthalmic Lenses—Recommendations. Merrifield, Virginia:Optical Laboratories Association, 1999:1-30.

8. Sheedy J and Shaw–McMinn P. Diagnosing and treatingcomputer-related vision problems. Woburn, Mass.: But-terworth–Heinemann, 2002.

9. Uemura T, Arai Y, Shimazaki C. Eye-head coordinationduring lateral gaze in normal subjects. Acta Otolaryngol1980;90:191-8.

10. Pedrono C, Obrecht G, Stark L. Eye-head coordinationwith laterally “modulated” gaze field. Am J Optom Phys-iol Opt 1987;64:853-60.

11. Selenow A, Bauer EA, Ali SR, et al. Assessing visual per-formance with progressive addition lenses. Optom Vis Sci2002;79:502-5.

Corresponding author:

James E. Sheedy, O.D., Ph.D.The Ohio State University

College of Optometry320 West Tenth Avenue

P.O. Box 182342Columbus, Ohio 43218-2342

[email protected]

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