Recent trends in progressive power lenses

Colin Fowler

Vision Sciences Department, Aston University, Birmingham B4 7ET, UK

Summary

Despite having been first marketed in the 1950s, new designs of progressive addition spectaclelens continue to appear. Some of the recent patent literature is reviewed on the design of suchlenses. As well as a number of improvements to general purpose designs, specifically toinclude aspheric surfaces for the distance portion of progressive lenses, the literature includesa recent patent on an improved version of the Alvarez lateral translation lens system. The op-timisation of single vision lens forms in order to extend the depth of field for early presbyopes isalso discussed. # 1998 The College of Optometrists. Published by Elsevier Science Ltd

Introduction

Progressive addition spectacle lenses have now beencommercially available for about 40 years. Manydi�erent design approaches have been tried over theyears and these have been reviewed by Sullivan andFowler (1988). New designs are being continually pro-duced, mostly having the same overall design concept:

1. Stable distance power over the top half of the lens.

2. Stable reading area in the centre of the lower partof the lens.

3. Progressive change of power down a central corri-dor from zones 1 to 2, with minimal aberrationalastigmatism.

4. Progressive power change takes place on one sur-face of the lens only, the other being worked spheri-cal or toroidal as demanded by the prescription.

5. Curvatures are blended to give a single vision typelens appearance.

6. Lens available in a wide range of distance prescrip-tions, with reading additions up to +3.50 D.

Some special purposes lens types have appeared,designed for particular purposes. For example:

(a) Plano distance, for emmetropic early presbyopes.

(b) Progressive lens with top bifocal addition for occu-pational requirements.

(c) Special designs for shallow spectacle frame shapes.

(d) Long progression and wide intermediate corridorfor use with display screen equipment.

(e) Bifocal lens with a progressive segment.(f) Single vision lens with progressive component giv-

ing enhanced depth-of-®eld.

The last of these is interesting as it perhaps has awider application than some of the other designs.Aberrational astigmatism and mean sphere plots ofone such design are shown in Figure 1 and Figure 2.However, it is the general purpose designs as

described above which predominate, and this is alsotrue of the patent literature. Bearing in mind the con-straints that the concepts described above place on ageneral purpose design, it is perhaps surprising that somany new designs continue to appear. One reason,given by Guilino (1993), is that newer manufacturingtechniques, based on numerically controlled machinetools, enable much more complex surfaces to be accu-rately produced than was possible at one time.

Recent patents

The patent literature is a useful guide to trends inprogressive lens design, as lens designers do not other-wise publish much detail of lenses, for obvious com-mercial reasons.Kitchen (1995) describes a lens design philosophy

where aspheric surfaces are used in order to reducelens aberrations. In addition, the lens is made in verti-cally symmetrical form, so that left and right versionsare not required. But it is claimed that when the lens isrotated to give the required inset, the aberrations stillremain low. The patent (assigned to BMC Industries,Inc.) is interesting in that the key lens material quoted

Ophthal. Physiol. Opt. Vol. 18, No. 2, pp. 234±237, 1998# 1998 The College of Optometrists. Published by Elsevier Science Ltd

All rights reserved. Printed in Great Britain0275-5408/98 $19.00+0.00

PII: S0275-5408(97)00069-0

Received: 28 August 1997

234

is polycarbonate, although it is claimed that equally

satisfactory versions can also be made in CR39 and

glass. One feature of the polycarbonate design is a

requirement for a constant edge thickness uncut, to fa-

cilitate injection moulding, and a mathematical func-

tion is given to provide this requirement. In optical

terms, the aim of this design is primarily to reduce the

distortion of vertical and horizontal lines. It is not

possible to comment on the quality or otherwise of the

optical design, as no numerical values are given in the

patent.

For many years, it has been traditional to illustrate

the bene®ts of a particular design by showing aberra-

tional power plots of a lens with a plano distance area.

A trend in recent years has been to produce lenses

where the design has been optimised for di�erent dis-

tance prescriptions, and the patent of Kato (1995)

assigned to the Seiko Epson Corporation is an

example of this. Here examples are given where the

lens is aspheric on the front or back surface, or both

in some instances. This is a departure from common

practice, where the lens has the progression and any

asphericity on the front surface only. To make a bi-

aspheric lens requires the use of atoroidal surfaces if

prescription cylinders are to be incorporated.

A further claim of this patent is an improvement inprism thinning techniques. Prism thinning has been

used for many years to improve the cosmetic appear-ance of progressive lenses by generally applying base

down prism, the amount usually being simply a func-tion of the reading addition. However, unless the dis-

tance prescription is taken into account, the results canbe unpredictable, particularly in the minus range. Thus

a prism (Pt) according to this patent is worked at base908 depending on the minus power of the distance(PW) such that:

ÿ��4:0ÿADD�=9��PWRPtRÿ ��8:0ÿADD�=9��PWSome examples of the calculated prism required areshown in Figure 3.

Harsigny et al. (1996), in a patent assigned to

Essilor International, describe a more conventionaltype of approach, where the progressive surface is on

the front of the lens in all cases. Two speci®c claimsare made: ®rst of all the maximum gradient of mean

sphere down the principal meridian being located inthe intermediate vision portion, and secondly that the

surface cylinder gradient on the progressive surface iscontrolled to a low value, this depending on the near

addition. As might be expected, the low cylinder valuesare gained by compromising the stability of the dis-

tance zone, so that the 0.5 D cylinder isopter makes anangle of up to 308 relative to the horizontal axis of thelens. As examples of expected sphere and cylinder gra-

dients, an example is given in the patent of a +1.50addition, where the maximum mean sphere gradient is

0.13 D/mm, and the maximum astigmatic gradient is0.23 D/mm.

One particularly interesting feature of this patent is

that `®eld widths' are quoted for the design at variousnear additions. This describes the separation of the

0.50 D cylindrical contours at a point 14 mm belowthe geometric centre of the lens. This point is the one

generally used by this manufacturer as the near powerchecking point on progressive lenses.

These values underline the point that the e�ective

Figure 1. Iso cylinder plot of Rodenstock Cosmolit P lens.40�40 mm aperture. Values in dioptres. See Fowler andSullivan (1990) for measurement method.

Figure 2. Mean sphere plot of Rodenstock Cosmolit P lens.40�40 mm aperture. Values in dioptres. See Fowler andSullivan (1990) for measurement method.

Figure 3. Prism thinning values in relation to negativepower distance prescription, plotted from Kato (1995) data.

Progressive power lenses: C. Fowler 235

near zone in a progressive addition lens shrinks withincreasing addition (Table 1).It is normally a design criterion of progressive

lenses that astigmatism is kept to a minimum downthe umbilical meridian, and also that the progressionpower increases to the design near power and thenstabilises. A design by Ueno et al. (1996) breaks boththese conventions. The lens makes use of asphericcurves across the whole of the front surface, and oneof the design aims is to reduce the aberrations in thelower periphery of the lens, particularly on the nasalside. This design allows a certain amount of surfaceastigmatism at the top of the progression, and also inthe reading area, but it is possible that the eye willnot notice this when the lens is in the `as worn pos-ition'. However not enough detail is given in thepatent to be able to verify this. In addition, the ad-dition power reduces at the bottom of the lens, pre-sumably with the design aim of reducing aberrationalastigmatism.The same group of authors (Umeda et al., 1996)

patented a lens where complex horizontal asphericcurves are used. In the original aspheric surface pro-gressive addition lenses (e.g. Varilux 2), horizontalfront surface sections were limited to variable conicsections. But in this patent, the curvature in the hori-zontal meridian changes in a complex manner. Anexample of this type of design for a lens with a +2.50add is given, which shows a maximum surface aberra-tional astigmatism of 2.00 D over a small area, com-pared with the 2.50 D of a `conventional' lensillustrated for comparison. It should however bepointed out that this reduction in surface astigmatismis gained at the expense of a smaller stable distancevision area.

In the third of this group of patents assigned to theNikon Corporation (Umeda and Takahishi, 1996), amerit function is described which controls the meanpower at any point on the lens as a function of the dis-tance power and the reading addition.

All the above patents have described di�erentapproaches to the design of general purpose progress-ive addition lenses. But Alvarez (1967) proposed adi�erent method for producing a progressive powerlens for presbyopia. Two lens elements were used,

which when slid laterally to one another produced a

variable power across the whole e�ective aperture of

the system. Mukaiyama et al. (1997) describe an

improved version of this type of lens system. As

pointed out in the patent, this type of lens can be cru-dely simulated by taking two conventional progressive

lenses, arranged in contact so that the umbilical meri-

dians are parallel, but with one lens inverted relative

to the other. By sliding one lens relative to the other,

an increase in plus power will take place in the centre,but the edge of the system will exhibit considerable

aberrations.

The system described in the patent has two lenses

where the power distribution is illustrated diagramma-tically in Figure 4. As the cylinder distribution of the

rear lens (A) and the front lens (C) cancel each other

out when superimposed, there is no unwanted astigma-

tism. The sphere values of the rear lens (B) and frontlens (D) will combine in the same way to give an over-

all e�ect of ÿ1.00 D. It will be apparent that a vertical

displacement of one element relative to the other will

vary the sphere element without inducing any cylindri-

cal component. Unlike Alvarez (1967), no detail of thegeometry of surfaces used for the lens elements is

given.

One particular problem with such a device as a spec-

tacle lens would be the considerable re¯ections fromthe four surfaces used. An intriguing suggestion by

Mukaiyama et al. (1997) is that a ¯uid should be

placed between the lens elements with a similar refrac-

tive index to the lens material. This would eliminate

re¯ections from the adjoining surfaces, but would alsoremove their e�ect optically. The practical problems of

arranging such a lens pair that would slide laterally

Table 1.

Addition Field width (mm)

+1.00 15+1.50 12+2.00 11+2.50 9+3.00 9

Figure 4. Schematic diagrams of power distribution in lenselements described by Mukaiyama et al. (1997) A: Spherepower (D) rear lens. B: Cylinder power (D) rear lens. C:Sphere power (D) front lens. D: Cylinder power (D) frontlens. E: Arrangement of lens element movement relative topupil (P).

236 Ophthal. Physiol. Opt. 1998 18: No 2

whilst maintaining a ¯uid ®lm between would seemconsiderable.

Alternatives to progressive lenses?

Although progressive addition lenses work well formany presbyopes, they cannot be considered the idealsolution for all wearers. It should be remembered thatthe optimum use of single vision lenses can delay therequirement for a multifocal correction (Fowler, 1997).Monovision is used extensively for contact lens wearers(Back et al., 1992), but there seems to be only anecdo-tal evidence of its use with spectacles.Another possibility is to utilise the aberrations of a

single vision lens to give a progressive type e�ect. It isgenerally considered that spherical surface single visionlenses have constant power. If such a lens is movedacross the aperture of a focimeter, the reading doesnot change signi®cantly. But the situation is verydi�erent when a lens is worn, as the eye here is rotat-ing behind the lens. The angle of view becomes moreand more oblique as the eye rotation angle increases,and the vertex distance increases. This can cause sig-ni®cant changes in power, as is well known. But whatis often forgotten is that the lens performance at nearcan be quite di�erent to that at distance, particularlyin higher power lenses. An example of this is shown inFigure 5, where a ÿ6.00 DS single vision lens behaveslike a concentric progressive, having an e�ective ad-dition to +1.32 D at 358 eye rotation, for a nearobject at 250 mm from the spectacle plane. This usefule�ect is very sensitive to lens form, so that early pres-byopes should have their lens type changed with care.This is particularly the case in medium to high ametro-pias.

Conclusions

Progression addition spectacle lenses are now thecorrection of choice for many presbyopes. Despite theconsiderable restriction on design innovation imposedby cosmetic requirements, new types continue to beproduced. Manufacturers continue to explore suchvariable as progression length, base curve and aspheri-city to try and extend the acceptability of their lensesacross the prescription range.The patent literature is of mixed usefulness in under-

standing these developments. While patents may illus-trate the line of thought from a given design group,there is often not enough detail given to be able tofully evaluate the `invention'. Also, many lens designsare never patented, as some manufacturers feel thatpublishing a patent may give away too much infor-mation.There is some evidence that di�erent technologies,

such as the use of lens systems, may become morecommon in the future, but these developments stillhave a long way to go before they can be mass-mar-keted in a form which is robust and cosmeticallyattractive.

References

Alvarez, L. W. (1967). Two-element variable-power sphericallens. United States patent 3305294.

Back, A., Grant, T. and Hine, N. (1992). Comparative visualperformance of three presbyopic contact-lens corrections.Optom. Vis. Sci. 69, 474±480.

Fowler, C. W. (1997). Single vision spectacle lenses for earlypresbyopes. In: Vision Sciences and Its Applications, 1997OSA Technical Digest Series, Optical Society of America,Washington DC, pp. 10±13.

Fowler, C. W. and Sullivan, C. M. (1990). Automaticmeasurement of varifocal spectacle lenses. Ophthal.Physiol. Opt. 10, 86±89.

Guilino, G. H. (1993). Design philosophy for progressiveaddition lenses. Appl. Opt. 32, 111±117.

Harsigny, C., Miege, C., Chauveau, J-P. and Ahsbahs, F.(1996). Progressive multifocal ophthalmic lens, UnitedStates patent 5488442.

Kato, K. (1995). Progressive power lens, United Statespatent 5455642.

Kitchen, G. A. (1995). Progressive power lens, United Statespatent 5446508.

Mukaiyama, H., Kato, K. and Komatsu, A. (1997). Variablefocus type eyesight correcting apparatus, United Statespatent 5644374.

Sullivan, C. M. and Fowler, C. W. (1988). Progressiveaddition and variable focus lenses: a review. Ophthal.Physiol. Opt. 16, 402±414.

Ueno, Y., Umeda, T. and Takahashi, F. (1996). Progressivemultifocal lens, United States patent 5506630.

Umeda, T., Ueno, Y. and Takahashi, F. (1996). Progressivemultifocal lens, United States patent 5523807.

Umeda, T. and Takahashi, F. (1996). Progressive power lens,United States patent 5557348.

Figure 5. Single vision lens behaving as concentric pro-gressive addition lens. F' vÐback vertex power (D); F2Ðrear surface power (D); tÐcentre thickness (mm); zÐcen-tre of rotation distance (mm); nÐlens refractivee index;p1Ðfront surface asphericity (here spherical); p2Ðrear sur-face asphericity (here spherical); S'vÐsagittal vertex spherepower (D); T'vÐtangential vertex sphere power (D); LÐin-cident vergence (mÿ1).

Progressive power lenses: C. Fowler 237