CA2265705C - Set of progressive multifocal ophthalmic lenses - Google Patents

Abstract

The invention concerns a set of progressive multifocal ophthalmic lenses, ea ch having a first progressive multifocal surface and a second surface preferabl y spherical. It proposes to define the set of lenses with regard to the optical characteristics of the lenses, and particularly wearer power and oblique astigmatism, in wor n conditions. For this purpose, the invention defines an ergorama associating with each sight direction in worn conditions a target object point, and a given power. This ergorama supplies a power target for a definition by optimisation of the lenses, and is used in a radii plotting programme for calculating the optical characteristics during optimisation. The set of lenses has substantially identical optical performances for a given addition, whatever the power of the far vision reference point.</SDOAB >

Description

xfl1O70‘LflLflCA 02265705 1999-03-15Translation of WO 98/12590 as filedSET OF PROGRESSIVE MULTIFOCAL OPHTHALMIC LENSESThe present invention relates to a set of progressivemultifocal ophthalmic lenses; it also relates to a methodfor determining an ergorama for a set of progressivemultifocal ophthalmic lenses, said ergorama. providing" anassociation, for each lens, between a point towards whichthe glance is directed and each direction of glance in theconditions under which the lens is actually worn. Finally,it relates to a method for defining a progressiveophthalmic lens.Progressive multifocal ophthalmic lenses are now well-known. They are used for correcting long—sightedness andallow wearers of spectacles to look at objects over a largerange of distances without needing to take their glassesoff.situatedSuch lenses typically comprise a far vision regionin the upper portion. of the lens, and. a nearvision region situated in the lower part of the lens and anintermediate vision region linking the near vision regionand the far vision region, together with a main meridian ofprogression which passes through these three regions.French Patent 2,699,294 discusses,in its preamble, thevarious elements of such aprogressive multifocalophthalmic lens together with the work carried out by thepresent applicant to improve the comfort of wearers of suchlenses. Reference should be made to that document for moreinformation on these various points.The applicant also proposed, for example in UnitedStates Patent 5,270,745 or 5,272,495 to cause the meridianto vary, and notably to off—center it towards a near visioncontrol point, as a function of power addition andametropia.Applicant has also proposed, in order to better satisfythe viewing requirements of long~sighted people and improveCA 02265705 l999-03- l5progressive multifocal lens comfort, various improvements(French Patents 2,683,642, 2,699, 294, 2,704,327).Usually, these progressive multifocal lenses comprise afront aspherical face which is the face that faces awayfrom the wearer of the spectacles, and a rear spherical ortoroidal face directed towards the wearer of thespectacles. This spherical or toroidal face makes itpossible to adapt the lens to the user's ametropia,meaning that a progressive multifocal lens is onlygenerally defined. by its aspherical surface. As is wellknown, such a aspherical surface is generally defined bythe height or altitude of all points thereon. Parametersare also used consisting of maximum and minimum curvatureat each point, or, more frequently, their half—sum andtheir difference. The half—sum and difference, multipliedby a factor n ~ 1, n being the refractive index of the lensmaterial, are called mean sphere or power, and cylinder.Families of progressive multifocal lenses can bedefined, each lens in a family" being" characterised. by apower addition, corresponding to a variation in powerbetween the far vision region and the near vision region.More precisely, power addition, referred to as A,corresponds to the variation in power between a point L inthe far vision region and a point P in the near visionregion, which are respectively referred to as the farvision control point and near vision control point, andwhich represent points where the glance intersects thesurface of a lens for viewing to infinity and for readingvision.Within the same family of lenses, power addition variesfrom one lens to another in the family between a minimumand maximum value of power addition. Usually, the minimumand maximum power addition values are respectively 0.75<iiopters and 3.5 diopters, and. power addition. varies byCA 02265705 l999-03- 150.25 diopters steps from one lens to the next one in thefamily.Lenses having the same power addition differ by theirvalue of mean sphere at a reference point, also known asthe base. One can for example decide to measure the base atthe far vision control point L.Thus, by choosing pairs, (power addition, base) a setof front aspherical faces for progressive multifocal lensesare defined. Usually, one can thus define five values forthe base and 12 values for power addition, giving a totalof 60 front faces. In each one of the bases, optimizationis carried out for a given power.Using, with one of these front faces, a rear face whichis spherical or toroidal and near to the rear face used foroptimization, makes it possible to cover all of therequirements of progressive multifocal lens wearers. Thisknown method makes it possible, starting from semi—finishedlenses of which only the front faces is shaped, to preparelenses suited to each wearer, by simply machining aspherical or toroidal rear face.This method suffers however from the disadvantage ofonly being an approximation; consequently, the resultsobtained with a rear face different from the one used foroptimization are not as good as those corresponding to therear face used for the optimization.United States Patent 5,444,503 discloses a progressivemultifocal lens in which the rear face is adapted to eachwearer, and is constituted by an aspherical surface. Thisaspherical surface is a not multifocal and appears to becalculated so as to provide the optical power necessary atcertain reference points. In that Patent, it is consideredthe solution would make it possible to overcome the defectsarising from replacing the rear space used for optimizationby a rear face approximating it.CA 02265705 l999-03- 15This solution has the disadvantage of considerablycomplicating" lens manufacture: it implies measurement offollowed byof an aspherical rear face.the position of the lenses on the the wearer,determination,Theand machining,invention provides a multifocalprogressiveophthalmic lens having improved aesthetic appeal and whichhas improved performance over existing lenses while makingtheit possible to preserve ease with which the semi-finished lenses can be adapted to 23 wearer. While stillconserving this ease of implementation, the inventionprovides adaptation of lenses by simple machining of therear face which does not lead to a defect in vision evenwhen the rear face is different from the rear space usedfor optimization.More precisely, the invention provides a set ofprogressive multifocal ophthalmic lenses determined bymeans of ergoramas which associate, for each lens, a pointtowards which the glance is directed with each direction ofglance, under wearing conditions,in which, for a lens under the conditions in which itis worn, a wearer power is defined in a direction of glanceand for an object point, as the sum of the the degree ofnearness of an object and the degree of nearness of theimage of said object point,in which each one of said lenses has:— a first and a second surface, said first surfacebeing a progressive multifocal surface;~ a far vision region, a near vision region and a mainmeridian of progression passing through said two regions,said far ‘vision region, near‘ vision region and xneridianbeing sets of directions of glance under the wearingconditions;— ea power addition Ix equal to ea variation jjl wearerpower for the point towards which the glance is directed inthe between a reference direction ofergorama, glance inCA 02265705 l999-03- 15the far vision region and a reference direction of glancein the near vision region;— and in which variations in wearer power along saidmeridian, for the said point towards which the glance isdirected ixi the ergorama are substantially identical foreach one of the lenses of a set having the same poweraddition.According to one embodiment, said lenses each have aprescribed power addition selected within a discrete set, adifference in power addition A between two lenses of saidset having the same prescribed power addition being lessthan or equal to 0.125 diopters.Advantageously, with astigmatism aberration in adirection of glance, under wearing conditions, beingdefined for an object point,— for each lens, along said meridian, astigmatismaberration for a point towards which the glance is directedin the ergorama is less than or equal to 0.2 diopters.According to a further embodiment, with astigmatismaberration in a direction of glance, under wearingconditions, being defined for an object point,~ for each one of said lenses under wearing conditions,angular width in degrees between lines for whichastigmatism aberration for points on the ergorama is 0.5diopters, at 25° below a mounting cross on said lens, has avalue greater than 15/A + 1, A being the power addition.Advantageously, with astigmatism aberration in adirection of glance under wearing conditions being definedfor an object point,— for each one of said lenses under wearing conditions,whichis 0.5an angular width in degrees between lines forastigmatism aberration for points on said ergoramadiopters, at 35° below a lens mounting cross, has a valuegreater than 21/A +10, A being the power addition.CA 02265705 l999-03- 15According to one embodiment, with astigmatismaberration in a direction of glance under wearingconditions being defined for an object point,v for each one of said lenses under wearing conditions,a solid angle bounded by lines for which astigmatismaberration for points in said ergorama equals 0.5 diopter,and points situated at an angle of 45° with respect to amounting cross on said lens has a value greater than 0.70steradiansAccording to a further embodiment, for each one of saidlenses under their wearing conditions, wearer powerdifference in the far vision region, in each direction ofglance, between a point towards which the glance isdirected in said ergorama and object points the degree ofnearness of which differs from the degree of nearness ofsaid point towards which the glance is directed by between0 and 0.5 diopters, is less than or equal to O.l25 dioptersas an absolute value.Advantageously, for each one of said lenses under theirwearing conditions, wearer‘ power difference in the nearvision region, in each direction of glance, between a pointtowards which the glance is directed in said ergorama andobject points the degree of nearness of which differs fromthe degree of nearness of said point towards which theglance is directed. by an absolute value of less than 1diopter, is less than or equal to 0.125 diopters as anabsolute value.According to a further embodiment, in which, withastigmatism aberration and direction of glance underwearing conditions being defined for an object point,for each one of said lenses under wearing conditions, adifference in astigmatism aberration in the far visionregion, in each direction of glance, between a pointtowards which the glance is directed in said ergorama andobject points the degree of nearness of which differs fromCA 02265705 l999-03- 15the degree of nearness of said point towards which theglance is directed by between 0 and 0.5 diopters, is lessthan or equal to 0.125 diopters as an absolute value.Advantageously, with astigmatism aberration anddirection of glance under wearing conditions being definedfor an object point,for each one of said lenses under wearing conditions, adifference in astigmatism aberration in the near visionregion, in each direction of glance, between a pointtowards which the glance is directed in said ergorama andobject points the degree of nearness of which differs fromthe degree of nearness of said point towards which theglance is directed. by" an absolute value of less than 1diopter, is less than or equal to 0.125 diopters as anabsolute value.A. method for determining an ergorama for a set ofprogressive multifocal ophthalmic lenses is also provided,said ergorama associating, for each lens, a point towardswhich the glance is directed with each direction of glanceunder actual wearing conditions, comprising the steps of— defining standard characteristics of 51 wearer, andnotably ametropia and power addition;— defining an environment in the form of a set ofobject points to be looked at, for the standard wearer;— calculating the direction of glance for a referenceobject point for near vision, using a thin lensapproximation, for a power calculated from said ametropiaand said power addition;— calculating accommodation from the direction ofglance for said reference object point for near vision andfrom the distance between pupils;— determining the wearer's Donders curve, fromaccommodation and convergence for said reference objectpoint for near vision;CA 02265705 l999-03- l5— determining a direction of glance for other objectpoints in an environment, using an iterative process, for athin lens approximation, based on said Donders curve.The step of determining the direction of glance forother object points in said environment can comprise, foreach one of said other points:— calculating a convergence without a lens;— calculating accommodation from the Donders curve;— calculating a power using a thin lens approximation;~ repeating, to convergence to one direction of glance,the steps consisting of:— determining deviations brought about by an thinlens of the calculated power;— determining a direction of glance making itpossible to compensate said deviations with saidthin lens of the calculated power;— calculating a convergence from the new directionof glance;— calculating a power, using a thin lensapproximation, from the new convergence and theDonders curve.Additionally, for each lens, a wearer power can beassociated with each direction of glance under wearingconditions, said wearer power being a last powercalculated, using a thin lens approximation, during saidsteps of repetition until convergence is reached.A method for defining a progressive ophthalmic lens byoptimizing the optical characteristics of an ophthalmiclens is also provided, said optical characteristics beingCalculated during optimization using a ray tracing program,under wearing conditions.These optical characteristics can be a wearer power andastigmatism aberration, under wearing conditions.CA 02265705 l999-03- l5Wearer power for an object point can be defined as thesum of degree of nearness of the image and the degree ofnearness of an object.In one embodiment, optimization consists of minimizing,by iterations, differences between optical characteristicsof the lens and target values, and in which values ofwearer power obtained according to the method of claim 14are used as target values for wearer power using, as targetvalues for astigmatism aberration, values for astigmatismfor a lens having a first known progressive surface with asurface power addition equal to a target wearer poweraddition for an ophthalmic lens to be defined, and a secondspherical surface such that power at a far vision referencepoint is zero.As a starting lens for optimization, a lens can beemployed having a first known progressive surface with asurface power addition equal to a target wearer poweraddition for an ophthalmic lens to be defined, and a secondspherical surface.Finally, optimization can consist in varying a variableaspherical layer that is added to said starting lensFurther advantages and characteristics of the inventionwill become more clear from a reading of the descriptionwhich follows of some embodiments of the invention providedby way of example, and with reference to the attacheddrawings in which:FIG. 1 showes an eye and lens optical system;FIG. 2 is a flow chart of the various steps in definingthe reference point for near ‘vision and calculating theDonders curve for the wearer.FIG. 3 shows both eyes diagrammatically and thedirection of glance.FIG. 4 illustrates Donders law graphically.CA 02265705 l999-03- l5l0FIG. 5 is a flow chart of the various steps incalculating ergorama outside the reference point for nearvision.FIG. 6 shows a typical pattern for an ergorama, indiopters.FIG. 7 is an example of a basic cut which can be usedfor providing lenses according to the invention.FIG. 8 shows variation in optical power along ameridian, for different power additions from 0.75 to 3.5diopters.FIG. 9 shows curves corresponding to those in FIG. 8,for a prior art lens.FIG. 10 is a similar view to that in FIG. 8, in whichhowever graphical indications of wearer optical power havebeen added for lenses corresponding to extreme opticalpower for each base.FIG. 11 shows the same graphical indications as FIG. 10for a prior art lens.FIG. 12 shows the results obtained. according to theinvention, in terms of variation with respect to ergorama.FIG. 13 shows results corresponding to those on FIG.12, for astigmatism aberration, for the same lens.FIGS. 14 to 16 show optical power, astigmatismaberration and optical power along the meridian for a knownlens.FIGS. 17 to 19 show optical power, astigmatismaberration and optical power along the meridian for a firstlens according to the invention.FIGS. 20 to 22 show optical power, astigmatismaberration and optical power along the meridian for asecond lens according to the invention.In a manner known per se, we define a mean sphere forevery point on the surface, given by the formula:D = n — I (_l + I )2 R1 R2 CA 02265705 l999-03- 15llwhere R1 et R2 are the maximunm and Ininimuni radii ofcurvature expressed in meters, and. n is the refractiveindex of the material constituting the lens.A cylinder D is also defined, given by the formula(f=(n—1).|_1 —_1|R1 R2The invention proposes defining the characteristics oflenses not only in terms of a mean sphere or cylinder, butrather to take the situation of the wearer of the spectacleFIG. llenses into consideration.shows, to illustratethis, an eye and lens optical system showndiagrammatically.In FIG. 1, the mean plane of the lens is inclined withrespect to the vertical by an angle which corresponds tothe usual inclination of spectacle frames. This angle isfor example 12°.We shall call Q‘ the center of rotation of the eye,and define a sphere of vertices centered on Q’ of radiusg‘, which is tangential to the rear face of the lens at apoint on the horizontal axis.As an example, a common value for radius q‘ is 27 mm,and this gives satisfying result when the spectacles areworn.A given direction of glance corresponds to a point J onthe sphere of vertices and can also be defined in space, inspherical coordinates, by two angles alpha and beta.Q‘Jwhile angle betaAnglealpha is the angle between straight line and thehorizontal plane passing through point Q’is the angle between the straight line Q‘J and the verticalplane passing through point Q‘.A given direction of glance corresponds to a point J onthe sphere of vertices or to a pair* of values (alpha,beta). In the object space, for a point M on thecorresponding light ray, the degree of nearness of anCA 02265705 l999-03- 1512object, PO, is defined as being the inverse of the distanceMJ between point M and point J on the sphere of vertices:PO 2 1/MJThis makes it possible to calculate the degree ofnearness of an object in the framework of a thin lensapproximation at every" point of the sphere of vertices,which. is used for determining’ the ergorama, as will beexplained below. For an actual lens, it is possible, usinga ray tracing program, to consider the degree of nearnessof an object as being the inverse of the distance betweenthe object point and the front surface of the lens, on thecorresponding ray. This is described in more detail belowin the description of the optimization process.Still for the same direction of glance (alpha, beta),the image of the point M having a given degree of nearnessis formed between two points S and T corresponding,respectively, to a minimum and maximum focal length (whichwould be the sagital and. tangential focal length. in thecase of surfaces of revolution). The quantity:is called the degree of nearness of the image of pointBy analogy with the case of the thin lens, one thusdefines, in a given direction of glance and for a givendegree of nearness of an object, i.e. for a point in objectspace on the corresponding light ray, an optical power asbeing the sum of of the degree of nearness of the image andthe the degree of nearness of an object.Using the same notation, in each direction of glanceand for a given the degree of nearness of an object, anastigmatism aberration (aberration due to astigmatism) AA.is defined as beingCA 02265705 l999-03- 1513This astigmatism aberration corresponds to theastigmatisn1 of the light beam created. by the asphericalfront surface and the rear spherical surface.Thus, two possible definitions of optical power andastigmatism aberration of the lens are obtained accordingto the invention, under wearing conditions. Otherdefinitions could also be used, but the above have theadvantage of being defined simply, and. of being" readilycalculated using a ray tracing program for a given lens.Additionally, according to the invention, an ergoramais defined which gives, for each direction of glance, thedegree of nearness of an object and a wearer power. Theergorama is defined for a given situation of the wearer, inother words for an ametropia~power addition pair.The ergorama is thus a function that maps fourvariables: ametropia, power addition and direction ofglance in the form of angles alpha and beta, to two values:the degree of nearness of an object and a wearer power.The definedergonomicergorama thus can be determined byphysiological, and postural tests, and byknowledge of the laws of optics. One can notably consider:deviation thethe— prismatic introduced by powerencountered on lens determined by Prentice's rule(prism=H * Power). Prismatic deviations modify the positionof the eyes and of the head in different ways depending onametropia;~ subjective accommodation used as a function of theprescribed power addition, of ametropia and of the degreeof nearness of an object. This accommodation is calculatedusing the improved Donder's law that relate convergence (orapparent nearness of the object) and accommodation therebyensuring normal binoculars vision; for more details of thislaw, reference can be made to FIG. 4 described below;CA 02265705 l999-03- 1514— a reduction in visual acuity as a function of agewhich can be reflected by a moving nearer of near visiondistance when the power addition increases;— posturalpreferences of wearers in agivenenvironment which determine the position of the head and ofthe eyes for near vision work and the visual—motor strategyemployed to describe this environment.shallforBy way of describetheexample, we one way ofdetermining ergorama, any given ametropia, forexample by a power at point L for near vision, and for agiven power addition.According to the invention, one can proceed in thefollowing manner: firstly, the direction of glance and thedeterminedtheThis,power are for viewing the near vision point.the half—lineassociated with a scanning strategy makesFrom this, slope of Donders can beobtained.it possible to determine power for other directions ofglance.We shall now explain in more detail this way ofdetermining an ergorama, with reference to FIGS. 2 to 5,for a left—hand lens. FIG. 2 is a flow chart showing thevarious steps in defining the near vision control point andfor calculating the Donders curve for the wearer.As explained above, we shall start out by determiningthe direction of glance and the power in order to view thenear vision point.we choose the characteristics of thein FIG. 2.To achieve this,standard wearer, step 10 One can for exampleconsider that the standard wearer has the followingbotheyes accommodate by the same amount and move by the samecharacteristics: he is isomemetropic and orthophoric,amount in case of version and in a symmetrical manner inthe case of vergenc; the distance between the pupils is 65mm, the distance between the lens and the center of eyeballrotation is 27 mm, the distance between the center ofCA 02265705 l999-03- 1515rotation of the eyeball and the center of rotation of thehead. is 100 mm., and the pantoscopic angle is 12°. Thecenter of eyeball rotation is the point marked Q'on FIG. 1.Head inclination is given by the position of theFrancfort plane with respect to the horizontal, asexplained in French Patent applications 2,683,642 and2,683,643 is the name of the present applicant.For near vision work,is 33°the standard lowering of the eyesand lowering of the head is 35° so that the workplane can be parallel to the mean vertical horopter.Next, FIG.2, in order to position there the standard wearers chosena working environment is chosen, step 20,in step 10. One can for example choose a place of work inan office described by a document of known format (A4 forexample) placed on a horizontal work surface also of knowndimensions. The central point M situated two—thirds of theway up from the bottom of this documenttheis a place wherefall willconstitute a first point of reference for near vision.Thiswearer naturally lets his glance andpoint M istheplaced. at the near vision distancetotalthe sum of 33°given by power addition and for a standardinclination of the glance of 68°, i.e. and35°, with respect to the horizontal.We have thus positioned the standard wearer in a givenenvironment. This positioning only depends on thecharacteristics of the wearer and notably ametropia andpower addition.At step 30 in FIG. 2, a lens is introduced and thevariations in the direction of glance brought about by thepresence of this lens are calculated. The correspondingcalculations are made using a thin lens approximation, atall points on the sphere of vertices. In other words, atevery’ point on the sphere of vertices, an infinitesimalthin lens is considered the axis of which passes throughthe optical center of rotation of the eye.CA 02265705 l999-03- l5l6FIG. 3 shows diagrammatically the right eye OD and theleft eye OG and, in dotted lines, the direction of glancefor looking at the near vision reference point M. It isknown that the power addition of thewearer, in thedirection corresponding to the glance for point M, is equalto the sum of far vision power power and power addition.when a thin lens having a corresponding power is introducedinto the path,shown in dotted lines, of the glance on thesphere of vertices, the rays undergo deviation as shown bythe solid line on FIG. 3, and so that the point on whichthe glance falls is no longer point M but rather point M‘.FIG.3 is a plan view: it is however clear that deviationis brought about not only in a horizontal plane, but alsoin a vertical plane.The inclination determined at step 20 is thus modifiedby introducing glass into the optical path, as a result ofprismatic deviations induced by the power encountered atthe lens which should be equal to the power VL prescribed,to which the prescribed power addition is added.At step 40 in FIG. 2, eye movement and possibly headmovement, making it possible to correct these prismaticdeviations, are determined. For example, it can beconsidered that vertical prismatic deviation is compensatedfor by a vertical movement of the eyes and by movements ofthe head in order to fix the glance on this point.thatThe parteach plays in this compensation depends on thewearer's ametropia. For powers VL less than -2 diopters, itis considered that compensation is totally provided for bythe eyes. For an ametropia of +2 diopters and above, thepart that the head plays in compensation is total, in otherwords the eyes do not move. For powers VL between -2diopters and +2 diopters, it is considered that the partthe head plays in compensation increases linearly: in otherwords, for a powers VL of -1 diopter, vertical prismaticCA 02265705 l999-03- 1517deviation is compensated for to an extent of 75% by eyemovement and 25% by head movement.We consider that horizontal prismatic deviation isfully compensated for by eye movement bringing about amodification to convergence.The calculations are again performed with the thin lensapproximation at every point on the sphere of vertices, asexplained with reference to step 30.At the end of step 40, the eye movements and, possibly,head movements making it possible to correct theseprismatic deviations have been determined and,consequently, the direction of glance for viewing the nearvision reference point.Thanks to this, those points on the lenses where powershould be equal to the prescribed power V L supplemented bythe prescribed additions are known. Also, the exactpositioning of the wearer is known through the eye and headmovements needed for the compensation. We have alsodetermined the positions of the centers of rotation of theeyes and of the head with respect to the document and thusto the working surface.At step 50 in FIG. 2, the wearer‘s subjectiveaccommodation. is calculated from the position thusdetermined. In fact, the power at the lens, the position ofthe lens in front of the eye and the object distance forviewing the near vision reference point are known.Subjective accommodation. is deduced. with. the help ofthe formula:wearer power 2 degree of nearness of an object ~accommodationwhich corresponds to the thin lens approximationperformed at every point on the sphere of vertices.At step 60 in FIG. 2, knowing this accommodation, andthe direction of glance, the Donders law applicable to theCA 02265705 l999-03- 1518wearer is determined. This law supplies, as a function ofage, a relation between convergence and accommodation.FIG. 4 shows Donders law graphically. The X-axis showsconvergence in m‘l, and the y—axis shows accommodation indiopters. The dashed-line curve shows the relation betweenthese two values for a younger spectacle wearers (age 25).The solid line curve and dash—dot curves respectively showthe relation between these two values for spectacle wearersaged 41 and SO.Knowing, for the near vision reference point,accommodation and convergence, the slope of the linearportion of the Donders curve is calculated.The limit of the horizontal portion of the Donderscurve is given by the wearer's maximum accommodation whichdepends on age. Age is linked to power‘ addition. on thebasis of clinical studies.One hence knows, following step 60 in FIG. 2, thestraight—line Donders curve which will now allow us torelate subjective accommodation to convergence, for theselected wearer.This enables a complete definition of the wearer and ofhis position in his environment to be obtained. Next, forevery direction of the eyes and thus for every point on thelens, an associated power and the degree of nearness of anobject are obtained, by scanniing over the wearer'senvironment, as explained with reference to FIG. 5.For this, a strategy is set for scanning theenvironment and for the rules of compensation of theprismatic deviations by the head and eye movements. Inorder to scan a document, one can say in general terms thatthe wearer only moves his head in order to compensate forthe vertical prismatic deviation according to the rule setout above. The major part of the document scanning is thusachieved by eye movements.CA 02265705 l999-03- l519Above the document, the head and the eyes movesimultaneously in order to reach a final position so thateye inclination is zero when head inclination is zero.Additionally, beyond the working surface, objectdistance is linearly interpolated at the vertical positionof the eyes in orbits between the distance of the edge ofthe work surface and infinity, which is the object distancefor far vision (zero head and eye inclination).A strategy for scanning the environment is thusdefined, in other words a set of points that are viewed inthe environment, and associated eye and head positions.For each one of these points, knowing the degree ofnearness of an object, the direction of glance and thenecessary power are determined, as will now be explainedwith reference to FIG. 5.At step 100, a point in the environment is taken.Advantageously, the environment is described in terms ofangular coordinates the origin of which is the center ofrotation of the eye for the lens for which the calculationsare being performed, and the scanning is done in anincremental fashion one degree at a time, starting out fromthe lowest possible position (800) in the sagittal plane.At step 110, for this point in the environment,convergence is calculated. in the absence of a lens: ineffect, the distance between the point and the center ofrotation of the eyes as well as the distance between thewearer's pupils are known.At step 120, knowing this convergence and the wearer'sDonders curve, an accommodation is determined and thenecessary power at the lens is calculated. In fact, theDonders curve gives accommodation as a function ofconvergence; power is calculated using the thin lensapproximation as explained above with reference to step 50of FIG. 2.CA 02265705 l999-03- 1520Step 130 in FIG.in FIG. 2.5 corresponds nmtatis nmtandis tostep 30 At step 130, the introduction of thelens having the power determined at step 120 leads toprismatic deviation which requires, in order to compensateeye and head movement, a modification in the distance ofthe point viewed and of convergence. Just like in step 30,the calculations are performed using the thin lensapproximation at every point on the sphere of vertices.At step 140, the new accommodation and the new power,brought about by these modifications, are calculated.Next, there is a return to step 130, using‘ the newpower calculated. By means of repeated iterations, in otherwords by repeating steps 130 and 140, errors in directionof glanceare minimised and the final result is a power for whichthe system is stable. In practice, the calculationsconverge generally after 10 to 15 iterations.For this power, there is a corresponding head positionand eye position in the orbits which gives the place on thelens where this power should be situated in order to viewthe point in the environment selected at step 100.theWe have thus determined, for a direction of glance,degree of nearness of an object and a lens power, making itpossible to view a given point in the environment.At step once 150, we proceed to the next point in theenvironment following the scanning strategy explainedabove, before returning to step 110.In this way, at the end of scanning, a table of valuesis obtained for the right eye and a table of values for theforleft eye containing, each angular position of the eyein the orbit and thus for each point on a lens, a power andan associated object distance.in aWe have thus calculated, for the standard wearer,given. environment, and for any given ametrpia and poweraddition, power and nearness for each direction of glance.LNCA 02265705 l999-03- 1521In this way, the ergorama can be determined. Forexample, the calculations described above are performed forpower‘ addition values varying in 0.25 per steps between0.50 and. 3.5 diopters, and for far vision power valuesvarying in 0.50 steps between -12 and +12 diopters.Thus, the ergorama can be determined for varyingametropias and power additions. To sum up, one proceeds asfollows:— the standard characteristics of a wearer are defined,notably ametropia and power addition;— an environment, in other words a set of points of tobe Viewed, is defined;— the direction of glance for the near vision point iscalculated, using the thin lens approximation at everypoint on the sphere of vertices, for a power deduced fromametropia and power addition;— from this, the wearer's Donders curve is deduced,relating accommodation and convergence;— the direction of glance and power for other points inthe environment are determined. by an iterative process,using" the thin lens approximation. at every’ point on thesphere of vertices, using the Donders curves.This definition of the ergorama additionally makes itpossible to define, on a lens, a main meridian ofprogression from a set of directions of viewing. The mainmeridian of progression is advantageously defined from theergorama and corresponds, for a given ametropia and poweraddition, to the set of directions of glance correspondingto points in the environment situated in the sagittalplane.One can obviously’ use other definitions of the mainmeridian of progression. 4FIG. 6 shows the typical form of an ergorama along themeridian, M1 diopters, as ea function cflf the angle alphabetween the direction of glance and the horizontal planeCA 02265705 l999-03- 1522passing through the point Q’, for an ametropiacorresponding to zero power in far vision, and a poweraddition of 2.00 diopters. On FIG. 6, only variations inthe ergorama as a function of the angle alpha have beenshown. In fact, as a first approximation, one canreasonably consider that the ergorama is only a function ofangle alpha and that it only varies slightly as a functionof angle beta. Typically, the ergorama is zero at the farvision control point and has a value of the order of 2.5 to3.5 diopters at the near vision control point.The invention proposes that one considers, foroptimizing the aspherical face of a lens, values of opticalpower‘ and. astigmatism aberration rather than values ofmean sphere and cylinder. Taking these optical values intoaccount rather than surface values provides for 51 betterdefinition of the aspherical face of lenses and betterpreservation of the optical properties of the lenses, for aconstant power addition, with differing powers.FIG. 7 shows an example of the base cut which can beemployed for providing lenses according to the invention,FIG. 7 shows, on the y—aXis, the value of the bases and ontthe x—axis, the corresponding optical power values at thereference point. As shown on FIG. 7, one can use, fordefining lenses, base values of 2, 3.5, 4.3, 5.3 and 6.2diopters. For a value of optical power, in other words fora value on the x—axis of FIG. 7, the base value shown onFIG. 7 is taken as the appropriate value. This choice offive basic values hence makes it possible to cover allwearer optical powers between -6 and +6 diopters.The invention proposes defining lenses using a programfor optimizing optical parameters of lenses, with thefollowing characteristics.A merit function to be used for optimization isselected. by choosing a target and weighting the variousregions of the target.CA 02265705 l999-03- 1523The target is described, for a given ametropia, by achoice of the far vision power, and for a given poweraddition.Regarding power, the wearer power given by theergorama, for the selected ametropia and power addition, isconsidered as a target.Regarding astigmatism aberration, the results suppliedby performing measurements on a prior art lens can be usedas a target, such as for example,thethe lenses marketed bythe applicant under "Comfort" trademark. Moreprecisely, for the given ametropia and power addition, weconsider a known lens having the same power addition, andzero power at the far vision control point. This gives us aknown lens having the selected power addition andametropia.Using the ray~tracing program, measurements are made onthis lens of astigmatism aberration as defined above, underwearing conditions and, starting out from the nearnessvalues given by the ergorama. We can consider the wearingconditions defined in FIG. 1. We thus obtain, in eachdirection of glance, given by a value pair (alpha, beta) avalue for astigmatism aberration.These values can be nmdified, for further increasingthe performance, by decreasing the values for astigmatismaberration obtained in the lateral regions, for wideningthe far vision region and the near vision region.We then consider a target lens having the followingoptical characteristics:— along the meridian, defined by the ergorama, a powergiven by the ergorama and zero astigmatism aberration;— away from the meridian, a power given by the ergoramaand an astigmatism aberration measured on the correspondinglens of the prior art, modified if necessary.CA 02265705 l999-03- 1524We thus have, for each direction of glance, a valuefor wearer power and astigmatism aberration, provided bythe target lens.The aim of the optimization program, starting out froma lens to be optimized, is to come as near as possible tothe target lens. For this, we can consider a merit functionwhich represents the deviations between the lens to beoptimized and the target lens, defined as follows. For aset of points on the lens, or on the sphere of vertices,or, yet again, of the directions of glance, identified by aconsider the merit function in the form:where pi is a weighting of the point i;variable i, weVij is the value of the j—th type of parameter at thepoint i;Cij is the target value of the j—th type of parameterat the point i;wij is the weighting of the j—th type of parameter atthe point i.One can for example achieve suitable results by taking(70a set of 700 points distributed. along the meridianpoints) and over the remainder of the lens, with a higherconcentration around the meridian.The value j can be set to 2 and we can use parameterswhichdefined above.are wearer power and astigmatism aberration, asThe weighting pi of points i makes it possible to applya higher or lower weighting to various regions of the lens.It is preferable to apply a higher weighting at themeridian and to decrease the weighting as one moves awayfrom the meridian.The value Vij is measured for point i by a ray tracingprogram, using the definitions of wearer power and ofastigmatism aberration given above, starting from the valueof degree of nearness supplied by the ergorama. Vii is theCA 02265705 l999-03- 1525value of wearer power measured at the point i and Vi2 isthe value of astigmatism aberration measured at point i.More precisely, we can proceed in the following manner.In the alpha, beta direction. of point i, using" the raytracing program, we construct the ray departing from thecenter of rotation of the eye and which enters through therear face of the lens, passes through the lens and. outthrough the forward face, terminating in the object space.Next, the object point located on the ray thus traced at adistance from the front face of the lens equal to theinverse of the the degree of nearness of an object given bythe ergorama for the alpha, beta direction is considered.Starting from of this object point, a plurality of rays,for example three are traced towards the lens, in order toreconstruct the points J and T in FIG. 1; it this way, anexact evaluation of the image obtained for a given objectpoint is achieved. In this way, degree of nearness of theimage and astigmatism aberration Vi2 are calculated. Usingthe ergorama and calculated degree of nearness of theimage, the wearer power Vil in the direction alpha, beta isdetermined.The values Cij are target values: in the example, Cilis the wearer power value and Ci2 is the value ofastigmatism aberration at the point i. wij is theweighting of the j-th type of parameter at point i. One canthus privilege, for a given point, wearer power orastigmatism.In this way we define a target and a nerit functionthe deviations of therepresentative of opticalcharacteristics of a lens compared to this target. Such amerit function is clearly positive and should be minimisedduring the optimization process.To proceed with optimization, all one needs to do is toselect a starting lens and a method of calculation makingCA 02265705 l999-03- 1526it possible to decrease, by iterations, the value of themerit function.For the method of calculation, a damped least squaresmethod can be used or any other optimization method knownper se.For a given ametropia and power addition, we can takeas the starting lens, aof thelens from the prior art having anaspherical face and a basesame power addition,value at the far vision control point equal to that givenby the curve in FIG. 7; we associate with this asphericalface, a spherical face that makes it possible to obtain thedesired ametropia,Thus,for‘ a given thickness at the center.for a given power, the lens of the invention has aflatter front surface than a conventional prior art lens.In order to proceed with optimization, one canadvantageously start out from this starting lens, adding alayer to be optimized to a spherical surface and then onlymodifying this layer in the optimization process. Forexample, a Zernike polynomial model can be used for thislayer; this makes it possible to facilitate calculation inthe ray tracing, the Zernike polynomial being rewritten interms of altitude at the end of the optimization process,thereby providing a map of the altitudes of points on theaspherical face.One thus obtains,for the given ametropia and for agiven power addition, an optimized lens, after iterationsof the optimization program.method,Using a damped least squaresthe merit function defined above and such astarting lens, some tens of iterations are sufficient toachieve, in the majority of cases, a lens having excellentperformance.To avoid the need to proceed with optimization for eacheach ametropia—power addition pair of values, one maychoose to only" carry out optimization for central powervalues at each horizontal step of the carve in FIG. 7.}OLflCA 02265705 1999-03-1527Then, for neighbouring powers, an aspherical layercalculated in the optimization program is simply added tothe starting lens. A posteriori, a check is carried out toensure that the optical performance obtained is correct, inother words near to the corresponding target. Thus,in theexample of the base cut in FIG. 7 , optimization is onlydone for all wearer optical powers between -2.5 and Odiopters, which correspond to a base value of 3.50diopters.The invention makes it possible to obtain practicallyidentical results regardless of the wearer optical power,for a given power addition.The FIGS. below show examples of lenses according tothe invention. and known lenses. In the description. thatfollows, we shall use the following definition of the farvision region, intermediate vision region, and near visionregion: these regions are defined as all of the directionsof glance or all corresponding points on the lens for whichastigmatic aberration is below 0.50 diopters, The term iso-astigmatism line refers to lines consistingwhichof points forastigmatism aberration has a constant value. Theviewing area in the far vision region is then the surfacein the in otherswept by the glance far‘ vision. region,words between 0.5 diopter iso—astigmatism lines, the edgeof the lens and above the geometric center thereof.The width of the near vision field is then the angularwidth. at near ‘vision control point height on the lens,between 0.5 diopter iso—astigmatism lines.FIG. 8 shows variation in optical power along themeridian, for different power additions, from 0.75 to 3.5diopters. The x-axis on FIG. 8 shows elevation of theglance, or the angle alpha, in degrees. The y—axis showsvariation in optical power compared to optical power at thereference point: this variation is zero at the referencepoint, located on the front face of the lens 8 mm above theCA 02265705 l999-03- 1528geometric center, equivalent to an angle alpha of around8°. For each power addition, FIG. 8 shows the variousoptimization optical powers, which are those defined above(-4.5, ~1.5, 1.3 and 4.75 diopters).It will be noted that for a given power addition,optical power along the meridian is practically identicalregardless of the power at the far vision reference point,in other words the invention makes it possible to provide"optical single-design" in other words optical performancefor‘ the wearer‘ in the intermediate space, i.e. in thelens—eye space, which is independent of the far visionpower. From FIG. 8, the target values of optical poweralong the meridian, which are practically achieved, can bereadily distinguished.FIG. 9 shows, by" way of comparison, a correspondinggraphical representation of a prior art lens. On FIG. 9, itwill be seen that there is a wider spread in the values ofoptical power, for a given power addition, as a function ofvarious far vision powers. How the invention improvesperformance of known lenses is well illustrated here.FIG.10 is a view similar to that in FIG. 8, in whichhowever wearer optical powers have been added for lensescorresponding to the limiting optical powers for each base,for exampleIt will be noted in FIG.-2.25 and O diopters for the 3.5 diopter base.10 that the variations in opticalpower along the meridian still substantially correspond tothe inventionthe target values in FIG. 8. In other words,still yields comparable optical performance even when therear faces employed for optimization are replaced bydifferent rear faces.FIG. 11 shows, by way of comparison, correspondinggraphical representations for a prior art lens. On FIG.it will be noted that therell,is a far greater spread ofoptical power‘ values, for a given power addition, as afunction of the various far vision powers.H0CA 02265705 1999-03-1529Corresponding results are obtained for astigmatismaberration. One obtains notably, according to theinvention, an astigmatism aberration which is below 0.2diopters on the meridian, regardless of the far visionpowers and the power addition,FIG. 12 theand for all optical powers.shows results obtained. according to theinvention, in terms of variation with respect to theergorama. The various curves in FIG.12 show variation inoptical power along the meridian when one departs from theergorama by a value comprised between +2.00 diopters and —2.00 diopters,FIG.in 0.25 diopter steps. The bottom curve on12 corresponds to a difference of +2.00 diopters, andthe topmost curve corresponds to a difference a of -2.00diopters. Variations with respect to the the target opticalpower are shown on the y—axis, the x—aXis showing elevationthe FIG. 12corresponding to a lens according to the invention having aof glance (angle alpha), curves inpower addition of +3.50, and an optical power of 5 dioptersat the reference point (From FIG.6.2 diopter base).12 it can be seen that variation in opticalpower remains below 0.125 diopters when deviation withrespect to the ergorama is between 0 and 0.5 diopters inthe far vision region (angle alpha between -300 and 0°).In the near vision region20° and 40°),(angle alpha comprised betweenvariation. in optical power remains below0.125 diopters, where the absolute value of the deviationfrom the ergorama istheless than 1 diopter.For same lens, FIG. 13 gives the correspondingresults for astigmatism aberration. Like FIG. 12,variations in the astigmatism aberration remain low, evenwhen there is a deviation from the ergorama used in theinvention. In far vision, variations in astigmatismaberration stay below 0.125 diopters for deviations fromthe ergorama ranging from 0 to 0.50 diopters. In nearvision, departures from the ergorama may reach 1. diopterCA 02265705 l999-03- 1530without astigmatism aberration varying by more than 0.125diopters.Similar results are found in terms of the width offield at the near vision measuring point: the field widthdoes not vary by more than 15% from a nominal value whenthe deviation from the ergorama is less than 1 diopter.In far vision, the viewing area (the area within the0.5 diopter iso—astigmatisn1 line) does not vary" by xnorethan 15% when deviation from the ergorama is below 1diopter.In other words, and with respect to the target ergoramaused in the definition of optical power, deviations arepossible, while still ensuring optical power andastigmatism aberration, near vision field width or farvision viewing area only suffer slight variations. Even ifthe spectacle wearer does not have an ergoramacorresponding" to the one used in the invention, resultsfrom the lens of the invention remain satisfactory:comparable optical performances are provided for alloptical power base values, for a given addition, whilepreserving simplicity of machining of the rear lens face.FIGS. 14 to 22 show results obtained using theinvention, compared with the results from prior art lenses.On FIGS. 14 to 22, a line for optical power level (orastigmatism aberration), i.e. lines formed from pointshaving identical optical power (or identical astigmatismaberration) are shown for a prior art lens and for lensesaccording to the invention. The lines are shown for valuesof optical power (or astigmatism aberration) increasing in0.25 diopter steps; only" integer values or half—integervalues for optical power (or astigmatism aberration) areindicated on these drawings, intermediate values (0.25,0.75, 1 25, etc) not being indicated.CA 02265705 l999-03- 1531The lenses are shown in a spherical coordinatereference frame, the x—aXis being angle beta and the y—axisangle alpha.On FIGS. 16, 19 and 22, optical power along themeridian is shown in solid lines; also, the dashed linesshow minimum and maximum optical power, correspondingrespectively to:~ the sum of the ergorama and 1/JS;— the sum of the ergorama and 1/JT.On FIGS. 16, 19 and 22, the y—axis is the angle alphain degrees,FIGS.and the x—axis shows optical power in diopters.14 to 16 show a 70 mm diameter prior art lens;the front of this lens is a progressive multifocal surfaceof base 2 diopters,and 2 diopter power addition. The rearface is chosen so as to have a far vision optical power of-4.5 diopters,the lens having a prism of 1.36°. The planeof the lens is inclined with respect to the vertical by120, and it is 1.2 mm thick at the center. We haveconsidered a value for q'of 27 mm as mentioned withreference to a FIG. 1.FIG. 14 shows lines of equal optical power and FIG. 15lines of equal astigmatism aberration.FIG. 16shows optical power‘ and Ininimuwm and. maximumoptical power along the optical meridian. At the far visionmeasurement point, as indicated, optical power is -4.5diopters. Astigmatism aberration is 0.26 diopters. At thenear vision measurement point, optical power is -2.10diopters. Astigmatism aberration is 0.19 diopters. Actualoptical power addition is thus 2.4 diopters.FIGS. 17 to 19 show the same corresponding views for afirst lens according to the invention, of diameter 70 mm;the front face of this lens is a pmogressive multifocalsurface of 2.0 diopter‘ base and power‘ addition. 2Ø Therear‘ face is chosen. so as to have a far vision opticalpower of -4.5 diopters, and the lens has a prism of 1.36°.CA 02265705 l999-03- 1532The plane of the lens is inclined with respect to thehorizontal by 12°, its thickness at the center being 1.2mm. We have considered the value for q'of 27 mm asmentioned with reference to FIG. 1.FIG. 17 shows lines of equal optical power, and FIG. 18lines of equal astigmatism aberration.FIG. 19 shows optical power‘ and minimum and maximumoptical power along the optical meridian. At the far visionmeasurement point, as indicated, optical power is -4.50diopters. Astigmatism aberration is a 0.02 diopters. At thenear vision measurement point, optical power is -2.50diopters. Astigmatism aberration is 0.01 diopters. Actualoptical power addition is thus 2.00 diopters.FIGS. 20 to 22 show corresponding views for a secondlens according to the invention, identical to the first buthowever with a base of 5.3 diopters, a power addition of 2diopters and an optical power of three diopters, andthickness at the center of 4.7 mm.FIG. 20 shows lines of equal optical power and FIG. 21lines of equal astigmatism aberration.FIG. 22 shows optical power, and maximum and minimumoptical power along the optical meridian. At the far visionmeasurement point, as indicated, optical power is 3diopters. Astigmatism aberration is 0.02 diopters. At thenear vision measurement point, optical power is 5.06diopters. Astigmatism aberration is 0.01 diopters. Actualoptical power addition is thus 2.06 diopters.A comparison of these figures clearly shows theadvantages of the inven:ion.Firstly, when compared to the prior art, the inventionInakes it possible to take good. account of the differentrear faces, and to obtain satisfactory" results for thewearer not in terms of mean sphere and cylinder, but ratherin terms of optical power and astigmatism aberration.Notably, a sharp drop in astigmatism aberration along theCA 02265705 l999-03- 1533meridian will be noticed, the curves for optical power,minimum optical power and maximum optical power practicallycoinciding in the lenses of the invention. More precisely,for the two lenses of FIGS. 17 to 22 like for the otherlenses of the inventicwn it is ensured. that astigmatismaberration remains below 0.2 diopters along the meridian.Further, the invention makes it possible to ensure, fora given power addition, practically comparableperformances: FIGS. 18 and 21 have similar shapes andensure the presence of far vision and near vision regionswhich are substantially identical, and which are moreextensive than those in the lens of FIG. 15.Near vision field width, in the two lenses of theinvention, is respectively 24° and 26°. In the prior artlens, ii: is only 18°. Field with gm; higher 111 the twolenses of FIGS. 17 to 22 just like in the other lenses ofthe invention at 21/A +10 degrees, where A is the poweraddition.Qualitatively, the invention provides a set of lensesin which optical performance of the various lenses issubstantially identical for a given power addition,independently’ of optical power‘ at the far vision regionmeasurement point: this corresponds to an "optical single-design”.More precisely, according to the invention, the farvision viewing area, defined above, shows a variation ofless than 15% for a given power addition, regardless of thevalue of optical power at the far vision region measurementpoint.According to the invention, the near vision fieldwidth, also defined above, shows a variation of less than15% for any given power addition, regardless of opticalpower at the far vision region measurement point.Obviously, it is possible to reverse the front face andthe rear face, i.e. to arrange for the multifocalCA 02265705 l999-03- 1534aspherical surface of the lens to be directed towards thewearer‘ without this modifying the invention in any way.Also, the optimization method and the starting surface canbe changed or, yet again, other definitions of opticalpower and astigmatism aberration may be employed.

Claims (20)

WHAT IS CLAIMED IS:

1.- A set of progressive multifocal ophthalmic lenses determined by means of ergoramas that associate to each direction of glance a degree of nearness of an object and a wearer power, for a given ametroptia and a given power addition of a standard wearer, in which, for a lens under the conditions in which it is worn, the wearer power is defined in a direction of glance and for an object point, as the sum of the degree of nearness of an object and the degree of nearness of the image of said object point, in which each one of said lenses has:
- a first and a second surface, said first surface being a progressive multifocal surface;
- a far vision region, a near vision region and a main meridian of progression passing through said two regions, said far vision region, near vision region and meridian being sets of directions of glance under the wearing conditions;
- a power addition A equal to a variation in wearer power for the point towards which the glance is directed in the ergorama, between a reference direction of glance in the far vision region and a reference direction of glance in the near vision region;
- and in which variations in wearer power along said meridian, for the said point towards which the glance is directed in the ergorama are substantially identical for each one of the lenses of a set having the same power addition.

2.- The set of lenses according to claim 1 in which said lenses each have a prescribed power addition selected within a discrete set, a difference in power addition A between two lenses of said set having the same prescribed power addition being less than or equal to 0.125 diopters.

3.- The set of lenses according to claim 1 or 2 in which, with astigmatism aberration in a direction of glance, under wearing conditions, being defined for an object point, - for each lens, along said meridian, astigmatism aberration for a point towards which the glance is directed in the ergorama is less than or equal to 0.2 diopters.

4.- The set of lenses according to one of claims 1 to 3, in which, with astigmatism aberration in a direction of glance, under wearing conditions, being defined for an object point, - for each one of said lenses under wearing conditions, angular width in degrees between lines for which astigmatism aberration for points on the ergorama is 0.5 diopters, at 25°
below a mounting cross on said lens, has a value greater than 15/A
+ 1, A being the power addition.

5.- The set of lenses according to one of claims 1 to 4 in which, with astigmatism aberration in a direction of glance under wearing conditions being defined for an object point, - for each one of said lenses under wearing conditions, an angular width in degrees between lines for which astigmatism aberration for points on said ergorama is 0.5 diopters, at 35°
below a lens mounting cross, has a value greater than 21/A +10, A
being the power addition.

6. - The set of lenses according to one of claims 1 to 5, in which, with astigmatism aberration in a direction of glance under wearing conditions being defined for an object point, for each one of said lenses under wearing conditions, a solid angle bounded by lines for which astigmatism aberration for points in said ergorama equals 0.5 diopter, and points situated at an angle of 45° with respect to a mounting cross on said lens has a value greater than 0.70 steradians.

7.- The set of lenses according to one of claims 1 to 6 in which, for each one of said lenses under their wearing conditions, wearer power difference in the far vision region, in each direction of glance, between a point towards which the glance is directed in said ergorama and object points the degree of nearness of which differs from the degree of nearness of said point towards which the glance is directed by between 0 and 0.5 diopters, is less than or equal to 0.125 diopters as an absolute value.

8.- The set of lenses according to one of claims 1 to 7, in which, for each one of said lenses under their wearing conditions, wearer power difference in the near vision region, in each direction of glance, between a point towards which the glance is directed in said ergorama and object points the degree of nearness of which differs from the degree of nearness of said point towards which the glance is directed by an absolute value of less than 1 diopter, is less than or equal to 0.125 diopters as an absolute value.

9.- The set of lenses according to one of claims 1 to 8, in which, with astigmatism aberration and direction of glance under wearing conditions being defined for an object point, for each one of said lenses under wearing conditions, a difference in astigmatism aberration in the far vision region, in each direction of glance, between a point towards which the glance is directed in said ergorama and object points the degree of nearness of which differs from the degree of nearness of said point towards which the glance is directed by between 0 and 0.5 diopters, is less than or equal to 0.125 diopters as an absolute value.

10.- The set of lenses according to one of claims 1 to 9 in which, with astigmatism aberration and direction of glance under wearing conditions being defined for an object point, for each one of said lenses under wearing conditions, a difference in astigmatism aberration in the near vision region, in each direction of glance, between a point towards which the glance is directed in said ergorama and object points the degree of nearness of which differs from the degree of nearness of said point towards which the glance is directed by an absolute value of less than 1 diopter, is less than or equal to 0.125 diopters as an absolute value.

11.- A method for determining an ergorama for a set of progressive multifocal ophthalmic lenses, said ergorama associating for each lens and for each direction of glance a degree of nearness of an object and a wearer power, for a given ametroptia and a given power addition of a standard wearer, comprising the steps of - defining standard characteristics of the wearer, and notably the ametropia and the power addition;
- defining an environment in the form of a set of object points to be looked at, for the standard wearer;
- calculating the direction of glance for a reference object point for near vision, using a thin lens approximation, for a power calculated from said ametropia and said power addition;
- calculating accommodation from the direction of glance for said reference object point for near vision and from the distance between pupils;
- determining the wearer's Donders curve, from accommodation and convergence for said reference object point for near vision;
- determining a direction of glance for other object points in an environment, using an iterative process, for a thin lens approximation, based on said Donders curve.

12.- The method according to claim 11, in which said step of determining the direction of glance for other object points in said environment comprises, for each one of said other points:
- calculating a convergence without a lens;
- calculating accommodation from the Donders curve;
- calculating a power using a thin lens approximation;
- repeating, to convergence to one direction of glance, the steps consisting of:
- determining deviations brought about by a thin lens of the calculated power;
- determining a direction of glance making it possible to compensate said deviations with said thin lens of the calculated power;
- calculating a convergence from the new direction of glance;
- calculating a power, using a thin lens approximation, from the new convergence and the Donders curve.

13.- The method according to claim 12, in which additionally, for each lens, a wearer power is associated with each direction of glance under wearing conditions, and in which said wearer power is a last power calculated, using a thin lens approximation, during said steps of repetition until convergence is reached.

14.- A method for defining a progressive ophthalmic lens by optimizing the optical characteristics of an ophthalmic lens, said optical characteristics being calculated during optimization using a ray tracing program, under wearing conditions for a standard lens wearer, in which optimization consists of minimizing, by iterations, differences between optical characteristics of the lens and target values, and in which values of wearer power obtained according to the method of claim 13 are used as target values for wearer power.

15.- The method according to claim 14, in which said optical characteristics are a wearer power and astigmatism aberration, under wearing conditions.

16.- The method according to claim 15, in which wearer power is defined for an object point as the sum of degree of nearness of the image and the degree of nearness of an object.

17.- The method according to claim 15 or 16, in which optimization consists of minimizing , by iterations, differences between optical characteristics of the lens and target values, and in which values of wearer power obtained according to the method of claim 14 are used as target values for wearer power.

18.- The method according to claim 15, 16 or 17, in which optimization consists in minimizing , by iterations, differences between optical characteristics of a lens and target values, and in which use is made, as target values for astigmatism aberration, of values for astigmatism for a lens having a first known progressive surface with a surface power addition equal to a target wearer power addition for an ophthalmic lens to be defined, and a second spherical surface such that power at a far vision reference point is zero.

19.- The method according to one of claims 15 to 18, in which as a starting lens for optimization, a lens is employed having a first known progressive surface with a surface power addition equal to a target wearer power addition for an ophthalmic lens to be defined, and a second spherical surface.

20.- The method according to claim 19, in which optimization consists in varying a variable aspherical layer that is added to said starting lens.