# Patent

## Title
Pair of progressive ophthalmic lenses

## Abstract

A pair of progressive ophthalmic lenses (1, 2) meets special conditions for improving binocular vision of a wearer, while avoiding discomfort for peripheral vision. A first one of the conditions relates to height values of far vision fields, intermediate vision fields and/or proximate vision fields, for indicating that the fields are different enough in height between both lenses. A second one of the conditions sets a maximum value for the relative difference in mean refractive power gradient between both lenses.

## Background
TECHNICAL FIELD OF THE INVENTION
The invention relates to a pair of progressive ophthalmic lenses, and to a method for providing such lens pair to a wearer.
In the context of the present invention, a pair of ophthalmic lenses means two eyeglasses which are intended to be provided to one and same wearer when both eyeglasses are mounted into one spectacle frame. Each lens of the pair is therefore dedicated to correcting both an ametropia and a presbyopia of one of the wearer's eyes in accordance to an ophthalmic prescription obtained for this eye. Visions of both right and left eyes are thus corrected simultaneously.
The invention specifically relates to provide improved binocular vision to the wearer, by selecting appropriately the progressive lenses to be paired.
BACKGROUND OF THE INVENTION
Progressive ophthalmic lenses allow sharp vision of objects which are located at varying distances in front of the lens wearer, the wearer being presbyopic. For this purpose, each progressive ophthalmic lens has a mean refractive power which varies along a meridian line of the lens. Here, the meridian line corresponds to the main gaze direction used by the wearer to see at varying distances. The vision distances may be defined by using an ergorama. As a general rule, the vision distance decreases from more than 2 meters for a far vision gaze direction, to about 40 centimeters for a proximate vision gaze direction which is oriented below the far vision gaze direction.
But out of the meridian line, a progressive ophthalmic lens has mean refractive power values and resultant astigmatism values which are not equal to those on the meridian line at constant height value for the gaze direction lowering angle. As a consequence, the optical powers which are actually produced by each lens do not match exactly the prescription out of the meridian line and out of lens zones dedicated respectively to far vision and proximate vision. Many efforts have been spent for years to increase the far- and/or proximate vision areas in combination with reducing the correction deviations out of these areas and the meridian line. But these improvements relate in optimizing each lens separately.
In addition, it has been discovered that the wearer's binocular vision can be improved by selecting appropriately both lenses which pertain to one and same pair, each with a different lens design. For example, document FR 2 704 327 proposes selecting one of the ophthalmic progressive lenses of the pair with a large far vision zone, and the other ophthalmic progressive lens of the pair with a large proximate vision zone. As a consequence, the lens with large far vision zone has a reduced proximate vision zone, and the one with large proximate vision zone has a reduced far vision zone. But binocular vision allows the wearer to select the one of his eyes which has sharper vision for each binocular gaze direction. Thus, each of the largest far- and proximate vision zones between both lenses appears to be actually effective for providing sharp binocular vision, whereas the smallest far- and proximate vision zones are useful for providing stereoscopic vision perception.
U.S. Pat. No. 7,083,277 discloses another example of lens pair with both lenses being different in far- and proximate vision field widths.
However, vision discomfort may still arise when both lenses of the pair are thus selected with different designs. In particular, such discomfort relates to peripheral gaze directions with respect to the meridian lines of the lenses.
Starting from this situation, an object of the present invention consists in improving binocular vision of a wearer of ophthalmic progressive lenses, while avoiding discomfort for peripheral vision.
SUMMARY OF THE INVENTION
For meeting this object or others, the present invention proposes a pair of progressive ophthalmic lenses, in which each lens has a prescribed far vision mean refractive power, a prescribed addition which is common to both lenses of the pair and, in as-worn conditions of the lens by a wearer, a main line, a fitting cross and a mean refractive power for each gaze direction through the lens corresponding to a lowering angle α and to an azimuth angle β relating to a cyclopean coordinate system. The main line for each lens corresponds to gaze directions for which a resulting astigmatism is minimum as a function of the azimuth angle β, and at constant value for the lowering angle α. In addition, the lowering angle α is measured from the gaze direction through the fitting cross, with positive values downwards.
Each lens further has:

    
    
        a first height, which is defined as a difference in the lowering angle α between a gaze direction on the main line where the mean refractive power equals the prescribed far vision mean refractive power plus 10% of the prescribed addition, and the gaze direction through the fitting cross; and
        a second height, which is defined as a difference between the lowering angle α equal to 36° and a gaze direction on the main line where the mean refractive power equals the prescribed far vision mean refractive power plus 85% of the prescribed addition;
    
    


The first height quantifies the level in the lens along a vertical direction in the as-worn conditions, at which the mean refractive power starts increasing.
The second height quantifies a spread of the proximate vision zone along the vertical direction, above the gaze direction corresponding to the value of 36° for the lowering angle α.
According to a first feature of the invention, the pair has at least one among a difference between the first heights of the two lenses greater than 8° in absolute value, and a difference between the second heights of the two lenses greater than 5° in absolute value. This first feature indicates that at least one of the spreads of the far vision zone and the proximate vision zone along the vertical direction is different between both lenses. Thus, a sharp vision zone which is vertically-increased for one of the two eyes is provided to the wearer for far or proximate vision. This provides to the wearer large perceived field of vision for each vision zone in binocular vision.
In addition, a maximum mean refractive power gradient is defined for each lens, for gaze directions corresponding to a cyclopean gaze direction which is comprised:

    
    
        inside a zone centered on the cyclopean gaze direction corresponding to a gaze direction passing through the prism reference point of this lens and containing all cyclopean gaze directions respecting the following inequality (|α|2+|β|2)1/2≦40°, and
        outside a central optical zone, this central optical zone comprising the main line ML_min (αmin, βmin) of cyclopean directions for which, for each angle αmin, the resulting astigmatism reaches its minimum, the central optical zone being delimited on either side of the main line by cyclopean gaze directions whose azimuth angle is equal to βmin±5°.
    
    


Then, according to a second feature of the invention, a relative difference in the maximum mean refractive power gradient between both lenses of the pair is less than or equal to 0.08 in absolute value. This second feature sets a threshold condition relating to the mean refractive power gradient, for both lenses not to have respective designs in peripheral part which are too different from each other, which can cause discomfort to the wearer. In particular, it avoids assigning a lens with hard design to one of the wearer's eyes together with another lens with soft design to the other one of the wearer's eyes.
Hence, the invention consists in an advantageous combination of two features which provide an improved binocular vision by using a specially selected progressive lens pair, while ensuring comfort in peripheral vision.
In the context of the invention, the cyclopean coordinate system denotes any coordinate system suitable for identifying a gaze direction which is originating from one point located between both eyes of the wearer, and which is oriented towards an object currently looked at by the wearer. Such direction is called cyclopean gaze direction, and separate gaze directions respectively for both eyes can be derived from the cyclopean gaze direction. But, when characterizing a lens pair according to the invention without a wearer of these lenses being individually identified, the cyclopean coordinate system to be used is defined by a separation distance which is equal to 64 mm (millimeter) between both pupils of a standard wearer, and for a cyclopean eye which is located at mid position between both eyes of this standard wearer.
According to a first optional improvement of the invention, the difference between the first heights of the two lenses may be greater than 12° in absolute value, and/or the difference between the second heights of the two lenses may be greater than 10° in absolute value. In this way, binocular vision of the wearer may be further improved.
According to a second optional improvement of the invention, the relative difference between the two lenses in the maximum mean refractive power gradient may be less than 0.025 in absolute value. In this way, comfort in peripheral vision may be further improved.
Optionally, the difference between the first heights of the two lenses and that between the second heights of the two lenses may be substantially equal to each other in absolute value and have opposite signs. Then, a first one of the two lenses has a far vision zone which is increased as compared with the other lens, and this latter lens has a proximate vision zone which is increased as compared with the first lens. In addition, both zone increases are similar in extent.
The invention may be advantageously applied when an absolute difference in prescribed far vision mean refractive power between both lenses is less than or equal to 1.0 diopter. Out of these cases, optical aberrations as power deviation or resulting astigmatism may not be fully mastered as for each vision zones, so that the design difference between both lenses as provided by the present invention can fully appear.
Similarly, the invention may be advantageously applied when an absolute difference in prescribed astigmatism values between both lenses is less than or equal to 1.0 diopter.
The present invention also proposes a method for providing a pair of progressive ophthalmic lenses to an identified wearer, which method comprises the following steps:

    
    
        /1/ for the wearer to whom both lenses are dedicated:
        
            obtaining prescriptions for both eyes including a prescribed far vision mean refractive power and a prescribed addition ADD which is common to both lenses of the pair;
            obtaining a value for a separation distance between both pupils of the wearer, and a position of a cyclopean eye for this wearer; and
            defining a cyclopean coordinate system for the wearer, based on the pupil separation distance and the cyclopean eye position;
        
        
        /2/ for each lens in standard as-worn conditions of the lens by the wearer, determining the main line, the fitting cross, the mean refractive power for each gaze direction, and defining the first and second heights as above, as well as the maximum mean refractive power gradient for the lens;
        /3/ optimizing at least one lens of the pair, so that at least one among the difference between the first heights of the two lenses is set to a value greater than 8° in absolute value, and the difference between the second heights of the two lenses is set to a value greater than 5° in absolute value, and so that the relative difference between the two lenses in the maximum mean refractive power gradient is set to a value less than 0.08 in absolute value; and
        /4/ manufacturing both lenses of the pair in accordance with an optimization result of step /3/.
    
    


Conveniently, the optimizing process of step /4/ may be implemented using computer means.
In some implementations of a method of the invention, step /3/ may be based on data linked to the wearer. These data linked to the wearer may be obtained themselves before step /3/, for example at a same time as step /1/ is carried out.
Possibly, both lenses may be symmetrical with respect to each other, in accordance with right and left sides in the as-worn conditions, but a lens design is shifted vertically for one of the lenses with respect to the other lens, with a non-zero shifting length. In this way only one lens design is actually required for obtaining both lenses of the pair.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the invention will become apparent from the detailed specification below, which is provided only for illustration purpose without inducing any limitation, and based on the appended drawings now listed:
FIGS. 1a to 1c illustrate optical parameters related to a lens pair according to the invention;
FIG. 2 is a diagram for illustrating a particular implementation of the invention;
FIGS. 3a and 3b are mean refractive power maps respectively for a right lens and a left lens of a pair according to an embodiment of the invention;
FIGS. 3c and 3d are maps of resulting astigmatism respectively for the right lens and the left lens of FIGS. 3a and 3b; and
FIGS. 4a to 4d correspond respectively to FIGS. 3a to 3d for another embodiment of the invention.



Same notations used in different ones of these figures have same meanings.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1a is a perspective view representing both lenses of a pair in as-worn conditions for a wearer, and FIG. 1c is a planar view of the lenses with parameters relating to the cyclopean coordinate system.
In FIG. 1a, reference OR denotes the right eye 10 of the wearer equipped with the lens 1, and reference OL denotes the left eye 20 of the same wearer equipped with the lens 2. Hence, both lenses 1 and 2 are worn simultaneously by the wearer to provide him a corrected binocular vision.
The wearing conditions for both lenses 1 and 2 may be the usual ones which are known in the art. In particular, both lenses 1 and 2 are mounted together within a spectacle frame (not shown) so that the back surface of each lens may be located at a distance of about 25.5 mm (millimeter) from the rotation center of the corresponding eye. ROR and ROL denote respectively the rotation centers of the eyes 10 and 20. The pantoscopic angle for each lens 1 and 2 may be 8° (degree), with each lens top edge leaned ahead with respect to the wearer's face. The wrap angle of each lens 1 and 2 may have a mean value of about 5°, this angle corresponding to a tilt of each lens about a vertical axis so that the temporal edge of the lens is shifted backwards with respect to its nasal edge.
Progressive lenses comprise micro-markings that have been made mandatory by a harmonized standard ISO 8980-2 published on 2004 Feb. 1 (ISO 8980-2:2004). Temporary markings may also be applied on the surface of the lens, indicating positions of control points on the lens, such as a control point for far vision, a control point for near vision, a prism reference point PRP and a fitting cross FC for instance. If the temporary markings are absent or have been erased, it is always possible to a skilled person to locate the control points on the lens by using a mounting chart and the permanent micro-markings. Indeed, item 7.1 c) of the standard ISO 8980-2 specifies that the manufacturer's name must be permanently indicated on the progressive addition lens, and item 8.1 of the same standard ISO 8980-2 indicates additional information which must be indicated on the lens packaging or accompanying documents. It refers to Article 6 of ISO 14889:2003 for reciting this additional information or information which must be available on request to the lens manufacturer. This latter comprises lens alignment rules for recovering the locations of all reference points and measurement points which may be non-permanently marked on the lens, including the fitting cross, the prism reference point, the far vision control point, the proximate vision gaze direction (item 6.2 f) of ISO 14889 referring back to item 7.2 a) to e) of ISO 8980-2). Therefore, all these points are set unambiguously for any progressive ophthalmic lens, from the initial design and the manufacturing of this lens, and cannot be selected in a different way when examining this lens later.
The prism reference point PRP is considered here at the midpoint of the straight segment which connects the circle-like micro-markings. When the lens is mounted in front of the eye, the fitting cross is placed before the pupil or before the eye rotation center of the eye for a primary gaze direction. The primary gaze direction corresponds to the situation where a wearer is looking straight ahead. In the chosen frame, the fitting cross FC corresponds thus to a lowering angle α of 0° and an azimuth angle β of 0°.
Then, each lens may be mounted within the spectacle frame so that the gaze direction of each eye 10, 20 passes through the fitting cross FC of the corresponding lens 1, 2 when the wearer is approximately looking horizontally at an object located in front of and far away from him, with the wearer's head itself approximately vertical. GFC denotes this gaze direction, which is commonly called primary gaze direction. Additionally for each one of the lenses 1, 2, GPRP denotes the gaze direction which passes through the prism reference point PRP of this lens.
N and T denote respectively the nasal side and the temporal side of each lens 1, 2. Both nasal and temporal sides are separated from each other by a meridian line ML within each lens. Within each lens, the meridian line ML is the track of the gaze direction of the corresponding eye when the wearer is looking successively at objects located at varying distances in front of him, as defined by an ergorama. To determine this meridian line ML by ray tracing, one can consider that for each gaze direction, the wearer is looking at an object located at the distance given by the ergorama and contained in the sagittal plane of the wearer. Usually, manufacturers will match this meridian line ML of an ophthalmic lens with approximately the line ML_min which contains the gaze directions that corresponds to the minimum of resulting astigmatism, or the line which is located at almost equal distance from two gaze directions through the lens respectively on the nasal side and the temporal side, with same values for the lowering angle and also same values for the resulting astigmatism. According to the monocular coordinates system, each meridian line ML or each line ML_min is contained in a vertical plane above the fitting cross FC, and deflected towards the nasal side N below the fitting cross FC. According to the cyclopean coordinates system, the meridian line ML is contained in a vertical plane for each lens containing the fitting cross FC. The line ML_min is called main line for each lens.
Each lens 1, 2 is selected based on a prescription obtained for the wearer and indicating a prescribed far vision mean refractive power SPH_MVL, a prescribed astigmatism value CYLVL and a prescribed addition ADD suitable for correcting the ametropia and presbyopia of each eye. The mean refractive power is obtained by summing the half value of the prescribed astigmatism to the prescribed sphere: SPH_MVL=SPHVL+CYLVL/2. For the present invention, the prescribed addition ADD is equal for both eyes 10 and 20. Then, the mean refractive power for each eye for proximate vision is obtained by summing the prescribed addition ADD to the far vision mean refractive power SPH_MVL prescribed for the same eye: SPH_MVP=SPH_MVL+ADD. For each lens, the prescribed far vision mean refractive power SPH_MVL is produced for a gaze direction intersecting the lens front surface at a far vision control point VL. For example, this far vision control point VL may be located so that the corresponding gaze direction is 8° above the gaze direction GFC within a common vertical plane.
Also for each lens 1, 2 separately, the calculated proximate vision mean refractive power SPH_MVP is produced for a gaze direction for which the lowering angle is positive, that is to say below the fitting cross FC. For the invention implementations which are described here, a proximate vision gaze direction GVP is defined on the meridian line ML where the mean refractive power has reached 100% of the prescribed addition ADD, with respect to the prescribed far vision mean refractive power. Usually, for a regular progressive lens, 85% of the prescribed addition is reached between 22° and 26° from the fitting cross FC, and generally at 24° below the fitting cross FC, and 100% of the prescribed addition is reached between 34° and 38° from the fitting cross FC, and generally at 36° below the fitting cross FC. The intersection of GVP with the front surface is called the proximate vision point VP and does not necessarily match the proximate vision control point as indicated by the manufacturer of the lens.
Any gaze direction G through one of the lenses 1, 2 is originating from the rotation center ROR or ROL of the corresponding eye 10 or 20 respectively. It is identified using a lowering angle value αm and an azimuth angle value βm.
Actually, the lowering angle αm and the azimuth angle βm as just defined relate to each lens separately, but do not suit for describing binocular vision and assessing binocular vision performances. So the above-defined angles αm and βm which relate to monocular vision for each eye are to be replaced with a common angular system which defines the respective gaze directions of both eyes when operating together for binocular vision. FIG. 1b illustrates the definition of such system suitable for binocular vision, as known from WO 2011/061267 for example.
The rotation centers ROL and ROR of both eyes are represented as before, and a cyclopean eye denoted CE is defined on the straight segment which connects ROL and ROR. The location of the cyclopean eye CE between both rotation centers ROL and ROR may be determined in several alternating manners. It may be at the middle position between ROL and ROR, or shifted along this segment depending on the wearer. In particular, the Man skilled in ophthalmics knows methods for determining the location of the cyclopean eye CE from measurements performed on the wearer. Such a method may be based on the measurement of a dominance between the right eye 10 and the left eye 20. A dominance rate comprised between −1 and +1 may be determined for the wearer in this way. The cyclopean eye CE is then superposed with the right eye rotation center ROR when the measured value of the dominance rate equals −1, and it is superposed with the left eye rotation center ROL when the measured dominance rate equals +1. Then, for a measured value of the dominance rate which is intermediate between −1 and +1, the cyclopean eye CE is moved from ROR towards ROL proportionally to the measured value.
When the wearer is looking at an object point OP contained in his visual environment, a cyclopean gaze direction GCE connects the cyclopean eye CE to the object point OP. This cyclopean gaze direction GCE is identified itself using a lowering angle α and an azimuth angle β defined as before but with the cyclopean eye CE used as the angle apex. Then, lowering angle values and azimuth angle values are defined for both eyes, respectively αR and βR for the right eye 10 and αL and βL for the left eye 20, corresponding to the respective gaze directions of the eyes when the wearer is looking at the object point OP through the lenses 1 and 2. Thus, the respective gaze directions of the eyes 10 and 20 for binocular vision are defined by the values of α and β relating to the cyclopean gaze direction. These values are used for the maps of FIGS. 2a to 2d. According to the cyclopean coordinate system, gaze directions which are oriented downwards with respect to the fitting cross FC have positive values for the lowering angle α, and gaze directions which are oriented towards the right side according to the point of view of the wearer with respect to the fitting cross FC or meridian line ML have negative values for the azimuth angle β. Both angles α and β are zero for gaze direction GFC.
Each lens 1, 2 is of progressive type for presbyopia-correction. Then it produces a mean refractive power PPOα, β and a lens astigmatism ASTα, β which vary continuously depending on the gaze direction G through the lens. So the PPOα, β- and ASTα, β-values vary as two functions of the α- and β-angle values for each lens 1, 2. The resulting astigmatism ASRα,β of one lens for any gaze direction G through this lens is the difference between the actual astigmatism value ASTα,β of the lens for this gaze direction and the prescribed astigmatism for the same lens. It is also commonly called unwanted astigmatism.
For each one of the lenses 1 and 2 independently, two gaze directions denoted G1 and G2 are defined both on the main line ML_min. The gaze direction G1 corresponds to the prescribed far vision mean refractive power increased by 10% of the prescribed addition. The gaze direction G2 corresponds to the prescribed far vision mean refractive power increased by 85% of the prescribed addition. Therefore, the gaze direction G2 appears below the gaze direction G1. α1 and α2 denote the respective values of the lowering angle for the gaze directions G1 and G2 in the cyclopean coordinate system.
With reference to FIG. 1c, the two height pairs below are defined:

    
    
        first heights: denoted V_OR_VL for the right lens 1 and V_OL_VL for the left lens 2, and equal to α1 for the lens of concern; and
        second heights: denoted V_OR_VP for the right lens 1 and V_OL_VP for the left lens 2, and equal to 36°−α2 for the lens of concern.
    
    


Then, the following height differences are calculated, for quantifying the discrepancies between both lenses 1 and 2:

    
    
        first height difference: Delta_V_VL=V_OR_VL−V_OL_VL
        second height difference: Delta_V_VP=V_OR_VP−V_OL_VP
    
    


FIG. 2 illustrates a simple embodiment of the invention, where the progressive lens design on the front surface of one of the lenses 1 and 2 is shifted along the vertical direction with respect to the other lens. Before shifting, both lenses may have same design. FIG. 2 is a diagram which indicates the mean sphere values along the meridian line ML for both lenses 1 and 2, as a function of a length coordinate in millimeters along the meridian line ML on the front surface of the lenses. The zero value of the length coordinate along the meridian line ML is at the prism reference point (PRP). The fitting cross FC is positioned at +4 mm above the prism reference point (PRP), and positive values for the length coordinate relate to above the fitting cross. The design shift between both lenses is 3 mm, corresponding to a shift in the lowering angle α of about 6°. For easy understanding, the respective progression curves of both lenses are superposed on the same diagram in FIG. 2, with the 3 mm translation length therebetween along the vertical direction. The diagram zero point corresponds to the fitting cross location which is effective for each lens when fitted into a spectacle frame so as to produce the 3 mm design shift. Therefore, for this first embodiment of the invention, Delta_V_VL=−6° and Delta_V_VP=+6°. Notations 10% and 85% in the diagram indicate respectively the points in each curve which correspond to the gaze directions G1 and G2, for the lens 1 or 2 relating to this curve.
Obviously, invention embodiments similar to that of FIG. 2 can be obtained with any length value for the vertical design translation which is implemented for one lens of the pair with respect to the other lens. For implementing the invention, the vertical translation length is sufficient for producing an angular shift which is higher than 5° between the respective gaze directions G2 of both lenses. When the vertical translation length is also sufficient for producing an angular shift higher than 8° between the respective gaze directions G1 of both lenses, then both height criteria which are involved in the invention are met simultaneously.
In another invention exemplifying embodiment characterized by the maps of FIGS. 3a to 3d, the prescribed far vision mean refractive power is 0.0 diopter for both right lens 1 and left lens 2. The prescribed addition ADD is 2.0 diopters for both lenses 1 and 2. From the maps of FIGS. 3a and 3b, the respective lowering angle values α1 and α2 for the gaze directions G1 and G2 are:

    
    
        for the right lens 1: α1=0.3° and α2=20.4°
        for the left lens 2: α1=10.3° and α2=28.7°
    
    


Hence, for the right lens 1:

    
    
        V_OR_VL=0.3°
        V_OR_VP=15.6°
    
    


and for the left lens 2:

    
    
        V_OL_VL=10.3°
        V_OL_VP=7.3°
    
    


Therefore, the height differences are:

    
    
        first height difference: Delta_V_VL=−10.0°
        second height difference: Delta_V_VP=8.3°
    
    


For each one of the lenses 1 and 2 separately, a channel is defined for each lens 1, 2 as the set of all gaze directions which are separated from the line ML_min by less than 5° in the azimuth angle β according to the cyclopean coordinate system. The side boundaries of the channel are indicated in broken lines in FIGS. 3a to 3d and the channel itself denoted CH.
A mean refractive power gradient is also calculated for each gaze direction G through any of the lenses 1 and 2 independently from the other lens. It is the module of the two-dimension vector with first component equal to the α-derivative of the mean refractive power, namely the derivative of PPOα, β with respect to the lowering angle α, and second component equal to the β-derivative of the mean refractive power PPOα, β. Both derivatives are calculated for the same gaze direction G for obtaining the mean refractive power gradient relating to this gaze direction.
Then, a maximum mean refractive power gradient is determined for each one of the lenses 1 and 2 separately, outside the corresponding channel CH but with an angular distance of less than 40° from the gaze direction GPRP. In the maps of FIGS. 3a to 3d, the circles denoted CC correspond to all gaze directions which are separated exactly by 40° in angular distance from the gaze direction GPRP. Therefore, the maximum mean refractive power gradient is assessed for each one of the lenses 1 and 2 inside the circle CC but outside the channel CH. It can be denoted Max_Grad_PPOα, β_OR for the right lens 1 and Max_Grad_PPOα, β_OL for the left lens 2.
For lens 1 and from FIG. 3a, the maximum mean refractive power gradient Max_Grad_PPOα, β_OR, is 0.986 diopter/°, produced for α=40° and β=24°.
For lens 2 and from FIG. 3b, the maximum mean refractive power gradient Max_Grad_PPOα, β_OL, is 0.965 diopter/°, produced for α=34° and β=24°.
Therefore, the relative difference in the maximum mean refractive power gradient between both lenses 1 and 2, namely Ratio_Grad=(Max_Grad_PPOα, β_OR−Max_Grad_PPOα, β_OL)/(Max_Grad_PPOα, β_OR+Max_Grad_PPOα, β_OL), is 0.011, thus less than 0.08 in absolute value.
In still another exemplifying embodiment of the invention, which is characterized by the maps of FIGS. 4a to 4d, the prescribed far vision mean refractive power is again 0.0 diopter for both right lens 1 and left lens 2, and the common prescribed addition ADD is again 2.0 diopters. From the maps of FIGS. 4a and 4b, the respective lowering angle values α1 and α2 for the gaze directions G1 and G2 are:

    
    
        for the right lens 1: α1=6.9° and α2=31.1°
        for the left lens 2: α1=1.0° and α2=22.3°
    
    


Hence, for the right lens 1:

    
    
        V_OR_VL=6.9°
        V_OR_VP=4.9°
    
    


and for the left lens 2:

    
    
        V_OL_VL=1.0°
        V_OL_VP=13.7°
    
    


Therefore, the height differences are:

    
    
        first height difference: Delta_V_VL=5.9°
        second height difference: Delta_V_VP=−8.8°
    
    


In addition, the relative difference in the maximum mean refractive power gradient between both lenses 1 and 2 for this latter embodiment, namely Ratio_Grad=(Max_Grad_PPOα, β_OR−Max_Grad_PPOα, β_OL)/(Max_Grad_PPOα, β_OR+Max_Grad_PPOα, β_OL), is 0.035 in absolute value.
A pair or progressive ophthalmic lenses according to the invention may be designed by optimizing at least one of the lenses 1 and 2 with respect to at least one of the height differences and the relative difference in the maximum mean refractive power gradient between both lenses. Such optimization may be performed by implementing computer means, in a manner well-known in the art. In particular, at least one merit function may be used for quantifying a combination of deviations each between an actual value for one design parameter and a target value for this parameter. For implementing the invention, the difference in the first and/or second heights between both lenses may be handled each as one design parameter for the optimization step.
Furthermore, the optimizing step may take into account additional data such as data linked to the wearer.
In first design method implementations, the optimization step may comprise the following substeps:

    
    
        obtaining at least one data linked to the wearer;
        based on this data linked to the wearer, determining one of the wearer's eyes as a main eye or determining a value of a dominance rate for both eyes;
        obtaining a relationship between a modulation parameter for at least one of the differences in first heights and second heights between both lenses on one hand, and the main eye or value of the dominance rate on the other hand; and
        performing the optimization using the modulation parameter for the at least one of the differences in first heights and second heights.
    
    


Data linked to the wearer means any data related to the wearer or a combination thereof, such as data linked to at least one eye of the wearer, data linked to optometric measurements carried out on the wearer or data linked to lifestyle, posture or handedness of the wearer.
In such first implementations, data linked to the eye means either one of the following data or any combination of several of them:

    
    
        prescription data such as prescribed optical power, prescribed astigmatism with module and axis orientation, mean power also called mean sphere and equal to the prescribed optical power added to half of the prescribed astigmatism, prescribed prism with value and orientation, etc;
        data related to the high order aberrations of the eye;
        data related to the eye's biometry such as the position of the eye rotation center, the cornea, the pupil, or the pupil diameter;
        binocular data related to the binocular behavior of both eyes, and allowing to identify one of the eyes as the main eye: which one of the eyes is the dominant one, the sighting one, the eye with the best acuity, the preferred eye, the eye with the highest or lowest amount of high order aberration, the eye which is more sensitive to blur and/or contrast, etc.
    
    


“Ocular dominance” is defined for example in the Dictionary of visual science, 4th edition, D. Cline, H W Hofstetter, J R Griffin. “Visual acuity” refers to acuteness or clearness of vision, which depends in turn on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative capability of the brain. “Blur sensitivity” represents the acuity loss produced by a defocus introduced in front of the eye. “Contrast sensitivity” refers to the ability to detect contrast, and may be determined by measuring the minimum contrast level which can be detected.
When data linked to the eye are used, one of the wearer's eyes may be determined as the main eye or a value of a dominance rate for both eyes may be determined.
When prescription data are used, the main eye may be that one of the eyes which has the lowest mean power value, in absolute value.
When binocular data are used, the main eye may be the dominant eye or that of the eyes which is more blur-sensitive or has better acuity.
Then, the lens with the largest value for the first height, namely the lens corresponding to the largest value among V_OR_VL and V_OL_VL, may be assigned to the main eye.
The modulation parameter may be a weight parameter used for the height differences concerned in the merit function involved in the optimization process. Alternatively, it may be a distribution parameter of design targets.
The optimization step itself may be computer-implemented. But it may also involve a selection of a semi-finished blank for each eye, among blanks having respective designs different from each other but corresponding to the prescribed addition. Then, the face of each selected blank opposite to that provided with the desired design is machined so as to produce the optical power and astigmatism prescribed for the eye of concern.
When data linked to optometric measurements are used, one of the wearer's eyes may be determined as the main eye or a value of a dominance rate for both eyes may be determined.
Data linked to lifestyle of the wearer means data related to jobs and hobbies of the wearer. Different wearers, though, typically have different lens usage requirements, some of which being related to the wearer's vocational and avocational activities. Different vocational and avocational activities may encompass different ranges of accommodative demands due to, for example, variation in the working distance requirements of the activity and the nature and relative motion of the visual targets associated with the activity. Thus, different activities way warrant different considerations in the selection of an appropriate lens design. Data linked to lifestyle can be obtained by methods such as interviews and surveys at an optometrist, a store or the like.
When data linked to lifestyle are used, one of the wearer's eyes may be determined as the main eye or a value of a dominance rate for both eyes may be determined. Particularly, when data linked to lifestyle are segmented, one of the wearer's eye may be determined as the main eye. If data linked to lifestyle relate to a continuous criterion, a value of a dominance rate for both eyes may be determined. Data linked to lifestyle are advantageously used in combination with other data linked to the wearer, particularly data linked to posture or handedness.
Data linked to posture of the wearer means data related to positions and orientations of body parts (head, trunk, etc.) of the wearer to perform a task (reading, writing, watching computer screen, etc.) in determined environmental conditions (relative positions and orientations of objects). From posture adopted by an individual can derive knowledge of distances of object seen as a function of the gaze direction (ergorama).
When data linked to posture are used, one of the wearer's eyes may be determined as the main eye or a value of a dominance rate for both eyes may be determined. Particularly, when data linked to posture are segmented, for example if an object is seen either on the left side or on the right side of the sagittal plane of the wearer, one of the wearer's eye may be determined as the main eye. If data linked to posture relate to a continuous criterion, for example the distance from the object to the sagittal plane, a value of a dominance rate for both eyes may be determined.
When data linked to posture are used in combination with data linked to lifestyle, for example if the wearer mainly performs near vision tasks and if his posture is such that the objects seen in near vision are mainly on the right side of his sagittal plane, then the design of the right eye lens may have a larger height in near vision than that of the left eye lens.
Data linked to handedness of the wearer means data related to manual laterality, or laterality, of the wearer. Right-handed persons and left-handed persons behave very differently when executing certain near vision tasks. The specific near vision task of writing on a sheet of paper is usually considered for determining laterality. The laterality of a person can thus be defined by the hand used for writing on a sheet of paper, or more accurately, by calculating an handedness score consisting in asking questions about the hand used in everyday tasks. The Edinburgh Handedness Inventory is an example of such a way to determine a handedness score (Oldfield R. C. (1971), “The assessment and analysis of handedness: The Edinburgh Inventory”, Neuropsychologia, vol. 9, p. 97-113).
When data linked to handedness are used, one of the wearer's eyes may be determined as the main eye.
When data linked to handedness are used in combination with data linked to lifestyle, for example if the wearer mainly performs near vision tasks and if he is right-handed, then the design of the right eye lens may have a larger height in near vision than that of the left eye lens.
In second design method implementations, the optimization step may comprise the following substeps:

    
    
        obtaining target designs respectively for both lenses of the pair;
        based on data linked to the wearer, determining one of the wearer's eyes as a main eye or determining a value of a dominance rate for both eyes;
        obtaining a relationship between a modulation parameter for at least one of the differences in first heights and second heights between both lenses on one hand, and said main eye or value of the dominance rate on the other hand;
        modifying the target designs using the modulation parameter for the at least one of the differences in first heights and second heights; and
        performing the optimization using the modified target designs.
    
    


The difference between the first and second design method implementations relates in the use of the modulation parameter during the optimization process itself for the first implementation, whereas it is used to define the optimization target for the second implementation.

## Claims

1. A method for providing a pair of progressive ophthalmic lenses to an identified wearer, said method comprising the following steps:
/1/ for the wearer of both lenses:
obtaining prescriptions for both eyes including a prescribed far vision mean refractive power and a prescribed addition for each of the two lenses, said prescribed addition being common to both lenses of the pair;
obtaining a value for a separation distance between both pupils of the wearer, and a position of a cyclopean eye for the wearer; and
defining a cyclopean coordinate system for the wearer, based on the pupil separation distance and the cyclopean eye position;

/2/ for each lens in standard as-worn conditions of the lens by the wearer:
determining a main line, a fitting cross, a prism reference point, and a mean refractive power for each gaze direction through the lens corresponding to a lowering angle α and to an azimuth angle β within a cyclopean coordinate system,
the main line for each lens corresponding to gaze directions for which a resulting astigmatism is minimum as a function of the azimuth angle β, and at constant value for the lowering angle α, and
the lowering angle α being measured from the gaze direction through the fitting cross, with positive values downwards;

defining a first height as a difference in the lowering angle α between a gaze direction on the main line where the mean refractive power equals the prescribed far vision mean refractive power plus 10% of the prescribed addition, and the gaze direction through the fitting cross;
defining a second height as a difference in the lowering angle α between 36° and a gaze direction on the main line where the mean refractive power equals the prescribed far vision mean refractive power plus 85% of the prescribed addition;
defining a maximum mean refractive power gradient for gaze directions corresponding to a cyclopean gaze direction which is comprised:
inside a zone centered on the cyclopean gaze direction corresponding to a gaze direction passing through the prism reference point of said lens and containing all cyclopean gaze directions respecting the following inequality (|α|2+|β|2)1/2≦40°, and
outside a central optical zone, said central optical zone comprising a main line of cyclopean directions for which, for each angle αmin, the resulting astigmatism reaches its minimum, the central optical zone being delimited on either side of the main line by cyclopean gaze directions whose azimuth angle is equal to βmin±5°;


/3/ optimizing at least one lens of the pair,
so that at least one among a difference between the first heights of the two lenses is set to a value greater than 8° in absolute value, and a difference between the second heights of the two lenses is set to a value greater than 5° in absolute value, and
so that a relative difference between the two lenses in the maximum mean refractive power gradient is set to a value less than 0.08 in absolute value; and

/4/ manufacturing both lenses of the pair in accordance with an optimization result of step /3/.

2. The method according to claim 1, wherein both lenses are symmetrical with respect to each other, in accordance with right and left sides in the as-worn conditions, but a lens design is shifted vertically for one of the lenses with respect to the other lens, with a non-zero shifting length.

3. The method according to claim 1, wherein step /3/ further comprises:
obtaining at least one data linked to the wearer;
based on said data linked to the wearer, determining one of the wearer's eyes as a main eye or determining a value of a dominance rate for both eyes;
obtaining a relationship between a modulation parameter for at least one of the differences in first heights and second heights between both lenses on one hand, and said main eye or value of the dominance rate on the other hand; and
performing the optimization using the modulation parameter for the at least one of the differences in first heights and second heights.

4. The method according to claim 1, wherein step /3/ further comprises:
obtaining target designs respectively for both lenses of the pair;
based on data linked to the wearer, determining one of the wearer's eyes as a main eye or determining a value of a dominance rate for both eyes;
obtaining a relationship between a modulation parameter for at least one of the differences in first heights and second heights between both lenses on one hand, and said main eye or value of the dominance rate on the other hand;
modifying the target designs using the modulation parameter for the at least one of the differences in first heights and second heights; and
performing the optimization using the modified target designs.