Note: Descriptions are shown in the official language in which they were submitted.
CA 02250615 1998-10-15
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MULTIFOCAL OPHTHALMIC LENS
BACKGROUND OF THE INVENTION
The present invention relates to a multifocal
ophthalmic lens, having an aspherical surface which has a
mean sphere and a cylinder at every point thereon.
Such lenses are well known; among multifocal lenses
one can distinguish lenses known as progressive lenses
adapted to vision at all distances, and lenses that are
more specifically dedicated to near vision and
intermediate vision.
Progressive multifocal ophthalmic lenses comprise a
far vision region, a near vision region, an intermediate
vision region, and a main meridian of progression passing
through the three regions. French patent application
2,699,294 describes, in its preamble, the various
elements of a progressive multifocal ophthalmic lens
(main meridian of progression, far vision region, near
vision region, etc..), as well as the work carried out by
the applicant to improve wearer comfort of such lenses.
Applicant has also proposed, in order to better
satisfy the visual needs of long sighted people and to
improve comfort of progressive multifocal lenses, to
adapt the shape of the main meridian of progression, as a
function of the power addition value A (French patent
application FR-A-2,683,642).
For such lenses, the power addition value A is
defined as the variation in mean sphere between a
reference point in the far vision region and a reference
point in the near vision region.
Such progressive lenses are generally prescribed as a
function of the wearer's ametropia and the power needed
for near vision.
Lenses also exist which are more specifically
dedicated to near vision; such lenses do not have a a far
vision region with a defined reference point like
conventional progressive lenses have. Such lenses are
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prescribed depending on the near vision power the wearer
needs, independently of the far vision power. Such a lens
is sold commercially by the
applicant under the Essilor Delta trademark; this lens is
also as simple to use and easy to wear as a progressive
lens, and it is attractive to long-sighted people not
fitted with progressive lenses. This lens is also
described in French patent application FR-A-2,588,973. It
has a central portion which is equivalent to the single-
focus lens that would normally have been employed for
correcting longsightedness, in order to ensure
satisfactory near vision. It additionally has a slight
decrease in power in the upper portion, ensuring that the
wearer also has sharp vision beyond the usual near vision
field. Finally, the lens has a point at a power value
equal to the nominal power for near vision, a higher
power region at the lower portion of the lens, and a
lower powered region in the top portion of the lens.
Existing multifocal lenses, whether they be
progressive or dedicated to near vision can still be
further improved as regards their foveal vision
performance, in order to improve wearer comfort. Wearers
of multifocal lenses do in fact sometimes feel
uncomfortable with dynamic vision. Such lenses can also
be improved by preserving a near vision region that is
sufficiently high to ensure optimum wearer comfort;
finally, it is important that wide visual fields be
provided in near, intermediate and far vision.
SUMMARY OF THE INVENTION
The present invention provide a multifocal lens which
overcome the disadvantages of prior art lenses and which
provides wearers with improved peripheral vision while
still ensuring foveal vision is good, thereby ensuring
ease of adaptation of wearers to their lenses. The
invention nevertheless ensures rapid progression of mean
sphere, ensuring the presence of a large near vision
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region. It also provides balanced distribution of
isosphere and isocylinder lines.
The invention provides a multifocal ophthalmic lens
comprising an aspherical surface having at every point
thereon a mean sphere and a cylinder, and comprising a
far vision region VL, a near vision region VP, an
intermediate vision region VI, a main meridian of
progression MM' passing through said three regions,
in which a principal length of progression, defined
as a ratio between power addition and maximum slope of,
mean sphere along said meridian is less than 16 mm;
in which sphere varies in a monotonous fashion as a
function of angle on a 20 mm radius circle centered on a
geometric center of.the lens at both sides of said
meridian,
and in which the far vision region delimited in an
upper portion of said lens by lines formed of points for
which cylinder is equal to half power addition includes
an angular sector having its origin at the geometric
center of the lens with an included angle greater than
150 .
Advantageously, the main meridian of progression is
made up by mid-points of horizontal segments joining
respective lines formed by points where cylinder is 0.50
diopter.
In one embodiment, the near vision region, delimited
in a lower portion of said lens by lines formed by
points where cylinder is equal to half power addition has
a width that is greater than 12 mm at a point of
reference for near vision..
In another embodiment, said included angle has a
value comprised between 160 and 170 , preferably of the
order of 165 .
Preferably, the modulus of the derivative dS/d6 of
mean sphere with respect to angle on said circle is
comprised between 0.005 and 0.015 when said angle 0 is
comprised in the ranges (30 ; 100 1 and (270 ; 325 1.
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Advantageously, the modulus of the derivative dS/dO
of mean sphere with respect to angle on said circle is
comprised between 0.01 and 0.04 when said angle 0 is
comprised in the ranges [125 ; 180 ] and [2000; 25001.
In one embodiment, the lens is a multifocal lens
dedicated to near vision and intermediate vision, said
lens having a power addition defined as a difference
between maximum and minimum values of mean sphere on said
meridian of progression, inside a 20 mm radius circle
centered on the geometric center of said lens.
In another embodiment, the lens is a progressive
multifocal lens having a reference point for a near
vision region, a reference point for a far vision region,
and a power addition defined as a difference between the
values of mean sphere at these two points.
Further features and advantages of the present
invention will become more clear from the description
which follows of one embodiment of the invention provided
by way of non-limiting example with reference to the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatical front view of a multifocal
progressive lens.
FIG. 2 shows graphically variation in power along the
meridian of the lens according to the invention.
FIG. 3 is a front view of the lens in FIG. 2, showing
the main meridian of progression and lines indicating the
level of mean sphere.
FIG. 4 is a front view of the lens in FIG. 2, showing
the main meridian of progression and lines indicating
cylinder level.
FIG. 5 is a three-dimensional view of the slopes of
sphere on the lens of FIG. 2.
FIG. 6 shows, diagrammatically, sphere on the 20 mm
radius circle centered on the geometric center of the
lens, as function of angle for the lens in FIG. 2 and for
the lens in FIGS. 7 to 12.
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FIGS. 7 to 9 are similar views to those in FIGS. 2 to
4, for a power addition of 2 diopters.
FIGS. 10 to 12 are similar views to those in FIGS. 2
to 4, for a power addition of 3 diopters.
5 DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Below, we shall employ an orthonormal coordinate
system in which the x-axis corresponds to the horizontal
axis of the lens and the a y-axis corresponds to the
vertical axis; the centre 0 of the reference frame is the
geometric center of the lens.
FIG. 1 is a diagrammatical front view of a known
progressive ophthalmic lens, showing the various elements
thereof.
FIGS. 2 to 6 show the optical characteristics of the
lens according to the invention, this lens having a
diameter of about 60 mm. In FIGS. 2 to 6, we have
described a lens having a power addition of one diopter.
FIGS. 7 to 12 show a similar view, for lenses having
a power addition of 2 or 3 diopters.
With reference to FIG. 1, the various elements of a
multifocal ophthalmic lens will now be described. Such a
lens generally has an aspherical face shown in FIG. 1
and a second face which can be aspherical or toroidal.
For every point on the aspherical surface, a mean
sphere D is defined from the formula:
D = n - 1 ( 1 + 1
2 R1 R2
in which:
Rl and R2 are the maximum and minimum radii of
curvature expressed in meters, and
n is the refractive index of the lens material.
Cylinder C is defined by the formula:
C = (n - 1) 1 1 - 1
Rl R2
Isosphere lines are lines constituted by the
projection onto a plane tangential to the progressive
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surface at the geometric centre 0 of points on the lens
surface having the same value of mean sphere. Similarly,
isocylinder lines are lines constituted by the projection
onto this same plane of points having the same cylinder.
Conventionally, the lens 1 comprises in its upper
portion a far vision region VL, in its lower portion a
near vision region VP and, between these two regions, an
intermediate region VI. For a progressive lens, a
reference point P is defined in the near vision region at
which near vision is measured and a reference point L at
which far vision is measured. For a lens dedicated to
near vision, a reference point P is defined in the near
vision region for measuring near vision; however, no
corresponding reference point is defined for the far
vision region.
On FIG. 1, the main meridian of progression 2 of the
lens is shown, passing through the far vision region, the
intermediate vision region and the near vision region.
This meridian is defined as the locus of the mid-points
of horizontal segments delimited by the 0.50 diopter
isocylinder line. In the example of FIG. 1, the meridian
is essentially composed of three segments, the first one
extending substantially vertically from the top of the
lens, passing through the point L, down to a point D,
referred to as the fitting centre, and located between
the far vision control point L and the geometric centre
0. The second segment extends from point D obliquely
towards the nasal side of the lens, and the third segment
starts from the end C of the second segment and passes
through the near vision control point P. Other forms of
meridian are possible.
In the case of progressive multifocal ophthalmic
lenses, a power addition is defined in a manner known per
se, this being the difference in mean sphere between a
reference point P in the near vision region and a
reference point L in the far vision region.
For multifocal lenses dedicated to near vision and
intermediate vision, minimum and maximum values of sphere
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are measured on the meridian thus defined within the
bounds of a 20 mm radius circle centered on the geometric
center of the lens. The power addition is now the
difference between these minimum and maximum values of
sphere; this definition is substantially equivalent, for
progressive lenses, to the conventional definition of
power addition and being the difference in sphere between
the reference points for near and far vision.
With these definitions, it is generally considered
that the limit of the far vision region in the upper
portion of the lens is formed by isocylinder lines of a
value equal to half the power addition. Similarly, the
limit of the near vision region in the lower portion of
the lens is set by isocylinder lines of a value equal to
half the power addition.
The inner circle shown on FIG 1. represents the
region scanned by the eye when performing everyday tasks.
The size and position of this portion, known as the
foveal vision region has been determined by numerous
series of measurements carried out in the applicant's
laboratories; reference can example be made to IEEE,
Portable eye movement recorder by T. Bonnin and N. Bar,
Proceedings of the 14th annual international conference
of the IEEE Engineering in Medicine and Biology Society
1992, part 4, pages 1668 to 1669, to AAO 1993, to
"Optimization of ophthalmic aspheric lenses: recording of
eye movement for everyday tasks", N. Bar, T. Bonnin and
C. Pedreno, Optometry and vision science 1993, No. 12s,
volume 70 page 154, or yet again to ECEM 93, "The use of
visual space", a poster by N Bar. This region covers a 30
mm diameter disc centered on the mounting center.
To ensure maximum visual comfort for the wearer, we
consider the 40 mm diameter disc centered on the
geometric center of the lens, which encompasses the
foveal vision region and we have set out to limit
tangential variations in sphere along this circle.
Controling variations in sphere along this circle makes
it possible to master deformations in the optical
14794Gfi f)(X- 21 septembre 1998 - 7lI8
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characteristics of the multifocal surface; the wearer's
peripheral vision is thus improved. It is also desirable
to maintain a sufficiently wide visual field in the
foveal vision region. The present invention makes it
possible to obtain a balanced distribution of the
isosphere and isolcylinder lines; The relevant circle is
also shown on FIG. 1.
In prior art lenses, and in particular in the case of
the applicant's lenses, vision in the region around the
main meridian of progression is completely satisfactory.
In order to improve the gentleness of progression of
the lenses, and comfort in the foveal vision region, the
present invention and sets out to consider a new
definition of the characteristics of the lens surface,
explained with reference to the following figures. The
figures cover the case of progressive multifocal lenses;
the invention applies mutatis mutandis to multifocal
lenses dedicated to near vision.
FIG. 2 is a a graph showing power along the meridian
of the lens according to the intervention, the power
addition of this lens being one diopter. The y-axis
coordinates of the graph of FIG. 1 are the y-axis
coordinates on the lens; the x-axis coordinates give the
difference in power, in diopters, from the reference
point in the far vision region.
The point having the value y = 8 mm on the y-axis
along the meridian corresponds to the reference point L
for far vision, which, in the case of FIG. 2, is the
point of minimum sphere; at this point, mean sphere is
5.2 diopters and cylinder is 0; the point having a y-axis
value of 14 mm on the meridian is the reference point P
for near vision; at this point, mean sphere is 6.22
diopters and cylinder is 0.02 diopters.
A principal length of progression is defined as the
ratio between power addition A as defined above and the
maximum value of the slope of mean sphere along the
meridian; this ratio is written as:
Lpp = A/Pmer
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For a progressive multifocal lens, we have:
Lpp = (SVP SVL)/Pmer
where SVP and SVL are respectively the values of mean
sphere at the near and far vision control points and Pmer
is the maximum value of the slope of sphere along the
meridian ; this slope of sphere corresponds to the
maximum modulus of the gradient of sphere with respect to
x or to y.
For a progressive multifocal lens dedicated to near
and intermediate vision, we have:
Lpp = (Smax - Smin)/Pmer
where Smax and Smin are respectively the maximum and
minimum values of sphere on the meridian, and Pmer is as
defined above.
This ratio is equivalent to a length, and represents
the length over which mean sphere increases by a value
corresponding to power addition.
FIG. 2 shows that, at first, sphere remains
substantially constant in the far vision region above
point L. It shows also that sphere remains substantially
constant in the near vision region, around point P.
Finally, it shows that the principal length of
progression, equal to 12.50 mm, is low, and is thus less
than 16 mm. This ensures satisfactory near vision in a
region extending above the near vision control point,
obviating the need for the wearer to move his head.
Comfortable and extensive near vision is thereby ensured.
Maximum slope of sphere on the meridian is 0.08 diopters
per mm.
FIG. 3 is a front view of the lens in FIG. 2, showing
the main meridian of progression and lines of equal mean
sphere. Those elements shown on FIG. 2 will be found
also in FIG. 3 with the addition of isosphere lines. The
isosphere lines in FIG. 3 are the lines 11, 12, 13 and 14
representing respectively mean sphere which is greater by
0.25, 0.5, 0.75 or 1 diopter to mean sphere at the far
vision control point L. Finally, a 40 mm diameter circle
centered on the geometric center of the lens is shown.
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FIG. 4 is a front view of the lens in FIG. 2, showing
the main meridian of progression and lines of equal
cylinder. Those elements shown in FIG. 2 are also
present in FIG. 4. As cylinder is low along the main
5 meridian of progression, there are two isocylinder lines
for each value of cylinder. The isocylinder lines in
FIG. 4 are the lines 16 and 16', and 17 and 17',
representing, respectively, a cylinder of 0.25, and 0.50
diopters.
10 As indicated above, in the upper portion of the lens
the border of the far vision region is substantially
constituted by the 0.5 isocylinder lines 17 and 17'. The
lens of the invention thus has a wide far vision region
which extends over almost all of the top half of the
lens. Quantitatively, the far vision region includes an
angular sector defined by two half-lines 20 and 21'
originating from the geometric center of the lens with an
included angle greater than 130 ; in FIG. 4, the angle
between the half-lines 20 and 20' is of the order of
160 .
In the lower portion of the lens, the border of the
near vision region is also substantially constituted by
the 0.5 isocylinder lines 17 and 17'.
FIG. 5 is a three-dimensional representation of
slopes of sphere on the lens of FIG. 2; FIG. 5 shows the
slope of mean sphere, in diopters per mm, as a function
of position on the lens, in the reference frame defined
above.
FIG. 6 shows variation in mean sphere on the 40 mm
diameter circle centered on the geometric center of the
lens for various values of power addition; the y axis is
graduated in diopters and the x axis shows the angle 0 in
a system of polar coordinates the center of which is the
geometric center of the lens and the angles of which are
measured starting from an upwardly-directed vertical
half-line originating from the geometric center of the
lens; in other words, the x-axis represents the angle 0
between, firstly, an upwardly-directed vertical half-line
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originating from the geometric center of the lens and,
secondly, a half-line originating from the geometric
center of the lens and passing through the point on said
circle where sphere is being measured. The lower curve
in FIG. 6 corresponds to the variation in sphere on the
40 mm diameter circle for the one-diopter power addition
lens shown in FIGS. 2 to 5; the middle and upper curves
on FIG. 6 respectively show these same variations for
lenses of power addition two and three diopters.
FIG. 6 shows that variations in sphere on the 20 mm
radius circle centered on the geometric center of the
lens is monotonous, when one moves on the circle from a
point of intersection with the meridian to another point
of intersection of the circle with the meridian.
Stated in other terms, on FIG. 6, the point of x-axis
value 0 or 360 corresponds to the point having
coordinates x = 0 mm, y = 20 mm in the orthonormal
reference frame defined above, and also substantially
corresponds to the point of intersection of the meridian
and the circle, in the upper portion of the lens.
The point on the x-axis where 0 = 187 on FIG. 6 is
the point for which sphere has a maximum value; this
point corresponds to the intersection of the circle with
the meridian in the lower portion of the lens, and has,
in the orthonormal reference frame defined above, the
coordinates x = 3.47 mm and y = -19.70 mm.
When one moves around the circle from the point of
angle 0 = 0 towards the point of angle 0 = 187 , sphere
is an increasing function of angle; when one moves around
the circle from the point of angle 0 = 187 towards the
point of angle 0 = 0 , sphere is a decreasing*function of
angle.
This condition regarding monotonous variation of
sphere over the circle at both sides of the meridian
ensures there is very gentle and uniform progression of
the optical characteristics of the lens, both inside the
foveal region as well as outside it.
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The lens in FIGS. 2 to 6 thus ensure progression is
very gentle, ensuring much easier adaptation on the part
of the wearer of the lenses.
Quantitatively, this is reflected by the following
conditions .
(1) the far vision region comprises an angular sector
with its origin at the geometric center of the lens, with
an included angle of at least 150 ;
(2) the principal length of progression i.e. the
ratio between power addition and the maximum slope of
mean sphere on the meridian is less than 16 mm, and
(3) variation in sphere on the 20 mm radius circle
centered on the geometric center of the lens is
monotonous at both sides of the meridian.
Relation (1), as explained above, sets a lower limit
for the surface of the far vision region.
Relation (2) reflect the fact that the principal
length of progression of the lens is low, and thus the
fact that the near vision region is sufficiently high on
the lens to ensure optimum comfort for the wearer in near
vision.
The third relation ensures, through monotony of
variations in mean sphere at the edge of the foveal
region, and taking account of the continuity and
derivability properties of the progressive surfaces, well
known to those skilled in the art, good mastery of
variations in optical parameters inside as well as
outside this region.
The combination of these three conditions ensures a
good distribution of the isosphere and isocylinder lines
over the surface of the lens, thereby ensuring very
gentle progression.
The combination of these three conditions is not
satisfied by any of the prior art multifocal ophthalmic
lenses tested by the applicant. The invention provides,
for the first time, such a distribution of isocylinder
and isosphere lines.
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FIGS. 7 to 9 are similar views to those in FIGS. 2 to
4 but for a lens of power addition 2 diopters; FIGS. 10
to 12 are similar views to those in FIGS. 2 to 4, but for
a lens of power addition 3 diopters. Isosphere lines
with a 0.25 diopter step are shown on FIGS. 8 and 11;
isocylinder lines in 0.25 diopter steps are shown on
FIGS. 9 and 12. These diagrams also show the half-lines
tangential to the A/2 isocylinder lines, in the far
vision region.
For each of these lenses, the three conditions are
satisfied. In the case of the lens in FIGS. 2 to 5, we
have, as indicated above:
angle at the center of the angular sector included in
the far vision region: 163 ;
Lpp = 12.50 mm.
For the lenses of power addition 2 and 3 diopters,
the values of the angle at the center and the principal
length of progression are the same.
The invention provides additional advantageous
characteristics, which, combined with these three
conditions, make it possible to improve the performance
of the lens according to the invention.
According to the invention, the near vision region
has, at the level of the reference point for near vision,
a width of at least 12 mm, and preferably a width greater
than 13 mm; this width is measured at the y-axis
coordinate of point P, between the A/2 isocylinder lines,
where A is the power addition as defined above. As can
be seen on FIG. 3, in the case of a power addition of one
diopter, the width of the near vision region is 13.5 mm.
For a power addition of two or three diopters, this value
remains substantially the same.
In one embodiment of the invention, the angle at the
center of the angular sector contained in the far vision
region is comprised between 160 and 170 , and is
preferably near to 165 ; in the example shown in the
figures, this angle at the center is substantially 163
14784(;{t I))(?+ scptembre 1998 - 13/18
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for the one diopter power addition lens, and is the same
for the two or three diopter power addition lenses.
Advantageously, limits are also imposed on the slope
of mean sphere on the 20 mm radius circle; this slope is
in fact the derivative dS/dO of the function represented
on the curves of FIG. 6.
The table below gives average values for the absolute
value of slope for different angles and for different
power additions.
power 30-100 125-180 200-250 270-325
addition\
dS/dO
one 0.005 0.012 0.013 0.005
diopter
two 0.010 0.025 0.026 0.01
diopters
three 0.015 0.036 0.040 0.015
diopters
In all cases, the absolute value of slope is
comprised between 0.005 and 0.015 for values of angle 0
in [30 ; 100 ] or [270 ; 325 ] ; it is comprised between
0.01 and 0.04 for values of angle 0 in [125 ; 180 ] or
[200 ; 25001.
We shall now give details of the various
characteristics that make it possible to provide the
various lenses according to the invention. As known per
se, the surface of the lenses is continuous and
continually derivable three times. As known to those
skilled in the art, the surface of progressive lenses is
obtained by digital optimisation using a computer,
setting limiting conditions for a certain number of lens
parameters.
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Combinations of the three conditions defined above,
with, if appropriate, one or several of the other
criteria defined above can be used as limiting
conditions.
5 These criteria apply both for a conventional
progressive multifocal lens with a reference point in the
far vision region and a reference point in the near
vision region, as well as for a multifocal lens that is
dedicated to near vision.
10 One can advantageously start by defining, for each
lens of the family, a main meridian of progression. For
this, the teachings of French Patent Application FR-A-
2,683,642 mentioned above, is used. Any other definition
of the main meridian of progression can be used, for
15 applying the teaching of the invention.
Obviously, this invention is not limited to what has
been described: among other things, the aspherical
surface could be the surface facing the wearer of the
lenses. Additionally, although there was no mention in
the description of lenses which can be different for both
eyes, this of course obviously applies.
147846U.UO(' - S uctobre 1998 - 15/18
