U.S. patent application number 11/453342 was filed with the patent office on 2007-05-10 for ophthalmic lens.
Invention is credited to Jean-Pierre Chauveau, Bruno Decreton, Gilles Le Saux.
Application Number | 20070103640 11/453342 |
Document ID | / |
Family ID | 36649152 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070103640 |
Kind Code |
A1 |
Chauveau; Jean-Pierre ; et
al. |
May 10, 2007 |
OPHTHALMIC LENS
Abstract
Progressive multifocal ophthalmic lens having a power addition
prescription and presenting a complex surface having a fitting
cross and a principal meridian of progression. The lens has, when
being worn, a normalized reduced root mean square deviation of less
than 0.025 microns per dioptre over a zone that includes the far
vision control point and covering a sector whose apex lies
4.degree. below the fitting cross with an angular aperture of
between 150.degree. and 160.degree.; and a progression length of
25.degree. or less, the progression length being defined as the
angle of lowered of the view direction from the fitting cross down
to the point on the meridian for which the wearer's optical power
reaches 85% of the addition prescription. The lens is suitable for
increased far vision with good accessibility to near vision.
Inventors: |
Chauveau; Jean-Pierre;
(Paris, FR) ; Decreton; Bruno; (Charenton Le-Pont,
FR) ; Le Saux; Gilles; (Paris, FR) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
36649152 |
Appl. No.: |
11/453342 |
Filed: |
June 14, 2006 |
Current U.S.
Class: |
351/159.42 |
Current CPC
Class: |
G02C 7/025 20130101;
G02C 7/061 20130101; G02C 7/065 20130101 |
Class at
Publication: |
351/168 |
International
Class: |
G02C 7/06 20060101
G02C007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2005 |
FR |
05 11 328 |
Claims
1. Progressive multifocal ophthalmic lens having a power addition
prescription and presenting a complex surface having: a fitting
cross; a far vision zone with a control point, a near vision zone
with a control point and an intermediate vision zone; a principal
meridian of progression passing through these three zones, the lens
having, when being worn and relative to a plane far vision
prescription by adjustment of the radii of curvature of at least
one of its faces: a reduced root mean square deviation, normalized
to the addition prescription, of less than 0.025 microns per
dioptre over a zone that includes the far vision control point and
covering a sector whose apex lies on the meridian of progression at
approximately 4.degree. below the fitting cross with an angular
aperture of between 150.degree. and 160.degree., the reduced root
mean square deviation being calculated by setting to zero the 1 st-
and 2nd-order coefficients in the Zernike polynomial expansion of a
wavefront passing through the lens; and a progression length of
25.degree. or less, the progression length being defined as the
angle of lowered viewing from the fitting cross down to the point
on the meridian for which the wearer optical power reaches 85% of
the addition prescription.
2. Lens according to claim 1, characterized in that the normalized
reduced root mean square deviation zone of less than 0.025 .mu.m/D
covers an angular aperture sector of 155.degree..
3. Lens according to claim 1 or 2, characterized in that the
normalized reduced root mean square deviation zone of less than
0.025 .mu.m/D covers a sector having a mid-axis approximately
coincident with the principal meridian of progression in the far
vision zone.
4. Lens according to claim 1, characterized in that the normalized
reduced root mean square deviation zone of less than 0.025 .mu.m/D
covers a radius sector of between 35.degree. and 45.degree..
5. Lens according to claim 4, characterized in that the normalized
reduced root mean square deviation zone of less than 0.025 .mu.m/D
covers a radius sector of approximately 40.degree..
6. Visual equipment comprising at least one progressive multifocal
ophthalmic lens having a power addition prescription and presenting
a complex surface having: a fitting cross; a far vision zone with a
control point, a near vision zone with a control point and an
intermediate vision zone; a principal meridian of progression
passing through these three zones, the lens having, when being worn
and relative to a plane far vision prescription by adjustment of
the radii of curvature of at least one of its faces: a reduced root
mean square deviation, normalized to the addition prescription, of
less than 0.025 microns per dioptre over a zone that includes the
far vision control point and covering a sector whose apex lies on
the meridian of progression at approximately 4.degree. below the
fitting cross with an angular aperture of between 150.degree. and
160.degree., the reduced root mean square deviation being
calculated by setting to zero the 1st- and 2nd-order coefficients
in the Zernike polynomial expansion of a wavefront passing through
the lens; and a progression length of 25.degree. or less, the
progression length being defined as the angle of lowered viewing
from the fitting cross down to the point on the meridian for which
the wearer optical power reaches 85% of the addition
prescription.
7. Method of correcting the vision of a presbyopic subject,
comprising the supplying to the subject, or the wearing by the
subject, of a visual equipment comprising at least one progressive
multifocal ophthalmic lens having a power addition prescription and
presenting a complex surface having: a fitting cross; a far vision
zone with a control point, a near vision zone with a control point
and an intermediate vision zone; a principal meridian of
progression passing through these three zones, the lens having,
when being worn and relative to a plane far vision prescription by
adjustment of the radii of curvature of at least one of its faces:
a reduced root mean square deviation, normalized to the addition
prescription, of less than 0.025 microns per dioptre over a zone
that includes the far vision control point and covering a sector
whose apex lies on the meridian of progression at approximately
4.degree. below the fitting cross with an angular aperture of
between 150.degree. and 160.degree., the reduced root mean square
deviation being calculated by setting to zero the 1 st- and
2nd-order coefficients in the Zemike polynomial expansion of a
wavefront passing through the lens; and a progression length of
25.degree. or less, the progression length being defined as the
angle of lowered viewing from the fitting cross down to the point
on the meridian for which the wearer optical power reaches 85% of
the addition prescription.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Pursuant to U.S.C. .sctn.119, this application claims the
benefit of French Patent Application 05 11 328, filed Nov. 8, 2005.
The contents of the prior application is incorporated herein by
reference in its entirety.
TECHNICAL FIELD
[0002] This invention relates to an ophthalmic lens.
BACKGROUND
[0003] Any ophthalmic lens intended to be worn in a frame is
associated with a prescription. In ophthalmics, the prescription
may comprise a power prescription, either positive or negative, and
an astigmatism prescription. These prescriptions correspond to
corrections to be provided to the wearer of the lenses in order to
correct defects in his vision. A lens is fitted into the frame
according to the prescription and the position of the wearer's eyes
relative to the frame.
[0004] In the simplest cases, the prescription is nothing more than
a power prescription. The lens is said to be a unifocal lens and
exhibits symmetry of revolution. It is simply fitted into the frame
so that the principal viewing direction of the wearer coincides
with the axis of symmetry of the lens.
[0005] For presbyopic wearers (long-sighted subjects), the value of
the power correction in far vision is different from that in near
vision, owing to the difficulties of accommodation in near vision.
The prescription is therefore made up of a far vision power value
and an addition (or power progression) representative of the power
increment between the far vision and the near vision; this amounts
to a far-vision power prescription and a near-vision power
prescription. Lenses suitable for presbyopic wearers are
progressive multifocal lenses; these lenses are described for
example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No.
5,272,495, FR-A-2 683 642, FR-A-2 699 294 or FR-A-2 704 327. These
progressive multifocal ophthalmic lenses comprise a far vision
zone, a near vision zone and an intermediate vision zone, a
principal meridian of progression passing through these three
zones. They are generally determined by optimization on the basis
of a number of constraints imposed on various characteristics of
the lens. These lenses are general-purpose lenses in that they are
adapted to the different needs of the wearer.
[0006] Families of progressive multifocal lenses are defined in
which each lens of a family is characterized by an addition, which
corresponds to the power variation between the far vision zone and
the near vision zone. More precisely, the addition, denoted by A,
corresponds to the power variation between a point FV in the far
vision zone and a point NV in the near vision zone, which are
called the far-vision control point and the near-vision control
point, respectively, and which represent the points of intersection
of viewing with the surface of the lens for vision at infinity and
for reading vision.
[0007] In any one family of lenses, the addition varies from one
lens to another of the family, between a minimum addition value and
a maximum addition value. Usually, the minimum and maximum addition
values are 0.75 dioptres and 3.5 dioptres respectively, and the
addition varies from 0.25 dioptres in 0.25 dioptre steps from one
lens of the family to the other.
[0008] Lenses of the same addition differ by the value of the mean
sphere at a reference point, called here the base. For example, it
is possible to choose to measure the base at the far-vision
measurement point FV. Thus, by choosing an addition/base pair, a
set of aspherical front faces is defined for progressive multifocal
lenses. Usually, five base values and twelve addition values may
thus be defined, i.e. sixty front faces. In each of the bases, an
optimization for a given power is carried out. This known method
makes it possible, starting from semi-finished lenses, only the
front face of which is conformed, to prepare lenses suitable for
each wearer by simply machining a spherical or toric rear face.
[0009] Thus, progressive multifocal lenses usually have an
aspherical front face, which is that face of the spectacles on the
opposite side from the wearer, and a spherical or toric rear face,
turned towards the person wearing the spectacles. This spherical or
toric face allows the lens to be adapted to the user's ametropia so
that a progressive multifocal lens is generally defined only by its
aspherical surface. As is well known, such an aspherical surface is
generally defined by the height of all its points. The parameters
formed by the minimum and maximum curvatures at each point, or more
usually their half-sum and their difference, are also used. This
half-sum and this difference, when these are multiplied by a factor
(n-1), n being the refractive index of the material of the lens,
are called the mean sphere and the cylinder, respectively.
[0010] A progressive multifocal lens may thus be defined, at any
point on its complex surface, by geometrical characteristics
comprising a mean sphere value and a cylinder value, these being
given by the following formulae.
[0011] As is known, a mean sphere D at any point on a complex
surface is defined by the formula: D = n - 1 2 .times. ( 1 R 1 + 1
R 2 ) ##EQU1##
[0012] where R.sub.1 and R.sub.2 are the local maximum and minimum
radii of curvature, expressed in metres, and n is the index of the
constituent material of the lens.
[0013] A cylinder C is also defined by the formula: C = ( n - 1 )
.times. 1 R 1 - 1 R 2 . ##EQU2##
[0014] The characteristics of the complex face of the lens may be
expressed by means of the mean sphere and the cylinder.
[0015] Moreover, a progressive multifocal lens may also be defined
by optical characteristics, taking into consideration the situation
of the person wearing the lenses. This is because the optical
ray-tracing laws result in optical defects when the rays move away
from the central axis of any lens. These known defects, which
include amongst others a power defect and an astigmatism defect,
can generically be called ray obliquity defects.
[0016] Ray obliquity defects have already been well identified in
the prior art and improvements have been proposed. For example,
document WO-A-98/12590 describes a method of determining, by
optimization, a set of progressive multifocal ophthalmic lenses.
That document proposes to define the set of lenses by considering
the optical characteristics of the lenses, and especially the
wearer power and the oblique astigmatism under wearing conditions
of the lenses. The lens is optimized by ray tracing on the basis of
an ergorama associating, with each viewing direction under the
wearing conditions, a target object point.
SUMMARY
[0017] It is also possible to consider optical aberrations called
higher-order aberrations, such as spherical aberration or coma,
involving the distortions suffered by a non-aberrant spherical
wavefront passing through the lens.
[0018] It is considered that the eye rotates behind the lens in
order to scan its entire surface. Thus, an optical system, composed
of the eye and the lens, is considered at each point, as will be
explained in detail later on with reference to FIGS. 1 to 3. The
optical system is thus different at each point on the surface of
the lens, since the relative positions of the principal axis of the
eye and of the lens are actually different at each point owing to
the rotation of the eye behind the lens.
[0019] In each of these successive positions, the aberrations
undergone by the wavefront passing through the lens and limited by
the eye's pupil are calculated.
[0020] The spherical aberration arises for example from the fact
that the rays passing through at the edge of the pupil do not
converge on the same plane as the rays passing through close to its
centre. Moreover, coma represents the fact that the image of a
point located off-axis will have a "comet-like" trail, due to the
power variation of the optical system. The reader may refer to the
article by R. G. Dorsch and P. Baumbach, "Coma and Design
Characteristics of Progressive Addition Lenses", Vision Science and
its Applications, Santa Fe, February 1998, which describes the
effects of coma on a progressive multifocal lens.
[0021] The distortions of this wavefront may be described overall
by the root mean square or RMS deviation. The RMS deviation is
generally expressed in microns (.mu.m) and denotes, for each point
on the complex surface, the deviation of the resulting wavefront
relative to a non-aberrant wavefront. The invention proposes a
progressive multifocal lens defined by its optical characteristics
under wearing conditions, guaranteeing good visual acuity of the
person wearing the progressive lenses, especially in terms of far
vision, while allowing good accessibility to the power levels
needed for near vision.
[0022] The invention consequently proposes a progressive multifocal
ophthalmic lens having a power addition prescription and presenting
a complex surface having:
[0023] a fitting cross;
[0024] a far vision zone with a control point, a near vision zone
with a control point and an intermediate vision zone;
[0025] a principal meridian of progression passing through these
three zones, the lens having, when being worn and relative to a
plane far vision prescription by adjustment of the radii of
curvature of at least one of its faces:
[0026] a reduced root mean square deviation, normalized to the
addition prescription, of less than 0.025 microns per dioptre over
a zone that includes the far vision control point and covering a
sector whose apex lies on the meridian of progression at
approximately 4.degree. below the fitting cross with an angular
aperture of between 150.degree. and 160.degree., the reduced root
mean square deviation being calculated by setting to zero the 1 st-
and 2nd-order coefficients in the Zemike polynomial expansion of a
wavefront passing through the lens; and
[0027] a progression length of 25.degree. or less, the progression
length being defined as the angle of lowered viewing from the
fitting cross down to the point on the meridian for which the
wearer optical power reaches 85% of the addition prescription.
[0028] The lens according to the invention has one or more of the
following characteristics, depending on the embodiment:
[0029] the normalized reduced root mean square deviation zone of
less than 0.025 .mu.m/D covers an angular aperture sector of
155.degree.; [0030] the normalized reduced root mean square
deviation zone of less than 0.025 .mu.m/D covers a sector having a
mid-axis approximately coincident with the principal meridian of
progression in the far vision zone;
[0031] the normalized reduced root mean square deviation zone of
less than 0.025 .mu.m/D covers a radius sector of between
35.degree. and 45.degree.; and
[0032] the normalized reduced root mean square deviation zone of
less than 0.025 .mu.m/D covers a radius sector of approximately
40.degree..
[0033] The invention also relates to visual equipment comprising at
least one lens according to the invention and a method of
correcting the vision of a presbyopic subject, comprising the
supplying to the subject or the wearing by the subject of such
equipment.
DESCRIPTION OF DRAWINGS
[0034] Other advantages and features of the invention will become
apparent on reading the following description of the embodiments of
the invention, these being given by way of example and with
reference to the drawings which show:
[0035] FIG. 1, a diagram of a lens/eye optical system, seen from
above;
[0036] FIGS. 2 and 3, perspective diagrams of a lens/eye
system;
[0037] FIG. 4, a wearer optical power graph along the meridian of a
lens according to the invention;
[0038] FIG. 5, a wearer optical power map of the lens according to
the invention;
[0039] FIG. 6, an oblique astigmatism amplitude map of the lens
according to the invention; and
[0040] FIG. 7, a normalized reduced RMS map of the lens according
to the invention.
DETAILED DESCRIPTION
[0041] Conventionally, characteristic optical quantities, namely a
power and astigmatism, are defined for a given lens under the
conditions in which it will be worn. FIG. 1 shows a diagram of a
lens/eye optical system in side view, and shows the definitions
used in the rest of the description. The centre of rotation of the
eye is called Q'. The axis Q'F' shown in the figure by the dot/dash
line is the horizontal axis passing through the centre of rotation
of the eye and extending in front of the wearer--in other words the
axis Q'F' corresponds to the primary viewing direction. This axis
cuts, on the front face, a point on the lens called the FC (fitting
cross), which is marked on lenses in order to allow them to be
positioned by an optician. The fitting cross is generally located 4
mm above the geometric centre of the front face. Let the point O be
the point of intersection by this axis Q'F' on the rear face. A
vertex sphere, with centre Q' and radius q', is defined, which
sphere cuts the rear face of the lens at point O. As an example, a
value of the radius q' of 27 mm corresponds to a standard value and
provides satisfactory results when the lenses are worn. The cut of
the lens may be drawn in the (O,x,y) plane defined with reference
to FIG. 2. The tangent to this curve at the point O is inclined to
the (O,y) axis at an angle called the pantoscopic angle. The value
of the pantoscopic angle is typically 8.degree.. It is also
possible to draw the cut of the lens in the (O,x,z) plane. The
tangent to this curve at the point O is inclined to the (O,z) axis
at what is called the curving contour. The value of the curving
contour is typically 0.degree..
[0042] A given viewing direction--shown by the solid line in FIG.
1--corresponds to a position of the eye rotating about Q' and to a
point J on the apex sphere. A viewing direction may also be
identified, in spherical coordinates, by two angles .alpha. and
.beta.. The angle .alpha. is the angle made between the Q'F' axis
and the projection of the straight line Q'J on the horizontal plane
containing the Q'F' axis, this angle appearing in the diagram of
FIG. 1. The angle .beta. is the angle made between the Q'F' axis
and the projection of the straight line Q'J on the vertical plane
containing the Q'F' axis. A given viewing direction therefore
corresponds to a point J on the apex sphere or to a coordinate pair
(.alpha.,.beta.).
[0043] In a given viewing direction, the image of a point M in the
object space, located at a given object distance, is formed between
two points S and T corresponding to minimum and maximum distances
JS and JT (which would be the sagittal and tangential focal lengths
in the case of surfaces of revolution and of a point M at
infinity). The angle .gamma., identified as the astigmatism axis,
is the angle made by the image corresponding to the shortest
distance with the (z.sub.m) axis in the (z.sub.m,y.sub.m) plane
defined with reference to FIGS. 2 and 3. The angle .gamma. is
measured in the anti-clockwise direction when looking at the
wearer. In the example of FIG. 1, the image of a point in the
object space at infinity is formed, on the Q'F' axis, at the point
F'. The points S and T are coincident, which amounts to stating
that the lens is locally spherical in the primary viewing
direction. The distance D is the rear frontal plane of the
lens.
[0044] FIGS. 2 and 3 show perspective diagrams of a lens/eye
system. FIG. 2 shows the position of the eye and the reference
frame associated with the eye, in the principal viewing direction,
.alpha.=.beta.=0, called the primary viewing direction. The points
J and O are then coincident. FIG. 3 shows the position of the eye
and the reference frame that is associated therewith in a direction
(.alpha.,.beta.). Shown in FIGS. 2 and 3 are a fixed reference
frame {x,y,z} and a reference frame {x.sub.m,y.sub.m,z.sub.m}
associated with the eye in order to show clearly the rotation of
the eye. The reference frame {x,y,z} has as origin the point Q' and
the x-axis is the Q'F' axis--the point F' not being shown in FIGS.
2 and 3 and passes through the point O. This axis is directed from
the lens to the eye in correspondence with the direction of
measurement of the astigmatism axis. The {y,z} plane is the
vertical plane. The y-axis is vertical and directed upwards. The
z-axis is horizontal, the reference frame being a direct
orthonormal coordinate system. The reference frame
{x.sub.m,y.sub.m,z.sub.m} associated with the eye has the point Q'
as centre. The x.sub.m axis is defined by the viewing direction
JQ', and coincides with the {x,y,z} reference frame in the case of
the primary viewing direction. Listing's law gives the
relationships between the {x,y,z} and {x.sub.m,y.sub.m,z.sub.m}
coordinate systems for each viewing direction--see Legrand, Optique
Physiologique, Volume 1, published by Revue d'Optique, Paris
1965.
[0045] Using these elements, it is possible to define a wearer
optical power and astigmatism, in each viewing direction. An object
point M at an object distance given by the ergorama is considered
for a viewing direction (.alpha.,.beta.). The points S and T
between which the image of the object is formed is determined. The
image proximity IP is then given by: IP = 1 2 .times. ( 1 J .times.
.times. T + 1 J .times. .times. S ) ##EQU3## while the object
proximity OP is given by: OP = 1 M .times. .times. J . ##EQU4##
[0046] The power is defined as the sum of the object and image
inverse distances, i.e. P = OP + IP = 1 M .times. .times. J + 1 2
.times. ( 1 J .times. .times. T + 1 J .times. .times. S ) .
##EQU5##
[0047] The amplitude of the astigmatism is given by: A = 1 J
.times. .times. T - 1 J .times. .times. S . ##EQU6##
[0048] The angle of the astigmatism is the angle .gamma. defined
above: this is the angle measured in a reference frame associated
with the eye, relative to the z.sub.m direction with which the
image T is formed in the (z.sub.m,y.sub.m) plane. These power and
astigmatism definitions are optical definitions, under wearing
conditions and in a reference frame associated with the eye.
Qualitatively, the power and the astigmatism thus defined
correspond to the characteristics of a thin lens which, placed
instead of the lens in the viewing direction, would locally provide
the same images. It should be pointed out that the definition
provides, in the primary viewing direction, the classical
astigmatism prescription value. Such a prescription is produced by
the ophthalmologist, in far vision, in the form of a pair
consisting of an axis value (in degrees) and an amplitude value (in
dioptres).
[0049] The power and the astigmatism thus defined can be measured
experimentally on the lens using a frontofocometer. They may also
be calculated by ray tracing under wearing conditions.
[0050] The invention proposes a progressive multifocal ophthalmic
lens having the advantages of enlarged far vision, with also good
accessibility in near vision. The lens provides good visual acuity
in far vision with a clear field, limiting optical aberrations over
a section lying below the fitting cross and covering a large angle
in the far vision zone. The proposed solution thus provides good
accessibility to the powers needed for near vision, allowing the
wearer to see satisfactorily at distances of about 40 cm without
having to lower his eyes substantially, the near vision zone being
accessible from 25.degree. below the fitting cross. The lens is
thus a lens adapted to enlarged far vision and to near vision. The
lens has a prescription such that the power values prescribed to
the wearer in far vision and in near vision are achieved on the
lens.
[0051] The lens will be described below with reference to one
embodiment suitable for presbyopic wearers having a power
progression prescription of 2 dioptres.
[0052] FIGS. 4 to 7 show a 60 mm diameter lens with a progressive
multifocal front face and having a prism of 1.15.degree. of
geometric base, oriented at 270.degree. in the TABO coordinate
system. The plane of the lens is inclined at 8.degree. to the
vertical and the lens has a thickness of 2 mm. A q' value of 27 mm
(as defined with reference to FIG. 1) is considered for the
measurements on the lens of FIGS. 4 to 7.
[0053] The lens shown in FIGS. 5 to 7 is in a spherical coordinate
system, the angle .beta. being plotted on the x-axis and the angle
a being plotted on the y-axis.
[0054] The lens has an approximately umbilical line, called the
meridian, on which the astigmatism is virtually zero. The meridian
is coincident with the vertical axis in the upper portion of the
lens and is inclined on the nasal side in the lower portion of the
lens, the convergence being more pronounced in near vision.
[0055] The figures show the meridian and reference marks on the
lens. The fitting cross FC of the lens may be geometrically located
on the lens by a cross or any other mark, such as a dot surrounded
by a circle traced on the lens, or by any other appropriate means.
This is an alignment point physically placed on the lens, which is
used by the optician for fitting the lens into the frame. In
spherical coordinates, the fitting cross has the coordinates
(0.degree.,0.degree.) since it corresponds to the point of
intersection of the front face of the lens with the primary viewing
direction, as defined above. The far vision control point FV is
located on the meridian and corresponds to a viewing elevation of
8.degree. above the fitting cross. The far vision control point FV
has the coordinates (0.degree.,-8.degree.) in the predefined
spherical coordinate system. The near vision control point NV is
located on the meridian and corresponds to a lowering of the
viewing direction by 35.degree. below the fitting cross. The near
vision control point NV has the coordinates (6.degree.,35.degree.)
in the predefined spherical coordinate system.
[0056] FIG. 4 shows a graph of the wearer optical power along the
meridian. The angle .beta. is plotted on the y-axis and the power
in dioptres is plotted on the x-axis. The minimum and maximum
optical powers, corresponding to the abovementioned quantities 1/JT
and 1/JS respectively, are shown as the dashed curves and the
optical power P as defined above is shown as the bold curve.
[0057] The figure shows that there is an approximately constant
wearer optical power around the far vision control point FV, an
approximately constant wearer optical power around the near vision
control point NV, and a uniform progression of the power along the
meridian. The values are shifted to zero at the origin, where the
optical power is in fact -0.03 dioptres corresponding to a plane
far-vision lens prescribed for emmetropic presbyopic subject.
[0058] In the case of a progressive multifocal lens, the
intermediate vision zone generally starts in the region of the
fitting cross FC; this is the point where the power progression
starts. Thus, the optical power increases from the fitting cross to
the near vision control point NV, for 0 to 35.degree. values of the
angle .beta.. For angle values greater than 35.degree., the optical
power then becomes approximately constant again, with a value of
2.23 dioptres. It should be noted that the wearer optical power
progression (2.26 dioptres) is greater than the prescribed power
addition A (2 dioptres). This difference in power value is due to
oblique effects.
[0059] A progression length, denoted PL in FIG. 4, may be defined
on a lens, this being the angular distance--or the difference in
ordinates--between the optical centre of the lens, or the fitting
cross FC, and a point on the meridian at which the power
progression reaches 85% of the prescribed power addition A. In the
example shown in FIG. 4, an optical power of 0.85.times.2 dioptres,
that is to say 1.7 dioptres, is reached for a point with the
angular coordinate .beta.=24.5.degree. approximately.
[0060] The lens according to the invention thus exhibits
accessibility to the powers needed for near vision with a moderate
lowering of the glance of 25.degree. or less. This accessibility
guarantees comfortable use of the near-vision zone.
[0061] FIG. 5 shows the level lines of the wearer optical power
defined along one viewing direction and for an object point. As is
usual, the isopower lines have been plotted in FIG. 5 in a
spherical coordinate system. These lines are formed from points
having the same optical power value P. Isopower lines of 0 dioptres
to 2.25 dioptres have been shown.
[0062] FIG. 5 shows a far vision zone with no power variation,
extending below the fitting cross. The value of the wearer optical
power is therefore approximately constant around the fitting cross
FC. This almost zero power variation around the fitting cross
allows a certain tolerance in positioning the lens when fitting it
into the visual equipment, as will be explained later.
[0063] FIG. 6 shows the level lines corresponding to the oblique
astigmatism amplitude under wearing conditions. As is usual, the
isoastigmatism lines are plotted in FIG. 6 in a spherical
coordinate system; these lines are formed from points having the
same astigmatism amplitude, as defined above. The 0.25 dioptre to
2.50 dioptre isoastigmatism lines have been shown.
[0064] It may be seen that the far vision zone is relatively
clear--the isoastigmatism lines above 0.25 dioptres open broadly so
as to free the far vision field. It may also be seen that the
isoastigmatism lines widen, in the lower portion of the lens, at
the height of the near vision reference point NV. In the lower
portion of the lens, the 0.75 and 1 dioptre isoastigmatism lines
are almost parallel and vertical, defining a zone containing the
near vision reference point NV.
[0065] FIG. 7 shows the reduced RMS level lines normalized to the
calculated addition prescription under wearing conditions. The RMS
is calculated for each viewing direction, and therefore for each
point on the glass of the lens using a ray-tracing method. A wearer
pupil diameter of approximately 5 mm is considered. The RMS shows,
for each point on the lens corresponding to a viewing direction,
the deviation between the resultant wavefront and a non-aberrant
spherical reference wavefront corresponding to the best sphere
passing through this resultant wavefront. The RMS values have been
calculated for the lens of FIGS. 4 to 6, that is to say for a plane
far-vision lens having a power addition prescription of 2 dioptres,
prescribed for emmetropic presbyopic subjects.
[0066] One possible arrangement for measuring the aberrations of a
wavefront passing through the lens, as perceived by the wearer's
eye, is described in the article by Eloy A. Villegas and Pablo
Artal, "Spatially Resolved Wavefront Aberrations of Ophthalmic
Progressive-Power Lenses in Normal Viewing Conditions", Optometry
and Vision Science, Vol. 80, No. 2, February 2003.
[0067] As is known, a wavefront that has passed through an
aspherical surface may be expressed by a Zemike polynomial
expansion. More precisely, a wave surface may be approximated by a
linear combination of polynomials of the type: z .function. ( x m ,
y m , z m ) = i .times. a i .times. p i .function. ( x m , y m , z
m ) ##EQU7##
[0068] where the P.sub.i are the Zernike polynomials and the
a.sub.i are the real coefficients.
[0069] The Zernike polynomial expansion of the wavefront and the
calculation of the aberrations of the wavefront have been
standardized by the Optical Society of America, the standard being
available at the Harvard University website
ftp://color.eri.harvard.edu/standardization/Standards_TOPS4.pdf.
[0070] The RMS is thus calculated under the conditions when the
lens is being worn. The RMS is then reduced, that is to say the 1
st-order and 2nd-order coefficients in the Zernike polynomial
expansion of the wavefront are set to zero. The optical power and
astigmatism defect aberrations are therefore not included in the
reduced RMS calculation. The RMS is then normalized, that is to say
divided by the prescribed power addition.
[0071] In FIG. 7, the normalized reduced RMS is shown, expressed in
microns per dioptre. The iso-RMS lines from 0.01 .mu.m/D to 0.05
.mu.m/D have been shown. Drawn on FIG. 7 is a sector whose apex
lies on the principal meridian of progression at 4.degree. below
the fitting cross FC and with an angular aperture of 155.degree..
Depending on the optical optimization criteria used, the angular
aperture of this sector may be between 150.degree. and 160.degree..
In the zone of the lens covered by this sector, which includes the
far vision control point FV, the normalized reduced RMS is limited
to 0.025 .mu.m/D. This zone having a normalized reduced RMS of low
value ensures that the wearer has optimum visual perception in far
vision.
[0072] For reasons of symmetry of the lenses, the sector thus
defined may have a median axis approximately coincident with the
principal meridian of progression in the far vision zone.
[0073] In FIG. 7, the sector having a normalized reduced RMS
limited to 0.025 .mu.m/D has a radius of 40.degree.. However,
depending on the optical optimization criteria used, this radius
may be between 35.degree. and 45.degree..
[0074] The lens according to the invention therefore has a very
clear far vision zone with limited optical aberrations.
[0075] The lens according to the invention is prescribed when
considering far-vision and near-vision wearer prescriptions,
thereby determining the necessary addition. When the complex
surface is on the front face of the lens, the necessary power may
be obtained, as in the prior art, by machining the rear face in
order to ensure that the power is identical to the prescribed
power.
[0076] The fitting of the lens into visual equipment may be
accomplished in the following manner. The horizontal position of
the wearer's pupil in far vision is measured, i.e. only the pupil
half-distance, and the total calibre height of the visual equipment
frame is determined. The lens is then fitted into the visual
equipment with the fitting cross positioned at the measured
position.
[0077] The reader may refer on this point to Patent Application
FR-A-2 807 169 which describes a simplified method of fitting
ophthalmic lenses into a frame. That document describes in
particular the various measurements made by opticians and proposes
to measure only the pupil half-distance in order to effect the
fitting of the lenses into the frame using the total calibre height
of the frame.
[0078] Fitting the lens therefore requires only a conventional
measurement of the far-vision pupil half-distance, and a measure of
the calibre height of the frame, in order to determine the height
at which the fitting cross must be placed in the frame. Next, the
lens is machined and fitted into the frame, so that the fitting
cross is at a defined position. The vertical position of the
fitting cross may of course be determined conventionally by making
a measurement of the fitting height, by measuring the fitting
height by measuring the position in the frame of the viewing
direction when the subject is looking in far vision. This
measurement is performed conventionally, the subject wearing the
frame and looking at infinity.
[0079] The lens according to the invention may have an improved
tolerance to the fitting described above. This tolerance is
provided by limiting the optical aberrations around the fitting
cross. In particular, the wearer power and oblique astigmatism
values are approximately constant around the fitting cross.
Furthermore, the normalized reduced RMS value is limited around the
fitting cross.
[0080] The lens described above may be obtained by optimizing a
surface using the optimization methods known per se and described
in the documents of the prior art that were mentioned earlier
relating to progressive multifocal lenses. In particular,
optimization software is used to calculate the optical
characteristics of the lens/eye system with a predetermined figure
of merit. For the optimization, one or more of the criteria
presented in the above description with reference to FIGS. 4 to 7
may be used, and especially:
[0081] a normalized reduced root mean square (RMS) deviation of
less than 0.025 .mu.m/D over a zone that includes the far vision
control point FV and covering a sector whose apex lies on the
meridian of progression at approximately 4.degree. below the
fitting cross with an angular aperture of between 150.degree. and
160.degree.; and
[0082] a progression length of 25.degree. or less, the progression
length being defined as the angle of lowered viewing from the
fitting cross down to the point on the meridian for which the
wearer optical power reaches 85% of the addition prescription.
[0083] These criteria may be combined with others, and especially
with a radius of the sector of the normalized reduced root mean
square deviation zone of less than 0.025 .mu.m/D between 35.degree.
and 45.degree..
[0084] The choice of these criteria makes it possible to obtain a
lens by optimization. A person skilled in the art would readily
understand that the lens in question does not necessarily have the
values corresponding exactly to the criteria imposed. For example,
it is not essential for the upper value of the normalized reduced
RMS to be reached or for the apex of the limited normalized reduced
RMS sector to be exactly at 4.degree. below the fitting cross.
[0085] In the above optimization examples, it has been proposed to
optimize only one of the faces of the lenses. It is clear that, in
all these examples, the role of the front and rear surfaces may
easily be changed over, whenever optical objectives similar to
those of the lens described are achieved.
* * * * *