Note: Descriptions are shown in the official language in which they were submitted.
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CA 02232539 1998-03-18
TITLE OF THE INVENTION
GRADIENT-INDEX MULTIFOCAL LENS
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention generally relates to spectacle
lenses (or eyeglass lenses) and more particularly to a
progressive power multifocal lens for presbyopia.
j 2. Description of The Related Art
Generally, in a progressive power multifocal lens,
there are a zone designated as "a far (or distance) vision
viewing portion" (hereunder sometimes referred to simply
as a distance portion or as a farsight portion) for viewing
long-distance places, another zone designated as "an
intermediate vision viewing portion" (hereunder
sometimes referred to simply as a middle portion or as an
intermediate portion) for viewing middle-distance places
and still another zone designated as "a near vision viewing
portion" (hereunder sometimes referred to simply as a
reading (or near) portion or a nearsight portion) for
viewing short-distance places. Incidentally, the term
"middle-distance" referred herein often designates
distance ranging from 50~ centimeters (cm) to 2 meters (m)
approximately. FurtlZer, the term "long-distance"
frequently designates (:namely, means) distance longer
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than the middle-distance. Moreover, the term "short-
distance" often designates distance shorter than the
middle-distance. However, in some case, the term "long-
distance" designates only infinite distance in some cases.
Furthermore, sometimes, the term "short-distance"
designates distance ranging from 30 cm to 33 cm. Thus, as
matters stand, there are no definite definitions of these
terms.
Originally, there are no apparently clear boundaries
among these zones on a progressive power multifocal lens.
Therefore, even though these terms are not defined
definitely, there is no inconvenience in actually wearing
progressive power mult;ifocal lenses. However, when a
lens is designed, manufactured, inspected and further put
into a rim or frame, Nome reference points, which are
precisely defined on the lens, become necessary. Among
such points, presently most common points are the
following three points: a far vision power measuring
position (namely, a position for measuring the (refractive)
power of a lens in the c;~se of a far vision) F; a near vision
power measuring position (namely, a position for
measuring the (refractive) power thereof in the case of a
near vision) N; and a position E through which a visual line
(namely, a line of sight) of an eyeglass wearer (namely, a
person wearing the lens)
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passes when his or her eyes are in a frontal vision condition
(namely, in a front viewing condition).
The determination of the far vision power measuring
position F and the near vision power measuring position N
of a lens is unavoidable for checking whether or not the
lens is within specifications determined according to ISO
(International Standards Organization) standards, JIS
(Japanese Industrial Standards) or the like. Further, the
position E, through which a visual line (namely, a line of
sight) of a wearer passes, is indispensable for determining
the vertical or horizontal direction when a lens is put into
a rim or frame.
In addition, for example, a position Q for measuring
the (prism) refractive power of a lens is necessary. These
points, however, are often made to coincide with the
geometric center (or central point) G thereof.
Incidentally, in the case of a lens on which the position F is
preliminarily inwardly offset (or deviated) towards the
nose of an eyeglass wearer, it is usual that each of the
positions Q, N and E is also deviated inwardly from the
normal position thereof' by a distance being equal to the
deviation (or offset) of the position F. Further, the
starting and end points of a progressive change in
refractive power are :important. It is, however, not
mandatory to indicate the starting point and the end point
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on the surface of the lens. Moreover, it is difficult to
specify the locations of these points
through actual measurement thereof. Thus, such starting
and end points are somewhat unsuitable for reference
points to be used for describing techniques employed in an
invention. Furthermore, it is often that the positions F
and N of a lens are shifted upwardly and
downwardly therefrom by a distance (namely, 2 to 4
millimeters (mm) or so), which corresponds to the radius
of an aperture portion o~f a lensmeter (or lensometer),
resp ectively.
Meanwhile, the quality of a progressive power
multifocal lens has been discussed according to whether
or not the optical conditions (for example, a change in
surface astigmatism, a change in axial surface astigmatism,
a change in average additional surface refractive power, a
change in the horizontal direction of a prism refractive
power and a change in the vertical direction of the prism
refractive power) of a lens surface of the progressive power
multifocal lens are .appropriate. For instance, in
Japanese Patent Publication Nos. 49-3595/1974 and 5-
20729/1993 Official Gazettes, it is described that a range of
micro- spherical-surfaces, which is named as "an umbilical
meridian", is placed along a main or principal fixation line
in an almost central portion of the lens and that "because
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the umbilical meridian is a range of micro- spherical-
surfaces, no astigmatisms is caused and a good field of view
is obtained". However, it holds just for a surface of the
lens that "no astigmatism is caused owing to the
sphericalness of the micro-spherical- surfaces of the
umbilical meridian". Namely, the lens is not put into a
state in which no astigmatism is caused by what is called
"transmitted light" that has been transmitted by the lens
and then reached an eyeglass wearer. The same goes for
the average refracting power. Thus, even when the average
surface refracting power distribution is uniform as in the
case of the "spherical surface", the surface refracting power
distribution in the case of the transmitted light cannot be
uniform. This tendency is notable in peripheral portions
such as the nearsight portion of the lens and in the case of
high far-vision power. The distributions of average
refractive powers and astigmatism in the case of the
"transmitted light" having actually reached the eyes of an
.>
eyeglass wearer are largely different from those of the
aforementioned average "surface" refractive powers and
the aforesaid "surface" astigmatism, respectively.
Further, the case of the "transmitted light" is
referred to in Japanese Patent Publication No. 47-
2394311972 Official Ga2;ette and Japanese Patent National
Publication No. 4-50~D87011992 Official Gazette and
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Japanese Patent Unexamined Publication No. 6-
1882311994 Official Gazette.
However, each of the aforesaid Japanese Patent
Publication No. 47-23943/1972 Official Gazette and
Japanese Patent National Publication No. 4-500870/1992
Official Gazette refers only to the astigmatism occurring on
the principal fixation line. From the viewpoint of
providing an eyeglass wearer with a good "broad field of
view" only by regulating astigmatism occurring on a single
line, the lenses disclosed in these Official Gazettes are
incompetent as a progressive power multifocal lens.
Further, Japanese Patent Unexamined Publication
No. 6-18823/1994 Official Gazette discloses a lens that has
a first surface (namely, the front surface) formed as a
"progressive (power) surface" and a second surface
(namely, the back surface) by which all of the
inconveniences of the distribution of optical conditions
of the lens in the case of using "transmitted light" are
obviated. The second surface (namely, the back surface)
of this lens is disclosed therein merely as an
"aspherical surface without point symmetry and axial
symmetry". Moreover, no practical calculation method is
disclosed therein.
Furthermore, no practical method for changing a
parameter concerning the optical conditions of the lens
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in the case of using "transmitted light" is also disclosed
therein. Especially, note that both of the improvement of
the transmission average refractive power and the
obviation of the transmission astigmatism cannot always
be achieved at the same time. As a consequence,
an attempt to achieve the balance between the transmission
average refractive power and the transmission astigmatism
should be made. Methods for achieving the balance
therebetween are inherent in the inventions disclosed therein
but are not referred to therein at all.
Here, in the case where the second surface (namely,
the back surface), which is a prescribed surface, is an
aspherical surface, it is obvious that the time and cost
required to produce a lens would increase owing to the
aspherical surface processing. Further, because this
aspherical surface is an aspherical one, the lens has to
be produced after a reception of an order. Namely, a
-) method of preliminarily receiving an order cannot be
employed. Therefore, in addition to the time and cost
required to produce a lens, delivery time after reception
of prescribed values is disadvantageous to this method,
in comparison with the currently-used method which has
previously been described.
The present invention is accomplished to eliminate
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the drawbacks of these methods.
It is, accordingly, an object of the present
invention to provide a progressive power multifocal lens,
by which an eyeglass wearer can obtain a substantially
good "broad field of view", without increasing time and
cost required to produce a prescribed surface thereof.
SUMMAR'i~ OF THE INVENTION
To achieve the foregoing object, in accordance with
the present invention, there is provided a progressive
power multifocal lens th;~t belongs to a group of progressive
power multifocal lenses designed under a certain rule in
such a manner that fundamental elements such as a far
vision power measuring position F and a near vision power
measuring position N of a progressive power multifocal
lens meet a common wearing object. In the case of this
lens, a surface refractive power (in units of diopters) at the
far vision power measuring position F is employed as a base
curve (Bi). Further, a difference in surface refractive
power between the far vision power measuring position F
and the near vision power measuring position N is
employed as an addi~~tion Di (in units of diopters).
Furthermore, let W(Di, Bi) denote a width of a region in
which values of a surface average additional refractive
power along a horizontal section line extending below the
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near vision power measuring position N are not less than
Di/2.
In such a case, when arbitrary two progressive power
multifocal lenses, whose additions are Da and base curves
are B 1 and B2, respectively, are extracted or selected from
the group of the progressive power multifocal IgrISGS the
following relation holds for B 1 > B2:
W(Da, B1) > W(Da, B2).
Thus, there can be obtained a progressive power
multifocal lens by which an eyeglass weaxer can obtain a
substantially good "broad field of view", without
increasing time and cost required to produce a prescribed
surface thereof.
Moreover, preferably, in the case of an embodiment
of the hereinabove-mentioned progressive power multifocal
lens of the present invention, when a single curve passing
through at least both of the far vision power measuring
position F and the near vision power measuring position N
is employed as a principal fixation line, a horizontal
deviation H of an arbitrary point P on the principal fixation
line towards the nose of a wearer with respect to the far
vision power measuring position F is given by:
H=K.Dp/Di
where K is an arbitrary constant meeting the following
inequality ~,Q<K~5.0 : Dp the additional surface
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refractive power at the point P; and Di an addition.
Furthermore, preferably, in the case of an
embodiment of any of the aforementioned progressive
power multifocal lenses of the present invention, a change
in optical conditions along a horizontal (or transverse)
section curve intersecting with the principal fixation line
at an arbitrary point P .occurs in such a manner that in a
portion where the principal fixation line is not horizontally
(or laterally) deviated from the horizontal (or lateral)
location of the far vision power measuring position F, the
optical conditions are symmetric with respect to a plane
which contains the point P and is perpendicular to the
section curve and serves as a plane of mirror symmetry and
that in another portion where the principal fixation line is
horizontally (or laterally) deviated to the nose of a wearer
from the horizontal (or lateral) location of the far vision
power measuring position F, the change in the optical
conditions along a horizontal section curve extending from
the point P tothe nose thereof is larger than the change in
the optical conditions along another horizontal section
curve extending from the point P to an ear thereof.
Furthermore, preferably, in the case of an
embodiment of any of the aforementioned progressive
power multifocal lenses of the present invention, the
addition (Di) has a value ranging from 0.75 diopters to 3.00
CA 02232539 2003-04-08
diopters. Here, let W(Di, X) (millimeters (mm))
represent a width of a region in which the value of
astigmatism along the horizontal section curve passing
through the near vision power measuring position N is not
more than X.
In this case, when arbitrary two progressive power
multifocal lenses, whose additions are Da and Db,
respectively, and base curves are the same with each
other, are selected from the group of the progressive
power multifocal lenses, the following relation holds for
the addition Da > Db:
W(Da, X) ~ W(Db, X.Db/Da ).
where X = 1.00 diopter.
Fuxther, preferably, in the case of any of the
progressive power multifocal lenses of the present
invention, an arbitrary point P on a part of the
principal fixation line, which is other than the far
vision power measuring position F and the near vision
power measuring position N, has two different principal
curvatures (namely, maximum and minimum curvatures
corresponding to this point P).
Hereinafter, the present invention will be described
more detailedly.
As for the distribution of distinct vision area
among the "distance portion", the "middle portion" and
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the "reading portion" in an ordinary progressive power
multifocal lens, the area of the distinct vision region
in the "distance portion" is largest, though the ratio
among the areas of the distinct vision regions respectively
provided in the three portions varies more or less with the
kind of the progressive power multifocal lens. This is
because the progressive power muitifocal lenses should
cope with the extremely high frequency of using the far
vision in daily life. Moreover, the sensitivity of human
eyes to astigmatism has a tendency to become highest in
the case of using the far vision and to become dull as the
eyesight to be used is changed from the intermediate vision
to the near vision.
Wearing tests conducted in the inventor's own way
reveal that the distinct region in the case of using the
far vision should have astigmatism which is not more than
about 0.5 diopters and that in the case of using the near
_ vision, an object can be distinctly viewed if the value of
astigmatism ranges from about 0.75 diopters to about 1.00
diopter. It is, therefore, judged as being unreasonable to
make a simple comparison among the areas of the distinct
vision regions respectively provided in the portions at a
certain value of astigmatism.
Further, the quality of a progressive power
multifocal lens should be discussed according to whether
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or not the distribution of optical conditions (or
properties) of the entire lens corresponding to the
field
of view are suitable. The present invention, thus, aims
to improving the "optical conditions" of the lens in
the
case of using "transmitted light" by grasping the difference
thereof from the "optical conditions on the lens surface"
through a conjecture based on the "optical conditions
on the
lens surface", which is logically insufficient or imperfect,
and through the prediction of the distributions of the
average refractive power. and the astigmatism of the
lens in
the case of using the "transmitted light" having
substantially reached an eye of an eyeglass wearer in
addition to the condition of astigmatism on a single
line
and by then feeding back the difference to the "optical
conditions on the lens surface".
This object of the present invention itself is similar
to
an object of the invention disclosed in the aforementioned
Japanese Patent Unexamined Publication No. fi-
1882311994 Official Gazette. Such a mere desire is not
all
what the present application describes. The present
application further proposes a practical improvement
method, by which the object of the present invention
can
be attained without increasing the time and cost required
to produce a prescribed surface.
Namely, first, similarly as in the case of the
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conventional lens, the prescribed surface is shaped like
a spherical surface or what is called an astigmatic
surface, which is relatively easily produced so that the
time and cost required to produce the prescribed surface
do not increase. Thus, when preparing a semi-finished
goods which employ "progressive (power) surfaces" as
first (or front) surfaces and have several kinds of base
curves, the range of the far vision power, at which the
semi-finished goods are used, is preliminarily
determined. Moreover, the shape of the progressive
surface of each of the semi-finished goods is adjusted to
that which is most suitable for the corresponding far
vision power range. Thereby, a substantially good "broad
field of view" can be ensured for an eyeglass wearer
without increasing time and cost required to produce the
prescribed surface thereof.
Meanwhile, the differences among various kinds of
progressive power multifocal lenses are those in the
"average refractive power distribution" and the
"astigmatism distribution". The ease-of-use of each of
the lenses varies with the differences in these distributions
therebetween. Incidentally, the term "an
average refractive power distribution" (of a lens)
designates a distribution of additional refractive power
used to compensate for deficiency of the amplitude of
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accommodation of an eyeglass wearer, and more
specifically, designates "'a surface average refractive
power distribution" obtained by subtracting the base
curve of a lens, namely, a surface refractive power at
the far vision power measuring position F from the
average refractive power distribution on the surface of
the lens. Further, the term "an astigmatism distribution"
designates a difference between two principal curvatures
on the surface of a lens, namely, designates a "surface
astigmatism distribution".
The conventional progressive power multifocal lenses
have been evaluated by representing optical information
on the lens surface in the form of a distribution chart or
diagram, and then discussing whether or not the
distribution represented by the distribution diagram is
suitable for an eyeglass wearer.
However, light actually reaching an eye of the
eyeglass wearer is "transmitted light" that has been
transmitted and refracted by a spectacle lens.
Therefore, no matter how superior the "diagram for
illustrating the optical information distribution on the
surface of a lens" is, this does not make sense if a
"diagram for illustrating the optical information
distribution in the case of using light transmitted by
the lens" is not superior. Namely, matters of importance
CA 02232539 1998-03-18
are not the "surface average refractive power distribution"
and the "surface asi;igmatism distribution" but a
"transmission average refractive power distribution" and a
"transmission astigmatism distribution". There is an
approach to obtaining "diagrams for illustrating these
optical information distributions in the case of using
transmitted light" by actually measuring or observing
these distributions. Judging from the viewpoint of the
feedback of such information to a lens design, this
approach is impracticall. Consequently, in accordance
with the
present invention, the "diagrams for illustrating the
optical information distributions in the case of using
transmitted light" are drawn by obtaining whole data by
calculation.
As to parameters necessary for this calculation, all
factors respectively determining the shape of spectacle
lenses and the position al relation between each of the
eyeballs of an eyeglass wearer and an object are needed,
in addition to the refractive index of the material of
the lens.
As shown in FIG. f.2, actual lenses, however, are
fitted into an eyeglass frame or rim. Further, the
eyeglass lenses are worm by a wearer in such a manner
that each of the lenses is at a distance of about 12 to
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15 mm or so forwardly away from the corresponding eye of
the wearer and is bent forwardly from the vertical
direction at an angle of 5 to 10 degrees or so (in the
case of FIG. 22, 7 degrees are employed as such an
angle). Actually, the aiforementioned factors are an
angle formed between a visual line and each of the two
surfaces of the lens, the thickness of the lens at a
place where each of the surfaces of the lens intersects
with the visual line, the refractive powers respectively
corresponding to the two surfaces of the lens, the vertex
distance from the vertex; of (the anterior surface of) the
cornea of each eye of the wearer to the lens
(incidentally, in the case of FIG. 22, 12 mm is employed
as this distance), the distance from the vertex of the
cornea to the center of rotation of each eyeball
(incidentally, in the case of FIG. 22, 13 mm is employed
as this distance), the distance from the lens to the
object, prismatic thinning correction data (incidentally,
in the case of FIG. 22, the prism power is brought dowm
by 1 prism diopter) and so forth.
Further, the optical information in the case of
using the transmitted Eight depends on what an eyeglass
wearer views, namely, depends on the "object distance
(namely, objective distance)". Thus, it is also necessary to
obtain the "object distance". Here, note
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that the "object distance" does not depend on the far
vision (or farsight) power and the addition which
correspond to the eyeglass wearer. Namely, the term "a
long-distance place", which the eyeglass wearer views,
usually designates a place at an infinite distance from
the wearer. Furthermore, the term "near (or short-
distance) place" designates a place at what is called a
"reading distance", namely, a distance ranging from 30 to
-) 33 cm or so. Moreover, although there is no general
criterion for defining the "object distance" in visual
field regions other than long-distance and short-distance
places, the "object distance" in the case of such visual field
regions can be calculated in a proportional
distribution manner from the addition of progressive
power multifocal lenses worn by the eyeglass wearer and
from the surface average additional refractive power
distributions thereof on the assumption that the surface
average power distributiions of the progressive power
multifocal lenses worn by the eyeglass wearer are
correct, namely, serves l;he purpose of wearing these
eyeglass lenses.
To obtain the "object distance", first, the
reciprocal (or inverse) PX (hereunder sometimes referred
to as the "objective power" (in units of diopters)) of the
"object distance" is found. Namely, let Di, Pn and SDi
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denote the (basic) addition (in units of diopters) of the
progressive power multifocal lens, the reciprocal of the
short-distance (in units of diopters) to be given and the
surface average additional refractive power (in units of
diopters) of the lens at a place corresponding to the
"objective power" to be obtained, respectively. Thus, the
"objective power" PX is obtained by
PX = Pn.SDi/Di -~
For example, when the basic addition of the
progressive power multifocal lens is 2.00 diopters; the
reciprocal of the short-distance to be given 3.00
diopters (corresponding to 33 cm); and the surface
average additional refractive power of the lens at a
place corresponding to the "objective power" to be
obtained is 1.50 diopters, respectively, the "objective
power" PX is obtained as follows:
PX = 3.00 . 1.50 ~ 2.00 = 2.25 diopters.
This "objective power" is equivalent to the "object
distance" of about 44.4 cm.
A comparison between the "diagrams fox illustrating
the optical information distributions in the case of using
transmitted light", which is obtained as a result of
performing a calculation by using these parameters, and
the "diagram for illustrating the optical information
distribution on the surface" of the progressive power
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multifocal lens, on which the calculation is based, reveals
the following matters.
In the case of the "distribution of the transmission
average additional refractive power", the width W of a
region, in which values of a surface average additional
refractive power along a horizontal section line
extending below the near vision power measuring position
N are not less than one-half the addition, becomes
narrower when the far vision power is positive, and the
width W becomes wider when the far vision power is
negative, in comparison with that in the case of the
"surface average additional refractive power
distribution".
Hence, the width VV in the case of the positive far
vision power is set in such a manner as to be wider than
that in the conventional case, while the width W in the
case of the negative far vision power is set in such a
way as to be narrower than that in the conventional case.
Thereby, the "transmis:>ion average additional refractive
power distribution", which is closer to the suitable
distribution serving lthe essential purpose, can be
obtained.
Here, note that the value of the base curve of a
semi-finished lens used in the case of a positive far
vision power is larger than that of the base curve of a
CA 02232539 1998-03-18
semi-finished lens used i.n the case of a negative far
vision power, in general.
Progressive power multifocal lenses designed by
taking these respects into consideration are superior in
the transmission average power distribution and the
transmission astigmatism distribution to the conventional
lenses. As a consequence, it reveals that the designed
progressive power multifocal lenses have the following
properties (or characteristic features).
Namely, in the case of a progressive power
multifocal lens that belongs to a group of progressive
power multifocal lenses designed under a certain rule in
such a manner that the i"undamental elements such as the
far vision power measuring position F and the near vision
power measuring position N of the progressive power
multifocal lens meet a c:ommon wearing object, a surface
refractive power (in units of diopters) at the far vision
power measuring position F is employed as a base curve
(Bi). Further, a difference m surface refractive power
between the far vision power measuring position F and the
near vision power measuring position N is employed as the
addition Di (in units of diopters). Furthermore, let W(Di,
Bi) denote the width of a region in which values of a surface
average additional refractive power along a horizontal
section line extending below the near vision power
21
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a
measuring position N are not less than Di/2.
In such a case, when arbitrary two progressive power
multifocal lenses, whose additions are Da and base curves
are B1 and B2, respectively, are extracted or selected from
the group of the progressive power multifocal lenses, the
following relation holds for B 1 > B2:
W(Da, B 1) > W(Da, B2).
Moreover, it is necessary for further facilitating
j the progressive power multifocal lens of the present
invention to determine the position of the principal
fixation line on the lens in such a manner that when a
single curve passing through at least both of the far
vision power measuring position F and the near vision
power measuring position N is assumed and named the
"principal fixation line", the horizontal deviation H of
an arbitrary point P on the principal fixation line
towards the nose of a wearer with respect to the far
vision power measuring position F is obtained by:
H = K.Dp / Di
where K is an arbitrary constant meeting the following
inequality 1.p<K_<_5.Q ; Dp an additional surface
refractive power at the point P; and Di an addition.
The purpose of increasing the additional surface
refractive power along the principal fixation line is to
view a nearer object. When viewing a nearer object, each
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of the visual lines of the left and right eyes comes
nearer to the nose of a wearer (namely, the convergence
action of his or her eyes is enhanced). It is, thus,
necessary for meeting the convergence action to increase
the deviation of the principal fixation line toward his or her
nose. Consequently, the horizontal deviation H of an
arbitrary point P on the principal fixation line is
proportional to the value obtained by dividing the
j additional surface refractive power Dp by the addition Di.
Additionally, the reason for allowing the value of the
arbitrary constant K to have a permissible range is that
when passing through the lens, the visual line is refracted
by the prism action (or effects} of the horizontal component
of the transmission refractive power of the lens at the
position thereof corresponding to the deviation H, and that
thus, it is preferable that when the transmission refractive
power is negative, the value of the constant K decreases,
and in contrast, when the transmission refractive power is
positive, the value of the constant K increases. In the case
that the transmission re:f'ractive power is 0, the value of the
constant K is preferably 2.5 or so.
Moreover, the progressive power multifocal lenses of
the present invention can be further improved by applying
the following techniques to the contents of the
aforementioned "design of the laterally asymmetric type"
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so as to make these lenses more convenient to use.
Namely, it is necessary for obtaining good binocular
vision to use a lens for the right eye of a wearer and a lens
for the left eye thereof, which are matched with each other
in the following respects of: the astigmatism in the case of
the lenses through which the visual lines of the wearer
pass; the direction of what is called the axis of the
astigmatism; what is called the average power of the lens
(namely, the arithmetic mean of what is called the
spherical power (or diopter) thereof and what is called the
astigmatic power (na;mely, the cylindrical diopter)
thereof ; the horizontal .component of the prism refractive
power (namely, the prism diopter) thereof; and the vertical
component of the prism diopter thereof.
Here, note that in the case where an object to be
viewed is placed in front of the wearer, it is enough for
such improvement of the progressive power multifocal
lenses to take only the placement of the aforementioned
principal and the distribution of the surface refractive
power into consideration.
However, when they object to be viewed moves to the
side of the wearer, the visual line of one of his or her
eyes moves to his or her ear, whereas that of the other
eye moves to his or her :nose. Thus, the lenses, through
which the visual lines of his or her left and right eyes
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CA 02232539 2004-04-19
pass, respectively, are not always in the same optical
conditions.
If the object to be viewed is in the infinite
distance from the wearer, the deviation angles of the
visual lines of his or her left and right eyes at the
time of changing the viewing condition from the front
viewing condition to the side (or periphery) viewing
condition are equal to each other. It is, therefore,
preferable that the optical conditions on the lenses are
(namely, the distribution of an optical characteristic
quantity on the lens are) symmetry with respect to a plane
which contains the aforementioned principal fixation Iine
and is perpendicular to the section curve (for example, to
the horizontal direction) and serves as a plane of mirror
symmetry (incidentally, this distribution of the optical
characteristic quantity on two lateral halves of the lens are
not simply symmetric with (an arbitrary point on) the
principal fixation line but are symmetrical with respect to
the plane of mirror symmetry as seen by putting a mirror on
the principal fixation Iine or curve including the arbitrary
,,
paint P (additionally, the reason for employing such a
symmetric distribution is that the "optical characteristic
properties" include directional properties such as a change
in the direction of what is
called the axis of the astigmatism (or astigmatic
CA 02232539 1998-03-18
difference)).
In contrast, if the object to be viewed is in the
finite distance from the wearer, the visual lines of his
or her left and right eyes go nearer to his or her nose
by the convergence action of his or her eyes, respectively.
When changing the viewing condition from the front
viewing condition to the side (or periphery) viewing
condition during his or her eyes are in this state, the
deviation angles of the visual lines of his or her left and
right eyes are equal to each other if the distance between
the object and the wearer (namely, his or her eyes) is
invariant. However, a.s can be readily understood by
considering, for example, the case where the object is in the
immediate vicinity of the wearer, the distance
therebetween usually increases when changing the viewing
condition from the front viewing condition to
the side viewing condition. As a result, the convergence
action of his or her eyes becomes weak and the visual lines
of his or her eyes become nearly parallel.
Thus, if the object to be viewed is in the finite
distance from the wearer, the deviation angles of the
visual lines of his or her left and right eyes are different
from each other upon changing the viewing condition from
the front viewing condition to the side viewing condition.
Namely, the angular deviation of the visual line moving to
2G
CA 02232539 1998-03-18
his or her ear is larger tJhan that of the visual line moving
to his or her nose. In the case of the spectacle lenses
turning together with the head of a wearer, this tendency is
further increased or enb.anced owing to the turn of his or
her head in the side viewing condition (incidentally, the
head turns nearly a half of an angle required to change the
viewing condition from i;he front viewing condition to the
side viewing condition and moreover, his or her eyeballs
turn by the remaining part of such an angle, and thus this
tendency becomes noticeable. Consequently, it is
preferable for viewing an object placed in the finite
distance from a wearer that the optical conditions on a
portion, in which the corresponding part of the principal
fixation line is deviated ltoward his or her nose with respect
to the aforementioned position F, of each lens are laterally
asymmetric with a plane, which includes an arbitrary
point on the principal fixation line, in the lateral (or
.> horizontal) direction.
In the case of the progressive power multifocal
lenses, the optical conditions (namely, the distributions
of the optical characteristic properties) in the lateral
(or horizontal) direction of an arbitrary point on the
principal fixation line tlhereof changes usually. It is,
therefore, preferable for realizing same (or similar)
optical conditions (namely, the symmetric distribution of
27
CA 02232539 1998-03-18
the optical characteristic property) on the two lateral
half parts of the lens, through which the visual line
passes, the change in the optical conditions along a
horizontal section curve extending from the arbitrary
point P to the nose thereof is larger than that in the
optical conditions along another horizontal (or lateral)
section curve extending from the point P to an ear
thereof.
In summary, it is preferable that in a portion where
the principal fixation line is not horizontally (or
laterally) deviated from (namely, with respect to) the
horizontal (or lateral) location of the far vision power
measuring position F, a change in optical conditions (at
least one of a change in the astigmatism along the
horizontally (or laterally) sectional curve intersecting
with the principal fixation line at an arbitrary point P
on the principal fixation. line, a change in the direction
of what is called the axis of the astigmatism (or the
astigmatism axis), a change in the average power thereof,
a change in the horizontal component of the prism diopter
(or refractive power) thereof and the vertical component of
the prism diopter thereof) occurs symmetrically with
respect to a plane whiich includes the point P and is
perpendicular to the sectional curve and serves as a plane
of mirror symmetry. Moreover, it is desirable that in
28
CA 02232539 1998-03-18
another portion where the principal fixation line is
horizontally (or laterally) deviated to the nose of a wearer
from the horizontal (or :lateral) location of the far vision
power measuring position F, the change in the optical
conditions along a horizontal (or lateral)
section curve extending from the point P to the nose
thereof is larger than that in the optical conditions
along another horizontal. (or lateral) section curve
extending from the point; P to an ear thereof.
Furthermore, in view of the fact that progressive
power lenses having large additions (Di) become necessary
with advancing age, the countermeasures against problems
occurring in the case of large additions (Di) are studied in
order to make the progressive power multifocal lens of the
present invention more convenient to use.
Namely, those who wear spectacle lenses having
relatively small additions (Di) are relatively young and
thus lead active visual lives. Such wearers,
accordingly, require the stability of a (dynamic) visual
field at the time of moving their heads and visual lines
largely. Conversely, those who wear spectacle lenses
having relatively large additions (Di) are of relatively
advanced age and thus lead inactive visual lives. Such
a wearer, therefore, requires the stability of a (static)
visual field at the time when he or her does not move his
29
CA 02232539 1998-03-18
or her head and visual lines so largely. Consequently,
it is preferable that the contents of the design, namely,
the distributions of the astigmatism of the progressive
power multifocal lens, tile directions of what is called
the axis of the astigmatism, the average power thereof
(namely, the arithmetic mean of the spherical power (or
diopter) thereof and the astigmatic power (namely, the
cylindrical diopter) thereof), the horizontal component
of the prism refractive power (namely, the prism diopter)
thereof and the vertical component of the prism diopter
thereof are changed in such a manner as to meet the
aforementioned requirernents.
Moreover, the results of the test independently
conducted by the inventor of the present invention have
revealed that there is little correlation between the
limitation astigmatism in the distinct vision zone of the
present invention in the case of using the far vision and
the addition (Di) and that a distinct vision can be
obtained or achieved if t;he astigmatism is within a range
of about 0.75 diopters to 1.00 diopter.
Therefore, if the same design is employed
correspondingly to any value of the addition (Di)
similarly as in the conventional manner, the distinct
vision zone in the case of using the near vision
inevitably has a tendency to narrow when the addition
CA 02232539 2003-04-08
c
(Di) is large. Such a tendency, however, can be
decreased if the design is changed in such a way that as
the addition (Di) is increased, the width W of the
astigmatism, which is less than about 1.00 diopters, is
increased so as to widen the distinct vision zone in the
case of the near vision.
In summary, the tendency of the distinct vision
zone, which is obtained by using the near vision, to
narrow in the case of the large addition (Di) can be
weakened if the width W(Di, X) (mm) of a zone, in which
the value of the addition (Di) ranges from 0.25 to 5.00
(at least 0.75 to 3.00) diopters and the value of the
astigmatism along a horizontal (or lateral) section curve
including the near vision power measuring position N is
not more than X (diopter), meets the following relation:
W(Da, X) ~ W(Db, X,Db ~Da)
(incidentally, X = 1. 00 diopter)
-> in the case that the "additions" of two kinds of lenses
meet the inequality Da > Db where Da and Db denote the
additions (Di) of two kinds A and B of lenses,
respectively. Incidentally, when the addition (Di)
becomes large, the astigmatism in the side of the near
portion or zone increases if the astigmatism is decreased
in the near portion. Thus the static visual field
becomes more stable, while the dynamic visual fields
31
CA 02232539 2004-04-19
becomes unstable* Namely, if a design to stabilize the
dynamic visual field is applied to the progressive power
multifocal lens having a relatively small "addition" and
the aforementioned method is applied to the progressive
power multifocal lens having relatively large "addition",
the static visual field of the progressive power
multifocal lens having a relatively large "addition"
becomes stable and the aforesaid requirements can be
simultaneously satisfied.
In the case of the lens of the present invention,
the "astigmatism" may be transmission astigmatism.
Further, the "average power" may be transmission average
power. Moreover, the "prism refractive power" may be
calculated from the deviation angles of visual lines.
Incidentally, concerning only the "addition", the
"additional surface refractive power" has especially been
used because this is the essential definition of the
"addition" (Di) used in the art. Additionally, if the
conventional definition ..(namely, "a curve on which no
surface astigmatism occurs (that is, what is called "an
,.,
umbilical meridian")) of the "principal fixation line" is
employed, the progressive power multifocal lens of the
present invention can be realized.
BRIEF DESCRIPTION OF THE DRAWINGS
32
CA 02232539 1998-03-18
Other features, objects and advantages of the
present invention will become apparent from the following
description of preferred embodiments with reference to the
drawings in which like reference characters designate like
or corresponding parts throughout several views, and in
which:
FIG. 1 is a front view of a progressive power
multifocal lens 1 (which is 70 mm in diameter) for the
) left eye of an eyeglass wearer, according to Embodiment
1 of the present invention;
FIG. 2 is a diagram for illustrating the surface
average power distribution in the case of a basic design
lens corresponding to Embodiment 1 of the present
invention;
FIG. 3 is a diagram for illustrating the
transmission average power distribution in the case of
the basic design lens corresponding to Embodiment 1 of
the present invention;
.)
FIG. 4 is a diagram for illustrating the surface
average power distribution in the case of a progressive
power multifocal lens according to Embodiment 1 of the
present invention;
FIG. 5 is a diagram for illustrating the
transmission average pe~wer distribution in the case of
the progressive power m.ultifocal lens according to
33
CA 02232539 1998-03-18
Embodiment 1 of the present invention;
FIG. 6 is a diagrams for illustrating the surface
astigmatism distribution in the case of the basic design
lens corresponding to Enabodiment 1 of the present
invention;
FIG. 7 is a diagraff~ for illustrating the
transmission astigmatism distribution in the case of the
basic design lens corresponding to Embodiment 1 of the
present invention;
FIG. 8 is a diagram for illustrating the surface
astigmatism distribution in the case of the progressive
power multifocal lens according to Embodiment 1 of the
present invention;
FIG. 9 is a diagram for illustrating the
transmission astigmatism distribution in the case of the
progressive power naultifocal lens according to
Embodiment 1 of the present invention;
FIG. 10 is a diagram for illustrating the surface
average power distribution in the case of a basic design
lens corresponding to Embodiment 2 of the present
invention;
FIG. 11 is a diagraim for illustrating the
transmission average power distribution in the case of
the basic design lens corresponding to Embodiment 2 of
the present invention;
34
CA 02232539 1998-03-18
FIG. 12 is a diagram for illustrating the surface
average power distribution in the case of a progressive
power multifocal lens according to Embodiment 2 of the
present invention;
FIG. 13 is a diagram for illustrating the
transmission average power distribution in the case of
the progressive power multifocal lens according to
Embodiment 2 of the present invention;
FIG. 14 is a diagram for illustrating the surface
astigmatism distribution in the case of the basic design
lens corresponding to Embodiment 2 of the present
invention;
FIG. 15 is a diagram for illustrating the
transmission astigmatism distribution in the case of the
basic design lens corresponding to Embodiment 2 of the
present invention;
FIG. 16 is a diagram for illustrating the surface
astigmatism distribution in the case of the progressive
power multifocal lens according to Embodiment 2 of the
present invention;
FIG. 17 is a diagram for illustrating the
transmission astigmatism distribution in the case of the
progressive power multifocal lens according to
Embodiment 2 of the present invention;
FIG. 18 is a diagram for illustrating the surface
CA 02232539 1998-03-18
astigmatism distribution in the case of a progressive
power multifocal lens according to another embodiment of
the present invention;
FIG. 19 is a diagram for illustrating the surface
astigmatism distribution in the case of a progressive
power multifocal lens according to still another
embodiment of the present invention;
FIG. 20 is a diagram for indicating the width W 1 of
a region, in which value:; of a surface average additional
refractive power along a horizontal section line
extending below the near vision power measuring position
N are not less than Dil2, in the diagram illustrating the
surface average power distribution of the progressive
power multifocal lens according to Embodiment 1 of FIG. 4;
FIG. 21 is a diagram for indicating the width W2 of
a region, in which values of a surface average additional
refractive power along a horizontal section line
extending below the near vision power measuring position
N are not less than Dil2, in the diagram illustrating the
surface average power distribution of the progressive
power multifocal lens a<;cording to Embodiment 2 of FIG.
12;
FIG. 22 is a diagram for illustrating the position al
relation between a spectacle lens and an eyeball; and
FIG. 23 is a table for showing the tendencies of the
3G
CA 02232539 1998-03-18
transmission distributions relative to the surface
distributions.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Hereinafter, the preferred embodiments of the
present invention will be described in detail by referring to
the accompanying drawings.
Embodiment 1
FIG. 1 illustrates a front view of a progressive
power multifocal lens 1 (70 mm in diameter) for the left
eye of an eyeglass wearer, according to Embodiment 1 of
the present invention.
As shown in this figure, in the case of the
progressive power multifocal lens 1 of this embodiment,
a far vision power measuring position F is located at a
place which is 8 mm upwardly away from the geometric
center G thereof. Further, a near vision power measuring
position N is disposed at a place which is deviated
downwardly from this geometric center G by a distance of
16 mm and also deviated therefrom laterally towards the
nose of a wearer by a distance of 2.5 mm. Moreover, a
position E, through which a visual line of the eyeglass
wearer passes when his or her eyes are in a frontal vision
37
CA 02232539 1998-03-18
condition (namely, in a front viewing condition), is located
at a place which is 2 mm upwardly away from the geometric
center G.
Incidentally, in the case of this embodiment, the
refractive power in the case of using the far vision is
S-5.50 diopters; the addition (ADD) +2.00 diopters; and
an employed base curve 2 diopters. Further, diethylene-
glycol-bisallyl-carbonate is employed as the material of the
lens, and the refractive :index (n d) thereof is 1.499.
FIG. 4 is a diagram for illustrating the surface
average power distribution in the case of a progressive
power multifocal lens according to Embodiment 1 of the
present invention. Further, FIG. 8 is a diagram for
illustrating the surface astigmatism distribution in the
case of the progressive power multifocal lens according
to Embodiment 1 of the ;present invention.
The progressive power multifocal lens having such
distributions is designed as follows.
Namely, first, optical information on the lens
surface is represented in distribution-diagram form by
using conventional techniques. Then, it is studied
whether or not such disi;ributions are best suited to the
eyeglass wearer. A lens having optimum "surface average
power distribution" and "surface astigmatism
distribution" is obtained on the basis of a result of the
38
CA 02232539 1998-03-18
study as a basic design lens.
FIG. 2 is a diagrams for illustrating the surface
average power distribution in the case of the basic
design lens corresponding to Embodiment 1 of the present
invention. FIG. G is a diagram for illustrating the surface
astigmatism distribution in the case of the basic design
lens corresponding to Embodiment 1 of the present
invention. Incidentally, in the diagram of FIG. 2 for
illustrating the average power, "contour lines" are
respectively drawn correspondingly to the values of the
average power, which are determined at intervals of 0.5
diopters. Further, in the diagram of FIG. fi for
illustrating the astigmavtism, "contour lines" are
respectively drawn correspondingly to the values of the
astigmatism, which are .determined at intervals of 0.5
diopters. These "contour lines" are drawn in common in
each of distribution diagrams which will be described
hereinbelow.
Next, the "transmi;ssion average power distribution"
and the "transmission astigmatism distribution" of the
basic design lens are obtained by calculation from the
surface average power distribution and the surface
astigmatism distribution, which are obtained in this way.
Actually, this calculation is performed by running a
simulation of the power and astigmatism of the spectacle
39
CA 02232539 1998-03-18
lens, through which light rays are incident on an eye of
a wearer, through the u:;e of three-dimensional ray
tracing by taking all of the aforementioned factors into
consideration.
FIG. 3 is a diagram for illustrating the
transmission average power distribution in the case of
the basic design lens corresponding to Embodiment 1 of
the present invention. Further, FIG. 7 is a diagram for
illustrating the transmission astigmatism distribution in
the case of the basic design lens corresponding to
Embodiment 1 of the present invention;
As is seen from the comparison between the surface-
average-power distribution diagram of FIG. 2 and the
transmission-average-power distribution diagram of FIG.
3, the average power of, especially, the near vision viewing
portion (namely, the reading portion) increases
extraordinarily in a transmission state.
Similarly, as is seen from the comparison between
the surface astigmatism distribution diagram of FIG. 6
and the transmission astigmatism distribution diagram of
FIG. 7, the aberration of, especially, the near vision
viewing portion increases in the case of FIG. 7, in
comparison with the case of FIG. 6.
As a consequence, it is found that although the
basic design lens is superior in the surface average
CA 02232539 2004-04-19
power distribution and the good surface astigmatism
distribution, the basic design lens is rather inferior in
the transmission average power distribution and the
transmission astigmatism distribution that actually
affects the feeling of a wearer at the time when he or
shev wears the lens.
It is sufficient for causing the most comfortable
feeling of an eyeglass wearer, which is the originally
intended purpose of the basic design lens, to establish
the lens so that the transmission average power
distribution and the transmission astigmatism thereof
themselves become closer to the surface average power
distribution and the surface astigmatism distribution,
respectively, as much as possible_
Thus, in the case of this embodiment, in view of the
fact that the fax vision power is negative, the
improvement of the design is repeated by t,-ia1 and error in
such a manner that the width W (namely, the width of a
region in which values of -a surface average additional
refractive power along a horizontal section line
y
extending below the near vision power measuring position
N are not less than half of the addition) in the case of the
progressive power multifocal lens of Embodiment 1
becomes narrower than the width VV in the case of the basic
design lens. Further, the transmission average power
41
CA 02232539 1998-03-18
distribution and the transmission astigmatism
distribution of each of trial lenses are obtained by
calculation. Among such trial lenses, a progressive power
multifocal lens, the obtained transmission average power
distribution and the obtained transmission astigmatism of
which are closest to the :surface average power distribution
and the surface astigmatism distribution of the basic
design lens, respectively, is employed as that of
Embodiment 1. Incidentally, the repetition of the design
is practically performed by making full use of optimization
techniques utilizing a computer.
FIG. 5 is a diagram for illustrating the
transmission average power distribution in the case of
Embodiment 1 of the present invention. FIG. 9 is a
diagram for illustrating the transmission astigmatism
distribution in the case of Embodiment 1 of the present
invention. As is clear from the comparisons among these
figures and FIGS. 3 and 7, regarding the power
distribution, the average power in the case of the
transmission average power distribution of Embodiment 1
(especially, the average power in the near vision portion
(namely, the reading portion)) is low in comparison with
the case of the transmission average power distribution of
the basic design lens. Thus, the transmission average
power distribution of Embodiment 1 becomes close to the
42
CA 02232539 1998-03-18
corresponding target distribution, namely, the surface
average power distribution of the basic design lens of FIG.
2, and is, therefore, improved.
Further, regarding the astigmatism distribution, the
aberration in the case of the transmission astigmatism
distribution of Embodiment 1 (especially, the astigmatism
in the near vision portion (namely, the reading portion)) is
low in comparison with the case of the transmission
astigmatism distribution of the basic design lens. Thus,
the transmission astigmatism distribution of Embodiment
1 becomes close to the corresponding target distribution,
namely, the surface astigmatism distribution of the basic
design lens of FIG. 6, and is, therefore, improved.
As a consequence, 'taking various factors into
account, it is found that the progressive power
multifocal lens of Embodiment 1 is superior to the basic
design lens.
Embodiment 2
A lens 3 of Embodiment 2 of the present invention is
also designed according to the same lens design method
applied to the progressive power multifocal lens of
Embodiment 1 as illustrated in FIG. 1, and further uses the
same material as of the lens of Embodiment 1 of the present
invention. The lens 3 of Embodiment 2 is
43
CA 02232539 1998-03-18
different from the lens of Embodiment 1 in that in the
case of this Embodiment 2, the refractive power in the
case of using the far vision is set at S+4.50 diopters;
the addition (ADD) at +2.00 diopters; and an employed
base curve at 7 diopters.
FIG. 12 is a diagram for illustrating the surface
average power distribution in the case of a progressive
power multifocal lens according to Embodiment 2 of the
present invention. Further, FIG. 16 is a diagram for
illustrating the surface astigmatism distribution in the
case of the progressive power multifocal lens according
to Embodiment 2 of the present invention.
This progressive power multifocal lens of
Embodiment 2 of the present invention is obtained by first
determining a basic design lens and then performing the
improvement of the desi3;n on the basis of the basic design
lens, similarly as of Embodiment 1 of the present
_ invention.
FIG. 10 is a diagram for illustrating the surface
average power distributiion in the case of the basic
design lens corresponding to Embodiment 2 of the present
invention. Further, FIG. 11 is a diagram for
illustrating the transmission average power distribution in
the case of the basic: design lens corresponding to
Embodiment 2 of the present invention. Moreover, FIG.
44
CA 02232539 1998-03-18
14 is a diagram for illustrating the surface astigmatism
distribution in the case of the basic design lens
corresponding to Embodiment 2 of the present invention.
Furthermore, FIG. 15 is a diagram for illustrating the
transmission astigmatism distribution in the case of the
basic design lens corresponding to Embodiment 2 of the
present invention.
In contrast with this, FIG. 13 is a diagram for
illustrating the transmission average power distribution in
the case of Embodiment 2 of the present invention.
Further, FIG. 17 is a diagram for illustrating the
transmission astigmatism distribution in the case of
Embodiment 2 of the present invention.
As is obvious from the comparisons among these
figures, regarding the power distribution, the average
power in the case of the transmission average power
distribution of Embodiment 2 (especially, the average
power in the near vision portion (namely, the reading
portion)) increases, in comparison with the case of the
transmission average power distribution of the basic
design lens. Thus, the transmission average power
distribution of Embodiment 2 becomes close to the
corresponding target di:>tribution, namely, the surface
average power distribution of the basic design lens of
FIG. 10, and is, therefore, improved. Further, regarding
CA 02232539 1998-03-18
the astigmatism distribution, the aberration in the case of
the transmission astigmatism distribution of Embodiment
2 (especially, the astigmatism in the far
vision portion) decreases in comparison with the case of
the transmission astigmatism distribution of the basic
design lens. It is, thus, found that the transmission
astigmatism distribution of Embodiment 2 becomes close to
the corresponding targelt distribution, namely, the
surface astigmatism distribution of the basic design lens
of FIG. 14, and is, therefore, improved.
Embodiment 3
FIGS. 18 and 19 are diagrams respectively
illustrating the surface astigmatism distributions in the
cases of progressive pov~er multifocal lenses according to
other embodiments of the present invention.
Incidentally, these embodiments employ the same design
_ techniques as employed in Embodiment 1 and Embodiment
2. The descri tion of the common
p parts is omitted for
simplicity of description..
The embodiments of FIGS. 18 and 19 are different
from Embodiment 1 and Embodiment 2 in that the
embodiments of FIGS. 18 and 19 are lenses having the far
vision power of 0.00 dio:pter and that the addition (ADD)
of the embodiment of FIG. 18 is +2.00 diopters, and the
4G
CA 02232539 1998-03-18
addition (ADD) of the embodiment of FIG. 19 is +1.00
diopter, and the distribution of FIG. 18 is shown by
using "contour lines" respectively drawn correspondingly
to the values of the astigmatism, which are determined at
intervals of 0.25 diopters, and the distribution of FIG. 19 is
shown by using "contour lines" respectively drawn
correspondingly to the values of the astigmatism, which
are determined at intervals of 0.125 diopters. The
y placement of positions F, E and N described in FIGS. 18 and
19 is the same as in the cases of Embodiment 1 and
Embodiment 2. Further°, a single curve (represented by a
dotted or dashed line) extending in a nearly central portion
of each of FIGS. 18 and 19 from top to bottom thereof as
viewed in each of these figures is a principal (or main)
fixation line and passes through the three positions F, E
and N.
Further, in a region (illustrated as being laying
higher or above the position F in each of these figures)
in which the principal fixation line is not deviated in
the horizontal (or lateral) direction from the far vision
power measuring position F, the spacings between the
contour lines are laterally (or horizontally) symmetric
with respect to a plane o~f mirror symmetry. Moreover, in
another region (illustrated as being laying lower or
below the position F in each of these figures) in which
47
CA 02232539 1998-03-18
the principal fixation line is deviated to the nose of a
wearer from the position F, the contour lines are dense
in the "nose-side part (namely, the right-side part as
viewed in each of these figures)" but are sparse in the
"ear-side part (namely, l~he left-side part as viewed
therein)". Thus, the change in the astigmatism along the
part extending from the principal fixation line to the
nose of the wearer is larger than the change in the
astigmatism along the part extending from the principal
fixation line to the ear thereof. This feature or
tendency holds true not only for the astigmatism, but
also for the direction of what is called the axis of the
astigmatic of the lens, the average refractive power
thereof, the horizontal component of the prism refractive
power (namely, the prism diopter) thereof and the vertical
component of the prism diopter thereof.
Here, in the case of the progressive power
multifocal lens, which has the same base curve and the
addition of Di diopters, let W(Di, X) (mm) represent a
width of a region in which the values of astigmatism
along the horizontal section curve passing through the
near vision power measuring position N are not more than
X.
In this case, when arbitrary two progressive power
multifocal lenses, whose additions are Da and Db
48
CA 02232539 2003-04-08
(diopters), respectively, and base curves are the same
with each other, are selected from the group of the
progressive power multifocal lenses, the following
relation holds for the addition Da > Db:
W(Da, X) > W(Db, X: Db/Da).
where X = 1.00 diopter. In connection with this
relation, the widths W in the cases of the progressive
power multifocal lenses of FIGS. 18 and 19 are compared
with each other, and it is studied whether or not this
relation is satisfied.
Thus, the width W3 of the near vision portion
(namely, the reading portion) in the case of FIG. 18 is
obtained as follows:
W3 = W(2.00, 1.00).
Fuxther, the width W4 of the near vision portion (namely,
the reading portion) in the case of FIG. 19 is obtained
as follows:
W4 = W(1.00, 0.5).
If the design of the lens of FIG. 18 is the same as
that of the lens of FIG. 19, the astigmatism distribution
of the lens of FIG. 18 should be equivalent to that of a
doublet consisting of two lenses of FIG. 19, because the
addition of the lens of FIG. 18 is twice as much as the
addition of the lens of FIG. 19.
Namely, the width (W_4) in the case of FIG. 19, in
49
CA 02232539 2004-04-19
which the addition (Di) is x-1.00 diopter and the
astigmatism (X) is 0_50 diopters, should be equal to the
width {W3) in the case of FIG. 18, in which the addition
(Di) is +2.00 diopters and the astigmatism {X) is 1_00
diopter_
However, the comparison between the widths e~' two
arrows, which are respectively indicated in FIGS. 18 and 19
and pass through the positions N of the lenses of
FIGS. 18 and i9, reveals that W3 > W4, namely, W(2.00,
1_00) > W(1.00, 0.50), that the aforementioned relation
is satisfied and that such a design decelerates a
tendency for the distinct vision area in the case of
using the near vision to decrease when the addition is
increased_
Relation between Lenses of Embodiment 1 and Embodiment 2
Next, the relation between the lenses of Embodiment
1 and Embodiment 2 will be verified hereinbelow.
FIG_ 20 is a diagram for indicating the width W 1 of
a region, in which values of a surface average additional
refractive power along a horizontal section line
extending below the near vision power measuring position
N are not less than Di/2, in the diagram illustrating the
surface average power distribution of the progressive
power multifocal lens according to Embodiment 1 of FIG.
4. Further, FIG_ 21 is a diagram for indicating the
CA 02232539 2003-04-08
width W2 of a region, in which values of a surface
average additional refractive power along a horizontal
section line extending below the near vision power
measuring position N are not less than Di/2, in the
diagram illustrating the surface average power
distribution of the progressive power multifocal lens
according to Embodiment 2 of FIG. 12.
In the case of the progressive power multifocal
lenses of these figures, which have the base curve of Bi
diopters and the addition of Di diopters, let W(Di, Bi)
represent a width of a region in which the value of
surface additional refractive power along the horizontal
section curve passing below the near vision power
measuring position N is not less than Di. The width W 1
of FIG. 20 is represented by W 1(2.00, 2.00). Further,
the width W2 of FIG. 21 is represented by W2(2.00, 7.00).
Here, the comparison between the widths W 1 and W2
reveals that there is little difference therebetween in the
case where the regions respectively corresponding to the
widths W 1 and W2 are in the vicinity of the near vision
power measuring position N and that the relation W2 >W1
holds for the case where these regions are shifted
downwardly from the position N.
Thus, it is found that the lenses of Embodiment 1
and Embodiment 2, which have additions of 2.00 diopters
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CA 02232539 2003-04-08
but a base curve of 7 diopters and a base curve of 2
diopters, respectively (incidentally, the base curve of
7 diopters > the base curve of 2 diopters), satisfy the
following relation:
W2(2.00, 7.00) > W 1(2_00, 2.00).
Incidentally, the tendencies of the "transmission
distributions" relative to the "surface distributions",
which are known from the results respectively
corresponding to Embodiment 1 and Embodiment 2, are
shown in a table of FIG. 23.
Although the preferred embodiments of the present
invention have been be described above, it should be
understood that the present invention is not limited
thereto and that other modifications will be apparent to
those skilled in the art without departing from the spirit of
the invention.
The scope of the present invention, therefore,
should be determined solely by the appended claims.
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