Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HARDISOFT SUPERPOSITION PROGRESSIVE LENS DESIGN
Background of the invention:
This invention relates to progressive ophthalmic lenses. More particularly,
this
invention relates to a new and improved progressive lens design having
desirable
features of both hard and soft progressive lens designs.
It is commonplace in the field of progressive lenses to refer to hard and soft
progressive lens designs. The hard design, reminiscent of the common trifocal,
features
a spherical distance portion (DP) spanning the width of the lens and a large
spherical
reading portion (RP), the two zones being connected by an umbilic corridor of
progressive dioptric power. The inherent and unavoidable aberrations, i.e.,
astigmatism, of the hard design are concentrated in the peripheral zones
bordering the
corridor and spherical RP. Because the aberrations are concentrated in
relatively small
areas of the lens, their magnitude and rate of variation are highly noticeable
to the
wearer, giving rise to the term "hard" progressive. In the hard design,
maximum
distance and reading utility are obtained at the expense of overall visual
comfort
resulting from concentration of all of the astigmatism in the peripheral zones
bordering
the umbilic corridor and the spherical reading portion. A typical prior art
hard lens
design is shown in U.S. Pat. No. 4,056,311.
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T'he soft design lessens the visual impact of the inherent aberrations by
allowing
them to extend into the lateral areas of the DP and by reducing or minimizing
the width
of the RP. The soft design may be entirely aspherical in both the distance and
reading
portions. The magnitude and rate of change of the aberrations of such a design
are
markedly lower than those of the hard design, giving rise to the term "soft"
progressive.
In the soft design, enhanced visual comfort is obtained at the expense of
reduced acuity
in the peripheral areas of the DP and RP. Perhaps the ultimate soft design is
disclosed
in applicant's prior U.S. Pat. No. 4,861,153.
Summary of the Invention:
Based on the foregoing, it can be seen that the design of progressive lenses
can be
considered one of trade-off and compromise. For maximum distance and reading
utility
the hard design is indicated, whereas for maximum comfort one chooses a soft
design.
From a mathematical perspective, the hard and soft designs represent the
endpoints of a
continuous spectrum of possible designs distinguished from each other by
differing
degrees of hardness. A design of intermediate hardness might be obtained, for
example,
merely by selecting a spherical DP whose size is intermediate between those of
the
pure hard and pure soft designs. The algebraic description of a typical
surface of
addition value A (i.e., the power add of the lens) in the design continuum can
be
written in the functional form
( 1 ) Z ~A~ =f ~A~ (x,Y,P)
where z denotes the sag (elevation) of the progressive surface above the x-y
plane at
the point (x,y) and p stands for a continuously variable parameter or set of
parameters
(DP and RP size, for example) whose value controls the degree of hardness. Let
pH and
ps denote the parameter sets corresponding to the pure hard and pure soft
(endpoint)
designs. The functional forms of these two limiting cases are then given by
(2) ZHtA~ =~A~ (x~Y~Px)
and
(3) Zs(A> =I(A~ (x~Y~Ps)
respectively.
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With such an intermediate design, one might hope to capture some of the
advantages of both the hard and soft designs while reducing their
disadvantages.
However, this is a relatively simplistic approach, and it does not achieve the
benefits
that can be achieved by the present invention.
One aspect of the invention provides a progressive ophthalmic lens having: a
distance portion, a reading portion and an intermediate portion; the lens
having a
composite progressive power surface Z,o~A) defined by the equation:
Z~(A) =ZH(A-B> +Zs(B) - ZH(°)
Another aspect of the invention provides a method of defining a composite
progressive power surface of a progressive ophthalinic lens having a distance
portion, a
reading portion, and an intermediate portion, including the steps of defining
a first
design component of addition A-B, defining a second design component of
addition B,
and defining a composite progressive power surface Z~~A) according to the
equation:
Zc(n) =ZHcA-s) +ZstB> - ZHtO).
Where in both aspects:
Z~~A) = elevation of the progressive power surface above a reference plane;
ZH~A-B) = elevation of the progressive power surface above said reference
plane
for a hard, first design component of the lens;
Zs~B) = elevation of the progressive power surface above said reference plane
for a soft, second design component of the lens;
ZH~°) = elevation above said reference plane for a base curve
equivalent to the
base curve of said hard, first design component of the lens surface,
derived from said first design component by setting A-B=0;
A = the power addition of the composite lens;
B = the power addition of said soft, second design component; and
A-B = the power of addition of said hard, first design component.
In accordance with the present invention, a new design is disclosed for a
composite lens in which aspects of hard and soft lens designs are combined in
linear
superposition to form the composite lens.
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Description of the Drawings:
FIGS. 1 and 2 show, respectively, the surface astigmatism and mean power plots
for a prior art hard lens design having a 2.0 D add.
FIGS. 3 and 4 show, respectively, the surface astigmatism and mean power plots
for a prior art soft lens design having a 1.25 D add.
FIGS. 5 and 6 show, respectively, the surface astigmatism and mean power plots
for a prior art hard lens design having a 0.75 D add.
FIGS. 7 and 8 show, respectively, the theoretical surface astigmatism and
power
plots for a composite lens of this invention having an overall add of 2.0 D.
FIGS. 9 and 10 show, respectively, the theoretical surface astigmatism and
power
plots for a prior art hard lens design having a 3.0 D add.
FIGS. 11 and 12 show, respectively, surface astigmatism and power plots for a
prior art hard lens design having a 1.75 D add.
FIGS. 13 and 14 show, respectively, the theoretical surface astigmatism and
power plots for a composite lens of this invention having a 3.0 D add.
FIGS. 15 and 16 show, respectively, the surface astigmatism and power plots
for
an actual lens of this invention having a 2.0 D add.
Description of the preferred embodiment:
In accordance with embodiments of the present invention, composite progressive
lenses are produced having an intermediate degree of hardness. The design of
the
embodiments result in a composite lens having the functional advantages of the
pure
hard design but exhibiting reduced peripheral aberration (astigmatism). Rather
than
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selecting an intermediate design from the continuum of designs represented by
Equation (1), the present invention achieves a composite design, comprised of
a linear
. superposition of the hard and soft endpoint designs represented by Equations
(2) and
(3). The elevation zc(A), of the composite lens above the x,y plane is given
by
(4.) Z,C(A) - ZH(A_B) -f- ZS(B) - ZH(0)
In the composite, the hard component has addition A - B and the soft component
has
addition B, the combination of which yields a composite design of addition A.
To
avoid a doubling of the composite elevation due to the base curves of the hard
and soft
components, one of the base curves, here represented by a hard design of zero
add, is
subtracted from the linear superposition.
The present invention also makes use of a threshold effect, which is that the
eye
will not notice aberrations (astigmatism) below a certain level, especiaily if
the
aberrations are peripherally located. Accordingly, in carrying out the present
invention
there may be assigned to the soft component the highest addition value
possible
1 S consistent with unnoticability of the associated peripheral DP
aberrations. For most
wearers, the DP aberrations associated with a soft design of addition of B ~ I
.25 D will
be virtually unnoticeable.
The following examples set forth practical examples of lenses constructed in
accordance with the present invention.
EXAMPLE 1
An embodiment of the composite design having an overall addition value of
2.00 D will now be described. For purposes of comparison, the surface
astigmatism
and mean power of a pure hard design of addition 2.00 D is shown in FIGURES I
and
2; the pure hard design is characterized by a spherical DP and is to be
considered prior
art. The composite design will be composed of a linear superposition of (a) a
soft
component of addition I .25 D, whose astigmatism and mean power
characteristics are
depicted in FIGURES 3 and 4; and (b) a pure hard component of addition 0.75 D,
whose astigmatism and mean power characteristics are shown in FIGURES 5 and 6.
Note that the hard component has a spherical DP and is in fact an 0.75 D add
version of
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' the lens depicted in FIGURES 1 and 2. The astigmatism and mean power
characteristics of the composite design are shown in FIGURES 7 and 8. A
comparison
of FIGURES 7 and I shows that the contours of 0.50 D astigmatism of the
composite
and the pure hard designs are nearly coincident. This means that the utility
of the DP
and RP of the composite design matches that of the pure hard design. However,
as
indicated in Table 1, the maximum surface astigmatism of the pure hard design
is 2.93
D, whereas the maximum surface astigmatism of the composite design is only
1.89 D,
an astigmatism reduction of 35.5%. Notice, too, that the astigmatism and mean
power
gradients of the composite design are more gradual than those of the pure hard
design,
thus making the composite design the more comfortable design and the easier
one to
adapt to. This Iea.ds to the conclusion that the composite design principle
achieves the
stated objective of reducing aberration over that of the prior art sphericalDP
design
while retaining the tatter's DP and RP utility.
EXAMPLE 2
A second example of the invention will now be given for a composite lens
having an overall addition of 3.00 D. The astigmatism and mean power
characteristics
of a pure hard design lens of 3.00 D addition are shown in FIGURES 9 and I 0.
The
composite design will be composed of (a) the same soft component of addition
1.25 D
as was used in the previous example and whose astigmatism and mean power
characteristics are depicted in FIGURES 3 and 4; and (b) a pure hard component
of
addition 1.75 D, see FIGURES 11 and I2. The astigmatism and mean power
characteristics of the composite lens are shown in FIGURES I3 and 14. A
comparison
of FIGURES I 3 and 9 shows that the contours of 0.50 D astigmatism are nearly
coincident, meaning that, as in the case of the previous example, the DP and
RP utility
of the composite design compares favorably with that of the pure hard design,
Moreover, as indicated in Table 1, the maximum astigmatism of the pure hard
design is
4.42 D, whereas that of the composite design is only 3.22 D, an astigmatism
reduction
of 27.1 %. As in the case of the previous example, the reduced astigmatism and
power
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gradients of the composite design make it the more comfortable one and the
easier one
to adapt to.
Table 1, below, sets forth a summary of the astigmatism characteristics of the
lenses of Examples I and 2.
ExampleAddition AdditionMaximum Maximum/ Reduction
A B astigmatismastigmatismof reduction
of soft of soft of compositeof astigmatism
componentcomponentdesign hard lens relative
of to
Addition hard design
A of
1 2.00 1.25 1.89 2.93 1.04 35.5
2 ~ 3.00 ~ I.25 3.22 ~ 4.42 ~ 1.20 ~ 27.1
~
An approximate formula can be stated for the maximum astigmatism achieved
in a composite design having a soft component of addition B. The maximum
I O astigmatism contributed by the soft component alone is approximately 0.75
B, whereas
that contributed by the hard component is approximately 1.5{A-B). Hence the
total
maximum astigmatism of the composite design is
(5) astig(max) = 1.SA - 0.75B
and the reduction of astigmatism relative to that of a hard design of addition
A is thus
0.75B. Equation (5) assumes that the astigmatism maxima associated with the
hard and
soft components alone occur at the same point of the composite progressive
surface. It
is easily seen that the astigmatism values of Table I conform approximately to
Equation (5).
Note that if the soft component of a composite design series is the same for
all
additions of the series, then the ratio of the maximum astigmatism to the
addition varies -
with the addition. In current parlance the superposition design principle
yields a multi-
design lens series.
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Fx_A~pI,E 3
. An actual composite lens according to this invention was constructed having
an
overall addition value of 2.00 D. This composite lens is composed of a linear
superposition of (a) a soft component of addition 0.80 D and (b) a hard
component of
addition 1.2 D.
FIGURES 15 and 16 show the astigmatism and power plots of the actual lens of
Example 3. Pertinent data derived from these plots are:
Distance portion width at level of MRP between branches of 0.50 mean power
curve: 25.6 mm
Distance portion width at level of MRP between 0.50 and I .00 astigmatism
curves, respectively: I2.6 mm and 36.0 mm, respectively.
Intermediate portion width at level 7 mm below MRP between 0.50 and I .00
astigmatism curves, respectively: 3.7 mm and 9.3 mm, respectively.
Reading portion width at Ievel 22 mm below MRP between 0.50 and I .00
I5 astigmatism curves, respectively: 11.07 mm and 18.0 mm, respectively.
Corndor length from MRP to i.87 D power line: I9.0 mm
Maximum nasal astigmatism: I .90 D
Gradient of astigmatism by 6 point method and I2 point method, respectively:
0.I 7 D/mm and 0.14 D/mm, respectively.
The astigmatism plot shows very little astigmatism above the 0-180 degree
line,
a characteristic normally found only in the hard progressive designs. On the
other
hand, the maximum surface astigmatism is less than 2.00 D and the gradient of
astigmatism is Less than 0.20 D/mm, values characteristic of soft
progressives. Thus
the superposition design of this invention can be said to provide the utility
of a hard
progressive and the comfort of a soft design while minimizing the
disadvantages of the
pure hard and soft designs.
Of course the Iens of Example 3 is asymmetrical with respect to the corridor
meridian to ensure binocular compatibility of the Lens pair, as should be the
case with
any lens of this invention.
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While preferred embodiments have been shown and described, various
modifications and substitutions may be made thereto without departing from the
scope
of the invention as defined in the appended claims. Accordingly, it is to be
understood
that the present invention has been described by way of illustrations and not
limitation.
