Note: Descriptions are shown in the official language in which they were submitted.
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~ULTIFOC~ CONTACT LENS WITH A DIFFERENC13 POW3~ ON
THE BACK SURFACE
BACKGROUND OF THE IN~tENTION
This invention pertains to the subject of contact lenses,
and in particular contact lense~ containing more than one
optical power or focal length.
It is well known that as an individual ages, the eye is
less able to accommodate, i.e., bend the natural lens in
the eye in order to focus on objectc that are relatively
near to the observer. This condition is referred to as
presbyopia, and presbyopes have in the past relied upon
spectacles or other lenses having a number of different
regions with different optical powers to which the wearer
can shift his vision in order to find the appropriate
optical power for the object or objects upon which the
observer wishes to focus.
With spectacles this process involves shiftin~ one's field
of ~ision from typically an upper, far power to a
different, near power. With contact lenses, however, this
approach has been less than sati~factory. The contact
lens, working in conjunction with the natural lens, forms
an image on the retina of the eye by focueing light
incident on each part of the cornea from di~fer~nt ~ield
angle~ onto each part oP the retina in order to form the
image. ~his ie demonstrated by the ~nct that as the pupil
contracts in re~ponse to brighter light, the image on the
retina does not shrink, but rather light coming through a
smaller portion of the lens i6 used to construct the
entire image.
It is known in the art that under certain circumstances
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that the bra~n can discri~inat~ separate competing image~
by acc~pting the in-focu~ image and reject1ng the out of
focus image.
One example of this type of lens used for the correction
of presbyopia by providing simultaneou6 nea~r and far
vision i8 described in U.S. 4,923,296 to Erickson.
Described therein is a lens 6ystem which comprises a pair
of contact lenses having one eye with a near upper portion
lo and a distant lower portion while the other eye contains
a distant upper portion and near lower portion. Together
these are said to provide at least partial clear images in
both eyes, and through suppression by the brain of the
blurred images, allows alignment of the clear i~age to ~ `~
produce an in-focus image. This system however requires
a ballasting b~ peripheral prism, or weight, to ensure the
proper orientation of the lens on the eye6 to achieve the
above described affect. ~
,'''
U.S. Patent number 4,890,913 to de Carle describes a
bifocal contact lens comprising a nu~ber of annular zone~
having different optical power~. Whlle thi~ reference
6tates that the vision zones may be distributed between
posterior and anterior sur$aces, it does not describe a
lens wherein the front and back surfaces function together
to provide a difference in optical power for near and
distant vision correction.
Another attempt at providing a bifocal contact lens i~
described in U.S. Patent number 4,704,016 to de Carle.
The lens is constructed either by the use o~ different
materials having different refractive indicia to achieve
different optical powers or by having different vision
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zone~ formed strictly on the bac~ ~urface o~ tha len~.
This reference as well fails to teach a lens wherein the
front and back surfaces cooperate to provide a difference
in optical difference powers corresponding to near and far
S focal lengths.
Prior art len~es using zones of different refractive focal
lengths were typically theoretical design3 and not
manufactured. This failure to realize an actual product
is due to the inability to manufacture the type of lenses
conceived. The production of contact lenses is
traditionally performed by ~pin casting or precision lathe
cutting. These processes produce radially symmetric
lenses upon which it is extremely difficult to effect non-
annular areas having different focal lengths becausemachining different curvatures around the lens is
impossible, unless it is of an annular, "bull's eye"
design.
One attempt known in the art to provide a method of
compen~ating for presbyopia without complex lens
manufacture is known as "monovisionn. In th~ monovision
eystem a patient is fitted with one contact lens for
distant vision in one eye and ~ second contact lens for
near vision in the other eye. Although it has been found
that with monovision a patient can acceptably di~tinguish
both distance and near ob~ects, there is a subst~ntial
1098 0~ binocularity.
For these reaeone, although ~imple system~ such a~
monovi~ion are somewhat understood, more complex schemes
~or multifocal refractive lenses are primarily
theoretical.
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Another approach to producing a multifocal correcti~ eyo
len~ involves the use of diffractive optics. One of the
shortcomings of this approac~, as with previously
described types of multifocal lenses using radially
5 symmetric, concentric near and far distiance zones has been
deficiency in near visionr particularly at low light
level~. In a diffractive design only about 40~ of the
light inc~dent on the lens i8 used for near ~icion with
another 40% being used for far vision. The remaining 20%
is not used for either near or far vision, but rather is
lost to hiqher orders of diffraction and scatter effect.
This represents the best theoretical case and in
manufacturing reality even less light is available due to
manufacturing difficulties. Difficulty of manufacture in
lS general represents another shortcoming of diffractive
lenses since the diffractive surface must be to tolexances
on the order of the wavelength of light.
.
A more practical approach to providing a multi-focal
contact lens is described in co-pending application serial
number 728,903 filed on 7/1o/gl wherein there ls discloced
a non-oriented multifocal refractive lens made of a
plural~ty of seg~ents having at least two different
optical powers to effectively focu~ light on the retina of
the eye providing near and distant vision. The first
optical power is provided on a first set of segments to
provide distant vision while the ~econd set o~ ~egment~
provides a s,econd optical power to provide ne~r vision.
The segmentsiare preferably arranged ~o that the ratio~ o~
the areas of each optical power will remain con~tant
de~pite the changing diameter of the pupil. Thi~ i~
accomplished by havin~ boundaries between segments o~ the
len~ beqin at the middle of the lens and continue to the
outer edge of the optical region, either as a line segment
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or as a arcu~te path. A further ~spect of thi~ inv~ntion
i5 a method of producing suoh a multifocal len~,. Thi~ i~
accomplished by taking lens ~urfAce mold~ for different
optical powers and separating the surface~ into segments
along a path of the center of the surface mold to the
circumferential edge so that the segments are si~ilarly
~ized and are interchangeable. A multifocal lerl~ mold can
then be assembled and the segments of the irst and second
lens surface molds which are fitted together to form the
lo composite lens surface mold.
An improved multifocal segmented contact lens is described
in copending application serial number 7/827,199 filed on
1/28/92. In this application there is di~closed a
lS multifocal contact lens characterized by having a central
zone wherein one of the multifocal segments includes the
central zone of the lens. The boundary between the
seg~ents i9 defined by an arcuate path such as a semi-
circle having both ends of the path on the ad~oining
parameter of the near and distant segments. This design
ha6 the advantage of eliminating from the central optical
axi~ the ~egment boundaries including the central ~unction
point found in the above descr1bed lens.
While th~ lenses made according to the above described
applications are functional and the manufacturing
techniques described therein are a practical way of
molding contact lenses, two area~ are ob~ect~ of
improvement. ..
The first is wearer comfort. For a contact lens co~fort
is determined to a large degree by the smoothnees of the
~urface of the contact with the cornea of the eye and the
eyelid. While the lenses made according to the above
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applications anticipate that the optical ~ur~a~es will ba
placed on the front surface o~ the contact len~ in order
to avoid irritation of the cornea, a lens having two
different optical ~urfaces on the front of th~ lens may -
~till irritate the eyelid or cause the lens to move
because the difference in height between t:he optical
surface causes a step which can snag the in~ide o~ the
eyelid.
~ ,:
lo Furt~er, the method described for manufacturing the lenses
requires the careful fitting of the mold piece~ containing
the different optical powers in order to make the mold for
the lens. While this represents a possible manufacturing
technique, it would be desirable to eliminate the
requirement for careful matching of the optical curve
segments used to make the mold each time a different
distant/near combination is made.
It is an object of the present invention, therefore, to
provide a multifocal contact lens having the advantages of
the above described lenses in the above copending
applications, while at the ~ame time minimizing the
deleteriou5 ef~ects as50ciated with the step between the
height of the different optical surfaces.
It is a further object of the present invention to
minimize the flare associated with a multifocAl, multl-
powered optlcal lens as w ciated with th~ opticnl
diqcontinuity at the boundary between optical ~sgments.
It is a further object to provide a method of
manufacturing a segmented multi~ocal lens that doQs not
require the careful separation and matching of different
optical powered surfaces to make the mold for various
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combinations of opti~al power bifocal contact len~es each
time a new lens combination is to be made.
~ MMA~Y OF THE INVENTIQ~
The above object~ are achieved by const:ructing a
multifocal lens having a first optical power on a first
portion of the optical region of the back ~urface of tha
lens, and a second optical power on a second portion of
lo the optical region of the back surface portion of the
lens, (that is, the surface in contact with the cornea of
the eye), and a third optical power over the entire
optical region of the front surface of the lens. The
first optical power on the back surface, in combination
with the third optical power on the front surface,
produces a basic, distance corrective optical power while
the second optical power, ~ore positive than the first and
also on the back surface, produces in combination with the
third optical power on the front surface, a difference
corrective power to provide the appropriate near focal
length for an individual requiring bifocals.
Thi~ type of bifocal contact lens can be readily
manufactured by molding. A convex mold surface containing
the inverse of the first and second optical powers i8
sel~cted for the back surface of the lens. A concave mold
piece containing the inverse of the third optical power io
selected for the front sur~acQ of the len~ ~uch that
together wlth the first optlcal power, the desired basi¢
di~tance corrective power i8 provided, and with the second
optical power, the desired near corrective power i8 ' .
obtained. The desired optical power~ are then imparted on
a lens by molding a contact lens between the two mold
portions.
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Thi~ len~, h~ving ~ first and ~ second opt~cal power on
port~on of the back surface of the optical region of the
contact lens and a third power on the front sur~ace of the
optical region contains several distinct improv~!~ents over
S the previous nonoriented multifo~al contact lens.
An advantage is by having the step on the back ~urface of
the contact lens in contact with the eye, the
discontinuity between the ~egments is filled with tear
fluid providing a consistent transition between segment~
and reducing the problem of diffractive flare which
typically occurs with a lens having a transition between
optic zones.
Another advantage to having a lens according to the
present invention is that the step height is not in
contact with the inside of the eyelid. Therefore,
relative movement between the lens and human tissue,
di~comfort and decentration of the lens is no greater then
th~t found in a typical ~ingle power lens.
A third advantage to the present invention i8 that the
~anu~acturing process is greatly ~impliPied. Rather than
needing to construct a contact lens mold by using segments
of di~ferent power~ which must be carefully fitted
together to put both the individual distant and near
optical powers on a single surface, separate surface mold~
may be maintained and then matched to a particular bifocal
preaoription. That iB, the power diP~erence i~ produced
on the back mold surface which can then be mixed with any
number of easily produced front mold surPaces to
efficiently produce a multiplicity of finished lens
parameters.
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BRIEF DE5CRIPTION OF THE DRAWING~
Fig. 1 is a plan view showing the optical re~ion of the
~old ~ack sur~ace, molds front surface and resulting lens
in Figures a, b and ~ respectfully.
Fig. 2 is a plan view showing the same respective surfaces
as in Fig. 1, but with an alternate geometric ~s~bodiment.
Fig. 3 is a plan view showing the same respective surfaces
as in Fig. 1, but with an alternate geometric embodiment.
DESCRI~ION OF TH~ PREFERRED EMBODIMENT
Thi~ invention retains the fundamental advantage6 of the
embodiments found in the co-pending application~ .
identified above in that the lens of the present invent~on
requires and has no weights, ballasting or prism to orient
the lens in a particular radial orientation. Another
aspect of this invention is that the area of near and
distant focal lengths can be equal and independent of
pupil size. This pupil size independence can be realized
when the ratio of areas for near and di~tant vi~ion
remains the Bame ~or any circle within the lens concentric
with the lens.
Referring to Figs. la, lb and lc, the simpl4~t embodiment ;;.
of the invention i8 ~hown in a plan view o~ the mold
surface for the back curve of the contact len~, the mold
surface for the ~ront curve of the contact lens, and the
resulting contact lens, respectlvely. This embodiment
consisting of wedge-shaped, alternating near and di~tant
portion~ on the optical region 10 of the len6. Thi~
optical region i8 6urrounded by lenticular portion 20. It
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i~ to be understood that if desired by the d~i~igner, the
optical part of the lens may extend to the len~ periphery.
The multifocal nonoriented contact lens shown in Fig. lc
is a result of one surface of the contact len6, such ~8
the surface imparted ~y the mold for the back ~urface of
the csntact lens shown in Fig. la ~aving alternating first
and second segments of power, combined with the third
power of the front surface of the contact lens, guch as
the surface imparted by the mold for the back surface of
the contact lens æhown in Fig. lb and the refractive power
of the bulk of the lens material.
By way of example, a patient with a typical bifocal
pre~cription requirement of a minus 3.50 diopter distance
vision and requiring a plus 2.00 diopter add for near
vi~ion (resulting in a near corrective power of minus 1.50
diopters) would previously have required the selection of
the front 6urfaces of mold piece~ having a power of minus
3.50 diopter~ and minus 1.50 diopters. These pieces would
then need to be carefully fitted to produc~ the
combination required above.
With the present invention, however, $t would only be
nec~sary to use the typical ~ingle power di~tant
correction mold surface for minus 3.50 diopter~ for the
distance portion on the front of the contact lens mold,
then select tbe ~egmented back eur~ace ~ingle mold pie~e
¢ontaining the ~econd, optical pow~r e~fecting the
difference of plU8 2.00 d~opters compared to the u~ual
back surface first optical power.
Desaribed in more detail, the above specified lens would
have on a ~irst optical region portion of the back concave
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~i~7828
surfa~, ~ fir~t optic~l powor of -47.620 diopter~. Thi~
would result from a back surfac~ radius o* 8.400 mm on
that first portion, and would be the same as that used on
~ingle focal length lenses providing -~.50 diopter
S correction. T~e second portion of the back surface would
have a radius of 8.768 mm and produce an optical power in
this portion of -45.620 diopter~.
The front surface of the lens would have in the optical
region a convex surface with a radius of 9.086 mm,
effecting a third optical power of 44.022 diopters across
the entire optical region. Again, this curvature and
power would be the same as that for a single focal length
len~ having a net power of -3.50 diopters.
The lens of the present example, made of etafilcon A and
having a refractive index of 1.4 would have a thickness of
0.070 mm increase the overall optical power of the
combined lens surfaces by 0.098 diopters, due to the thick
len~ effect.
The net result i8 a len~ with a back vertex pow~r for the
fir~t (back sur~ace), third (front surface) and bulk
rQfractive optical power~ of -47.620 + 44.022 + 0.098 -
-3.500 diopters for distance vision, all typical for a
s$ngle focal length contact lens. The second back ~urface
optical power, however, is +a . oo diopter~ more positive
than the first back 6urfaoe optical power, and when
combined with the third (front surface) and thick lens
correction optical powers yields a net back vertex power
of -45.620 + 44.022 + 0.098 - -1.500.
Once a back surface mold is made with the corresponding
surfaces for the basic front optical power, whatever it
..
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~ay be, and the particular near ~add" power, the need for
care~ul fitting of mold piece~ i8 ell~inated and the
flex~bil~ty to mat~h any ~ront ~urface distant vision
corrective power with the appropriate "add" or di~ference
optical power on a back surface optical region lens mold
surface allows for the simplified production of any
distant and near power combination desired.
One skilled in the art can appreciate that the
fundamentally similar but crude approximation of the
segmented lens described herein, known as "monovision"
where the patient is fitted with one contact lens for
distant vision in one eye and a second contact lens for
near vision in the other eye, can allow a patient to
acceptably distinguish both distance and near objects, but
with a substantial loss of binocularity.
By having both distant and near focal length in both eye~,
the wearer of the lens according to the present invention
can not only have acceptable vision at ~oth distant and
near focal lengths, but also attains a ~air degree o~
stereoscopic vi~ion wherein binocularity i8 achieved.
Prior art attempts to provide a non-oriented multifocal
ophthalmic lens eliminated the need for ballasting by
having a lens with concentric distant and near lens
portion~. While lenses o~ this type can be made according
to the ~tructure and method o~ the pre~ent invention, thQ
de~lgn~ de~cribed herein are pr~erred because they
maintain a constant or nearly ~onstant ratio between the
near and far vision areas. As can be seen ~rom Figures 1
and 2, unlike prior art lens designs these segments
maintain equal areas of near and far tocal lengths because
an area within a circle concentric with the lens
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independent o~ the circle'a 8iZ~, analsgou~ to ths pUpil
o~ the eye as ~t dilates and contract~ with the amount o~
light incident upon eye.
S In this way the lens of the present invention has the
advantage that the ratio between the distant and near
portion of the len3 can either be constant w~th radius or
can be a controlled function of the pupil size.
An aspheric lens surface may be used for one or more
optical surfaces, with the advantage that the aspheric
shapes allows a design to be fabricated having a uniform
and minimal edge thickness and smooth junction between the
optic region 10 and the lenticular portion 20. This i6
not possible with spherical sections. Although it is
possible to design a len~ according to the present
invention with spherical sections that would meet optical
requirements, the use of the aspheric surfaces
particularly on the back surface area minimizes the step
height difference between the surfaces and possible
irritation to the eye.
The appropriate design of optic~l aspherical ~urfaces for
artif$cial eye lenses i8 given in U.S. Patent Number
5,050,981.
Another design techni~ue can be used to lessen the ~tep
height difference between nQar and di~tance ~egment~ ~or
either the a~pherical or ~pherical segment len~ design.
Referring to Figure 2, an arcuate boundary between the
difference (near minus distance) segments and the ~egments
on the back ~urface oP lens can be used to decrease the
height difference, particularly at intermediate points.
... ..... .
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Using an arcuate boundary between the ~egment~ decrease~
the step height by defining a path that is at ~n angle to
the gradient between the two segment heights. In
practice t the arc is drawn with one end of the arc at or
S near the lens center and the other at the edge of the
optic region. A typical arc segment would be one where
the radius i8 longer than the arc chord, fcr example, a
ratio of two to one between the arc radius an~l the chord
bisector. Ratios of two to one or greater would ~e
lo expected to yield good results, although a ratio of les~
than two to one may be used, with the limiting case being
a semicircle.
The arcs defining the boundaries would be placed upon the
lens as shown in Fig. 2, having the symmetric pattern
shown.
The arcuate boundary between segments of a multifocal lens
reduce the step height between segments by traversing a
path at a substantial angle to the gradient ~ormed by the
two different heights of lens material rather than having
a boundary that substantially follows the gradient between
the two heights of the lens 6egments.
Molding technology which allows precision molding of
corrective eye lenses with high guality and repeatable
optical surrace~ now makes posslble len~e~ wlth complex
curvatures and ~urface6. As can be appreciated by one
skllled ln the art, once the mold 1~ made vlrtually any
type of lens shape regardless of its complexity can be
made repeatedly and with very little increase cost over
simpler shapes.
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A l~n~ of the above type i~, therefore, preferably
manufactured by molding. In general, the mold~ng proces~
preferred is that described in U.S. Patent~ 4,495,313 and
4,889,664. In thi8 process, the lenc surface ~old to be
made is not made on the ~urface that will immediately mold
the lens but is made one 6tep removed on a metal surface
which is used to make a plastic styrene mold which is ~hen
uæed to make the len3. As used in this specification, the
word "mold" is used to refer to any previous generation of
mold used in ~aking the lens, that i8 not only the
surfaces used to make lens itself, but the surfaces used
to make the molds that ultimately make the lens.
.
The metal molds containing the multifocal segmented
surfaces are made by selecting the appropriate lens powers
from conventional spherical or aspherical molds. In the
above example, these would be the surfaces corresponding
to the -3.50 diopters for the front surface and the back
surface having a difference power corresponding to +2.00
diopters.
The back mold surface is made by taking a back mold piece
with a surface having one optical power and a back curve
mold surface having the ~1.50 difference power. These
mold surfaces would then be cut into segment~ which are
similar and interchangeable. Preferably, making segment
cuts which correspond to diameter~ o~ the len~ sur~ace
through the cent~r point o~ the len~. The~e metal molds
are precision cut with wire electrodynamic machining
devices to produce segments with very little material los~
and extremely close fit by optical pol$shing of the cut
wall~.
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Molde separated ~n thi5 way can be fitted toqether to
produce the desired back curve mold sur~ace and bonded to
produce a surface that can be used to ~ake a back curve
mold that, along with the basic front distance power,
ultimately makes the contact lens. Because the segmented
back curve ~urfaces may be usGd independently with any
basic front power, a set of these segments may be bonded
together and need not to be then separat~d for later
reuse.
Althouqh it is an advantage of this invention that equal
surface areas for both the near and distant focal lengths
can be maintained independent of pupil diameter, it is
possible to make a lens according to the present invention
having an unequal ratio of near and distant focal length
areas. This is sometimes advantageous because near vision
i8 particularly difficult in low light conditions.
Another embodiment of the invention is where the ratio
between the area of near and distant focal length can be
made to be a function of pup~l diameter. For instance,
where the pupil diameter i8 small, there ~ay b~ an equal
area of near and distant focal lengths. A~ the pupil
diameter increases, however, such a~ under low light
conditions, the ratio of near to di6tant focal length can
increase by changing the ratio of the segment area~ near
the periphery of the optical region. It iB ea5y to tailox
not only the ratio of areas between ne~r and di~tant focal
length but al80 the point at which a tran6ition i~ made.
Any configuration i~ easily manufactured by molding after
the first lens mold i8 constructed as described above.
Referring now to Figure 3, an embodiment i8 shown wherein
the boundaries between the near segments and distant
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segment defined by semi-circular path that ha~ both end~
of the path on the ad~oining perimeter of the near and
distant segments. In addition, the boundarie~ defined by
the path are outside the central zone contained within the
optical region of the lens.
This embodiment has the advantage of eliminating from the
central optical axi~ seqment boundaries, inc:luding the
central junction point found in the previously described
10 embodiments.
As a specific example, a contact lens is provided where
t~e distant optical segment preferably is chosen to be
the one that includes the central zone of the optical
region. The semi-circular boundary bstween the near and
distant s~gments has a diameter of 5.165 millimeters and
a center on the central region optic region periphery.
The lens has the typical diameter o~ 14 mill~meters, and
a minimum distance through the center axis between segment
boundaries of l.S millimeters.
In another embodiment, the use of the hyperbolic arc path
allows the lens to maintain a boundary-~ree central zone
in the lens and can be designed to retain egual areas of
near and di~tant optical portions.
In a lens with these particular dimen~ions, the equation
describing the hyperbolic arc path o~ thi~ embodiment i~
given by the equation:
x2 ' ~ .
r~r2- ~kllj X2
where: r, = 0.4535 and
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k c -l~25
Although the above eguation defines a parabola, the
resulting curve may be specified 8S any conic section,
including an ellipse, hyperbola and parabola, depending
upon the k value.
The offset or mini~um distance from the central axis of
the lens to the near/distant boundary i~ 0.75 millimeters.
In this embodiment, however, the hyperbolic arc is such
that the s1ight loss of near focus optical area in the
central zone of the len~ is offset by the increase in near
zone optical area at the periphery.
With the embodi~ent ~hown in Figure 3, it may be possible
to construct the back surface of the lens mold not only by
cutting the entire back surface of the lens ~old, but also
by machining the optical region of that surface of the
mold. It i8 clear to one practicing in the art that if
the optical ~urface is not machined as one piece, the lens
may be made by the above described process wherein the
mold~ having the appropriate optical pow~rs can be
preci~ion cut along the appropriate curved path with wire
electrodynamic machining devices and then polished. The
cuts made into the outer peripheral, non optical portion
of the lens mold are of little conseguence 80 long as they
are properly matched to form a smooth surface.
The above description i~ givsn by way of example only and
variation thereon can be practiced within ths scope of the
following claims.
~'.',.
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