Note: Descriptions are shown in the official language in which they were submitted.
16644
Spuncast Compound Lens
And Method Of Manufacture
Brief Description Of The Invention
A reproducibly-made, wholly spuncasted, non-truncated ballasted,
compound-surfaced soft contact lens and the method of making the same by
spincasting a polymer forming monomer mixture.
Background To The Invention
Toric lenses are simple lenses which have for one of their surfaces a
segment of an equilateral zone of a torus and consequently have different
refracting power in different meridians. A torus is a smooth rounded pro-
tuberance on a body part. A toric lens is used to correct astigmatism by the
use of a surface of compound curvature. Such lenses have been commercially
available for a long period of time. The compound curvature constitutes small
variations in thickness in the lens in the optic region to make adjustments
according to the prescription of the wearer of the lens to correct the
wearer's
astigmatism problem. The thickness differences in a contact toric lens in the
optic region may vary as little as 0.03 millimeters (mm) yet represent from
about 8 to about 30 percent of the depth or thickness of the lens.
Because the toric lens contains a surface of compound curvature, the
orientation of the lens relative to the orientation of the eye is important
for
effective visual correction of the astigmatic problem. Such orientation is a
more difficult technical problem in the case of contact toric lenses.
There is a class of bifocal contact lens which selects a predetermined
region removed from the lens' central axis to locate the near vision element.
-1-
16644
....
These lenses rely on refractive monofocal optics to generate the far and near
focal powers. Like the contact toric lens, the bifocal contact lens of this
type
relies on proper orientation of the lens on the eye to assure efficacy of the
lens.
In order for such contact lenses to orient appropriately on the eye)
they should have a thickness differential coupled to a sloping wedge-like
cross-sectional profile in the top midperiphery of the lens. This involves the
generation of sloping surfaces from about the periphery, especially the mid-
periphery, of the lens such that as the slope of the lens approaches the optic
region) the lens gets thicker. This sloping wedge-like profile in the contact
lens functions in concert with eyelid movement over the lens to force the
alignment of the lens in the eye such that the corrective toric or bifocal
surface
is properly aligned for visual benefit and correction. Because of the history
of
the contact lens art) the techniques has been frequently referred to as
'ballasting." Though somewhat inappropriate, that term is used herein to
refer to the aforementioned sloping wedge-like profile.
There are a number of ways of ballasting a contact lens. They include
placing a thin top peripheral cross-sectionally wedge-like section in the lens
with or without a Blabbed section at the bottom portion of the periphery of
the
lens. The optical axis of the lens may or may not provide the common axis of
curvature for the front and back surfaces of the lens. When it is not, then
the
thin top peripheral cross-sectionally wedge-like section in the lens is a
prism
ballast. A lens which possesses a thin top peripheral cross-sectionally wedge-
like section with or without a Blabbed section at the bottom portion of the
periphery of the lens is termed herein a "thin-sectioned ballasted contact
lens."
Thin-sectioned ballasted contact lenses have been used in the oph-
thalmic industry for correcting the visual orientation of lenses used to
correct
a number of vision defects such as astigmatism and presbyopia. In order to
correct these visual anomalies) the contact lens must maintain a preferential
-2-
.,_
16644
orientation on the user's eye. The thin-sectioned ballasted contact lens
cooperates with eyelid movement to maintain the required orientation.
Extraordinarily tight lens dimensional tolerances are required to
reliably predict the equilibrium position of a thin-sectioned ballasted
contact
lens on the user's eye. The consequence of inadequate dimensional confor-
mance is unpredictable lens orientation and, as a result, residual astigmatism
in the case of toric lenses and blurry vision in the case of bifocal lenses.
The
unpredictable orientation greatly compromises the correction of astigmatism
in the case of toric soft contact lenses and presbyopia in the case of bifocal
soft
contact lenses. Consequently, such unpredictable orientation frequently
compromises the ultimate visual acuity and stability of vision provided by
these modalities of vision correction.
The degree of accuracy required of the manufacturing process is much
higher for a ballasted contact lens than a spherical lens as a spherical lens
does
not need to maintain a preferential orientation on the user's eye. Ballasted
contact lenses are routinely manufactured using lathing and cast molding
techniques. Lathing processes used to manufacture prism ballasted torics are
complex and labor intensive as many cutting and polishing steps are needed to
generate the required lens geometry. The cast molding technique simplifies
and reduces the labor intensiveness of the manufacturing operation. The
complex geometry needed to produce a ballasted toric lens is placed on master
molds. The master molds are typically replicated with a thermoplastic using
injection molding.
. The fabrication of the master molds is complex and labor intensive.
However, the master mold can be used to reproducibly generate thousands of
injection molded parts. The toric or bifocal contact lens is cast against
convex
and concave injection molded parts. The process yields lenses which do not
require surface polishing. Toric lathing processes, as a result of their com-
-3-
issue
plexity and labor intensiveness) yield lenses which are substantially more
expensive and less reproducible than cast molded torics.
It is known in the contact lens art that spincasting provides a means
for reproducibly manufacturing spherical soft contact lenses at a low cost.
However, spincasting introduces technical issues, if used for the manufacture
of lens that have an irregular shape) that do not exist with lathing and
casting.
For example) a toric lens has compound curvature and a bifocal lens where the
near correction surface are off centered, has a multifaceted surface which is
another form of a compound curvature. In spincasting, these compound
surfaces have to be on the anterior surface of the lens and the posterior
should
have the requisite corneal cup shape. Thus, the female spincasting mold
surface for such compound surfaced lenses would have to incorporate such
compound surfaces in its design. In cast molding this poses no problem
because the posterior is separately controlled by the presence of a male mold
More of the lens-forming monomers can be fed to the mold and the compres-
sion of the two molding surfaces in cast molding assures the integrity of both
surfaces. In lathing, the lens button being lathed can have a perfectly shaped
corneal cup and lathing away of excess plastic from the anterior surface can
be
used to provide the compound anterior surface without disturbing the corneal
cup surface (posterior surface) of the lens.
A spincast contact lens is difficult to produce with ordered symmetri-
cally aligned surfaces. This can be appreciated from a consideration of the
dimensions of a contact lens. The hydrated lens typically had a diameter of
about 12 to about 16 mm. The center thickness may range from about .13 mm)
in the case of a -5 diopter lens, to about 0.35 mm, in the case of a + 4
diopter
lens. Such lenses taper of in thickness to the feathered edges at the
periphery.
The amount of resin used in spincasting the lens is extremely small. For
example, there is used about 30 microliters of the lens-forming monomers in
making each lens.
-4-
~p X96
These facts demonstrate the seeming improbability of using spin-
casting alone as a method for making lenses possessing a compound
anterior surface such as a tonic and bifocal lens. The expectation would be
that the centrifugation of the small amount of lens-forming monomers in the
cavity mold would sweep it across the mold's surface in a manner that the
posterior surface would replicate or possess a translation of the anterior
surface of the lens. This would result in a corneal bowl shape that also
possessed the compound surface that is on the anterior surface. This could
only result in the fit of the lens being uncomfortable to the wearer of the
lens and adversely affect the corrective capabilities of the lens. On the face
of it, spincasting would appear to be a poor choice of processes for making
contact lenses with compound surfaces.
Quite surprisingly, it has been determined that spincasting can be
used to make a lens with a compound anterior surface and with a corneal
bowl, posterior surface, that does not possess a translation of the anterior
surface.
The present invention is directed towards simplifying the
manufacturing and improving the accuracy of ballasted compound surfaced
lenses such as tonic and bifocal lenses by employing spincasting. The
present invention also is directed towards inexpensively producing
compound surfaced lenses by spincasting. The present invention further is
directed towards reproducibly producing compound surfaced lenses by
spincasting.
The Invention
This invention relates to a reproducibly-make, wholly spuncasted,
non-truncated, ballasted soft contact lens containing a compound anterior
surface and the method of making the same by spincasting a polymer
forming mixture. More particularly, the invention relates to reproducibly-
made, wholly spuncasted, non-truncated, ballasted tonic or bifocal soft
contact lenses containing compound anterior surfaces and the method of
making the same by spincasting polymer forming mixtures. In a preferred
5
~~9~~~
embodiment of the invention, the ballasting comprises thin sections at
peripheral sections of the lens.
In one aspect, the present invention provides a reproducibly-made,
wholly spuncasted, non-truncated, ballasted soft contact lens containing a
compound anterior surface, a thickness profile that is asymmetrical with
respect to its geometric center to provide a prismatic lens with the central
axis of its posterior surface symmetrical to its peripheral edge.
In another aspect, the present invention provides a process of
reproducibly making a wholly spuncasted, non-truncated, ballasted soft
contact lens containing a compound anterior surface which comprises spin
casting a polymer-forming, lens-forming monomeric mixture in a cavity
mold having a surface that is offset to allow for generation of a prismatic
ballast, a compound curvature and a central axis that is symmetrical to the
peripheral edge of the lens being formed, by rotating the monomer mixture
in the mold whereby the mixture replicates the mold's surface while at the
same time undergoes polymerization and forms a prismatic lens with the
central axis of its posterior surface symmetrical to its peripheral edge, a
corneal bowl free of the compound curvature and possessing a thickness
profile that is asymmetrical with respect to its geometric center.
The invention includes the spincasting of a polymer-forming, lens-
forming monomeric mixture in a cavity mold where the mold surface
possesses the compound curvature for a toric or bifocal soft contact lens and
thin-sectioned ballasting for that lens, rotating the monomer mixture in the
mold whereby the mixture replicates the mold's surface while at the same
time forms on polymerization a corneal bowl free of the compound
curvature and the thin-sectioned ballasting, and initiating the polymerization
of the lens-forming monomer mixture during such rotation whereby to form
a reproducible, non-truncated, thin-sectioned ballasted lens containing a
compound anterior surface.
6
.. ~~~~?~~G issue
Brief Description of the Drawings
Figure 1 is a top (plan) view of a spincasted toric lens containing the
compound toric curvature and thin-sectioned ballasting.
Figure 2 is a side cross-sectional view of a spincast mold where the
interior molding surface is the same as the anterior surface of the lens of
Figure 1
Detail Description Of The Invention
Briefly: Spincast molding begins with the fabrication of a convex
master mold. The master mold is replicated by an injection moldable ther-
moplastic. Liquid monomer mixture is dispensed onto the casting surface of
the concave injection molded part. The mold, containing the liquid polymer-
forming or lens-forming monomers (materials), is spun at a precise spin rate
which determines the radius of curvature of the spun surface. The monomers
are polymerized with thermal or ultraviolet energy. The spincast process
yields surfaces which do not require additional treatment.
In the fabrication of contact lenses by spincasting, the lens-forming
material is placed in the mold cavity having an optical concave surface wetted
by said material, and then intermittently and forced fed, one at a time) into
the
inlet end of a rotating polymerization column which desirably comprises a
"conditioning". zone near the inlet end and a polymerization reaction zone
toward the outlet end It is preferred that the molds be characterized by a
pretreated optical surface to increase its hydrophylicity or wettability in a
manner well-know in the art. The speed of rotation of the tube and the molds
is adjusted to cause and/or maintain radially outward displacement of the
lens-forming mixture to a predetermined lens configuration which when
subjected to the polymerization conditions employed in the tube will form the
desired shaped contact lens. Rotational speed of) for example) 300 r.p.m.) and
_7_
~0~9~8~
lower, to 600 r.p.m., and higher, can be conveniently used. The precise
rotational speed to employ in the operation is, of course, well within the
skill of
the artisan. Factors to be considered include the type and concentration of
the
components comprising the lens-forming material employed, the operative
conditions of choice, the type and concentration of catalyst) initiator,
and/or
radiation energy source, and factors discussed below and readily apparent to
the artisan.
Illustrative of relevant prior art that describes the practice of spincast-
ing are the following:
O. Wichterle, U.S. 3,660,545, patented May 2,1972
D. ftawlings) U.S. 4,517,138) patented May 14, 1985
D. Rawlings, U.S. 4,517,139, patented May 14, 1985
D. Rawlings, U.S. 4,551,086, patented July 11,1983
D. Rawlings, U.S. 4,468,184, patented August 28,1984
D. Rawlings, et al.) U.S. 4,534,723, patented August 13) 1985
The liquid lens-forming mixture can comprise monomer, prepolymer
or vulcanizable components. Particular suitable components are hydrophylic
monomers preferably including those which form slightly or moderately
crosslinked, three dimensional networks such as those disclosed in U.S)
3,822,089. Illustrative hydrophylic monomers include water soluble
monoesters or an acrylic acid or methacrylic acid with an alcohol having an
esterifiable hydroxyl group and at least one additional hydroxyl group such as
the mono- and polyalkylene glycol monoesters or methacrylic acid and acrylic
acid, e.g., ethylene glycol monomethacrylate ("hema'), ethylene glycol
monoacrylate) diethylene glycol monomethacrylate, diethylene glycol
monoacrylate, propylene glycol monomethacrylate) dipropylene glycol
monoacrylate) and the like; the N-alkyl and N,N-dialkyl substituted ac-
rylamides and methacrylamides such as N-methylacrylamide, N,N-
dimethylacrylamide) N-methylmethacrylamide, N,N-dimethylmethacrylamide,
_g_
and the like; N-vinylpyrrolidone; and the alkyl substituted N-vinyl
pyrrolidones, e.g., methyl substituted N-vinylpyrrolidone; glycidyl
methacrylate; glycidyl acrylate; the unsaturated amines; the alkyl ether
acrylates; solubilized collagen; mixtures thereof; and others known to the
art.
Hydrophylic monomers particularly useful in the practice of the
invention to manufacture contact lenses include hydrophobic acrylic esters,
suitably lower alkyl acrylic esters, preferably wherein the alkyl moiety
contains 1-5 carbon atoms, such as methyl acrylate or methacrylate, ethyl
acrylate or methacrylate, n-propyl acrylate or methacrylate, isopropyl
acrylate or methacrylate, isobutyl acrylate or methacrylate, n-butyl acrylate
or methacrylate, or mixtures thereof.
Other suitable monomers include the ethylenically unsaturaed
monocarboxylic acid esters, in particular, the methacrylic and acrylic acid
esters of siloxane monomers and polymers with/without a pendant hydroxyl
group. These monomers are well documented in the contact lens art; see,
for example, U.S. Pat. Nos. 4,139,548; 4,235,985; 4,152,508; 3,808,178;
4,139,692; 4,248,989; and 4,139,513.
Among the preferred monomeric mixtures are those which contain at
least one alkylene glycol monoester of methacrylic acid, especially ethylene
glycol monomethacrylate, and at least one crosslinking monomer such as
the alkylene glycol diester of methacrylic acid, especially ethylene glycol
dimethacrylate. Such mixtures may contain other polymerizable monomers,
desirably in minor amounts such as N-vinylpyrrolidone, methyl
methacrylate, acrylamide, glycidyl methacrylate, N-methylacrylamide,
diethylene glycol monomethacrylate, and others illustrated.
9
'~~8~
The above illustrated monomers, monomeric mixtures including
mixtures of hydrophobic and hydrophylic reactants) may be further admixed
with a minor proportion or di - or polyfunctional species include: divinylben-
zene, ethylene glycol diacrylate or dimethacrylate) propylene glycol
diacrylate
or dimethacrylate) and the acrylate or methacrylate esters of the following
polyols: diethanolamine, triethanolamine, glycerol, pentaerythritol, butylene
glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, mannitol,
sorbitol, and the like. Other crosslinking monomers can be illustrated by
N,N-methylene-bis-acrylamide or methacrylamide, sulfonated divinylbenzene,
and divinylsulfone.
Additional lens-forming materials which are suitable in the fabrication
of contact lenses are illustrated by one or more of the following U.S.
Patents:
2,976,576; 3,220,960; 3,937,680; 3,948,871; 3,949,021; 3,983,083; 3,988,274;
4,018,853; 3,875,211; 3,503,942; 3,532,679; 3,621,079; 3,639,524; 3,700,761;
3,721,657; 3,758,448; 3,772,235; 3,786,034; 3,803,093; 3,816,571; 3,940,207;
3,431,046; 3,542,461; 4,055,378; 4,064,086; and 4,062,627.
The polymerization reaction can be carried out in bulk or with an
inert solvent. Suitable solvents include water; organic solvents such as
water-soluble lower aliphatic monohydric alcohols as well as polyhydric
alcohols, e.g.) glycol, glycerol, furfural, etc.; and mixtures thereof.
Frequently,
the solvent component utilizes, to control solution or dispersion viscosity, a
minor amount of the reaction medium, i.e., less than about 50 weight percent.
That portion of the solvent can be termed a reactive diluent.
Polymerization of the lens-forming mixture may be carried out with
free radical catalysts and/or initiators or the type in common use in vinyl
polymerization. Such catalyst species can include the organic peroxides, the
alkyl percarbonates) hydrogen peroxides, and inorganic materials such as
ammonium, sodium) or potassium persulfate. Polymerization temperatures
can vary from about 20~C.) and lower) to about 100~C.) and higher.
-10-
16644
Polymerization of the monomer or prepolymer material can also be
effected using, for example) radiation (U.V., X-ray, microwave, or other
well-known forms of radiation) with/without the presence of well-known
initiators) and/or catalyst(s).
When using radiation as the catalyst in the polymerization process,
the polymerization column (tube), as typically used in spin casting, has to be
fabricated from a material that will not impede the transmission of the
radiation into the polymerization zone of the column. Glass) such as Pyrex,
would be a suitable material for the polymerization column when using long
wave U.V) radiation as the catalyst. When using other types of catalysts as
recited above, the polymerization column could be fabricated from various
types of metals such as steel, nickel, bronze, various alloys, and the like.
A feature of the invention involves the moderation of the polymeriza-
tion reaction. Because the mass of the lens-forming monomer mixture is so
small (about 30 microliters) and is spread out over a relatively large area so
as
to form a thin symmetrical film called the lens, the rate at which the
polymerization reaction is effected for the compound surfaced lens of the
invention is moderated as compared to the reaction rate of a comparable lens
but which is devoid of the compound surface. The reason for this is simple.
The compound surface presents a variability in the cross-sectional thickness
that is not found in the conventional essentially spherical lens of the prior
art.
This variability in thickness creates portions of the lens which will complete
its cure prior to other portions of the lens. If the reaction rate is not
moderated and the cure is effected in the same manner as the conventional
lens, the resultant lens will frequently be warped Therefore, to avoid such
warpage, it is desirable to slow down the polymerization. The appropriate
conditions for polymerization is a somewhat trial and error process because
they are predicated on a variety of factors such as the selection of the
monomers in the lens-forming monomer mixture, the curing agent or catalyst)
the initiator system, the thickness of the lens to be formed, and the like. As
a
-11-
16644
201965
rule) U.V. light is used to initiate the decomposition of the cure catalyst.
The
intensity of the light can be easily reduced to slow down the rate of
decomposi-
tion of the initiator, and this effectively reduces the rate of the &ee
radical
reaction, thus moderating the rate of the reaction. Therefore, the invention
comprises the carrying out of the reaction at conditions that avoid warpage of
the lens by too rapid rate of polymerization.
Various techniques are described in the art for the creation of a tonic
surface on a contact lens. The techniques described in the art are primarily
directed to lathing a plastic contact lens button to generate a tonic surface.
Such techniques are also useable for lathing the master mold from which is
made the plastic mold used in spincasting the tonic lens of the invention.
Illustrative of art describing such techniques are the following. U.S. Patents
Nos. 3,900,972, 3,902,277, 3,913,274, 4,178,720, and 4,680,998, and USSR
Patent No. 984,693.
Prism-free, thin-sectioned ballasted, spincast contact lenses, which
are symmetrical with respect to the geometric center) are generated from
master molds which are created by rotating the master mold, in the lathing of
it, about the axis which passes through the apex of the central arc on the
mold's casting surface. Additionally, the casting surface and central arc
should
be concentric with respect to the molds outer surface. A characterization of a
prism-free, thin-sectioned ballasted, spincast contact lens can be found in
U.S.
4,095,878. The patent fails to describe how or whether the toroidal surface is
formed by spincasting, but does indicate at col. 2, lines 45-47, that if the
lens is
formed by "centrifugal molding the flattening must be part of the mold"
Thin-sectioned prism ballasted, spincast contact lenses are generated
by offsetting the central radius of the master mold, on the proposed casting
surface, with respect to the edge of the casting surface and mold's outer
diameter) The resulting spincasting mold is rotated about an axis which
passes through the geometric center of the casting surface when making the
-12-
2~~~~~~
lens. Though the spincasting mold's surface is offset to allow for the genera-
tion of the prismatic ballast) the spuncast surface forms independently of the
surface it is cast against and is symmetrical with respect to the axis of
rotation.
In other words, the central axis of the mold is symmetrical to the peripheral
edge of the lens being formed.
As a result of the offset central arc on the mold's casting surface, the
prismatic ballasted lens' thickness profile is asymmetrical with respect to
its'
geometric center. The asymmetrical thickness profile acts as a prism and
displaces light passing through the lens. The prism ballasting creates a
thickness difl'erential on the lens which results in a preferential lens
orienta-
tion on the user's eye. The anterior surface geometry of the spuncast pris-
matic or prism-free ballasted toric may be similar to that found on several
commercially available lathed toric lenses. It comprises a toric optic zone)
which is decentered with respect to the apex of the base curve) possesses a
midperipheral curve which isolates the toric optic zone from the periphery,
and a flattened region at the bottom of the lens which enhances patient
comfort and orientation stability. The lens may possess a small band about its
peripheral edge located on the anterior surface which has a center of arc
which
is not offset, thus the center arc is the axis of rotation for the purpose of
yielding an edge geometry of uniform thickness about the lens. This edge
enhances user comfort.
The complex anterior surface geometry of the ultimate contact lens is
placed on a metal master mold, such as a stainless steel master mold, by
lathing. The master mold is thereafter accurately replicated with an injection
moldable thermoplastic. Many different thermoplastics may be employed for
the spincasting mold, such as polypropylene) polyethylene) polystyrene,
polyvinyl chloride, their various copolymers, and the like. Polyvinylchloride
is
preferred because it has a low level of shrinkage during injection molding) is
chemically compatible with lens materials in their monomeric and polymerized
states and readily transmits ultraviolet light. As pointed out above) it is
-13-
lg~
preferred that the molds be characterized by a pretreated optical surface to
increase its hydrophylicity or wettability in a manner well-know in the art.
Such treatment is not required of a polyvinylchloride mold.
The stainless steel master molds are fabricatable to a dimensional
accuracy superior to the accuracy of lathed and polished plastic surfaces
generated by a high volume toric lathing processes. Moreover) a single master
mold can be used to generate 50,000 -100,000 injection molded casts which are
the molds in which the spuncast lens is made. The spuncast lens accurately
replicates the mold's casting surface. The polymerized lens expands isotropi-
cally during hydration) thereby preserving the lens' proportions. These
factors
collectively contribute to the minimization of lens to lens variability. This
would be the case even with, for example, a 43 percent water content hema-
methacrylic acid copolymer spuncast lens. The lens of the invention may be
hydrated to a water content from about 30 to about 90 percent by weight.
With respect to Figure 1) there is shown a top view of a spincasted
toric lens containing the compound toric curvature (not specifically
depicted))
and thin-sectioned ballasting. Because of the thoughness of replication,
Figure
1 may be used as well to characterize the top view of the spincasting mold
surface, the anterior (female concave) surface of the injection mold and the
master mold used to form the injection mold. Lens 1 is provided with top
flattened (or prismatic) section 5) eliptical section 3 and slabbed off
section 7.
The toroidal surface is part of section 3 and generates an undulation in
section
3 which is not shown. The toroidal surface is specifically inputted in the
master mold surface to accomodate a particular astigmatism problem. The
stabbed off section 7, which accomodates to the lower lid of the eye, is
provided
between lines 9) 11 and 12. The steepest curvature within the toroidal surface
of the lens is located at R . In a typical lens (dehydrated), that may be a
curvature of about 8.07 mmhe R area could have a radius of curvature
of 8.42 mm. The bottom flat section 7 (the stabbed off section) may have a
thickness varying from about 0.35 mm at points A and B, to a thickness at
-14-
lg~
point C at the 6 o'clock position, of 0.2 mm. measured about 0.25 mm from the
edge. The conventional lens is provided with orientation markings 17 which
comprise slots that are about 0.3 to about 0.4 mm deep and about 0.15 long.
The figure gives conventional angles of orientation of the above sections and
components.
With respect to Figure 2, there is shown a side view of a spincast mold
where the interior molding surface is the same as the anterior surface of the
lens of Figure 1. As pointed out above, because of the thoughness of replica-
tion, Figure 2 may be used as well to characterize the profile of the lens,
the
anterior (female concave) surface of the injection mold and the master mold
used to form the injection mold A variety of radii are characterized that are
used in defining the surface 19 of the mold The cross-sectional side view view
indicates that the central radius is offset with respect to the Y-axis 15 by
0.3
mm. A 0.3 mm. offset will produce a lens having about 1.25 diopters of prism.
As a result of the prism, the thickness differential between the 12 and 4:30
o'clock positions and the 12 and 7:30 o'clock position on the lens is ap-
proximately 0.27 mm. The prism region of the mold extends from section 5 to
points A and B. The stabbed off section 7 makes the lens more comfortable to
the wearer.
- 15-
