CA2107828A1 - Multifocal contact lens with a difference power on the back surface - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is a multifocal contact lens having a first optical power on a portion of the optical region of the back surface of the lens, a second optical power on another portion of the optical region of the back surface portion of the lens, and a third optical power over the entire optical region of the front surface of the lens.
The first optical power, in combination with the third optical power produces a basic, distance corrective optical power while the second optical power, more positive than the first, produces in combination with the third optical power a difference corrective power to provide the appropriate near focal length for an individual requiring bifocals. This type of bifocal contact lens can be readily manufactured by molding with a convex mold surface containing the inverse of the first and second optical powers selected for the back surface of the lens and a concave mold piece containing the inverse of the third optical power selected for the front surface of the lens.

Description

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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 7 ~ ~ ~

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 7~

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.
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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 s~

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 :.

~ 1 ~ 7 8 ~ 3 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.
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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 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 ' ' ' VTN-44 .

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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 ~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 6~Qij~

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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2 ~ 3 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.
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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~.

~ ;7 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 ~1~7'~2~

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 ~''"

3~ ~ 2 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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Claims (7)

1. A multifocal contact lens for focusing light on the retina of said eye in combination with a presbyopic natural lens of the human eye, said lens comprising a first optical power on a first portion of the optical region of the back surface of said lens, a second optical power on a second portion of the optical region of the back surface of said lens, and a third optical power on the optical region of the front surface of said lens, wherein said first optical power on said first back portion in combination with said third optical power on the front surface produces a basic, distance corrective optical power, and said second optical power on said second back portion is an optical power more positive than that of the first optical power and in combination with said third optical power on the front surface, produces a near corrective optical power.

2. The multifocal contact lens of claim 1 wherein a boundary between the first portion of the back surface and the second portion of the back surface is a semi-circular path.

3. The multifocal contact lens of claim 1 wherein a boundary between the first portion of the back surface and the second portion of the back surface is a path defined by a conic section.

4. A method of producing a multifocal contact lens having a front surface and a back surface, both containing an optical region, comprising the steps of:
imparting a first optical power on a first portion of the optical region of the back surface of said lens;
imparting a second optical power on a second portion of the optical region of the back surface of said lens; and imparting a third optical power on the optical region of the front surface of said lens.

5. The method of claim 4 wherein said first, second and third optical powers are imparted by:
making a convex mold surface containing the inverse of said first and second optical powers on the optical region of a convex mold piece, making a concave mold surface containing the inverse of said third optical power on said portion of the optical region of a concave mold piece, and molding said contact lens between said concave and said convex mold pieces.

6. The method of claim 5 wherein a semi-circular path is formed as a boundary between the first portion of the back surface and the second portion of the back surface when imparting said first optical power and said second optical power on those portions, respectively.

7. The method of claim 5 wherein a path defined by a conic section is formed as A boundary between the first portion of the back surface and the second portion of the back surface when imparting said first optical power and said second optical power on those portions, respectively.