Multifocal ophthalmic lens

Abstract: An ophthalmic contact lens with a phase plate and a pure refractive portion within its optic zone.

Description: A multifocal lens with a zone plate and a pure refractive portion within its optic zone. This invention relates to an improvement in multifocal lenses, contact lenses and intraocular lenses possessing zone plate optics. A "zone plate", as employed herein and in the claims, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets in the zones said combination diffracts light to produce a specific wavefront which results in a specific intensity distribution of light at the various order (e.g.,0 th ,1 st , etc.) foci of the zone plate. This invention concerns multifocal lenses, in particular contact lenses, and more particularly contact lenses utilizing zone plate optics, such as zone plate bifocals and "tuned" Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. 4,210,391; 4,338,005; and 4,340,283. The lens design of Cohen, supra , provides that the radii "r n " of the annular and concentric zones are substantially proportional to √n and that the zones are cut so as to direct light to more than one focal point (herein called a "Cohen lens design"). The Cohen lens design with zone plate optics allows less constructions which are exceptionally thin. Contact lenses may be designed with zone plate optics in order to achieve a bifocal or multifocal effect. The specific chromatic properties of a zone plate may be incorporated in the design of a contact lens including a contact lens having multifocal properties. It has been determined that contact lenses with zone plate optics can generate some problems for the wearer. One is the glare that can result from the non-optical edges of the steps between the annularly arranged echelettes that make up a zone plate and appears through wave interference as a disconcerting, intense light to the contact lens user. Another potential problem stems from (i) the need in soft contact lenses to have sufficient mobility in the lens' fit to the cornea to allow tear fluid exchange to cleanse the surface of the eye of metabolic waste and (ii) the inability of the soft lens to move sufficiently during wearing to satisfy that needed mobility. The provision of a multiplicity of multifocal Fresnel echelettes in the annular zone plate arrangement of the Cohen lens design in a soft contact lens tends to limit the mobility of the lens. It would be desirable to incorporate into the design of such lenses sufficient mobility that the lens has the capacity of moving about 0.5 to about 1 millimeter of distance during wearing. This would enhance the lens' ability to allow management of the buildup of metabolic waste under the lens. In GB-A-2183246 a contact lens consisting of hydrophilic material having an interferometric pattern thereon is disclosed. As shown in Figures 3 and 4, the contact lens comprises a zone plate pattern arranged excentrically from the optical axis with a remaining purely refractive portion. It is the intent of this invention, amongst other things, to provide a multifocal contact lens design encompassed within the annular arrangement of the Cohen patents, supra, which minimizes the effects of glare from the non-optical edges and/or possessses the requiste mobility during use, as characterised above. The invention achieves these results and affects the multifocal utlitiy of a lens with the aforementioned annular arrangement. In accordance with the invention, there is provided multifocal ophthalmic lens comprising within its optic zone a multifocal zone plate having a first and a second diffractive power and further comprising a purely refractive portion within its optic zone, characterised in that the purely refractive portion is substantially monofocal and has a power corresponding substantially to either said first or said second diffractive power of said multifocal zone plate. More particularly, this invention is directed to an ophthalmic contact lens of the Cohen lens design with a zone plate and a purely refractive portion within its optic zone. The term "purely refractive portion", as used herein and in the claims, means one or more areas of the lens which operate in accordance with the Fundamental Laws of Geometrical Optics, see Fincham et al., Optics, 9th Ed.. page 22. "Geometrical optics, ignoring diffraction effects due to the wave nature of light, assumes: (1) Neighbouring rays of light are independent of one another. (2) The propagation of light is rectilinear, i.e., light travels in straight lines. (3) Law of reflection. (4) Law of refraction. These lens areas are typified by smooth or relatively smooth interior or posterior surfaces. Hereinafter, the purely refractive portion may be referred to as a "channel", purely refractive channel" and "phase channel". In a further embodiment, this invention is directed to a multifocal lens of the Cohen lens design containing a zone plate and a pure refractive portion which symmetrically retards the light passing therethrough (hereinafter termed for convenience "symmetrical channel"). The symmetrical channel diffracts light to constructively interfere with the light waves of the zone plate(s) to form or enhance an image at a desired focal point. The symmetrical channel may be regarded as providing essentially monofocal capabilities. In a preferred embodiment, the zone plate portion is located centrally and the purely refractive portion forms the remaining peripheral annular portion of the optic zone. In a more preferred embodiment of the invention, the purely refractive portion is displaced from the base curve of the lens so as to form a purely refractive channel. In the more preferred embodiment, the purely refractive channel is designed to parallel the base curve of the contact lens and focuses light at the zeroth (0 th ) diffractive focal point of the zone plate portion of the optic zone. In the most preferred embodiment of the invention, the zone plate occupies the central portion of the optic zone and the zone plate is designed with half-wave echelettes so as to split light equally between the zeroth (0 th ) and first (1 st ) order diffractive focal points. This is a convenient way to employ the Cohen lens design by providing each plate zone with multifocal capacity and relying on a plurality of them in conjunction with wave interference to achieve multi-focalization. In the most favorable aspect of the most preferred embodiment, the purely refractive channel is located peripherally of the zone plate and set at a quarter-wave depth below the base curve. In practicing preferred embodiments of the invention, a transition zone may be placed at the juncture between the purely refractive channel and the zone plate, and the peripheral end of the annular refractive channel is left open so as to allow the lens to drape to the eye. Embodiments of the invention are described hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 depicts a frontal view of a contact lens containing an optic zone,"OZ", which comprises a central zone plate with four zones and a peripheral refractive region. Figure 2 depicts a cross-sectional side view of a contact lens having an optic zone which contains annular zone plate echelettes, "PPE" cut into the central portion of its base curve, and a peripheral annular refractive portion,"R". It also depicts the resultant vector amplitudes at the zeroth (0 th ) order focal point. Figure 3 depicts a cross-sectional side view of a contact lens having an optic zone which contains annular zone phase plate echelettes "PPE" cut into the central portion of its base curve, and a peripheral annular refractive portion in the form of a phase channel, "PC". It also depicts the resultant vector amplitudes at the zeroth (0 th ) order focal point. Figure 4 depicts a cross-sectional side view of a contact lens having an optic zone which contains annular zone phase plate echelettes "PPE" cut into the central portion of its base curve, and a peripheral annular phase channel, "PC", separated by an annular transition zone, "TZ". It also depicts the resultant vector amplitudes at the zeroth (0 th ) order focal point. Figures 5A and B depict a cross-sectional side view of a soft contact lens "CL", resting upon a tear layer, "TL", showing a tear lens without drapage, and showing an air lens formed by the dimpled surface "D" when drapage occurs. Figure 6 depicts a cross-sectional side view of a contact lens having an optic zone which contains zone phase plate echelettes "PPE" cut into the central portion of its base curve, and a drapage-compensated peripheral phase channel, "CPC". Figure 7 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains a phase channel, "PC", riding upon the tear layer "TL" of eye "E". Figure 8 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains a drapage-compensated phase channel, "CPC", riding upon the tear layer "TL" of eye "E". This view depicts a dip, "D", in the anterior surface of the contact lens, "CL", that occurs when the contact lens drapes on the eye. Figure 9 depicts a cross-sectional side view of a contact lens having an optic zone which contains an alternate half wave zone plate cut into the central portion of its base curve, and a peripheral refractive phase channel. It also depicts the resultant vector amplitudes at the zeroth (0 th ) order focal point. Figure 10 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains zone plate echelettes "PPE" cut into its base curve "BC", and a phase channel, "PC". Figure 11 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains zone plate echelettes "PPE" cut into its base curve "BC", and a phase channel, "PC", separated from the zone plate by a transition zone, "TZ". Figure 12 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains zone plate echelettes "PPE" cut into its base curve "BC", and a single compensated phase channel, "CPC", which is formed as a composite transition zone and phase channel modified to account for drapage upon the eye. Figure 13 depicts a cross-sectional side view of a contact lens, "CL", having an optic zone which contains a positive zone plate placed centrally together with a peripheral drapage-compensated phase channel, "CPC", designed for first (1 st ) order diffraction. Figure 14 depicts a cross-sectional view of a contact lens, "CL", having an optic zone which contains a positive zone plate placed peripherally of a centrally placed phase channel, "PC", designed for zeroth (0 th ) order diffraction. Figure 15 depicts a cross-sectional view of a contact lens, "CL", having an optic zone which contains a negative zone plate placed peripherally together with a centrally placed drapage-compensated phase channel, "CPC", designed for zeroth (0 th ) order diffraction. This invention relates to multifocal lenses, in particular contact lenses specifically designed with zone plate optics located within an area smaller than the optics zone of the lens. This can be initially demonstrated by the lens of Figure 1, which breaks down the designated optics zone OZ into four (4) annular zones (r₁, r₂, r₃ and r₄) and a peripherally placed annular refractive region. The benefits from this geometric configuration of the optics zone of a contact lens are less glare being transmitted to the user, improved mobility characteristics for the lens, and added refractive power to the zone plate powder. The radii relationship of the zone plate zones follows the characterizations of the Cohen lens design, to wit: r m = 2 m w f m = integer representing the m th zone w= the wavelength of light f=focal length of 1 st order diffraction. The contact lens of Figure 2 has a continuous half-wave zone plate in the central region of the optic zone. This is a bifocal and splits the light equally between zeroth (0 th ) and first (1 st ) order diffraction. The depth "h" of the zone plate echelettes is given as h=w/2(n′-n) n′= refractive index of contact lens n = refractive index of tear layer of eye Further, the purely refractive region R of this lens continues with the same base curve as the zone plate portion, thereby focussing light toward the zeroth (0 th ) diffractive focus of the zone plate. As a general rule, when lenses are constructed with an optic zone larger than the area containing the zone plate optics, the refracted and diffracted light will undergo interference at the focal point or points of the lens. This can result in some cases in a degrading of the focussed images. This is demonstrated by Figure 2 where the amplitudes PPA formed by the zone plate echelettes PPE results in the amplitude RA formed by the refractive region R which is 90 degrees out of phase and results in the composite amplitude A. In a preferred embodiment of the invention, where a contact lens incorporates both a zone plate as well as a purely refractive channel within its optic zone, it is desirable to alter the channel to enhance rather than degrade a zone plate image. In the typical embodiment of the invention, the resultant phases of the various zones of the lens are aligned to achieve maximum constructive interference. For example, they may be arranged annularly as to each other, and such configuration may be changed to compensate for the shape of the lens and variations in the optic zone from the traditional variations so long as the objectives of the invention are achieved. A product of the invention serving to enhance focussed images and obtaining the other benefits of the invention, utilizes a symmetrical channel resulting in the alignment of its phase with the resultant phase of one of the zone plate focal points. The resulting lens containing this embodiment is provided with a zone plate with an adjacent purely refractive channel that alters by virtue of symmetrical phase shift the phase relationship existing between the zone plate and the channel. The channel desirably focuses light to ameliorate the impeding effects of the refracted and diffracted light. Such a channel may be located interior or exterior of the zone plate and the purely refractive surface. Figure 3 illustrates a lens designed with a phase channel to provide the aforementioned symmetrical phase shift enhancement to align the phases between the purely refractive and zone plate portions of the optic zone. In Figure 3, the amplitudes PPA formed by the zone plate echelettes PPE, and the amplitude PCA formed by the purely refractive phase channel PC are approximately in phase and result in the composite amplitude A. The phases were brought into alignment by placing the phase channel at a depth of w/4(n′-n)above the base curve of the lens. If desired, the zone plate and the phase channel may be separated by a transition zone. Figure 4 illustrates a lens with such a transition zone separating the zone plate from the phase channel. This transition zone, as shown in the vector diagram of Figure 4, has little effect in the overall resultant amplitude of light. As shown below, the transition zone allows the design of soft lens profiles than can compensate for those cases where the lens phase channel tends to drape to the eye. In soft contact lenses construction, it is assumed that a soft contact lens will drape upon and take the shape and form of the surface of the eye upon which it is fitted. The principles of draping are illustrated in Figures 5A and B. Figure 5A depicts a soft contact lens designed to have a center thickness d and a tear layer thickness e . However, because of drapage, it is redesigned in Figure 5B with a center thickness d ′ in order to account for the absence of the tear layer and the addition of an air lens formed by the depression D in the contact lens surface. The standard contact lens formula for drapage is: d′=d + e(n - 1)/(n′ - 1) Soft contact lenses with a transition zone may be smoothly draped upon the eye by adjusting the phase channel depth to compensate for drapage. This other embodiment of the invention is illustrated inter alias in Figure 6. In Figure 6, the lens is provided with a central half-wave zone plate and a phase channel at a depth of w/4(n′ - 1) above the base curve of the lens. In this lens the peripheral portion of the base curve is cut away in order to prevent the formation of a channel and to allow for full drapage. Other illustrations of this embodiment are shown in Figures 7 and 8. Figure 7 depicts a lens CL with a phase channel PC resting upon the tear layer TL of eye E. Figure 8, on the other hand, provides lens CL with a transition zone and drapage-compensated phase channel CPC, resting upon the tear layer TL of eye E. This lens provides an annular dimple D in the anterior surface of the lens, directly above the region of drapage. The invention may be applied to any known type of multifocal lens or contact lens that utilizes both diffractive as well as purely refractive elements within a single optic zone. As a particular example is the alternating half wave bifocal phase plate of Figure 9. In this lens example the zeroth (0 th ) order diffractive focus requires a channel that imparts a phase shift equal to 90 degrees + 57.5 degrees. In this case the channel depth is determined according to the following formula: depth = w(180 - 147.5)/360(n′ - n) = 0.09w/(n′ - n). Figure 10 illustrates a contact lens CL having a posterior surface matched to a base curve BC. Cut into the base curve centrally is a half wave zone plate with echelettes PPE that are designed to diffract light equally to zeroth (0 th ) and positive first (1 st ) order diffractive focal points. The lens is provided with a phase channel by cutting, into the base posterior curve, peripherally of the zone plate, a quarter wave phase channel which refracts light to the zeroth (0 th ) order diffractive focal point. A similar lens design which includes, in addition, a transition zone to separate the zone plate from the refracting channel is depicted in Figure 11. In this embodiment of the invention, contact lens CL whose posterior surface is matched to a base curve BC, has (i) cut into the base curve centrally, a half wave zone plate with echelettes PPE, (ii) cut into the base curve peripherally of echelettes PPE, a quarter wave phase channel PC, and (iii) at the point of juncture of PPE and PC, a transition zone TZ comprising one-half of a zone plate echelette. The embodiment of Figure 12 depicts a modification of the lens of Figure 11 by a reduction in depth of peripheral phase channel PC to compensate for lens drapage. The channel in this case has been left open in order to facilitate the desired lens drapage. Figure 13 depicts a cross-sectional view of a contact lens CL with a positive zone plate displaced centrally with a peripherally aligned drapage-compensated annular phase channel CPC designed for first (1 st ) order diffraction. The phase channel in this lens does not parallel the base curve of the lens. This follows because the phase channel provides focussed light at the first (1 st ) order rather than the zeroth (0 th ) order diffractive focal point illustrated in the preceding illustrations of the invention. This allows the purely refractive portion of the lens to contribute to the add power rather than to the distance power. The embodiment of Figure 14 depicts a cross-sectional view of a contact lens CL with a positive zone plate placed peripherally together with a central phase channel PC designed for zeroth (0 th ) order diffraction. The embodiment of Figure 15 has the contact lens CL provided with a negative zone plate placed peripherally of a central drapage-compensated phase channel CPC designed for zeroth (0 th ) order diffraction. It is to be noticed that the phase channel in this lens parallels the base curve of the lens and, yet, is still able to contribute to the add power rather than to the distance power. This follows from the fact that we are using a negative zone plate which provides focussed light at the zeroth (0 th ) and negative first (1 st ) order diffractive focal points. A desirable lens encompassed by this invention has an optic zone of about 6 to about 8 millimeters comprising a concentric zone plate of about 4 to about 5 millimeters in diameter, viz., about 4.5 millimeters in diameter, and an annular channel having a width if about 0.5 to about 2.0 millimeters. In those situations where the degree of mobility of the lens is extreme but the level of glare reduction achieved by the practice of the invention is considered most desirable, the lens may be modified by inclusion of the keel construction outside of the optic zone to decrease the level of mobility yet retain the advantages of the invention. The lens of the invention may be made conventional processes in the art. For example, anhydrous versions of the base phase of a soft contact lens may be ground to provide the lens structures of the invention. Lenses may be casted from molds replicating the lens structures of the invention. The lenses may be made of glass and the conventional plastics used for making contact lenses.

Claims: Multifocal ophthalmic lens comprising within its optic zone a multifocal zone plate having a first and a second diffractive power and further comprising a purely refractive portion (R) within its optic zone (OZ), characterised in that the purely refractive portion is substantially monofocal and has a power corresponding substantially to either said first or said second diffractive power of said multifocal zone plate. A multifocal lens according to claim 1 wherein the purely refractive portion is configured to bring light passing therethrough substantially into phase alignment at the diffractive focal point for said corresponding diffractive power of said multifocal zone plate with the light focused at said focal point by said zone plate. A multifocal lens according to claims 1 or claim 2 wherein said first and second diffractive powers correspond to the zeroth (0th) and first (1st) order diffractive focal points of said multifocal zone plate. A multifocal lens according to any one of the preceding claims wherein purely refractive portion (R) constitutes one or more areas rotationally symmetrical about the optical axis of the lens. A multifocal lens according to claim 4 which the purely refractive portion (R) comprises a substantially annular area. A multifocal lens according to claim 4 which the purely refractive portion (R) comprises a substantially circular area. A multifocal lens according to any one of claims 1 to 5 wherein the zone plate is located centrally and the purely refractive portion (R) comprises the remaining peripheral annular portion of the optic zone (OZ). A multifocal lens according to any one of the preceding claims in which the ophthalmic lens is in the form of one of a contact lens (CL) or an intraocular lens. A multifocal lens according to claim 3 or to any claim dependent thereon wherein the purely refractive portion (R) is designed to parallel the base curve (BC) of the lens thereby focusing light at the zeroth (0th) diffractive focal point of the zone plate portion of the optic zone (OZ). A multifocal lens according to any one of the preceding claims wherein the purely refractive portion (R) is displaced from the base curve (BC) of the lens. A multifocal lens according to any one of the preceding claims wherein the zone plate is designed to split light equally between the zeroth (0th) and first (1st) order diffractive focal points. A multifocal lens according to claim 11, wherein the pure refractive portion (R) is located peripherally of the zone plate, parallels the base curve (BC) of the lens and is set at a depth below the base curve (BC) that is one half of the depth of the zone plate echelettes.

Inventors: COHEN ALLEN L DR, COHEN, ALLEN L., DR.
Applicants: COHEN ALLEN L DR, COHEN, ALLEN L., DR.
Application year: 1988
Main classification: G02C7
IPC codes: G02C7, G02B5, A61F2