Multi-focal diffractive ophthalmic lenses

Abstract: A multifocal ophthalmic lens has optical power, at least a portion of the optical power being produced by diffraction. The lens has a plurality of diffractive zones including a circular central zone and a plurality of concentric annular zones. The central zone has a radius r₀ and the first annular zone has a radius r₁ where r₁² - r₀² is not equal to r₀².

Description: Field of the Invention The present invention relates to ophthalmic lenses having a plurality of focal lengths. Background of the Invention As used herein the term "ophthalmic lens" means vision correction lenses such as contact lenses and intraocular lenses. Other, less common, vision correction lenses such as artificial corneas and intralamellar implants are also included in this definition. Bifocal spectacle lenses have been known for hundreds of years. In such lenses a first region of the lens is typically provided with a first focal length while a second region of the lens is provided with a second focal length. The user looks through the appropriate portion of the lens for viewing near or far objects. More recently there has been interest in developing other types of multifocal ophthalmic lenses. Multi-focal contact lenses utilizing an approach similar to that used in spectacle lenses are described in Contact Lenses: A Textbook for Practitioner and Student , Second Edition, Volume 2 on pages 571 through 591. Such lenses have serious drawbacks, however, because they require that the lens shift on the eye so that different portions of the lens cover the pupil for distant and close vision. This design cannot be used for intraocular lenses or other implanted lenses, because such lenses cannot shift. Even for contact lenses the design is disadvantageous because it is difficult to insure that the lens will shift properly on the eye for the desired range of vision. In another design for a bifocal contact lens described in the above-referenced textbook, a central zone of the lens is provided with a first focal length and the region surrounding the central zone is provided with a second focal length. This design eliminates the necessity for shifting the lens by utilizing the phenomenon of simultaneous vision. Simultaneous vision makes use of the fact that the light passing through the central zone will form an image at a first distance from the lens and light passing through the outer zone will form an image at a second distance from the lens. Only one of these image locations will fall on the retina and produce a properly focused image while the other image location will be either in front of or behind the retina. The human eye and brain will, to a great extent, work together to ignore the improperly focused image. Thus the user of such a lens receives the subjective impression of a single well-focused image. A disadvantage of such a lens is that, if the central zone is made large enough to provide sufficient illumination in its associated image in low light situations, i.e. when the patient's pupil is dilated, the central zone will occupy all or most of the pupil area when the pupil contracts in a bright light situation. Thus bifocal operation is lost in bright light. Conversely if the central zone is made small enought to provide bifocal operation in bright light situations, an inadequate amount of the light will be directed to the image associated with the central zone in low light environments. Because the central zone is commonly used to provide distant vision, this can create a dangerous situation when the user of such a lens requires distant vision in low light situations such as when the user must drive a motor vehicle at night. United States Patents 4,210,391; 4,340,283; and 4,338,005, all issued to Cohen, teach the use of a plurality of annular regions that direct light to multiple foci and rely upon simultaneous vision to discard unfocused images. They teach the use of alternating concentric Fresnel zones, wherein each of those zones have substantially equal area. The use of such equal area zones causes the lens to provide a diffractive focus of the light. A first focus will occur for the zero order diffracted light while a second focus will occur for the first order diffracted light. Such a structure is known as a diffractive zone plate. A diffractive zone plate must be designed for light of a particular wavelength and will work most efficiently for light at that wavelength. The radius of the n th zone (r n ) in the diffractive zone plates taught in the Cohen patents will be equal to √n r₁ where r₁ is the radius of the central zone. To a reasonable approximation r₁ would be equal to √ λf where λ is the design wavelength and f is the focal length of the diffractive structure. Therefore the n th zone would have a radius equal to √ nλf . In designing a diffractive zone plate a design wavelength must be selected. When a desired focal length and wavelength are selected for a lens as taught in the Cohen patents, the area of each of the zones, and thus the location of the boundary of each zone, are determined. This rigid definition of the zones result in a disadvantage to the zone plate structure. In order to obtain an efficient diffractive bifocal operation, a sufficient number of zones must be used. However if the area of the central zone is too large, under bright light situations with the pupil constricted, only a single zone or very few zones will be utilized. Thus the efficiency of the multi-focal operation is greatly reduced. Summary of the Invention The present invention provides a multifocal ophthalmic lens having optical power and a design wavelength, at least a portion of the optical power being produced by diffraction. The lens has a plurality of diffractive zones, said plurality including a circular central zone and a plurality of concentric annular zones surrounding said central zone. The central zone has a radius r₀ and the innermost annular zone has an outer radius r₁ where r₁²-r₀² is not equal to r₀². The present invention recognizes that the central zone of an ophthalmic lens utilizing a phase zone plate need not have the same area as the other zones. In one embodiment the central zone is made smaller than the other zones in order to insure adequate multifocal operation. In another embodiment the size and refractive power of the central zone is adjusted to control the distribution of energy between the foci. Brief Description of the Drawings Figure 1 is a front view of a lens having a flat surface constructed in accordance with the present invention; Figure 2 is a schematic diagram for use in describing the invention; Figure 3 is a cross-sectional view of a single zone of the lens of Figure 1; Figure 4 is a cross-sectional view of a meniscus lens constructed in accordance with the present invention; Figure 5 is a cross-sectional view of a single zone of the lens of Figure 4; Figure 6 is a schematic diagram of a lens for use in describing the derivation of the equation for curved surface corrections to zone radii; and Figure 7 is a cross-sectional view of a biconvex lens constructed in accordance with the present invention. Detailed Description An ophthalmic lens, generally designated 10 in Figure 1, is provided with a diffractive zone plate including zones 12, 14, 16, 18, and 20. Although the drawing shows only five zones, more would typically be provided. The exact number would depend on the amount of change from the base optical power of the lens, the size of the lens and the design wavelength, among other factors. Typical lenses have between 20 and 30 zones. As will be described below the radii of the zones will be affected by several factors including the choice of a design wavelength. In a preferred embodiment the design wavelength is chosen in the spectral region of the greatest photopic sensitivity of the human eye. The lens of Figure 1 typically has a base optical power provided by refraction. An additional power is provided by diffraction. Alternatively the entire optical power could be provided by diffraction. The diffractive optical power is provided by separating the zones by optical steps. An optical step causes light rays passing immediately on each side thereof to experience different optical path lengths, where the optical path length is defined in terms of wavelengths of light of a design wavelength. One way of providing optical steps is to provide physical structures on a surface of the lens. Alternatively optical steps may be provided on a smooth surface by varying the index of refraction of the underlying material. Such variation of the index of refraction may be accomplished, for example, by removing portions of the lens material and filling the structures formed thereby with a material having a different index of refraction or by doping portions of the lens with a dopant that causes the index of refraction of the doped regions to change. The size of the optical steps is defined in terms of optical height. The optical height of a step should be an odd half integral multiple of the wavelength of light of the design wavelength. Typically the optical height is one half wavelength for light of the design wavelength. The term optical height, as used herein, refers to the difference in optical path length in terms of wavelengths of light, for adjacent light rays passing on each side of the step. Thus to provide an optical height of one-half wavelength, the actual height should be (λ/2)/(i i -i₂) where λ is the wavelength of the light in question, n₁ is the first index of refraction, typically that of the lens material, and i₂ is the second index of refraction, that of the medium bordering the lens or of the modified portion of the lens. In order for the lens to exhibit diffractive power, a required condition is that rays of light passing through the edges of the zones arrive at the image point in phase. Thus the optical path length difference for a ray of light passing through the outer edge of a zone should be one wavelength less than that for a ray of light passing through the outer edge of the next zone. As previously described this requirement has in the prior art led to the conclusion that the radii of all zones are uniquely determined when a design wavelength, a focal length, and the indices of refraction are chosen. This conclusion is unduly restrictive. Figure 2 will be used to show that more freedom is available in selecting zone radii than taught by the prior art. The example shown in Figure 2 represents the special case of diffractive zones provided on an otherwise flat surface. In some ophthalmic lenses, such as intraocular lenses, the zones may actually be provided on a flat surface. In others the radius of curvature of the surface is great enough that it may be neglected. In Figure 2, z represents the distance from an object to the lens along the optical axis of the lens and z′ represents the distance from the lens to the image location along that axis. The distance represented by s₀ is the distance from the object to the outer edge of the central zone and that represented by t₀ is the distance from the outer edge of the central zone to the image location. Similarly s n represents the distance from the object to the outer edge of the n th zone and t n represents the distance from the outer edge of the n th zone to the image. The effect of the unnecessary restriction of the prior art is to require that s₀ equal z and t₀ equal z′. Instead, as previously explained, the proper requirement is that the optical path length difference from the outer edge of one zone to the outer edge of the next zone must be one wavelength. In order for this condition to be met, the following relationship must hold: s₀µ₁+t₀µ₂+nλ=s n µ₁+t n µ₂ (1) where µ₁ is the index of refraction of the medium through which the rays s₀ and s n travel, µ₂ is the index of refraction of the medium through which the rays t₀ and t n travel and λ is the design wavelength. Using the Pythagorean theorem equation (1) may be rewritten as: where r₀ is the radius of the central zone and r n is the radius of the n th zone. Thus the radius of the central zone may be arbitrarily chosen and equation (2) solved to determine the radii of the remaining zones to provide the desired diffractive power. As an approximation, equation (2) may be expressed as: r n ²=r₀²+2nλf (3) where f is the focal length of the portion of the optical power of the lens provided by the diffractive structure. The ability to arbitrarily select the radius of the central zone while providing diffractive optical power can be expressed in terms of the relationships among the radii of the zones. If the radius of the central zone is designated r₀, the radius of the innermost annular zone is designated r₁ and the radius of the second annular zone is designated r₂, the conditions previously described may be expressed by saying that r₀² is not equal to r₁²-r₀² and r₂²-r₁² is equal to r₁²-r₀². In general, r n ²-r n-1 ² is equal to r n-1 ²-r n-2 ² for values of n greater than or equal to 2. Since the radius of central zone 12 of Figure 1 may be chosen arbitrarily, it may be made smaller than the prior art dictates, causing the remaining zones to be moved closer to the center of the lens. Then even when the pupil of the eye is constricted, as in a bright-light situation, a sufficient number of zones will be used to allow multifocal operation of the lens. If the design wavelength, the focal length and the pupil size of the patient are such that an adequate number of zones can be provided while making the central zone larger than the other zones, such a design is acceptable. If such a design is possible for a particular patient, other advantages may be achieved. For example a zone plate having a different focal length, design wavelength or both could be provided within the central zone. Such an additional zone plate could be used as a method of redistributing the light energy between the foci. Another advantage of the invention lies in the ability to redistribute the proportion of the light directed to each focus without the use of another diffractive structure in the central zone. By providing a different refractive power in the central zone than that provided in the remainder of the lens, light passing through the central zone can be directed to either of the foci produced by the combined refractive and diffractive power of the remainder of the lens. By adjusting the size of the central zone and the focus to which it directs light, the energy distribution between the foci may be optimized. Thus a patient who must drive a vehicle in low light conditions might require more energy in the far object focus while another patient who does delicate work might require that more energy be provided to the near focus. Figure 3 shows a cross-sectional view of a single zone of a lens as it would be shaped on the flat surface of the lens of Figure 1. Those skilled in the art will readily perceive that the vertical scales in Figures 3 through 5 and 7 are greatly exaggerated in order to more clearly show the nature of the structures. The anterior side 30 is smooth while the diffractive zones are provided on the posterior side 32. Posterior side 32 includes diffractive zone 34 and step 36. As previously described the optical height of step 36 is λ/2, where λ is the design wavelength. The diffractive zone formed by region 34 and step 36 leaves a cut out section 38 in the posterior side 32 of the lens. As previously described, region 38 may be left open or may be filled with a material having a different index of refraction from that of the lens body. The shape of the zone surface will affect the diffractive orders to which energy is directed by the structure and the energy distribution among those orders. In a preferred embodiment the shape of region 34 of the illustrated zone is parabolic. The precise shape of the zone is, however, less important to the performance of the lens than the locations of the zone boundaries. The key requirements are that the zone boundaries be properly located and that the zone curves smoothly. Since a spherical zone shape is generally easier to generate than a parabolic one using currently available techniques and a sphere is a reasonably close approximation to a parabola over a small region, a spherical zone shape may be used to approximate the parabola. The spheres that are used in the preferred embodiment are designed in such a manner that the proper step height will be provided between the zones and the center of the spheres lie on the optical axis of the lens. Other shapes may also be used as long as such shapes are a good approximation to a parabola. Figure 4 shows a cross-sectional view through the center of a curved lens 40 constructed in accordance with one embodiment of the invention. The lens is a meniscus lens having a smooth anterior side 42 and a diffractive zone plate formed by a structured posterior side 44 having a series of diffractive zones 46, 48, 50, 52 and 54. Alternatively the zones could be formed on the anterior side 40 of the lens, or even on both sides of the lens. As previously described the optical steps separating the diffractive zones such as optical step 56 could also be formed in other ways not requiring an actual physical step. Figure 5 illustrates the zone shape for the curved lens 40 of Figure 4. The zone shown in Figure 5 has a region 60 and a step 62. The optical height of step 62 is again λ/2 for the design wavelength. Also shown in Figure 5 is dashed line 64 that represents the base curve of the lens (i.e. the curve that the lens surface would follow if no diffractive zones were provided). The shape of region 60 is determined in a manner similar to that of region 34 of Figure 3. When the zones are provided on a curved surface, improved performance may be obtained by introducing a correction for the curvature. The derivation of the curved surface correction will be more readily understood by reference to Figure 6. A term δ is introduced representing the difference in optical path length experienced by a light ray traveling from the object to the edge of the central zone to the image location and a light ray traveling along the optical axis. The value of δ is given by: δ=s₀µ₁+t₀µ₂-(zµ₁+z'µ₂) (4) where s₀ is the distance from the object to the edge of the central zone, t₀ is the distance from the edge of the central zone to the image location, z is the distance from the object to the lens along the optical axis, z′ is the distance from the lens to the image location along the optical axis and µ₁ and µ₂ are the indices of refraction through which the s₀ and t₀ beams travel, respectively. The values of z and z′ are chosen for the case of an image of an object close to the eye being focused on the retina by the near object focal power of the lens. Typically the physical object is located 30 to 40 cm from the eye. The object distance for these equations, however, is the distance to the image produced by the refractive power of the lens in combination with the cornea. When the object location is on the same side of the lens as the image location, z takes on a negative value. For a contact lens the value of z would typically be about -32 mm and the value of z′ would be about 30 mm. For an intraocular lens the value of z would typically be about -20 mm and the value of z′ would be about 19 mm. As previously described the optical path length difference from the outer edge of a zone to the outer edge of the next zone should be λ, where λ is the design wavelength. From this the optical equation for the n th zone may be written as follows: zµ₁+z'µ₂+nλ+δ=s n µ₁+t n µ₂ (5) where s n and t n are the distances from the object to outer edge of the n th zone and from the outer edge of the n th zone to the image location, respectively. This equation can be rewritten as: zµ₁+z'µ₂+νλ=s n µ₁+t n µ₂ (6) where ν=n+δ/λ (7) From geometric considerations, it may be shown that and where h n is the distance from a plane tangent to the lens on the optic axis to the lens at the outer edge of the n th zone and may be calculated by where R c is the radius of curvature of the lens. Substituting the values from equations 8, 9, and 10 into equation 6 and squaring twice yields: r n ⁴(c₂²-c₃²)-r²[2c₁c₂+c₃²(d²+d'²)]+c₁²-c₃²d²d'²=0 (11) where d= (z+h n ), d′= (z′-h n ), c₁= (zµ₁+z'µ₂+νλ)²-d²µ₁²+d'²µ₂², c₂= µ₁²+µ₂², and c₃= 2µ₁µ₂ This equation may be solved by iterative techniques. As previously described the object and image are effectively on the same side of the lens for a typical diffractive structure used in an ophthalmic lens. Therefore µ₁ and µ₂ are equal and the symbol µ may be substituted for both. Using this substitution and other approximations it can be shown that the following equation provides a reasonable approximation to equation 11: where f is the focal length of the diffractive power of the lens. As an alternative approximation the zone radii may be calculated by solving the following equation for r n : r n ²=r₀²+2nλf/µ-k²f³(νλ/µ)² where Figure 7 is a cross-sectional view of a biconvex lens, designated generally as 70 utilizing the invention. A lens of the form of lens 70 could be used as an intraocular lens. Lens 70 has a first side 72 and a second side 74. Diffractive zones, such as central zone 76 and annular zones 78 and 80 are provided on side 74. As in the lenses of Figures 1 through 6 the radius of central zone 76 may be chosen arbitrarily in order to provide the best functionality for a particular patient. If side 74 is the anterior side of the lens, equation (11) or equation (12) may be used directly to calculate zone radii with correction for the radius of curvature of the surface. If side 74 is the posterior side, those equations may be used by regarding the radius of curvature of the surface as negative.

Claims: A multifocal ophthalmic lens (10) having optical power and a design wavelength, at least a portion of said optical power being produced by diffraction, said lens having a plurality of diffractive zones, said plurality including a central zone (12) and a plurality of concentric annular zones (16, 18, 20) surrounding said central zone, said central zone (12) having a radius r₀ and the innermost (14) of said annular zones having an outer radius r₁ characterized in that r₀² is not equal to r₁² - r₀². The ophthalmic lens of Claim 1 wherein said lens is an intraocular lens. The ophthalmic lens of Claim 1 wherein said lens is a contact lens. The ophthalmic lens of Claim 1 wherein said lens is an artificial cornea. The ophthalmic lens of Claim 1 wherein r₀² is less than r₁² - r₀². The ophthalmic lens of Claim 1 wherein r₀² is greater than r₁² - r₀². The ophthalmic lens of Claim 1 wherein said zones are separated by optical steps having an optical height of one half wavelength for light of said design wavelength. The ophthalmic lens of Claim 6 wherein said design wavelength is in the spectral region of the greatest photopic sensitivity of the human eye. The ophthalmic lens of Claim 1 wherein said diffractive zones are provided on a curved surface. The ophthalmic lens of Claim 9 wherein said zones have radii that are corrected for the curvature of said curved surface.

Inventors: FUTHEY JOHN A C O MINNESOTA MI, SIMPSON MICHAEL J C O MINNESOT, FUTHEY, JOHN A. C/O MINNESOTA MINING AND, SIMPSON, MICHAEL J. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: G02C7
IPC codes: G02C7, G02B5, A61F2