Multifocal diffractive ophthalmic lens

Abstract: An ophthalmic contact lens containing more than one phase plate within its optic zone, including contact lens, especially of a Cohen lens design, which possesses phase plate optics within its optic zone that progressively shift the intensity of light focussed by bifocal phase plates between the diffractive focal powers by utilizing echelettes in the phase plates of varying depths and heights which are cooperatively aligned relative to the lens base curve surface that they openly face.

Description: This invention relates to a multifocal diffractive ophthalmic lens. This invention relies on an improvement in zone plate optics embracing contact lenses and intraocular lenses. A "zone plate", as employed herein and in the claims, is a unitary optical region of a lens with optical facets in the zones such that light is diffracted to produce a specific wavefront which results in a specific intensity distribution of light at the various order (e.g., 0th, 1st, etc.) foci of the zone plate. This invention relates, for example, to contact lenses and more particularly to 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, 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 lens constructions which are exceptionally thin. Contact lenses may be designed with phase 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. There is also the need in multifocal lenses to have the design capability to vary the intensity of light through the lens to accommodate pupil dilation and constriction. It is known that a pupil can vary from 3 to 6 mm. depending upon the level of ambient illumination. It would be desirable to be able to vary the distribution of energy between distance and near focal point according to the users needs. For example, in dim illumination, the user of a contact lens will be typically engaged in distance viewing such in driving of an automobile. It would be desirable to have a contact lens which accommodates that condition. Conversely, a user may seek to have maximum orientation to the near focus in the lens yet would require a reasonable intensity of light for distance viewing. It would be desirable to have lenses that can be biased to a users requirements or to light intensity. The nature of the light intensity problem is illustrated by reference to Figures 1 and 2. The lens portion depicted in Figure 1 is a cross-sectional side view of a portion of a half-wave bifocal zone plate with the echelette depths h given by the equation: h = w/2(n'-n) where: w = wavelength of light n' = refractive index of the contact lens n = refractive index of tear layer of eye In Figure 1 the individual amplitudes of light a₀ and a₁ are formed by the individual echelettes E. The total resultant amplitudes of light A₀ and A₁ formed at the 0 th and 1 st diffractive foci are also shown in Figure 1. In this illustration, the parallel nature of vectors a₀ and a₁ demonstrates that the intensity of light is split equally between the two focal points. The intensities at the 0 th order and the 1 st order diffractive foci are given by: I 0 = sinc 2 (1/2) [Intensity at 0 th order diffractive focus] I 1 = sinc 2 (1/2) [Intensity at 1 st order diffractive focus] It is not necessary for a bifocal zone plate to split the incident light equally between its two diffractive foci when the vectors a₀ and a₁ are in parallel. This is shown in Figure 2 where the cross-section of a portion of a bifocal phase plate with the echelette depths d given by the formula: d = a . h where: h = w/2(n'-n) 0<a<2 In the case of the Figure 2 illustration, the intensity of light is not split equally between the two focal points. The intensities at the 0 th order and 1 st order diffractive foci of this example are derived from the following equations: I 0 = sinc 2 (a/2) [Intensity at 0 th order diffractive focus] I 1 = sinc 2 (1-a/2) [Intensity at 1 st order diffractive focus] In this case, the amplitudes of light A₀ and A₁ are shifted in phase to parallel non-vertical aligned amplitudes produced by a half-wave bifocal zone plate. The phase shift e is derived from the equation: e = (1-a)π/2 Though the current developments are significant improvements in the art, there is always a need to improve on the adaptability of the lenses to pupil-diameter variations and decentration. It is desirable to provide bifocal performance of a lens of the Cohen design with the feature that it can shift focussed light from distance to near in coordination with the human eye's pupil, which normally constricts during near viewing. It has been determined that contact lenses with zone plate optics may generate a few problems for the wearer. One is the glare that results 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 build-up of metabolic waste under the lens. It is another feature 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 possesses the requisite mobility during use, as characterised above. In US-A-4,340,283 a phase shift multifocal zone plate is disclosed. This only one zone plate comprises a plurality of alternating odd and even annular concentric zones with echelettes of different height. This document of prior art does not provide a lens comprising a plurality of zone plates in its optic zone wherein each of the zone plates occupy a separate radial extent within the optic zone. In accordance with an aspect of the present invention, there is provide a multifocal diffractive ophthalmic lens comprising within its optic zone a plurality of concentric zone plates, the dimensions of the zones in one zone plate being different from the dimensions of the zones in another zone plate, characterised in that each of the zone plates occupies a separate radial extent whereby, in use, a focus shift from distance to near is effected in response to a change in pupil size. Embodiments of the invention provide an ophthalmic lens such as contact and intraocular lenses containing more than one zone plate within its optic zone wherein each of said zone plates occupies a separate radial extent. A preferred aspect of the invention is directed to an ophthalmic contact lens containing at least two zone plates within its optic zone. The invention contemplates the creation of a resultant wavefront derived from the contribution through interference of the wavefronts emanating from each zone plate. In another aspect, this invention relates to a contact lens which possesses zone plate optics within its optic zone that progressively shift the intensity of light focused by bifocal zone plates between the diffractive focal powers by utilizing echelettes in the zone plates of varying depths and heights which are cooperatively aligned relative to the lens base curve surface that they openly face. In a preferred embodiment of such aspect, the invention relates to a lens of the Cohen lens design in which progressive shifting of the intensity of light focused by the bifocal plate is between the diffractive focal powers effected through echelettes in the zone plate of varying depths and heights which are cooperatively aligned relative to the lens base curve surface that they openly face. In a further preferred embodiment of the invention, the median of the height of at least a predominant number of the echelettes (of a Fresnel-type configuration) of a draped bifocal phase plate lens fall essentially on the same imaginary curved or straight plane within the lens, the points of which are essentially equidistant on a perpendicular Straight line from the cornea. In the context of the above, a "draped" bifocal zone plate lens is a lens on the cornea draped to the extent that the lens is capable of draping within its intended use range. In a more preferred embodiment, essentially all, most preferably all, of the echelettes fall essentially on the same imaginary curved or straight plane within the lens, the points of which are essentially equidistant on a perpendicular straight line from the cornea. Figure 1 illustrates a cross-sectional side view of a portion of a half-wave bifocal zone plate with echelettes E formed to the depth h . It characterizes the individual amplitudes of light a₀ and a₁ formed by the individual echelettes E . The vector analysis depicts the total resultant amplitudes of light A₀ and A₁ formed at the 0 th and 1 st diffractive foci. Figure 2 illustrates a cross-sectional side view of a portion of a bifocal phase plate with echelette E formed to the depth ah. It characterizes the individual amplitudes of light a₀ and a₁ formed by the individual echelettes E. The vector analysis depicts the total resultant amplitudes of light A₀ and A₁ formed at the 0 th and 1 st diffractive foci. Figure 3 illustrates a cross-sectional side view of a portion of a bifocal lens consisting of two separate phase plates with echelettes E and E ' formed to the depths h and ah, respectively (with 0<a<1). It characterizes the individual amplitudes of light A₀ and A'₀ formed at the 0 th diffractive focus and the total resultant amplitudes of light A₁ and A'₁ formed at the 0 th diffractive focus and the total resultant amplitudes of light A₁ and A'₁ formed at the 1 st diffractive focus. Figure 4 illustrates a cross-sectional side view of a portion of a bifocal lens consisting of two separate zone plates and a phase channel of depth bh. It characterizes the individual amplitudes of light a₀ and a'₀ formed by the individual echelettes E and E ' of the separate zone plates. It shows the total resultant amplitudes A₀ and A'₀ formed at the 0 th diffractive focus and the total resultant amplitudes A₁ and A'₁ formed at the 1 st diffractive focus. Figure 5 illustrates a cross-sectional side views of a soft contact lens CL resting upon a tear layer TL demonstrating, as noted, a tear lens without drapage and an air lens formed by the dimpled surface D when drapage occurs. Figure 6 illustrates a cross-sectional side view of a portion of a bifocal lens containing two separate zone plates with echelette depths of h and ah, respectively, and a phase channel of depth ch. Figure 7 illustrates a cross-sectional side view of a contact lens CL designed with base curve BC. This lens contains three separate zone plates each comprising three echelettes cut to a specific depth. The outer two zone plates are set back from the base curve of the lens to form phase shifting channels that are compensated to allow the lens to drape to the base curve according to the invention. Figure 8 illustrates a cross-sectional side view of a contact lens CL designed with base curve BC. This lens contains three separate zone plates each comprising three echelettes cut to a specific depth. The outer two zone plates are set back from the base curve of the lens to form phase shifting channels that are compensated to allow the lens to drape to the base curve according to the invention. Figure 9 illustrates a cross-sectional side view of a contact lens CL designed with a base curve BC. This lens contains a composite of zone plates with echelettes of continuously varying depths. The outer echelettes are the shallowest and are set back increasingly from the base curve of the lens to form phase shifting channels according to the invention. Figure 10 illustrates a cross-sectional side view of a contact lens CL designed with a base curve BC. This lens contains a composite of zone plates with echelettes of continuously varying depths. The outer echelettes are the deepest and are set back from the base curve of the lens sufficient to allow the lens to drape to the base curve according to the invention. Details Of The Invention The bifocal performance of the lens of this invention stems from the feature that it can shift focussed light from distance to near in coordination with the human eye's pupil, which normally constricts during near viewing. The lens can reduce the level of glare by reducing echelette depth at the lens periphery, and thereby reducing the non-refracting area of these echelettes. The invention comprises a contact lens of a Cohen lens design with an optic zone formed as a composite of more than one zone plate of different dimensions, preferably two or more different zone plates of different dimensions. Apart from Cohen, supra , there are no guiding principles in the art for incorporating multiple zone plates within the optic zone of a multifocal contact lens. Even though it may seem apparent that the phase relationships between the separately focussed portions of light has to be considered in effecting such an incorporation, the art is devoid of suggestions as to how to form a composite lens in a manner such that the various zone plates will interact in a functionally acceptable fashion to overcome the problems cited above. It has been determined that each zone plate should obey a common phase plate formula in order to have them all contribute light to the same foci. The radii relationship of the zone plate zones follows the characterizations of the Cohen lens design, to wit: r m = 2 m w f w=the wavelength of light m=an integer f=focal length of 1 st order diffraction. but with w and f identical for all of the phase plates. Specifically, the position of the m th zone of the composite lens depends on a single formula for r m , independent of the phase plate to which the zone belongs. It would be advantageous if the separate zone plates of a Cohen lens design or a separate zone plate plus part of another zone plate contributed light aligned in phase. One way to accomplish this is to have each zone plate associated with a phase channel that introduces a constant phase shift to correct to the phase shift e , observed in Figure 2. The Figure 3 bifocal contact lens has an optic zone which contains two separate zone plates. In that characterization, all of the zones are positioned according to a single zone plate spacing as mentioned above to be desirable. However, the characterization fails to account for the phase shift that occurs at the transition between the zone plates. Therefore, Figure 3 demonstrates that the individual amplitudes of light a₀ and a'₀ formed by the echelettes E and E' of the two zone plates are not in phase. As a result, the total resultant amplitude of light A₀, formed at the 0 th diffractive focus, is diminished from its maximum possible magnitude. The same situation occurs at the 1 st diffractive focus. Figure 4 also depicts a bifocal contact lens having an optic zone containing two separate zone plates. However, in this case, a channel of depth bh is provided to bring the phases of light into alignment. The factor b is determined from the formula: b = (1-a)/2 Because of this channel, the total resultant amplitudes of light A₀ and A₁ formed at the 0 th and 1 st diffractive foci achieve their maximum possible magnitude. It should be noticed that setting the channels at a depth d as described above is equivalent to requiring that the line through the centers of all of the resulting echelettes, essentially parallels the base curve and is set back from the base curve by one-half the depth of the deepest echelette. Soft contact lenses usually incorporate the phenomenon of draping. It is generally assumed and expected 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. Figures 5A and B illustrate a soft contact lens designed to have a center thickness T and a tear layer thickness L. However, because of drapage, it must be redesigned with a center thickness T ' 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. Thus, incorporating these factors into the standard contact lens formulas for drapage, there is obtained the expression: T' = T + L (n-1)/(n'-1). A soft contact lens designed as a composite of different zone plates, each with an associated channel, will allow the phase channels to smoothly drape upon the eye simply by adjusting the phase channel depths to compensate for drapage. The lens shown in Figure 6 with a phase channel of depth ch above the base curve of the lens, c is determined by the equation: c = b (n'-n)/(n-1). The embodiment of the invention shown in Figure 7 encompasses 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 three annular zones of echelettes which are one-half wavelength deep. Cut into the intermediate portion of the base curve is another zone plate with three annular zones. The echelettes of this zone plate are of slightly less depth and are set slightly back from the base curve of the lens. The outermost portion of the base curve is cut with still another zone plate with three annular zones. The echelette depths of this most peripheral zone plate are the shallowest and most set back from the base curve. According to this invention, the line through the centers of all of the echelettes parallels the base curve and is set back from the base curve by a quarter-wave. The embodiment of the invention shown in Figure 8 depicts a contact lens CL having a posterior surface matched to a base curve BC. This lens comprises three separate annular zone plates each containing three annular echelettes cut to a specific depth. In this lens, the outer two zone plates are set back from the base curve of the lens to form phase shifting channels that are compensated to allow the lens to drape to the base curve according to the invention. Hence, the line through the centers of all of the echelettes in this lens does not parallel the base curve, but rather is steeper until the lens is placed on the eye and allowed to drape. Then the centers will essentially parallel the base curve. The embodiment of the invention shown in Figure 9 is a contact lens CL designed with a base curve BC. This lens contains a composite of zone plates with echelettes of continuously varying depths. The outer echelettes are the shallowest and are set back increasingly from the base curve of the lens to form phase shifting channels according to the invention. The peripheral portion of the lens is cut back from the base curve at least to the point where it lays tangent to the outermost echelettes so as to allow for full drapage of the contact lens. Again, in accordance with this invention, the line through the centers of all of the echelettes will parallel the base curve. In this embodiment, the echelette depths are monotonically decreasing from the lens center outward. The embodiment of the invention shown in Figure 10 is a contact lens CL designed with a base curve BC and containing a composite of zone plates with echelettes of continuously varying depths. In this case, the outer e helettes are the deepest and are set back from the base curve of the lens to only that amount to allow the lens to drape to the base curve according to the invention. In this embodiment, the echelette depths are monotonically increasing from the lens center outward. In this case a line through the centers of all of the echelettes will be flatter than the base curve of the lens until it is draped over the eye. 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 keel construction 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: A multifocal diffractive ophthalmic lens (CL) comprising within its optic zone a plurality of concentric zone plates, the dimensions of the zones in one zone plate being different from the dimensions of the zones in another zone plate, characterised in that each of the zone plates occupies a separate radial range whereby, in use, a focus shift from distance to near is effected in response to a change in pupil size. The multifocal ophthalmic lens (CL) of claim 1 wherein one surface of the lens is defined by a base curve (BC) and the zone plates each include one or more echelettes (E, E'), the echelettes associated with individual ones of the zone plates all having equal step heights and depths from the base curve (BC) but differ from the step heights and depths from the base curve (BC) for echelettes (E, E') in other zone plates. The multifocal ophthalmic lens (CL) of claim 1 or 2 wherein the spacing of the concentric zones of each of the zone plates obeys the same zone plate formula. The ophthalmic lens (CL) of claim 3 wherein the zone plate formula is: r m = 2 m w f where: m = an integer w = a predetermined design wavelength of light f = focal length of 1st order diffraction. The ophthalmic lens (CL) of claim 2, 3 or 4 wherein the ophthalmic lens contains two concentric zone plates within its optic zone, each of the zone echelettes (E, E') in one of said zone plates having a step height "h" and each of the zone echelettes (E, E') in the other of said zone plates having a step height "ah", where: 0 < a < 1, h = w/2 (n' - n), w = a predetermined design wavelength of light, n' = the refractive index of the lens, and n = the refractive index of the tear layer of the eye. The ophthalmic lens (CL) of claim 5 wherein each of the zone echelettes (E, E') having a step height "ah" are set back a distance "bh" from the base curve, where b = (1-a)/2. The ophthalmic lens (CL) of claim 5 including a zone plate having echelettes (E, E') of a step height equal to ah which are set back a distance "ch" from the base curve, where c = b (n'-n)/(n-1) and b = (1-a)/2. The ophthalmic lens (CL) of claim 2, 3, 4, 5, 6, or 7 wherein the step height of echelettes (E, E') is greater in zone plates located radially outward from the centre of the lens (CL). The ophthalmic lens (CL) of claim 2, 3, 4, 5, 6 or 7 wherein the step height of echelettes (E, E') is less in zone plates located radially outward from the centre of the lens (CL). The ophthalmic lens (CL) of claim 2, 3, 4, 5, 6 or 7 wherein each echelette (E, E') is set back from the base curve (BC) a distance corresponding to drapage of the lens (CL) on an eye during use. The ophthalmic lens (CL) of claim 2, 3, 4, 5, 6 or 7 wherein the echelette depths are monotonically decreasing from the lens centre outward and where the peripheral portion of the lens (CL) is cut back from the base curve (BC) at least to the point where it lays tangential to the outermost echelettes so as to allow for drapage of the lens (CL) on an eye during use. The ophthalmic lens (CL) of claim 1 wherein the zone plates are positioned relative to one another so that the zone plates focus light passing through the zone plates to common foci.

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