Patent Publication Number: US-10758340-B2

Title: Intraocular lens that matches an image surface to a retinal shape, and method of designing same

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a divisional of and claims priority to U.S. patent application Ser. No. 14/196,762, filed Mar. 4, 2014, which claims priority to U.S. Provisional Application No. 61/776,184 filed on Mar. 11, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to intraocular lenses (IOLs), and more particularly to IOLs designed to produce an image surface that matches the curved surface of the retina. 
     BACKGROUND OF THE INVENTION 
     When light from an object in the field of view of a human subject&#39;s eye enters the eye through the pupil, the light is focused by refraction through the optical system of the eye onto the retina, forming an image of the object on the retina. Photoreceptor cells in the retina react to the light and send signals through the optic nerve to the brain, where the signals are perceived as the object in the visual field. The photoreceptor cells are not uniformly distributed on the retina, and the resolution of the perceived object is determined in part by the density of the photoreceptors at the location on the retina where the image of that object is formed. The resolution of the perceived object is also determined in part by how correctly the image is focused on the retina by the eye&#39;s optical system. The eye&#39;s refracting optical elements include the cornea and the lens. If those elements are not able to focus on the retina, the image formed will be distorted and the subject&#39;s vision will not be optimal. 
     The photoreceptor cells in the retina that detect light are called rods and cones. The cone cells, named for their conical shape, function best in relatively bright light and are responsible for detecting color. The cones are most densely packed near a point on the retina called the fovea, and become less dense moving away from the fovea. The fovea is a small pit-like structure disposed near the central axis of the optics of the eye. Thus, the cones are responsible for high resolution vision of well-lit objects in the center of the visual field. In contrast, the rod cells, named for their cylindrical shape, function well in dim light and are most densely concentrated at the periphery of the retina, becoming less dense moving toward the fovea. The rods thus provide for peripheral vision and night vision. 
     In a healthy eye, light from the center of the field of view enters the pupil and is sharply focused near the fovea, whereas light from the periphery of the field of view is focused at corresponding points on the periphery of the retina. However, any of a plurality of disorders can cause the optics of the eye to focus light improperly on the retina, causing the subject&#39;s vision to be blurred. 
     To resolve some disorders, such as clouding that develops in the eye lens (called a “cataract”), the natural lens may be surgically removed, and an intraocular lens (IOL) surgically implanted in the eye, typically placed at the former location of the natural lens of the eye. 
     IOLs are designed to work in conjunction with the optical elements of the eye (e.g., the cornea) to produce a sharp image only at the on-axis focal point on the retina and the paraxial region centered thereon. In general, no consideration is given to the off-axis optical properties of the retina or the optical system. Accordingly, even when a subject&#39;s vision is corrected at the center of the visual field, the peripheral image in general remains out of focus, resulting in a retinal image that is not optimal. As such, the subject&#39;s corrected vision is not as good as it could be, and peripheral vision and night vision are generally much worse than optimal. 
     Moreover, prior art IOLs do not correct any form of central vision loss, because the only portion of the retinal image that is in focus is located at the point where central vision occurs, i.e., where the central axis of the optics of the eye intersects the retina. In addition, some disorders involve changes in the retinal surface topography, such as substance buildup in the outer retina. Because prior art IOLs only focus light near the fovea, they do not correct for such changes in the diseased peripheral retinal surface. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Understanding of the herein described devices, systems, and methods will be facilitated by consideration of the following detailed description of the preferred embodiments in conjunction with the accompanying drawings, in which like numerals refer to like parts. In the drawings: 
         FIG. 1  is a simplified diagram of a healthy eye; 
         FIG. 2  illustrates an image formed in a healthy eye, with a shape that matches the shape of the retina; 
         FIG. 3  illustrates the difference between the shape of the image surface and the surface of the retina; 
         FIG. 4  illustrates the image surface shape being matched to the retinal surface shape in accordance with the disclosure; 
         FIG. 5  illustrates an embodiment of an optical system in accordance with the disclosure; 
         FIG. 6  illustrates a method of matching an image surface shape to an retinal shape in accordance with the disclosure; and 
         FIG. 7  is a simplified diagram of a system for carrying out the method shown in  FIG. 6 , in accordance with the disclosure. 
     
    
    
     SUMMARY 
     The present invention is and includes an intraocular lens (IOL), system, and method. The IOL, system and method may include having a base lens and a complementary lens selected to form a curved image surface matching a retina surface when placed in an eye&#39;s line of sight. 
     DETAILED DESCRIPTION 
     It is to be understood that the figures and descriptions provided herein may have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, other elements found in typical vision correcting lenses, lens systems, and methods. Those of ordinary skill in the art may recognize that other elements and/or steps may be desirable and/or necessary to implement the devices, systems, and methods described herein. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps may not be provided herein. The present disclosure is deemed to inherently include all such elements, variations, and modifications to the disclosed elements and methods that would be known to those of ordinary skill in the pertinent art. 
     The present disclosure describes a compound-lens IOL that may be part of an optical system and method, and that may, alone or in the optical system and method, match an image surface shape to the shape of an eye&#39;s retina. The IOLs, systems, and methods disclosed herein provide improved vision, including improved peripheral vision and night vision. 
     The disclosure also describes surgically implantable intra-ocular elements, contact lenses, spectacle lenses, and corneal inlays, as well as corneal reshaping procedures and combinations of the foregoing, and may provide the aforementioned matching. The present invention may also include other refractive corrections, such as accommodating ophthalmic corrections, higher order aberration corrections, adjustable refractive corrections, and multifocal refractive corrections, by way of non-limiting example, and that may also provide the aspects described herein. 
     The herein described devices, systems, and methods match an image surface to the shape of the retina. In particular, one or more intraocular lenses are used to achieve a desired image surface that matches the curved surface of the retina. Thereby, the subject&#39;s vision in well-lit environments is substantively improved over the prior art. In addition, the subject&#39;s peripheral vision and vision in dimly lit environments are dramatically improved. 
       FIG. 1  illustrates a normal eye  100 , including cornea  102 , lens  104 , and retina  106 . Light emanating from or reflecting off of objects in the eye&#39;s field of view enters the eye through the cornea, and is refracted by the cornea. The light then passes through an aperture in the center of iris  108 , called the pupil. The iris is opaque, and forms a stop to block light not passing through the pupil. The light passed then proceeds through lens  104 . The lens adjusts its shape in a manner that focuses the light onto the surface of retina  106 , forming an image thereon. These structures of the eye form an optical system with a central optical axis, OA-OA′, that intersects the retina near fovea  112 . 
     In a normal eye, the lens focuses light from objects in the entire field of view onto the curved surface of retina  106 , forming an image of the visual field thereon. Photoreceptor cells in the retina react to the light and send signals through optic disc  114  (the “blind spot”) to optic nerve  110 , and thence to the brain (not shown). The optic disc represents the beginning of the optic nerve  110  to the brain. In the brain, the signals are perceived as the objects in the visual field.  FIG. 2  illustrates an image being formed on the retina surface by the natural lens, wherein the image surface matches the shape of the retina surface. 
     During cataract surgery an IOL is surgically implanted in the subject&#39;s eye, typically at the location at which the natural lens resided. An IOL is made of a refractive material, and is designed to focus light entering the eye onto the retina. 
     In general a single IOL is used, and it is designed so that the field of view is in focus only at the point at which the central axis of the optical system of the eye intersects the retina, while ignoring the off-axis properties of the eye and its optical system. The result leads to suboptimal vision, due at least to uncorrected field curvature and oblique astigmatism of the cornea/lens system, also known as marginal or radial astigmatism. 
       FIG. 3  illustrates the typical situation where the IOL is designed to focus light entering the eye from the left (as shown) through the cornea onto the retina. The prior art practice is to design the IOL to focus light only at a single point, where the central axis of the optical system, including the cornea and IOL, intersects the retina, and thereby the off-axis properties of both the optical system and the retina are ignored. As illustrated in  FIG. 3 , the image surface forms in front of the retina except where the optical axis intersects it, i.e., the image surface is less flat than the retina (i.e., is “more curved”). In other cases (not shown), the image surface may be formed behind the retina except where the optical axis intersects it, i.e., the image surface is flatter than the retina. As a result, in either case the image formed on the rest of the retina is blurred. 
     Compound lenses comprising a plurality of lens elements can be used to correct for these effects. However, prior art compound lenses are typically designed to provide an image surface that is flat, not curved. For example, imaging devices such as film, charged couple devices (CCDs), etc. that used in cameras, telescopes, microscopes, and the like have a flat (i.e., not curved) surface on which the image is formed. The surface on which etched components are formed by focusing light with compound lenses in semiconductor manufacturing is also flat. Accordingly, the compound lenses for use in such applications are typically designed to form a flat image. In contrast, the image surface produced by the herein disclosed devices, systems, and methods is curved to match the curved surface of the actual retina of a particular subject&#39;s measured eyeball. 
     Retinal shapes vary greatly between individuals. For example, different refractive groups, such as those with myopia (nearsighted) and those with hyperopia (farsighted), have been determined to have generally different retinal shapes (see, e.g., Atchison D A, Pritchard N, Schmid K L, Scott D H, Jones C E, Pope J M,  Shape Of The Retinal Surface In Emmetropia And Myopia , Invest Ophthalmol Vis Sci 2005, 46:2698-2707, the entirety of which is incorporated herein as if fully set forth). Therefore biometry, including measuring the dimensions and retinal shape of a particular subject&#39;s eye, must be carried out before implanting an IOL. Preferably, such biometry can be performed using one or more of Optical coherence tomography (OCT), confocal Scanning Laser Ophthalmoscopy (cSLO), Magnetic resonance imaging (MRI), and the like. Once the biometry has been completed and the eye dimensions and retinal shape are known, IOLs can be selected or designed that will form an image surface that matches the shape of the retina surface of the measured eye. 
     The image surface of an optical system is determined by the tangential and sagittal surfaces arising from oblique astigmatism, together with field (Petzval) curvature. In embodiments, two or more lenses can be incorporated into the IOL design to account for this image surface. Depending on the image shape desired to be formed, different combinations of lenses having positive and negative powers can be incorporated into the IOL design. The Petzval surface for a positive lens (i.e., a convex lens) bends inward, while that of a negative lens (i.e., a concave lens) bends outward. Thus, positive and negative lenses can be combined into a single compound IOL or IOL system that matches the image surface to the measured retina surface. 
     In an exemplary method of selecting or designing an IOL or IOL system, a desired IOL power is calculated for a particular measured eye in accordance with well-known prior art practices. Then, a positive lens may be selected to be positioned, for example in the posterior chamber, and a negative lens may be selected to be positioned, for example, in the anterior chamber. Such a configuration is illustrated in  FIG. 4 . The positive and negative lenses are selected to have a combined power equal to the IOL power calculated for the eye&#39;s deficiencies. In addition, the lenses are selected to achieve a particular Petzval surface radius that matches that of the retinal surface. The expression for the Petzval curvature is: 
               1     R   p       =       -     n   N       ⁢       ∑     i   =   1     N     ⁢           ⁢         n   i     -     n     i   -   1             n   i     ⁢     n     i   -   1       ⁢     R   i                   
where R p  is the Petzval radius of curvature, and R i  and n i  are the radius of curvature and refractive index, respectively, of the i th  lens surface of the combined IOL lenses. It is desired to match R p  to the radius of curvature of the subject&#39;s measured eye. To do so, lenses are selected with lens surfaces that have R i  and n i  which, when combined in accordance with the equation above, result in the desired Petzval radius.
 
     In an exemplary operation, a subject&#39;s eye is measured, and it is determined that the retina&#39;s radius of curvature is about 17.0 mm, and the corneal anterior radius of curvature is about 7.8 mm, and the corneal posterior radius of curvature is about 6.4 mm. The cornea has a refractive index of 1.375. In accordance with standard prior art practices, it is determined that a standard 20 diopter (20 D) positive monofocal IOL results in the desired focus at the central optical axis on the retina. In addition, the optical system of the eye, including the cornea and the standard 20 D IOL, is determined to have a Petzval radius of about −17.7 mm, i.e., larger than the radius of curvature of the retina. This results in an image surface that is somewhat flatter than the measured retinal shape. Therefore, the image formed on the retina is out of focus except at the paraxial region. To match the image surface to the retinal surface so that the image formed is in focus everywhere on the retina, the Petzval radius of the optical system must be reduced. 
     To reduce the Petzval radius, the single standard IOL of the prior art can be replaced with two lenses. One lens is a positive lens that is stronger than 20 D, and is designed or selected (hereinafter collectively referred to as “selected”) for implantation at a select position in the posterior chamber. A second, negative lens is selected for implantation at a select position in the anterior chamber. The two lenses properly configured will result in an effective power of 20 D as before, but will have a Petzval radius of −17.0 mm so that the image surface matches the retinal surface. For example, a positive 40 D IOL may be selected for placement in the posterior-chamber, and a negative IOL may be selected for placement in the anterior chamber such that the resulting optical system has a Petzval radius of about −17.0 mm. 
     In the case where the Petzval surface&#39;s radius of curvature is smaller than the radius of curvature of the retinal surface, the opposite approach may be used. In an exemplary operation suitable for this case, a subject&#39;s eye is measured and it is again determined that the retina&#39;s radius of curvature is about 19.0 mm. As before, in accordance with standard prior art practices it may be determined that a standard 20 diopter (20 D) positive monofocal IOL results in the desired focus at the central optical axis on the retina. In this case, however, the optical system of the eye, including the cornea and the standard 20 D IOL, is determined to have a Petzval radius of about −18.3 mm. This results in an image surface that is less flat than the measured retinal shape, as illustrated in  FIG. 3 . The image formed on the retina is again out of focus except for the paraxial region. But in this case, to match the image surface to the retinal surface so that the image formed is in focus everywhere on the retina, the Petzval radius of the optical system must be increased. 
     To increase the Petzval radius, the single standard IOL can be replaced with two lenses. One lens is a positive lens that is weaker than 20 D, and is selected for implantation at a select position in the posterior chamber. A second positive lens is selected for implantation at a select position in the anterior chamber. The two lenses properly configured will result in an effective power of 20 D as before, but will have a Petzval radius of −19.0 mm so that the image surface matches the retinal surface. For example, a positive 10 D IOL may be selected for placement in the posterior-chamber, and a second positive IOL may be selected for placement in the anterior chamber such that the resulting optical system has a Petzval radius of about −19.0 mm. 
     Thus, in either case, two IOLs may be selected which, when properly configured, result in a Petzval radius that matches the shape of the retina. This assumes that the shape of the retina is such that it can be matched with only two lenses. Some more complicated retinal shapes may be matched using more elaborate optical systems comprising more than two lenses, and/or non-spherical lenses, or lenses formed with bumps or depressions disposed so that the resulting image surface formed matches the surface of the retina, as will be appreciated by those skilled in the art in light of the disclosure herein. 
     In various embodiments, one or more of the plurality of IOLs used may be fully refractive, fully diffractive (see, e.g., Morris G M, Buralli D A, Federico R J.  Diffractive Lenses For Vision Correction . USA: Apollo Optical Systems, LLC, Rochester, N.Y. 2006, the entirety of which is incorporated herein as if fully set forth), or have a combination of diffractive and refractive surfaces (i.e., a hybrid lens). Selecting appropriate combinations of refractive, diffractive, and/or hybrid lenses can extend the range of shapes to which an image surface can be tuned. Notably, unlike the fully refractive lenses commonly used in IOLs, which always have a curved image surface, diffractive lenses can have a flat image surface (see, e.g., Buralli D A, Morris G M, Rogers J R,  Optical Performance Of Holographic Kinoforms , Appl Opt 1989; 28:976-983, the entirety of which is incorporated herein as if fully set forth). Accordingly, using one or more diffractive surfaces on one or more of the lenses can enhance the range of retina surface shapes that can be matched. 
     Various embodiments include compound lenses in which a plurality of optical system characteristics may be modified to achieve an image surface that matches the shape of a particular measured retina. These characteristics can include, with regard to each lens: selecting a size, shape, and thickness of the lens; selecting each surface of the lens to be refractive or diffractive; selecting an index of refraction of a homogeneous material from which the lens is formed, or forming the lens from a material having one or more refraction index gradients, each gradient extending smoothly or in discrete steps from a low value to a high value; selecting a distance of the lens along the optical axis from a fixed point on the axis such as a surface of the cornea or the retina; selecting an orientation of each surface of the lens about the optical axis in a plane normal (perpendicular) to the optical axis; placing the lens in a select plane that is not normal to the optical axis, or using a lens having a surface that is not normal to the optical axis; or any other physical characteristic of the lens or its placement in the system. The characteristics can also include, with regard to combining a plurality of lenses: selecting a number of lenses to combine; selecting an order of the lenses from the cornea to the retina or vice versa; or any other physical characteristic of the lens system or its placement. 
     In an embodiment, once the IOL system is designed, each IOL of the system is individually implanted in the eye. In another embodiment, a plurality of lenses may be combined to form a single compound IOL unit, such as an enclosure or envelope that contains a plurality of lens elements the entirety of which can be implanted together. Such an enclosure can include structural elements to secure each lens element&#39;s position and orientation within the enclosure. Such a compound IOL can be inserted in a single operation into the capsular bag after the natural lens is removed. Further, one or more IOLs or compound IOLs may be used in conjunction with a non-implanted, i.e., an exterior, lens or lenses. 
     In various embodiments, the calculations performed to design an optical system that matches the shape of a retina can be simplified by standardizing at least some of the parameters of the optical system, and preferably as many parameters as is practicable to achieve an acceptable result. An acceptable result may be determined by how closely the image surface formed by the optical system matches the measured shape of the retina. In an exemplary embodiment, an IOL system may be designed with first and second discrete IOL elements that may be individually implanted in the eye. The first IOL may be a positive lens disposed at a “standard” location within the posterior chamber and formed of a homogeneous material having a “standard” index of refraction. The second lens may be disposed at a “standard” location within the anterior chamber, and may be formed of a homogeneous material. No diffracting surfaces, surfaces tilted from the optical axis, surfaces with bumps or depressions, or the like may be used in this example. In this simplified case, the parameters of the first lens that must still be selected include its height, width, shape, thickness at the center of the lens, and the radii of curvature of its two surfaces. The second IOL may be either positive or negative, and its height, width, shape, thickness at the center of the lens, and the radii of curvature of its two surfaces must also be selected. The second lens may also be formed of a material having the same index of refraction as the first lens. However, it is noted that the Petzval radius of curvature of a lens depends not only on the radius of curvature of each of its surfaces, but on the lens&#39; refractive index as well. Consequently, the range of Petzval radii that can be achieved by using the second lens, and hence by the entire optical system, can be extended by selecting a homogenous material for the second lens having a higher or lower refractive index than the material of the first lens. 
     In an embodiment, all lenses can be placed in the posterior chamber inside the capsular bag. It is noted that an IOL system having two optical components is described in U.S. Pat. No. 7,238,201, entitled ACCOMMODATING INTRAOCULAR LENS SYSTEM WITH ENHANCED RANGE OF MOTION, the entirety of which is hereby incorporated by reference as if fully set forth. That system comprises an anterior optic and a posterior optic coupled to anterior and posterior haptic arms, respectively. The &#39;201 patent relates to other than the matching of the image surface to the shape of the retina that is included the herein disclosed devices, systems, and methods. Rather, the haptic arms are arranged so that a forward movement of the posterior optic actuates a substantially larger forward movement in the anterior optic. This supports the ability of the eye to modify its refractive power when viewing objects at different distances, called “accommodation”. Nevertheless, in an embodiment, the shape and power of each optic in such a system may be selected to achieve a Petzval surface that matches the shape of the retina in accordance with the present disclosure. 
     In another embodiment, one lens may be placed in the anterior or posterior chamber, and the other lens may be a surface profile disposed on or within the cornea, the surface profile having been formed by a laser, (e.g., using a LASIK, LASEK, or PRK procedure) resulting in the reshaping of the cornea. 
     As shown in  FIG. 5 , in an embodiment an aperture can be added to the optical system as an artificial stop, such that it is the limiting aperture of the optical system of the eye instead of the pupil. Adding the aperture does not affect the Petzval surface. However, oblique astigmatism can be improved by selecting an appropriate stop size and position. 
     Referring now to  FIG. 6 , there is shown a method  500  for matching an optical system image surface to the surface of a retina. Method  500  includes measuring the dimensions of a subject eye and the shape of the surface of its retina,  510 . The lens power needed to focus an image at the intersection of the optical axis of the eye on the retina is calculated,  520 . The shape of the image surface produced at the retina by that lens can then be determined and compared to the shape of the retina,  530 . In the case where the image surface is flatter than the retina surface, a base lens power may be selected that has a higher power than the default lens power,  540 . In that case, a negative complementary lens may be selected or designed that adjusts the image shape of the optical system to match the retina shape,  550 . Conversely, in the case where the image surface is less flat (i.e., more curved) than the retina surface, a base lens power may be selected that has a lower power than the default lens power,  540 . In that case, a positive complementary lens may be selected or designed that adjusts the image shape of the optical system to match the retina shape,  550 . If needed, additional optical elements may be selected, for example, to correct any residual image aberrations,  560 . An IOL system can then be selected or assembled that includes the base lens, the complimentary lens, and any additional optical elements,  570 . The IOL system can then be implanted in the subject eye,  580 . 
     As illustrated in  FIG. 7 , the present invention may be implemented using a clinical system  300  that is capable of assessing the eye&#39;s biometry and of performing the calculations set forth in method  500 . The system  300  may include a biometric reader  301 , a computing processor  302 , and a computer readable storage device  304  in data communication with the processor  302 . The computer readable storage device may include a volatile or non-volatile memory, a magnetic or optical drive, or the like. Storage device  304  may store therein an array of values  308  such as properties of various optical elements in various configurations. The storage device may also store therein sequences of instructions  310  which, when executed by the processor  302 , cause the processor  302  to select or design an IOL system configured for implantation into the eye of the subject presenting the biometric readings to biometric reader  301 . The array of values  308  may comprise data used or obtained from method  500  or other methods consistent with embodiments of the invention. The sequence of instructions  310  may include one or more steps of method  500  or other methods consistent with embodiments of the invention. 
     The processor  302  may be embodied in a general purpose computer, such as a desktop or laptop computer, and/or the processing, storage, and/or other resources of a computing cloud accessible through a network. The processor  302  may comprise or be coupled to hardware associated with biometric reader  301  operable to select or design an IOL system for placement into the eye of the subject that matches an image shape to the retina shape. In embodiments, the system  300  may be configured to be coupled to or receive data from another device, such as one or more instruments for obtaining measurements of a subject eye. Alternatively, the system  300  may be embodied in a handheld device that may be adapted to be electronically and/or wirelessly coupled to one or more other devices. 
     In another embodiment, rather than using a dual lens system, a single lens may be used with two different refractive indeces. In the thin lens approximation with lenses placed tightly together, the Petzval sum is P1/n1+P2/n2+ . . . , where P is equal to power. In a single lens embodiment, one part of the lens may be comprised of a high refractive index and another part of the lens comprised of a low refractive index. By way of example, the anterior or posterior part of the lens may have a high refractive index and the opposing side may have a low refractive index. Refractive power can then be appropriately distributed to make the Petzval surface of the IOL plus the cornea match the retina. 
     In certain embodiments a single lens, single refractive index material may be used. Certain IOL designs rely on having a well-defined shape factor. Additionally, the power is determined to give optimal focus on the fovea. However, if the lens is designed with enough thickness, preferably in the shape of a meniscus lens, the Petzval curvature may be optimized to match the retinal radius of curvature. The procedure for designing such a lens may comprise of: 
     1) receiving a measurement of at least a shape of a retina of a subject eye, wherein the retina has a radius of curvature; 
     2) determining a desired shape factor for an IOL, wherein the shape factor is determined from optical factors, engineering constraints and/or anatomical constraints; 
     3) determining a total IOL power for focusing light onto the retina surface of the subject eye; 
     4) determining the power of each respective surface of the IOL from a thickness of the IOL, the desired total IOL power and the desired shape factor; and 
     5) comparing a Petzval sum to the retinal radius of curvature. If the Petzval sum does not match the retinal radius, the power of the two surfaces may be increased or decreased. This adjustment of the power of the surfaces may be done while keeping the shape factor constant. The change of surfaces will change the power of the IOL, so the thickness of the IOL may be adjusted to correct the power. This adjustment of IOL thickness does not impact the Petzval sum or the shape factor. An advantage of this embodiment is that a single refractive index material may be used. 
     Although the invention has been described and illustrated in exemplary forms with a certain degree of particularity, it is noted that the description and illustrations have been made by way of example only. Numerous changes in the details of construction and combination and arrangement of parts and steps may be made. Accordingly, such changes are intended to be included in the invention, the scope of which is defined by the claims.