For treatment of conditions such as natural eye lens cataracts, a typical eye surgery procedure requires removal of the cataracted lens through an incision in the wall of the cornea of the eyeball, and replacement with an artificial intraocular lens (IOL) as an internal implant lens. Intraocular lenses can be made of a flexible material which permits a reduction of their overall apparent girth by temporary deformation, facilitating their insertion through the cornea, thereby advantageously enabling the use of a corneal incision of concomitantly reduced size. See, e.g., U.S. Pat. No. 4,828,558--Kelman.
A Fresnel lens is normally described by starting with a standard refractive lens, for example the planoconvex lens 10 shown in FIG. 1A, and dividing it into circular zones as shown in FIGS. 1B-1C. These zones each have some of the extra thickness between the plano side of the lens and the convex side of the lens removed, as shown in FIG. 1B. Grooves are left behind, which, for manufacturing considerations, might be made up of flat surfaces to replace the convex portion of the standard lens 10. Normally, inferior image forming capabilities result, and therefore intraocular lenses (IOL's) using this type of lens have not been pursued to a great extent. Referring to FIG. 2, it can be seen that collimated light rays passing through a Fresnel lens converge at a focus point 110, but also exhibit a formation of a large blur "spot," as shown by the arrows, which is undesirable.
Others have attempted to provide vision correcting lenses have the above described unique properties. For example, U.S. Pat. No. 4,637,697--Freeman discloses multifocal contact lenses utilizing diffraction and refraction. Among the embodiments disclosed is one using a Fresnel Zone Plate as an optic element. As explained therein however, the Fresnel Zone Plate should not be confused with the Fresnel Lens, which has no diffractive power. The Fresnel Lens has facetted zones which have random equivalent phase differences between them. This is due largely to having actual phase differences, which are equal to about 100 wavelengths, so that the equivalent residual phase difference may be any value between 0 and 2.pi. because of random inaccuracies in the manufacturing method. Any amplitude addition across the lens is insignificant and no usable diffractive power is generated. The power of a Fresnel Lens is therefore determined solely by refraction at each of the facets of the lens, each of which forms an image of the object. With correct design these images are formed in the same place and final intensity of the image is found by adding the intensities of the component images.
U.S. Pat. No. 4,828,558--Kelman discloses a laminated optic with an interior Fresnel Lens surface. The Kelman patent discloses the use of a Fresnel Lens which is laminated to entrap gas between the steps or ridges on the surface of the lens. The reference points out that an unlaminated Fresnel Lens, once inserted into the eye, is surrounded by the aqueous humor which coats the exterior of the Fresnel Lens and detracts from its optical effectiveness. The reason set forth in the Kelman reference is that the aqueous humor has an index of refraction sufficiently close to the index of refraction of the intraocular lens material such that the optical characteristics of the Fresnel Lens are detrimentally offset. Thus, this reference teaches that the surface of the Fresnel Lens must be covered by at least a flat planar laminating surface or, alternatively two Fresnel Lens must be placed "ridge to ridge" to protect the ridges and entrap gases therebetween.
It has been found that a "tuned" Fresnel lens operates in a similar manner to a blazed diffraction grating by combining the refraction of the standard Fresnel lens with the coherent superposition of waves of the Fresnel zone plate. See, Vannucci, G., "A `Tuned` Fresnel Lens", Applied Optics, Vol. 25, No. 16, Aug. 15, 1986, which is incorporated in its entirety herein by reference. A standard Fresnel lens has its spot size, d.sub.S, limited by and equal to the groove width, d. This is true in the limited case where, d.sup.2 &gt;&gt;2F.lambda., and diffraction effects are not considered, where F is the focal length of the lens and .lambda. is the wavelength of light under consideration. When Fraunhofer Diffraction Theory is considered for the case where d.sup.2 &gt;2F.lambda., then the spot size is d.sub.S =2F.lambda./d. The intensity profile, I(.rho.), of the spot is then calculated based on the radial distance, .rho., from the center of the spot and the initial intensity, I.sub.0, incident on the lens, thus: ##EQU1##
For the intermediate case where, d.apprxeq.2F.lambda., the spot size is determined by the Fresnel Diffraction Theory, a good approximation of which can be derived by assuming: EQU d.sub.s .apprxeq.d when d.sup.2 .gtoreq.2F.lambda. EQU d.sub.s .apprxeq.2F.lambda./d when d.sup.2 .ltoreq.2F.lambda..
Therefore, optimization of lens performance at a given wavelength is achieved by choosing a groove width, d, that results in the smallest spot size.
In the preceding discussion it has been assumed that for Fresnel lenses in general, light rays from individual grooves are superimposed incoherently at the focal point. This is because the spot diameter from a single groove is the same as the spot diameter from the whole lens. If, however, the groove depth is an integral number of wavelengths in the lens material, then the light emerges from the flat side of the lens in a coherent manner as shown in FIG. 3. The light will therefore focus in a much smaller spot, known as the diffraction limit for the lens diameter, which is the same limit of a conventional lens.
Therefore, it would be desirable to provide a vision correcting lens which utilizes the principles of a Fresnel Lens as well as a Fresnel Zone Plate. Moreover, it would be desirable to provide a Fresnel Lens which may be made of a single layer of material, thereby facilitating insertion into the eye in the form of an intraocular lens. It would be further desirable to provide a Fresnel multifocal intraocular small incision lens which utilizes the principles of Fresnel diffraction theory to facilitate its being "tuned" to provide two or more focal lengths at one or more wavelengths of light.
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