Bifocal ophthalmic lens constructed from birefringent material

[54] BIFOCAL OPHTHALMIC LENS

CONSTRUCTED FROM BIREFRINGENT
MATERIAL

[75] Inventor: Joseph C. Marron, Brighton, Mich.

[73] Assignee: Environmental Research Institute of Michigan, Ann Arbor, Mich.

[21] Appl. No.: 324,827

[22] Filed: Mar. 17, 1989

[51] Int. CI.5 G02C 7/06; G02C 7/04;

G02B 5/30; A61F 2/16

[52] U.S. CI 351/168; 351/159;

351/161; 351/164; 351/171; 623/6; 359/494;

359/495

[58] Field of Search 350/374. 379, 400-407;

351/159, 163, 164, 168-172, 160 R, 160 H, 161,

162; 623/6

[56] References Cited

U.S. PATENT DOCUMENTS

3,520,592 7/1970 Leib et al 350/379

3.536,921 10/1970 Caulfield 350/379

3,592,533 7/1971 Braunhut 350/406

3,944.346 3/1976 Shindler 350/407

4,155.122 5/1979 Budmigar 350/374

4.302.081 11/1981 Tseutaki 351/161

4,550.984 11/1985 Raymond 350/404

4,637,697 1/1987 Freeman 351/161

4,641,934 2/1987 Freeman 351/159

4,642.112 2/1987 Freeman 623/6

4,655.565 4/1987 Freeman 351/159

4,783,152 11/1988 Nishimoto 350/379

OTHER PUBLICATIONS

W. N. Charman, "Diffractive Bifocal Contact Lenses", Contax, 11, (May 1986).

S. St. Cyr, "The Holographic Bifocal Contact Lens",
Holosphere, vol. 15, No. 5, (Fall 1988).

Primary Examiner—Scott J. Sugarman
Attorney, Agent, or Firm—Krass & Young

[57] ABSTRACT

A dual focal length ophthalmic lens is formed from a birefringent material with its fast and slow axes perpendicular to the user's visual axis. The dual focal property arises due to the differing indices of refraction of the birefringent material for light polarized parallel to the fast and slow axes. Light emanating from far objects having one polarization and light emanating from near objects having the opposite polarization are both focused onto the user's retina. Depending upon which object is being viewed, an in-focus and a blurred image appear simultaneously on the user's retina. The ability of the user's eye/brain system to distinguish between the two images provides bifocal action from a single lens. The invention may take the form of a spectacle, contact, or intraocular implant lens.

9 Claims, 2 Drawing Sheets

BIFOCAL OPHTHALMIC LENS CONSTRUCTED FROM BIREFRINGENT MATERIAL

FIELD OF THE INVENTION

The present invention generally relates to ophthalmic lenses and, more particularly, is concerned with ophthalmic lenses formed from birefringent materials which provide a dual focal length capability.

BACKGROUND OF THE INVENTION

Shortly after the age of forty, many people experience a decrease in their ability to accommodate or properly focus their vision on objects located at arbitrary distances. Most commonly, people lose the ability to see near objects within reading distance. This condition is technically known as presbyopia (from the Greek "old eye"), and results from changes in the physiology of the human eye. Presbyopia coupled with the rising average age of the general population has increased the demand for bifocal ophthalmic lenses capable of correcting failing vision.

Conventional ophthalmic bifocal lenses known to the prior art can be classified into two basic categories. The first is the segmented type lens which is composed of distinct parts having differing radii of curvature and/or differing indices of refraction. Thus, the lens has different refractive powers and the user may focus on near objects by viewing through one of the segments or focus on far objects by viewing through the remaining portion of the lens. The second type of conventional bifocal lens consists of a classical refractive lens to which a diffractive element or pattern has been applied. The diffractive element in combination with the refractive power of the lens produces one focal length for near objects, while the undiffracted light from far objects focuses according to the refractive power of the lens. This results in two images being simultaneously present on the user's retina. The user's eye/brain system is then capable of discerning between these two images depending upon whether a near or a far object is being viewed. Examples of the two types of conventional ophthalmic bifocal lenses are described and illustrated in U.S. Pat. No. 4,302,081, issued to G. F. Tsuetaki, and U.S. Pat.No. 4,642,112 issued to M. H. Freeman.

The primary disadvantage of both the segmented lens and the diffractive element lens lies in the difficulty of fabrication which requires special machining or structuring on the surface of the lens. In addition, the segmented lens has problems associated with alignment, particularly when used as a contact lens. Also, the diffracted image resulting from the diffractive element lens can have chromatic aberration and imaging noise due to scattering and higher order diffraction. Excellent discussions of the problems encountered with the prior art bifocal lenses can be found in W. N. Charman, "Diffractive Bifocal Contact Lenses", Contax, 11, (May 1986); and S. St. Cyr, "The Holographic Bifocal Contact Lens", Holosphere, Vol. 15, No. 5, 14, (Fall 1988).

Consequently, a need exists for improvements in bifocal ophthalmic lenses which will result in improved performance and decreased difficulty in manufacture.

SUMMARY OF THE INVENTION

The present invention is directed toward providing a bifocal ophthalmic lens which has improved performance and can be fabricated by conventional lens making techniques thereby avoiding special machining and

2

construction. More particularly, the bifocal diffractive power of the present invention is achieved by forming the lens from a birefringent material having its fast and slow axes perpendicular to the user's visual axis through

5 the lens. Due to the inherent properties of the birefringent material, the lens has one index of refraction for light polarized in the direction parallel to the fast axis, and a larger index of refraction for light polarized in the direction of the slow axis, the slow axis being orthogo

10 nal to the fast axis. Natural light which emanates from objects within the user's field of vision can be decomposed into equal amounts of the two polarizations which are parallel to the fast and slow axes of the birefringent material. As a result, one polarization compo

15 nent of light emanating from objects far from the user, and the opposite polarization component of light from near objects can both be focused through the birefringent lens onto the user's retina. Depending upon whether a near or far object is being viewed, an in-focus

20 and a blurred image appear simultaneously on the user's retina. Since the user's eye/brain system is capable of ignoring the blurred image, a bifocal action can be achieved with the use of a single lens.

25 The present invention may take the form of a spectacle lens for mounting in an eye glass frame, a contact lens for adhering to and floating on the cornea of the user's eye, or an intraocular lens for implanting and replacing the natural lens within the lens capsule of the

30 user's eye. Whichever form the invention takes, the curvature of the lens surfaces and the refractive indices of the birefringent material can be varied to achieve the required refractive corrections for far and near vision in conjunction with other lenses in the user's visual sys

35 tem

Alternative forms the invention may take are birefringent lenses fused or attached to non-birefringent lenses and birefringent lenses completely encapsulated within the bulk of other non-birefringent lenses. These combi40 nations increase the number of interfacing surfaces which can be varied, thus making it easier to achieve the proper refractive correction for the user.

BRIEF DESCRIPTION OF THE DRAWINGS

45 The invention will be more fully understood by reference to the following detailed description and accompanying drawings in which:

FIG. 1 is a perspective view of a bifocal spectacle lens constituting a preferred embodiment of the present

50 invention,

FIG. 2 is a sectional schematic view illustrating the passage of light through the invention embodied as a spectacle lens,

FIG. 3 is a sectional schematic view illustrating the 55 passage of light through the invention embodied as a contact lens,

FIG. 4 is a sectional schematic view illustrating the passage of light through the invention embodied as an intraocular implant lens, 60 FIGS. 5 and 6 are sectional schematic views of two alternative birefringent lens arrangements for practicing the invention.

DESCRIPTION OF THE PREFERRED 65 EMBODIMENTS

The present invention preferably takes the form of an ophthalmic spectacle lens as illustrated in FIGS. 1 and 2. Structurally, the nature of the lens 10 is identical to