Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Patent
1998-04-09
2000-07-25
Sugarman, Scott J.
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
351177, G02C 704
Patent
active
060928999
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a multifocal contact lens, having a reading part in the bottom part and another lens in the top part.
DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR
1.97 and 1.98
Such a lens is known from U.S. Pat. Specification 4,854,089. In the general prior act it is known to provide a part with smaller radius of curvature in the centre of the lens. This part with smaller radius of curvature is, as it were, placed on a larger part with a larger radius of curvature. This is a rotationally symmetrical lens where there is no need for stabilizing means. Due to the fact that there is a kink at the position of the R.sub.lees -R.sub.verte (reading-distance) transition, a great deal of reflection occurs in the direction of the optical axis, which means that the optical characteristics of this lens are inadequate.
Designs are also known from the prior art in which a reading part is moulded or cast into the lens material, or is disposed on the front side in a half moon shape. Such systems can work only if the lens can move fully from distant to close up. The reading part covers about 50% of the pupil if the lens is centred on the cornea. In order to obtain an undistorted image, the lens will have to move downwards until it is at or below the lower eyelid. The lens has to move up again for reading. These systems are characterized in that the optical axes do not lie level, with the result that an image discontinuity always occurs. This causes an unsteady image for the wearer, and leads to complaints of double vision and shadow images.
In another system (tangent streak) the optical axis does remain level. It lies precisely in the centre of the lens. The disadvantage of this lens is that the distance/close-up division is always 50%. The lens is divided exactly into two halves. This optical system is characterized in that the two outer radii for the reading and distance part are monocurved, i.e. they are spherical from the centre to the edge. This produces a very thick lens. This design also means that at the edge quite a large height difference arises between the two curves. Both have a very negative influence on wearing and vision comfort.
On the other hand, the reading part on the lens is designed as a sort of half moon. The characteristic feature of this lens is the enormous image discontinuity which occurs. The reason for this lies in the fact that the lens has to be prismatic, in order to obtain a stable position on the eye, on the one hand, and in order to leave sufficient material to be able to make the reading part, on the other hand. This prismatic effect is present optically in the distance part. However, the reading part is not provided in the second instance with prism effect. The optical centre points do not lie on the optical axis, nor can they be joined tangentially in the centre. A further disadvantage is that the shape of the `half moon` on the front side, which is the reading part, is not adjustable. The dimensions of this part are different in the case of each lens power combination, with the result that much too large a reading part and too small a distance zone are produced. The consequence is that this lens has many reflections for the user. The lens requires a great vertical movement in order to function. This causes a high degree of discomfort for the user.
It is therefore the object of the present invention to provide a multifocal lens which has bivisual features, without the presence of a kink or similar transition which causes troublesome reflections in the direction of the optical axis or an image discontinuity, and which lens has such features that no great movement on the cornea is needed for fulfilment of the distance and reading functions.
BRIEF SUMMARY OF THE INVENTION
The problems described above can be avoided according to the invention by providing a multifocal contact lens comprising: outer surface and bounded on all sides by the remainder of the contact lens and wherein the outer limit of the reading portion lies
REFERENCES:
patent: 4573775 (1986-03-01), Bayshore
patent: 4854089 (1989-08-01), Morales
Procornea Holding B.V.
Sugarman Scott J.
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