Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Patent
1995-10-05
1998-05-05
Sugarman, Scott J.
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
351177, G02C 704
Patent
active
057482829
DESCRIPTION:
BRIEF SUMMARY
This invention relates to multifocal (including bifocal) contact lenses, and more particularly to such lenses having diffractive power.
U.S. Pat. No. 4,637,697 describes a bifocal contact lens in which at least a portion of the light passing through the lens is focussed by asymmetric zone plate surfaces. Such zone plate surfaces comprise a plurality of concentric zones arranged so as to cause diffraction of light transmitted through the lens, each zone providing an asymmetric retardation of light across the zone width. In one form of that invention, the zones are defined and the asymmetric retardation is provided by the surface contour of the lens.
In particular, the zones may be defined by steps in the lens surface. The lenses of U.S. Pat. No. 4,637,697 are designed to operate as bifocal lenses and in one form the concentric zones are arranged so that the lens surface forming the zone is in the form of a slope whose profile is uniform all round the concentric zone. Zones of this form may be shaped directly by means of laser machining using a suitably shaped mask, or by directly cutting with a diamond tool. This form of zone plate commonly results in most of the light in the visible spectrum being directed into a zero power and one positive power image. In the type of lens where there is a need for some refractive power to correct for distance vision, the refractive power will be undisturbed by the zero-order image of the diffractive power. The power for the near image is provided by the diffractive effect, and the added power, which is the difference between the overall power for distance viewing and the overall power for near viewing, is provided entirely by the diffractive effect. In this type of lens there is no need for orientation on the eye as the bifocal lens operates in the so-called simultaneous vision mode with both near and distance viewing being available to the eye at all points on the optical portion of the lens.
Multifocal contact lenses are also known which do not rotate on the cornea and may be stabilized in position by several methods. The most common method is one involving some form of ballasting i.e. shaping the lens so that it is thicker and thus heavier at one portion of the edge. Lenses can be produced with a wedge or prism shape with the thicker portion at the bottom. Lenses can also be truncated or cut off so that a lower portion is wider and heavier than the rest of the lens and thus able to maintain a particular orientation on the eye. These lenses contain separate areas on the lens for distance, near and where necessary intermediate vision. The ballasting causes the lens to return to a stable lower position on the eye after blinking when the wearer is looking straight ahead. In this position, the distance portion of the lens is located so that it is aligned in front of the pupil. Moving the eye down to read causes the lens to be pushed up as it is contacted by the lower lid so that the pupil becomes aligned with the portion of the lens that contains the near viewing portion. Such lenses can suffer from jump i.e. the problem of a displaced image that occurs as the eye shifts from distance to near and passes the boundary between the near and the distance portion. Such jump can be eliminated by using so-called mono-centric bifocals but many patients needing bifocals may find problems in being fitted with such lenses.
Contact lenses have been made available in the marketplace which utilize the diffractive effect for focussing the near image. Such lenses, unlike the ballasted multifocal lenses referred to above, do not need to be oriented on the eye but nevertheless are subject to some movement on the eye particularly when the eye moves down to read. The users of the multifocal lenses in general have reached an age where the amount of light needed for reading and close work is substantially greater than that needed by e.g. a twenty year old. The ability to read and do close work is therefore influenced by the light intensity of the image. In the case of a lens which uses asymmetric
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Pilkington Barnes Hind Inc.
Sugarman Scott J.
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