Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Reexamination Certificate
2000-04-17
2002-05-21
Schwartz, Jordan M. (Department: 2873)
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
C351S16000R
Reexamination Certificate
active
06390624
ABSTRACT:
FIELD OF INVENTION
This invention relates to a rigid contact lens which acts as a varifocal device when worn on a specified eye, a process for the manufacture of such a rigid contact lens, a process for shaping the back surface of such a rigid contact lens and a rigid contact lens blank having a back surface prepared by such a process.
BACKGROUND OF THE INVENTION
Presbyopia is a condition of advancing age in which the ability to focus on near objects, such as newsprint, is diminished. Presbyopes require reading glasses and such reading glasses always have positively powered lenses. Presbyopia is an advancing condition in that, at first, presbyopes require small additional amounts of positive power but this requirement increases with advancing age. The condition typically becomes noticeable then about +0.75 dioptres of additional power is required to read. The amount of extra power required for reading correction is commonly referred to as the “reading addition” in spectacle prescriptions.
Presbyopia affects the entire population. However, a presbyope may already be wearing spectacles or contact lenses to correct another vision defect, such a myopia. In such cases, spectacles or contact lenses will be required which are capable of correcting both types of vision defect.
Bifocal lenses are known which have two clearly delineated segments, one correcting distance vision and one correcting near vision, and this type of lens has been utilised to produce bifocal spectacles and bifocal contact lenses.
Similarly, multifocal lenses are known which have two or more clearly delineated segments. Thus, in spectacles, it is reasonably common to have lenses which correct for distance, near and intermediate vision and such multifocal lenses are often worn by workers such as VDU operators who are frequently required to operate at all these distances.
Varifocal lenses are also known in the context of spectacles which are designed to correct presbyopia but do not have clearly delineated segments.
The surface of a contact lens which sits on the eye when the lens is worn is known as the back surface and all rigid lens design classification is based on the arrangement of the back surface parameters. Thus, lenses which have back surfaces derived from spheres are broadly referred to as “spherical” and lenses which include non-spherical zones are broadly described as “aspheric”.
The back surface of a contact lens can be divided into two distinct zones. The first zone, which is centrally located, is referred to as the back central optic zone and it is through this zone that the lens-wearer sees. The second zone surrounds the back central optic zone and determines the lens-cornea relationship, that is, how the lens sits on the eye and how it behaves dynamically. This zone is referred to as the peripheral zone and the associated radius and diameter are known as the peripheral zone radius and peripheral zone diameter. Often, a lens will have more than one contiguous peripheral zone, each peripheral zone having its own associated peripheral zone radius and peripheral zone diameter. Areas of the back central optic zone which lie away from the axis of the lens are referred to as marginal zones and the portion of the back central optic zone which would be used by a normally dilated pupil is referred to as the effective optic zone.
Spherical aberration is a natural aspect of spherical optical lenses in which rays of light close to the lens axis (the paraxial region) focus at a slightly different place to marginal rays.
FIG. 1
shows a convex (positive) lens in which the paraxial incident ray p focuses at fp and a marginal ray m focuses at a shorter distance from the back vertex of the lens at fm. Rays which are incident between p and m will focus in between. The lens will therefore not have a single point of focus but a “blur circle”. A positive lens has a “real” focal length and positive spherical aberration, that is, as the incident ray moves away from the centre, the power of the surface becomes increasingly positive.
FIG. 2
shows the spherical aberration of a negatively powered lens in a similar fashion to
FIG. 1. A
negative lens has a “virtual” focal length and negative spherical aberration, that is, as the incident ray moves away from the centre, the power of the surface becomes increasingly negative.
The cornea and the eye may for many purposes of optical analysis be considered as a single optical element and that convention will be used here. The cornea is itself subject to positive spherical aberration as shown in FIG.
3
. Thus, marginal rays focus in front of paraxial rays. However, the extent of corneal spherical aberration varies between individuals and depends on the shape and size of the cornea.
During the early 1990s, an instrument called the videokeratoscope has developed very rapidly due to impetus from the rapid development of the personal computer and, at the same time, the very rapid development of corneal surgery using lasers where it is vital to monitor detailed changes in corneal topography before and after the surgical procedure. The videokeratoscope measures the topography of a patient's cornea over approximately the central 10 mm by shining bright concentric circles, known as a “Placido disk”, on the eye and measuring the reflected light. The algorithms used to calculate the corneal topography are known as “reconstruction algorithms” and are highly computing intensive hence the recent advances as personal computers have greatly increased the speed at which they can perform large volumes of complex calculations.
Various studies have shown that the average human cornea is well-modelled by means of a general ellipsoid, particularly over the central 10 mm. Ellipsoids can be specified mathematically in terms of vertex radius and numerical eccentricity. The videokeratoscope can determine the values of vertex radius and numerical eccentricity which will most closely describe the cornea being measured. Thus, it can determine the central corneal curvature and also the shape of the eye. The availability of the videokeratoscope in clinical practice has therefore provided a reliable means of measuring the key parameters of central corneal topography.
When a rigid contact lens is placed on the eye, it is normal to have a small layer of tears form between the back surface of the lens and the front surface of the cornea. From the above discussion, it is evident that the combined optical system of lens, tears and cornea will also have spherical aberration but that the amount will depend on the power of the lens, the lens-cornea relationship and the shape of the cornea.
If a patient who has normal corneal topography is wearing a positively powered contact lens, there will often be significant positive spherical aberration within a diameter corresponding to a normally-dilated pupil, because the positive spherical aberration of the lens will add to the positive spherical aberration of the cornea. Such a situation is shown schematically in FIG.
4
. On the other hand, if a patient having normal corneal topography is wearing a negatively powered lens, there may be little or no combined spherical aberration in some cases because the negative spherical aberration of the lens will reduce or cancel out the positive spherical aberration of the cornea.
The present invention is based on the premise that, if the spherical aberration of the combined optical system of lens, tears and cornea could be engineered so that the power in the centre corrected the patient's distance vision and the marginal power at a defined distance from the centre corrected the patient's near vision, then the lens would function as a varifocal device if the on-eye dynamics of the lens were such that, when the patient looked down, the lens moved or “translated” so that the line of sight was through the part of the lens that was powered for the patient's reading correction.
It is normal to fit rigid lenses so that there is movement relative to the cornea during normal eye movements such as blinking or looking
No.7 Contact Lens Laboratory Limited
Pearne & Gordon LLP
Schwartz Jordan M.
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