BIFOCAL LENSES

This invention relates to bifocal lenses and in particular to bifocal and trifocal contact and intraocular lenses.

Various designs of bifocal contact lenses have been proposed but all require considerable amounts of chair-side time for satisfactory fitting of such lenses. Designs have been proposed in which the viewing area consists of a central circular zone, surrounded by concentric zones of alternating near (reading) and distant vision. US Patent No. 4,890,913 discloses such a lens. The underlying content in the above patent is to try to ensure that at every pupil size the amount of light transmitted through the reading and distant vision zones is substantially equal. It has been found that with such lenses when correctly fitted, the wearer is able to concentrate on the clearest image focused on the retina.

While the lenses described in the above US patent have made a useful contribution to the art, and can reduce the amount of fitting time required, there are still some problems to be overcome. Lenses manufactured in accordance with the above patent are most effective when manufactured as hard lenses, or from lens material which have a low water content. If, on the other hand, lenses are made in accordance with the prior patent in a soft, high water content lens material, the zones formed on a posterior lenticular surface tend to be "ironed out" by the pressure of the upper eyelid.

Another unrelated problem of some prior art lenses, is that in certain light conditions, reflections are seen from junctions between adjacent zones. This can have the effect of giving the appearance of rings around a light source.

The present invention is therefore, in one aspect directed to a solution to the above problems and the provision of a lens which can be used both as a contact lens, and as an intraocular lens from a variety of conventional lens materials.

According to the present invention there is provided a multifocal contact lens having a posterior base curve and a viewing area generally corresponding to the maximum pupil area of the wearer, said viewing area having a central circular refractive zone having a first focal length corresponding to distance vision or reading vision and a plurality of annular concentric refractive zones which alternate between a second focal length corresponding to the other of reading and distance vision and said first focal length as they extend outwardly from said central zone, wherein at each pupil size between its minimum and maximum the relative areas of the distance and reading zones is between 40:60 and 60:40, characterised in that the total number of zones comprises at least 30, and wherein the reading zones are flatter than the posterior base curve and the distance zones are steeper than the posterior base curve.

The provision of a very large number of alternating reading and distance zones in the viewing area is believed to be responsible for overcoming many of the problems of the prior art lenses. One significant advantage is that by providing a large number of zones, it is much easier to ensure that the relative areas of the distance and reading zones is close to the preferred 50:50 ratio at all pupil sizes. As a consequence, the lenses provide good distance and reading vision in all lighting conditions. The number of zones is at least 30 and preferably at least 50. There is no theoretical upper limit but there will be a practical upper limit which is determined by the limitations of the equipment or system used to form very large numbers of zones on the surface of a lens. Existing computer controlled lathes should be capable of forming up to about 70-80 alternate zones on the lenticular surface of a standard contact lens, in which the major viewing area is about 6-8 mm in diameter.

The alternating zones are formed on the rear surface of the lens. One advantage of forming the zones on the posterior surface of the lens is that any imperfection in the surface is largely compensated by the tear fluid which will fill the space between the undulating profile forming the zones and the cornea.

Preferably, the maximum tear thickness between the anterior lens surface and the cornea is about 0.007 mm and may be as little as 0.003 mm.

In general, the maximum thickness of material removed between every other zone to form the zones is about 10% of the total thickness of the lens.

Another, separate problem arises in fitting bifocal contact lenses. This relates to the observation that eyes suffer from the fact that the combined power of the cornea and the natural crystalline lens increases towards the periphery. As a result, when the pupil dilates in low lighting conditions, a patient with normal distance vision will experience improved distance vision when wearing corrective lenses with a negative power of about 0.5 dioptres.

Another aspect of the present invention is based on the realisation that improved vision with bifocal lenses can be achieved by introducing a progressive power variation in a multi-zonal concentric lens in which the negative power of one or both types of vision zones is increased towards the periphery of the viewing area of the lens. Such progression in negative power should not exceed about 1 dioptre, preferably not more than about 0.75 to 0.8 dioptre and, generally, will be in the range of 0.25 to 0.75 dioptre. A power change of about -0.5 dioptre from centre to periphery being preferred.

According to a second aspect of the present invention, therefore, at least one of the zones has a power which is more negative than a zone of the same character which is situated closer to the centre of the lens.

Preferably, at least one distance vision zone has a power which is more negative than a distance vision zone situated closer to the centre of the lens. Generally, the distance vision zones have powers which progressively become more negative towards the periphery of the lens viewing area, so that the zones towards the periphery have the highest degree of negative power.

In general, there is no further increase in clarity by increasing the negative power by more than about 1 dioptre. Normally, the increase in negative power will be limited to 0.75 to 0.8 dioptre. Usually, optimum benefit is obtained where the negative power increase is in the region of 0.5 to 0.6 dioptre. The maximum additional negative power at the periphery is limited by the onset of reverse spherical aberration.

In the manufacture of lenses in accordance with both aspects of the invention, the zones are formed by machining base curve of the lens. The power is produced by machining or moulding the power surface of the lens. In order to provide a smooth transition between zones, it is preferable to make the zones at least partially aspherical, generally by making some or all of the near vision zones aspherical, so that spherical lens surfaces merge smoothly into spherical lens surface with minimum discontinuity at the junction between two zones. Lenses produced in accordance with this concept are described in UK Patent Application No. 2,295,686 (de Carle).

It may also be advantageous to manufacture lenses in accordance with the invention in such a way that the optical centre of at least the major viewing part of the lens is displaced by a small distance from the geometric centre of the lens. The displacement is such that when the lenses are worn, the lens is offset nasally on the cornea. This concept is described in PCT/WO 98/53360 (de Carle) and in European Patent Application No. 0618474 (Menicon).

In accordance with the second aspect of the present invention, some or all of the zones may be provided by diffraction as described in US Patent No. 4,637,697 (Freeman). Lenses in accordance with the first aspect of the invention generally operate by refraction.

Various embodiments in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

• Figure 1 is a section through the viewing area of an embodiment of a contact lens in accordance with the first aspect of the invention.
• Figure 2 is a plan view of a lens not in accordance with the first aspect of the invention, but useful in explaining the second aspect of the invention,
• Figure 3 is a part section on the line X-X in Figure 2.

Referring to Figure 1, this shows a currently preferred embodiment which has an anterior surface (1) and a base curve (2). There is a central circular distance vision zone (3) and a series of closely spaced alternating distance and vision zones (4) and (5). The reading zones are formed by providing zones which are flatter than base curve and thus the cornea, while the distance vision zones are formed from zones steeper than the base curve and the cornea. The zones continuously merge one with the other, and by providing a very large number of zones, the effect of possible spurious reflections from the junction area (6) are reduced.

The design shown in Figure 2, half the reading addition effect is achieved by the portion which is flatter than the cornea (i.e. the reading zone) and the other half by the curve that is steeper than the cornea (i.e. the distance zone).

Lenses in accordance with the invention may be made from high water content soft lenses. Minus lenses often have a central thickness of the order of 0.06 mm. so to avoid the tendency for the inner surface of the lens to be pressed onto the cornea, thus eliminating most of the effect of the reading addition, as well as giving a poor optical effect, it is desirable the reduce the thickness of the gap between the junction (6) and the cornea, where the lens is the greatest distance from the cornea. Preferably, this distance should be less than about 0.007 mm, preferably between 0.003 and 0.006 or less.

It will be appreciated that by forming the reading correction on the rear surface of the lens, the same profile of the rear surface can be used with a variety of front surface curvatures, depending on the basic distance correction required. Thus, if the lenses in accordance with the invention are manufactured by casting a monomer composition, the same rear surface mould half can be used with a number of different front surface mould halves, thereby reducing the inventory required.

Lenses in accordance with the invention may be manufactured by machining or by moulding (casting). In the case of casting, the master moulds will be produced by machining the desired profile into the master mould half from which the casing moulds are produced.

In the case of the embodiment of Figure 1, the zones are conveniently formed on the posterior lens surface by continuously changing the radius of the cutting tool.

Referring to Figures 2 and 3, these show a simplified version of a lens useful in explaining a second aspect of the invention. The surface of a contact lens as shown in Figure 3 has a major viewing area P consisting of a plurality of concentric zones and an outer area Q. The area P is shown in section in Figure 4 and comprises a diameter which will cover the pupil over the range of light conditions.

Although the major viewing area P is shown divided into six zones, including a circular central zone (13), the central viewing area will be divided into a number of zones as described above in relation to Figure 1.

As shown in Figure 2, the central zone (13) is a distance vision zone and this is surrounded by further distance vision zones (12) and (12a) and by reading zones (11), (11a) and (13a). The outermost zone (12b) may also be a distance vision zone but, generally will not take much part in the correction of the patient's optical defect, except in very dim light conditions.

The distance zone (13) at the centre of the lens does not have the same power as the corresponding distance vision zones (12, 12a) towards the periphery of the major viewing area (P). Instead, the distance vision zones progressively increase in their negative power towards the periphery. The increase may be progressive, for example, the first zone may be 0.25 dioptres more negative than the central zone (13), while the third zone may be 0.5 dioptres more negative, and the final zone (12b) may be a further 0.25 dioptres more negative so that it is 0.75 dioptres more negative than the central zone. The increase in negative power towards the periphery may, however, not necessarily be even, and several distance vision zones towards the periphery may have similar negative powers.

The reading zones may also increase in their negative power towards the periphery by the same or different degrees of negative power. Each reading zone may, for example, have the same reading addition in relationship to an adjacent distance vision zone so that effectively the reading zones become more negative towards the periphery. Alternatively in a less complex design, the reading zones may have the same reading addition in relationship to the innermost distance vision zone.

Figure 3 shows diagrammatically how the zones are cut in order to form alternate distance and reading refractive zones. These may be formed directly using a high precision computer-controlled lathe or, alternatively, the powers are cut onto the surface of the mould from which the lenses are produced.

Although, in the specific embodiment described above, the central zone is designed for distance vision and the adjoining annular zone for reading, it will be appreciated that the situation may be reversed.

Preferably, the lenses manufactured in accordance with this invention will be produced from material of the hydrated hydrophilic polymer type, so that the lenses are formed by machining or moulding a xerogel and the resulting products are then swollen by hydration in isotonic saline to produce the final lenses.

Although the invention has been described in connection with its application to the manufacture of bifocal lenses, it will be appreciated that the invention is also applicable to multi-focal lenses in which the vision zones may be divided into alternate near, intermediate and distance vision zones.

The annular zones and/or the basic power of the lens may be provided by refraction. However, the invention includes the use of diffractive rings to provide some or all of the power of the near or distance vision zones. In this connection, reference is made to US Patent No. 4,637,697 (Freeman) for details of the formation of diffractive power on a contact lens having basic refractive power.