GB2247538A - Lens system for myopic correction - Google Patents

Abstract

An ophthalmic laminated lens system, which has an overall negative refractive power for the correction of myopia, is comprised of a spherical rear component (21) and an aspheric front plastic cover lens (22). The front surface of the cover lens (23) is rotation ally symmetrical about the optical axis and has progressively increasing positive powers towards the ions periphery. Additional corrective supplements such as bifocal, multifocal or radiation filtering characteristics can also be incorporated on the cover lens. Production of commercial negative lens series in these laminated forms is simplified by the fact that a single design of cover lens can be applied on to a rear component of various types of ophthalmic materials and correcting powers. Such a lens system enables the making of thinner lenses and gives better imaging properties according to the position of the ions sitting in front of the eye. <IMAGE>

Description

LENS SYSTEM FOR CORRECTION OF MYOPIA SPECIFICATION The present invention relates to an improvement of negative ophthalmic lenses for the power strength in the range of -3.00 D to -12.00 D by means of laminating a cast or molded plastic element onto a base lens element of the same or of a different optical medium. It enables the making of thinner negative lenses with better optical properties effectively and economically.

Background of the invention Patients who are myopic or shortsighted can be corrected by means of ophthalmic lenses with negative power. Because of the fact that the correcting lens will be sitting at a distance away from the centre of eye rotation, different amount of focusing errors will be introduced when one looks at objects away the optical axis of the lens. This distance is known as the centre-ofrotation distance or the stop distance. The off-axis focusing errors can be quantified by the tangential and sagittal powers, the astigmatic aberration and sphere errors. Ophthalmic lenses are designed so that these lenses should induce the minimum amount of off-axis focusing errors with an assumption of an average stop distance when the eyes look at objects away from the primary direction of sight. This average stop distance is usually taken as 27 mm.The lenses so designed will usually acquire a meniscus shape with the front and back surface curving away from the eyes. These lenses are known as the "Best Form" lenses.

The stop distance varies with the face profiles of the patients and the shape and size of spectacle frames. There are studies showing that the stop distance varies between 18 mm to 42 mm.

Consequently, the actual optical performance of a "Best Form" lens may often deteriorate from expectation. Because of the costs of manufacturing a large number of different shapes of lenses for different powers at different stop distances, it is not a usual practice in the ophthalmic industry to produce different lens shapes to meet the requirements of the possible variations of stop distances.

For economy of lens production, manufacturers usually divide lens powers into groups for each of which a particular front or back surface curvature will be assigned. These front or back surface curvatures for various groups of correcting powers are known as the "Base Curves". The use of base curves in ophthalmic lens manufacturing restricts the performance of "Best Form" lenses even when a single stop distance is considered. As a result, these so called "Best Form" lenses are compromised lenses and may not necessarily give the best possible corrected vision.

Negative lenses have a thin centre and a relatively thicker edge. The edge thickness and weight of negative lenses increase approximately proportional to the correcting power of the lenses.

The changes in lens diameter have even higher effects on edge thickness and weight, and they vary approximately respectively according to the square and power of four of the lens diameters.

Hence, most patients requiring corrections in the medium to high range of power may find the lens thickness of their spectacles aesthetically objectionable and they may also complain of the heavy weight, especially when large aperture lenses are most in demand with modern spectacle frames.

The direct method of reducing lens thickness is to use an optical material of high refractive index ( presently up to a value of 1.8 and generally 1.7 ), and of reducing lens weight is ta use a material of low specific gravity (for example, optical plastics ). Because of the limit of availability of satisfactory very high index ophthalmic glass, thin negative lenses up to power of about -6.00 D only can be made possible, often with the expense of heavier lens and/or having high dispersive chromatic effects. Plastic lenses, even with the recently available high index versions, are often too thick to be appealing in the medium and high range of power strength.

Reducing edge thickness of high power negative ophthalmic lenses by ground-out lenticulation of the periphery are available, though these lenticulars have never been received as an attractive alternative because of their unpleasant appearance.

Recently, improvements in lenticular types of high negative lenses by blending off the lenticulating steps (Canada patent 1167672) or by using aspheric curves (US patent 4279480,4561736) have become available. It is also possible to maintain the offaxis optical performance of these lenses with aspheric surfaces (US patent 4279480,4289387) which may bring about a minor reduction in the edge thickness at the same time. Aspheric negative plastic lenses can be cast-molded, but to maintain a reasonable off-axis optical performance, the final lenses usually remain thick at -the edges. The cost of fabricating aspheric glass lens is often prohibitive to make them commercially attractive.

A different approach (US patent 4577942) to reduce lens thickness is to use a principal lens of a high refractive index glass in combination with a plastic lamina which has a central portion of zero refractive power and peripheral zones of progressive negative powers. It is claimed that the peripheral negative powers of the lamina are used to compensate the off-axis positive sphere errors due to the increasing stop distance from the centre of the lens to the periphery. This approach suggests that the principal lens should be made of a material of very high refractive index ( equals 1.83 ) which however has a very high specific gravity and dispersive property.

On negative spherical lenses, the edge thickness varies with the lens shape. A lens can be made substantially thinner in the plano-concave form. However, this lens has poor off-axis optical performance, except in the extreme high power end, e.g. over -14.00 D. The edge of a negative lens can also be made thinner if the centre thickness can be reduced. The centre thickness of negative glass lenses is limited by eye safety reasons to avoid lens breakage and that of the plastic lenses is limited by the mechanical rigidity of the material against warpage of the lenses under pressure of spectacle rim in the frame.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lens system in the power strength between -3.00 D to -12.00 D and it has a thinner edge thickness and an optimal optical performance according to individual fitting of the spectacles.

According to the present invention there is provided an ophthalmic lens system having a molded or cast plastic cover lens bonded to a base lens. The molding or casting formed plastic cover lens is the front and the base lens is the rear component of the system. These two components are bound together optically and physically by a suitable optical adhesives.

The front surface of the base lens should have, for maximum reduction in edge thickness and ease of coupling with the front plastic component, zero power and a concave back surface facing towards the patient's eye. If considered to be optically and/or aesthetically beneficial, the front surface can be made slightly convex or concave in shape.

The base lens can be made of materials of any refractive indices, preferably a higher index glass material with a reasonably low specific gravity and dispersive power. The material of the base lens should also be readily available in the optical industry to ensure a lower cost of the product. In some cases we may wish to have a very light lens, optical plastics can also be used for the purpose.

The base lens can be fabricated using standard ophthalmic surfacing toolings. It can either be mass produced in factories or surfaced on individual orders in local ophthalmic laboratories. As with most modern ophthalmic lens design, the back surface can be made toric for the correction of astigmatic refractive error of vision by using standard ophthalmic toolings.

The centre thickness of the base lens should be made as thin as practical fabrication procedures allow. It is expected that the centre thickness for glass types of base lens may go down to about 0.5 mm. In the specified power range, because of the substance in the para-central and peripheral area, the lens should be rigid enough to prevent warpage when mounted in spectacle frames. A slightly bigger centre thickness may be required for some plastic base lenses in order to provide the necessary mechanical rigidity.

Different types of optical plastics can be used for the cover lenses. Perhaps the only requirements of them are that the optical plastics should be readily available in the ophthalmic industry and be shaped properly by molding or casting processes.

These are essential on economy of lens production. The plastics should also have good surface scratch resistance or a hardening coating can be readily applied onto in order to provide resistance to the ruthless handling of the spectacles on general uses. An optical plastics of higher refractive index is also desirable so that reflection of light in the mating interface can be reduced to a minimal level.

The front surface of the plastic cover lens is an aspheric surface which is symmetrical on rotation about the optical axis.

The back surface of the cover lens is the mating surface with the front surface of the rear component and should be made to have the sanse surface curvature which is, in most cases, a simple flat surface.

For different stop distances, in order to provide best possible image quality and/or minimum edge thickness for all the correcting powers in the specified range and for different lens diameters and for a range of different optical media of the base lenses, the present invention contemplates that a small battery of different designs of front plastic cover lenses will be required.

The centre of the cover lens should have a zero or small refractive power with a gradually and continuously increasing positive power as one goes towards the periphery of the cover lens. This continuous changing of refractive power in the cover lens is brought about by the aspheric design of the front surface.

The amount of peripheral asphericity can be introduced by blending the transitional boundaries of gradual and small steps of lenticulations, or it can be introduced by a continuous aspheric surface. In cases of using a continuous aspheric surface, an axial cross-section of the front surface can be described by the Equation (1) of: X = BY2+DY4+FY6+Hy8+Jy10+Ly12 where X is the horizontal displacement along the optical axis from the front vertex of the cover lens and Y is the vertical displacement from the optical axis, and D,F,H,J,L are constants.

The value B describes a conic section and the value of which is related by: B = Ro/(l+(l-p(Rox2)l/2) where Rg is the curvature of the front surface in the central portion. P is the conventional aspheric factor for conic sections. When P = 1 , Equation (1) describes a circular curve if all other constants are zero in values. If the centre of the lens cover has zero power and the back surface is flat, the value of B is assigned to be zero. Variations of value B and/or the other constants in Equation (1) will bring about different designs of the cover lens.

The thickness of the cover lens should be sufficient so that when combined with the base lens will prevent the composite system from breakage or warpage on general uses. According to one aspect of the present invention, the centre thickness of the cover lenses will be made constant while the edge thickness varies with the front surface designs, and the required lens diameters. This design will be used to provide correcting lenses with optimal optical properties across the whole functional lens surface according to the variation of stop distances. Some reductions in the edge thickness of the overall lens system will usually be achieved as well.

According to another aspect of the present invention, in order to maximize the reduction in edge thickness of the whole system, the plastic cover lens should be knife-edged for the lens diameter. For a correcting lens of bigger diameter, a cover lens of the same surface configuration will be made thicker so that it comes to knife-edged again at the rim of the bigger lens.

The present invention contemplates that a single plastic cover lens may be used to give satisfactory results for different lens materials at the same or different correcting powers.

The cover lens can be incorporated with additional vocational elements such as bifocals, multifocals and/or special radiation filtering elements.

Lens systems made under the present invention are relatively less sensitive to power changes and hence toric lenses, which correct astigmatism of the eyes, can be made relatively simpler and have better optics.

BRIEF DESCRIPTION OF INVENTION The present invention will be further described, by way of examples, with reference to the drawings, in which: DRAWINGS Figure 1 is a diagram of a negative lens having a ground-out lenticulation at the periphery.

Figure 2 is a diagram of a negative lens having a smoothly blended lenticulating back surface.

Figure 3 is a diagram of a negative lens with a back spherical surface and a front aspheric surface which also produces a smooth lenticulating effect at the far periphery.

Figure 4 is a diagram of a laminated negative lens described in the above mentioned US patent 4577942.

Figure 5 is a diagram according to one application of the present invention.

Figure 6 is a diagram according to another application of the present invention.

Figure 7 is a diagram illustrating an example of the possible variations of lenses developed under the present invention.

The lenticular lens (1) as shown in Figure 1 has its front (2) and back (3) surfaces being spherical in shape. The peripheral area of the lens is often described as the carrier which is not optically usable because the lenticulating periphery (4) in the back surface is having a surface curvature departing from the required configuration to produce the correcting power. This kind of lenticular lens can be made in thin edges and is light in weight. It is however aesthetically unpleasant and displays the so-called "bottom of the bottle" effect to outside observers.

The lens (5) shown in Figure 2 is often known as the blended lenticular. Again, only the central zone of the lens provides usable optics while the peripheral carrier is not optically usable, but provides a pleasing thin lens periphery. The paracentral blending zone (6) reduces the usable area in the central portion compared with a standard lenticular lens of the type as shown in Figure 1.

Figure 3 shows a lens (7) which has an aspheric front surface (8) of rotational symmetry and the back surface (9) is spherical.

This lens is not aesthetically pleasing because of the unconventional front concave surface and the bead-shaped appearance at the edge. Again, only the central area of the lens provides usable optics.

Figure 4 shows a laminated lens system (10). It is composed of a plastic lamina (11) and a principal glass lens (12) of a high refractive index. The whole system can be quite heavy because of the high specific gravity of the material. Furthermore, due to the higher dispersive power of the principal lens material, disturbing color fringes around visual objects may be induced.

The front cover (11) of the combination is a plastic element.

This front cover element has a zero power in the centre (13) and two zones (14,15) of progressively increasing negative power in different rates in the periphery.

Figure 5 and Figure 6 shows two examples of the lenses (16,21) developed from the present invention. They are laminated systems, the cover lenses (17,22) of which have front aspheric surfaces (18,23) and it has a zero power in the centre and an increasing positive power towards the periphery. As the power change is gradual and continuous, starting from the centre of the cover lens, demarcation of the power changes into different regions is not practical. The mating surfaces (19,24) are flat. The plastic cover lens (17) in Figure 5 has a finite edge thickness (20).

This design of the cover lens aims to provide good off-axis optical quality across the whole lens surface at a range of stop distances. The cover lens (22) shown in Figure 6 is knife-edged (25) so that no extra edge thickness will be imposed onto that of the rear component. This design will provide bigger edge thickness reduction. The optical properties of the latter lens (21) are monitored to a satisfactory level within 30 degrees half-zone angle of eye rotation.

The mating surfaces of the lens system developed under the present invention may assume a curved configuration as shown (28) in the lens (26) in Figure 7. This composition may provide a slightly better appearance in the expenses of edge thickness reduction. The cover lens (27) in the example as shown in Figure 7 also carries an extra element (29) which provides the additional power for near vision for presbyopic patients.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For traditional spectacle lens designs, the shape of the lenses are defined for by the correcting lens power. There is little freedom of selecting different lens designs for different stop distances among the so-called "stock lenses", which are mass produced in the industry. Some modifications of lens forms can be made available through local laboratories, who may choose different base curves from the general prescribed designs by using semi-finished or rough lens blanks. The making of these modified lens forms in the local laboratories depends on the availability of these lens blanks supplied by the ophthalmic glass manufacturers and the range of spectacle toolings a particular local laboratory has.

In cases in which it is needed to prescribe certain vocational additions, such as bifocals, multifocals or special radiation filtering properties on the lenses, the present alternatives are often limited to certain types of lens materials. The costs of these special lenses may sometimes be too expensive to be incorporated onto glass materials. The flexibility of shape forming through molding or casting process makes optical plastics a very gocd candidate for providing these optical supplements.

The edge thicknesses of standard plastic lenses are however often undesirable in the medium to high strength of correcting powers.

According to one aspect of the present invention, plastic covers of different aspheric designs with an optional vocational additions such as bifocal segments, multifocal designs and special radiation filtering effects can be made available at reduced cost. This is made possible through the lower cost of production of the cover lenses of various designs by casting or injection molding process of plastic materials. Incorporating suitable tinting or special radiation filtering effects into plastic materials are presently widely available in the industry.

Since the cover lens does not carry any significant power, while the base lens which corrects the principal refractive error of the patient is made very thin, the overall lens can often be made thinner than traditional lenses. An example of the finished lens with an incorporated supplement according to the present invention is illustrated in Figure 7.

The improvement in optical quality that the present invention makes possible is illustrated in the following examples of -6.00 D lenses in various optical materials. The specifications of traditional designs for these lenses in three general ophthalmic materials are listed in Table 1 while Table 2 shows the off-axis performance of these lenses at stop distances of 25 mm and 33 mm.

It is apparent that patients with stop distance deviating from the so-called average value of 27 mm are not offered the best possible corrections as they look away from the optical axis of these lenses.

The common supply of these lenses are in sizes of 65 mm or 70 inm.

The edge thickness and the weight of them are also shown in Table 2.

It should also be noted that though the above lenses are generally prescribed in this country, they are not considered to be safe lenses as all these lenses in glass materials are made too thin in the centre (in order to reduce edge thicknesses) to allow subsequent toughening process against shattering. To be an eye protector, spectacles need to be able to pass a drop-ball test under the recommendations of the British Standard (BS 2092:1987).

In order to improve the performance of these lenses, laminating lens systems with appropriate cover lenses and base lenses can be designed according one aspect of the present invention. Figure 5 shows an example.

The plastic cover lenses will be made with a zero or small refractive power in the centre and the power will be increasing in the positive direction towards the cover periphery.

Two examples of cover lens designs are given in the following and they have specifications of: Material: an optical plastic designated as HIP600 in the Tables.

Refractive index : 1.600 Abbe number(V) : 34 Specific Gravity (S.G.) : 1.38 Centre thickness: 1 mm.

Back surface configuration : flat Front surface configuration : described by the equations of Cover Lens 1: X=8.5x10-7Y4-8.0x10-10Y6+8.5x10-13Y8-6.5x10-16Y10+2.1x10-19Y12 Cover Lens 2: X=5.6x10-7Y4-4.0x10-10Y6+5.6x10-13Y8-4.7x10-16Y10+1.455x10-19Y12 where X is the horizontal displacement from the front vertex of the cover lens along optical axis and Y is the vertical displacement from the optical axis.

Edge thickness Cover Lens 1 : 0.50mm at size of 65mm and 0.36mm at size of 70mm.

Cover Lens 2 : 0.69mm at size of 65mm and 0.60mm at size of 70mm.

The base lenses are fabricated with flat front mating surfaces in these examples to give the maximum edge thickness reduction. The concave back surfaces of the base lenses carry all the correcting power.

To couple with the above described examples of cover lens, base lenses have specifications of: Material : same as those illustrated in Table 1.

Centre thickness : 0.5 mm Front surface configuration : flat Back surface configuration : spherical with refractive power of -6.00 D.

Appropriate cover lens will be laminated onto the base lens by suitable optical adhesives. The off-axis performance of these lens systems at stop distances of 25 mm and 33 mm is shown in Table 3 together with the edge thicknesses and weight in lens diameters of 65 mm and 70 mm. It is obvious that the lens systems designed according to the present invention are superior optically. They are also thinner and lighter in weight. Because of the plastic nature of the cover lenses, these lenses are not easily shattered, they are safer optical appliances.

When compared with traditional designs, these laminated lens systems have slightly higher transverse chromatic aberration, which is the dispersive effect of the lenses at different angles of observation. It is anticipated that these increases in dispersive effect will normally not introduce additional difficulties to most patients.

Examples in Table 4 show the off-axis optical performance of laminated lens systems at stop distance of 25mm when Cover Lens 1 is coupled with different base lenses for different correcting powers.

Due to the fact that a single cover lens can be used to provide desirable effects for different lens materials and for different correcting powers, the present invention contemplates that only a small number of different cover lens designs will be sufficient to provide satisfactory results for most lens materials at different stop distances and different correcting powers.

As demonstrated in Table 3 and 4, since these lens systems designed according to the present invention are insensitive to change in correcting powers and lens materials, toric lenses which have different correcting powers in different meridians, in order to correct astigmatic eyes, can be made to have superior optical quality.

The lens edge thickness and weight are reduced by utilizing the fact that spherical lenses can be made thinner if they are fabricated in the plano-concave forms. Another aspect of reducing edge thickness and weight is achieved by making the base lens having a minimum possible centre thickness. Part of the reduction in edge thickness is however neutralized by the edge thickness of the cover lens in this specific type of lens system design. Nevertheless, the reduction in edge thickness can usually reach 10% to 15%, which aspheric lenses with conic surfaces (e.g. US patent 4289387) strive to achieve, compared with traditional ophthalmic designs.

Another advantage of these lens systems is that they have zero shape factor for spectacle magnification for a large range of powers. Spectacle lenses may produce different amount of magnification according to different front lens surface configurations. It is often required to balance these magnifications by a careful selection of lens designs between the two eyes if they require different correcting powers. This difficulty is overcome by the designs developed under the present invention, because most lenses will have the same zero power front surface in the central region.

According to another aspect of the present invention, for a given lens material, the lens edge thickness can be further reduced.

Reducing the edge thickness will give a much better aesthetically pleasing appearance for spectacles having negative corrections.

Reduction of lens edge thickness will also provide additional freedom of lens material selection so that the best qualities of various optical materials can be utilized to meet patients specific requirements. Examples of these qualities are the specific gravity and the dispersion.

Prior attempts to reduce lens thickness and weight give lenses that are either aesthetically unpleasant (for example the lenticular lens in Figure 1) or much too costly to be used for glass lenses (examples are shown in Figure 2 and Figure 3).

According to another aspect of the present invention, the base lens may carry the same specifications as described earlier according the previous aspect of the present invention. In order to reduce the edge thickness further, the cover lenses will be designed so that they are knife-edged at the rim for each specific lens diameter. Figure 6 shows an example of this lens system design.

This aspect of the present invention is hereinbelow described in details by way of examples.

Two cover lenses with their design specifications are given in the following Front surface configurations can be described by the relations Cover Lens 3 X=8.5x10-7Y4-8.0x10-10Y6+8.5x10-13Y8-6.5x10-16Y10+5.696x10-19Y12 Cover Lens 4 X=4.5x10-7Y4-3.3x10-10y6+7.0x10-13y8,1.70-15y10+1.6.207x10-18y12 Both covers have a flat back surface and a centre thickness of 1.0 mm if they are to be used for lens diameter of 65 mm. For lens diameter of 70 mm, Cover Lens 3 should be made with a centre thickness of 1.85 mm and 2.43 mm for Cover Lens 4. These covers will be knife-edged at the rims.

To correct a refractive error of -6.00 D using base lens materials with physical properties as shown in Table 1, the comparative reductions of edge thickness are shown in Table 6.

For patients with a short stop distance, Cover Lens 3 can be employed to couple with the base lens to give satisfactory optical properties, otherwise Cover Lens 4 will be used instead.

The performance of these lens systems at stop distance of 25 mm and 33 mm are illustrated in Table 5 for lens diameter of 65 mm.

It is clear that the focusing errors remain very small within 30 degrees half-zone angle of eye rotation.

For bigger lens diameter, the change in centre thickness in the cover lenses will shift the tangential and sagittal powers and the sphere error towards the positive direction, but the effect on performance are not significant and hence are not separately illustrated.

Unlike Cover Lens 1 and 2 described previously, because the far periphery of these cover lenses are brought down very rapidly, the focusing properties of these composite lenses will degrade rapidly at half-zone eye rotation angles beyond 30 degrees. It is generally believed that 30 degrees half-zone angle of eye rotation will be more than sufficient for most visual activities.

There are patients who may find the weight of the glass lens disturbing. In these cases, base lens in optical plastics offers the solution. As an illustrative example, a lens system with plastic base lens designated by HIP600 in the tables can be prescribed. It remains thinner compared with conventional high index glass lens designated by HIG701 as shown in Table 6.

There are still some patients who are very sensitive to dispersive effects of correcting lenses and may find the color dispersion of -6.00 D conventional lenses made of either the high index glass or plastics unsatisfactory. The common alternative in ophthalmic prescriptions at the moment is to use the socalled spectacle crown glass. Spectacle crown glass is an ophthalmic glass material of long standing history. Lenses made of spectacle crown glass have about half the transverse chromatic aberration compared with lenses made of the high index glass (e.g HIG701) or plastics (e.g. HIP600). This spectacle crown lens of conventional design is however very thick in the edge because of its lower refractive index of 1.523.

According to the present invention, a lens system with base lens material of spectacle crown glass can be used. For example, the Cover Lens 3 can be used to couple with spectacle crown glass base lens. The performance of the system at a stop distance of 25 mm is relatively weaker, but remains satisfactory. The resultant lens system, at lens diameter of 65 mm, has an edge thickness of 6.79 mm which is about 18% thinner compared with the edge thickness of spectacle crown lens of traditional designs. At longer stop distances, however, the presently given examples of cover lens designs fail to give very satisfactory optics compared with that of the traditional crown lenses. This short-coming can certainly be corrected by an additional cover lens design.

Having the similar properties as with the type of cover lenses described previously with examples of Cover Lens 1 and 2, this type of cover lenses designed to give minimum edge thickness can also be used for different correcting powers and materials. This valuable property keeps the number of required cover lens designs to a minimum level and reduces the costs of providing these lens systems.

Table 1

Lens Material N V S.G. F1 T HIP600 1.600 34 1.38 +3.50 1.20 HIG701 1.701 31 2.99 +4.02 1.50 HIGB00 1.800 25.4 3.47 +4.59 1.50 HIP600 = A common ophthalmic plastics.

HIG701 = A common high inde@ ophthalmit giass.

HIG800 = A common extra high index ophthalmit @lass.

N = Refractive index; V = Abbe number; S.G. = Specific Gravity; F1 = Front lens surface power in diopters.

T = Lens center thickness in millimeters.

Table 2

Lens Stop Eye Material Distance Rotation F@t F@s Ast. Spt. TCA E(65) W(65) E(70) W(70) 15 -6.06 -5.98 -0.09 -0.02 -0.12 25 30 -6.16 -5.87 -0.28 -0.01 -0.28 (mm) 45 -5.86 -5.57 -0.30 +0.28 -0.52 HIP@00 7.11 18.51 8.15 24.10 15 -5.96 -5.95 -0.02 +0.05 -0.16 33 30 -5.75 -5.76 +0.01 +0.25 -0.35 (mm) 45 -4.99 -5.33 +0.34 +0.84 -0.64 15 -6.09 -5.98 -0.11 -0.03 -0.1@ 25 30 -@.26 -5.88 0.38 -0.07 -0.30 (mm) 45 -@.11 -5.57 -0.54 +0.16 -0.58 HIG701 @.4@ 39.75 7.75 45.70 15 -5.99 -5.95 -0.04 +0.03 -0.17 33 30 -5.@5 -5.7@ -0.09 +0.20 -0.36 (mm) 45 -5.19 -5.31 +0.13 +0.75 -0.72 15 -6.10 -5.95 -0.13 -0.04 -0.16 25 3@ @.33 -5.87 -0.46 -0.10 -0.37 (mm) @@ -@.@7 -5.55 -0.72 +0.09 -0.72 HIG@@@ 5.81 41.@8 6.5@ 52.@@ 15 -6.0@ -5.95 -0.0@ +0.02 -0.21 @@ 30 -5.92 -5.75 -0.16 +0.16 -0.47 (mm) 45 -5.33 -5.29 -0.04 +0.69 -0.8@ Eve rotation from optical axis in degrees.

F t = Tangential power in dio@tsrs.

F's = S@@@ital powers in diopters.

Ast.= Astigmati@ Aber@ation in dicpters.

Soh.= Sphere error in diopters from the far point sphere.

T@A = Transverse Chrometic Aberration in prism diopters.

E(65) = Lens edgs thickness in millimeters for lens diemeter of 65 mm.

W(65) = Lens weight in grams for lens diameter of 65 mm.

E(70) = Lens edge thickness in millimeters for lens diemeter of 70 mm.

W(70) = Lens weight in grams for lens d@ameter of 70 mm.

Talbe 3

Base Lens Stop Cover Eve E @* E W* Material D@stance Lens Rotation F@t F@s Ast. Sph. TCA (65) (65) (70) (70) 15 -@.00 -5.95 -0.04 +0.03 -0.12 25 1 70 -5.9@ -5.60 -0.16 +0.12 -0.29 6.4@ 19.16 7.19 24.58 (mm) 43 -5.82 -5.48 -0.30 +0.35 -0.@@ HIG600 15 -5.97 -5.98 -0.02 +0.04 -0.16 3@ 2 30 -5.79 -5.78 -0.01 +0.22 -0.38 @.@@ 19.18 7.47 @4.5@ (mm) 45 -5.57 -5.45 -0.12 +0.49 -0.2@ 15 -6.02 -5.95 -0.07 +0.01 -0.13 25 1 30 -6.08 -5.81 -0.29 +0.05 -0.32 5.61 32.26 6.23 41.69 (mm) 45 -6.18 -5.4@ -0.6@ +0.15 -@.@@ HIG701 15 -6.00 -5.95 -0.05 +0.0@ 0.1@ 33 2 30 -5.92 -5.79 -0.14 +0.14 -0.42 5.8@ @@.26 6.@7 41.@@ (mm) 45 -5.9@ -5.47 -0.49 +0.29 -0.94 15 -6.04 -5.95 -0.09 +0.00 -0.1@ 25 1 30 -6.17 -5.81 -0.@7 +0.01 -0.40 5.02 3@.3@ 5.54 43.0@ (mm) 45 -6.45 -5.49 -@.@5 +0.0@ -@.@@ HIG800 15 -@.0@ -5.95 -0.07 +0.01 -0.@@ 3@ 2 @0 -@.04 -5.79 -0.25 +0.@@ -@.@@ 5.@@ @@.@@ @.@@ @@.@@ (mm) 45 -@.25 -5.47 -0.78 +0.14 -1.22 W* = Estimeted lens weight @@ gre@s @@ edding the weight of the b@@@ le@@ and a c@rculatr flet plate of the co@er lens material ha@ing thic@@e@@ eou@l to the ce@ter th@chness of t@e co@@r lens.This will @@er- estimate the weight of the le@@neted le@s.

Table 4

Base Lens Lens Eye Mater@al Power Rotation F't F's Ast. Sph. TCA 15 -7.99 -7.94 -0.05 +0.03 -0.16 -8.00 30 -7.92 -7.75 -0.16 +0.16 -0.38 (D) 45 7.51 7.34 0.17 +0.58 -0.83 HIP600 15 -9.96 -9.93 -0.04 +0.06 -0.20 -10.00 30 -9.76 -9.68 -0.08 +0.28 -0.48 (D) 45 -8.95 -9.14 +0.19 +0.96 -1.10 15 -8.04 -7.95 -0.09 +0.01 -0.18 -9.00 30 -8.11 -7.77 -0.34 +0.06 -0.43 (D) 45 -8.00 -7.36 -0.64 +0.32 -0.96 HIG701 15 -10.20 -9.93 -0.09 +0.02 -0.22 -10.00 30 -10.01 -5.70 -0.31 +0.15 -0.54 (D) 45 -9.55 -9.17 0.38 +0.64 -1.29 15 -8.07 -7.95 -0.12 +0.01 -1.22 -8.00 30 -8.@5 -7.37 -0.42 -0.01 -0.53 (D) 45 -8.39 -7.37 -1.02 +0.12 -1.24 HIG800 15 -10.07 -9.95 -0.13 0.00 -0.2@ -10.00 30 -10.20 -9.71 -0.49 -0.04 -0.@7 (D) 45 -10.03 -9.1@ -0.@6 +0.40 -1.58 Table 5

B@@@ Lens @t@@ C@ver E,e E W* E Material @@stance Len@ R@tati@n F@t F s Ast. Sph. TCA (65) (65) (70) 15 -6.00 -5.95 -0.04 +0.@@ -@.1@ 25 @ (mm) 30 -5.94 -5.80 -0.14 +0.13 -@.29 HI@600 5.93 19.18 6.83 15 -5.96 -5.94 -0.01 +0.05 -0.1@ 33 4 (mm) 30 -5.82 -5.77 -0.05 +0.20 0.3@ 15 -6.02 -5.95 -0.07 +0.01 -0.13 25 3 (mm) 30 -6.06 -5.81 -0.26 +0.06 -0.32 HIG701 5.11 32.26 5.87 15 -5.99 -5.95 -0.04 +0.03 -0.18 33 4 (mm) 30 -5.96 -5.78 -0.18 +0.13 -0.42 15 -6.04 -5.95 -0.09 +0.00 -0.16 25 3 (mm) 30 -6.15 -5.81 -0.35 +0.02 -0.40 HIG800 4.52 33.37 5.18 15 -6.01 -5.95 -0.06 +0.02 -0.22 33 4 (mm) 30 -6.07 -5.79 -0.28 +0.07 -0.52 W* = Est@mat@d le@s we@@ht in @@ams by adding the we@@@t of the b@@e lens @n@ @ @@@@ulatr flat pl@ts @f the cover lens ma@er@al hevi@g th@c@@@@@ e@@@@ t@ t@@ @enter tr@@iness of the @@@er lens. This will @@@@- estimate the weight of the laminated lens.

Table @

Es@@ HIF@@@ HIG701 HIG800 @n@c@ness Red@ction (%) @5 7@ @5 70 65 70 65 15.@ @@.1 @@.4 HIP600 7@ 1@.@ 28.3 @@.8 65 8.2 @@.5 70.@ HIG701 7@ @.@ @@.1 29.5 @@ @@.@ 12.0 22.2 HI@@@@ @ @@.@ 10.5 21.0 Comparision of lens edge th@ckness of the laminineted lens @ystems in @@ffenent base lens materials with convention@l lens de@@@ns at diameters of 65 mm and 70 mm.

Claims (7)

1. An ophthalmic lens system for persons suffering from myopia, said lens system comprising: (a) an aspheric plastic anterior cover lens bonded physically and optically to a rear component of a spherical or toroidal base lens; (b) said base lens carried all or most of the ametropic correction in the posterior ocular-side surface; (c) said base lens can be fabricated in any ophthalmic glass or plastics; (d) said cover lens having a posterior surface which can be physically and optically bonded to the anterior surface of said base lens leaving no empty space between the two said components; (e) said cover lens having an anterior object-side surface which is rotationally symmetrical;; (f) said anterior surface of the said cover lens being fabricated by blending small steps of lenticulations and/or having a continuous configuration, in the latter case, a section of which can be described by the formula: X = BY2 + DY4 + Fy6 + Hy8 + ...

wherein the optical axis of the lens system being taken as the abscissa and an axis of orthogonal to the optical axis being taken as the ordinate and wherein the coefficients are numbers including zero; (g) the central region of a said cover lens carrying zero or a small refractive power, making up the difference between the said base lens and the ametropia of a person wearing the said lens system; (h) said cover lens having a progressively increasing positive refractive power towards the periphery; (i) said progressively positive refractive power totally or partially offsetting the oblique focusing errors of the said base lens;

2. In the lens system claimed in claim 1, wherein a said cover lens has a constant centre thickness and a finite edge thickness, the value of which depends on the overall diameter of a said lens system;

3. In the lens system claimed in claim 1, wherein a said cover lens has zero or a very small edge thickness and the centre thickness of a said cover lens depends on the overall diameter of a said lens system;

4. In the lens system claimed in claim 1, wherein a single said cover lens is applicable on to a said base lens of different ophthalmic lens materials;

5. In the lens system claimed in claim 1, wherein a single said cover lens is applicable on to a said base lens of different refractive power to correct different amounts of ametropia;

6. In the lens system claimed in claim 1, wherein a said cover lens has a vocational element incorporated on the anterior surface;

7. In the lens system claimed in claim 1 and claim 6, wherein said vocational element includes additional correction for intermediate and/or near working distances, and/or radiation filtering characteristics.