GB1571930A - Opthalmic lenses - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO OPHTHALMIC LENSES (71) We, AMERICAN OlTICAL CORPORATION, a corporation organised under the laws of the State of Delaware, 14 Mechanic Street, Southbridge, State of Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to improvements in an ophthalmic lens and more particularly to a method of manufacturing a meniscus lens in which correction of off-axis errors is at least partly effected by a radial gradation of refractive index between the optical centre and edges of the lens together with proper control of surface curvatures and centre thickness.

In designing corrected spectacle lenses, it is common practice for the designer to make extensive off-axis computation and select a base curve which achieves the best performance, i.e.

the "tool" used by the designer is the overall amount of bending employed in a given lens.

Off-axis errors of prime concern are astigmatism and curvature of field (power error) with a third aberration, distortion, being of lesser but important consideration.

Relatively recent approaches to improving off-axis correction in ophthalmic lenses have included the use of aspherics wherein non-spherical surface curvatures are used in conjunction with proper base curve selection to further reduce off-axis astigmatism and curvature of field with improvement in distortion as well.

While aspheric surface curvatures in off-axis correction situations are most easily and economi cally applied to lenses which can be cast, e.g.

resin lenses, this approach to oblique correction can also be used in glass.

The art, being presently limited to one or combinations of the aforesaid techniques for reducing off-axis aberrations in ophthalmic lens design, is needful of improved designs and methods of their application which can be implemented with greater ease and economy, especially on glass, and which can offer greater variety and versatility to designers in their selections of aberrations to be corrected and the order of priority or emphasis applied thereto in particular oblique correction situations.

The preferred embodiments of the present invention provide lenses having off-axis correction without aspheric surface treatment, but which are comparable to and/or improved over prior art accomplishments with aspherics.

According to one aspect of the present invention a method of manufacturing a meniscus lens comprises the steps of forming a lens blank of material having a known refractive index value adjacent its axis and a gradation of refractive index radially of the axis, and providing the lens with optically finished convex and concave opposite faces of preselected curvatures, the lens having a preselected centre thickness and the geometrical surface configuration of one of the faces being selected according to desired off-axis correction of at least one opthalmic lens aberration, such as power error, astigmatism or distortion, and the gradation of refractive index being selected according to desired off-axis correction of power error or astigmatism.

The gradation of refractive index is preferably combined with aspheric correction in lens design wherewith a higher degree of oblique correction can be achieved and further wherewith more than the usual prime aberrations of astigmatism and curvature of field may be successfully dealt with. It is thus possible for a selection of aberrations to be attended to with emphasis thereon in an order of the lens designer's choice and with greater versatility than the heretofore limited "tool" of controlled surface bending, although some surface bending is still required.

Gradation of refractive index provides for greater ease and versatility in the designing of corrected opthalmic lenses, improvements of substantial significance and importance in resulting lenses, greater economy in the implementation of corrections of prime concern aberrations and others of importance and greater freedom of manner of applying variables of base curve selection and asphericity in the correction of oblique aberrations.

According another aspect of the present invention, there is provided a lens having optically finished convex and concave opposite faces of preselected curvatures, a particular centr thickness and refractive index value adjacent its axis and a gradation of refractive index in the material of the lens radially from its axis, the geometrical configuration of one of the lens faces being selected, in conjunction with the centre thickness, the refractive index value and the curvature of the opposite face according to the off-axis correction desired for at least one opthalmic lens aberration, such as power error, astigmatism or distortion, and the gradation of refractive index being such as to provide a desired correction of the power error or astigmatism.

Utilising regular spherical and/or toric lens surfaces of preselected dioptric values, off-axis corrections of lens aberrations comparable to and improved over those which may be accomplished with aspheric surface design are possible. Thus, the relatively complex and expensive processes of applying aspheric corrections to glass lenses can be avoided without sacrifice of oblique correction quality. In this connection, a lens base curve may be chosen so as to minimize astigmatism and a refractive index gradient used to control curvature of field.

Still higher degrees of correction are contemplated by using an index gradient along with an aspheric surface, and, of course, with careful selection of base curve. Base curve selection may be made with reduction of distortion in mind, asphericity chosen to minimize astigmatism and refractive index gradient utilized to reduce curvature of field (power error).

In order that the present invention be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of traditional geometry and assumptions basic to spectacle lens design; Figure 2 is a chart indicating the optical performance of a conventional +3.00 diopter spherical lens prescription in terms of its tangential and sagittal power errors; Figure 3 is a chart similar to Figure 2, but illustrating the optical performance of a +3.00 diopter lens having off-axis corrections applied according to the invention; Figure 4 is a cross-sectional view of an ophthalmic lens with stippling included for purposes of illustrating a refractive index gradient; Figures 5 and 6 illustrate a techique useful in preparing lens blanks having radially directed gradations of refractive index;; and Figures 7 and 8 are illustrations of other techniques for accomplishing gradations of re fractive index in lens blanks.

For ease in understanding the present inven tion Figure 1 illustrates the traditional geometric assumptions which are basic to spectacle lens design. In Figure 1, point P is a point on the e reference sphere C at which it would be desirabl to present the same optical corrections as are present at the vertex V of lens L. Problems as sociated with this endeavor are classical and re ported in the literature, e.g. Bechtold, Edwin W.

"The Aberrations of Ophthalmic Lenses", Am.

Jl. of Op. and Arch. Am. Acad. Optom., 35 (1) 10-24, 1958; Davis, John K., Henry G. Femald, and Arline W. Raynor, "An Analysis of Oph thalmic Lens Design",Am. Jl. of Op. and Arch.

Am. Acad. Optom. 41(7)400-421, 1964; Davis, John K., Henry G. Fernald, and Arline W. Raynor, "The Design of a General Purpose Single Vision Lens Series" Am. Al. of Op. and Arch. Am. Acad. Optom. April 1965; and Davis, John K. "Stock Lenses and Custom Design" Am. Jl. of Op., December 1967.

Figure 2 displays the result of the traditional calculations and indicates in terms of the tangential (t) and sagittal (s) meridional power errors the performance possible for a +3.00 spherical prescription. Data is given for a com monly encountered 28.5 mm center-of-rotation (CR) distance and index of refraction of 1.56.

Curvatures of front surface, rear surface and center thickness are 6.72 diopters, -4 diopters and 3.76 mm respectively.

It is immediately obvious that for a concave base curve of approximately -4.00 diopters, the average field curvature (power error) is approxi mately "0", i.e. the sagittal error is about -0.09 diopters and the tangential error is about +0.09 diopters. In order to reduce this astigmatism to "0", however, a concave base curve slightly steeper than -6.00 diopters would be required and the field curvature (power error) would become increased to about -0.17 diopter. Thus, it can be seen that power error and astigmatism cannot both be reduced to zero by conventional lens design techniques. Accordingly, the differences in lenses produced by various manu factorers stem from differences in the type of compromise favored by their designers.

Table I which follows sets forth sagittal and tangential powers, sagittal and tangential errors and astigmatism occurring in the exemplary +3.00 diopter lens at various angles A (Fig. 1).

The optimum in off-axis ophthalmic lens correction would, of course, be to accomplish simultaneous correction of oblique astigmatism and field curvature (power error). This can be accomplished according to the present invention through the use of a gradient index of refraction in the lens blank used to prepare the lens as shown in Table II which follows.

Table II represents a lens having a front surfac curvature of +8.64 diopters, a rear surface cur vature of -6.00 diopters, thickness of 3.84 mm and index of refraction of 1.56 at its axis af TABLE I Index is 1.56 Index Increment is 0.0000 Center of Rotation Distance is 28.5 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig A" Power Power Error Error matism 5. 3.00 3.00 -0.00 0ss0 0.01 10. 2.99 3.01 -0.01 0.02 0.02 15. 2.98 3.03 -0.02 0.04 0.05 20. 2.97 3.06 -0.03 0.06 0.09 25. 2.95 3.08 -0.05 0.09 0.14 30. 2.92 3.11 -0.09 0.09 0.19 35. 2.88 3.12 -0.12 0.12 0.24 TABLE II Index is 1.56 Index Increment is 0.0050 Center of Rotation Distance is 28.5 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism Index 5. 3.02 3.02 0.02 0.03 0.00 1.5650 10. 3.04 3.04 0.04 0.05 0.01 1.5700 15. 3.05 3.06 0.05 0.06 0.01 1.5750 20. 3.04 3.06 0.04 0.06 0.02 1.5800 25. 3.03 3.05 0.03 0.05 0.02 1.5850 30. 3.00 3.02 -0.00 0.02 0.02 1.5900 35. 2.96 2.97 -0.04 -0.03 0.01 1.5950 fording an axial power of 3.00 diopters. The lens is provided with a refractive index gradient increasing from center to edge by increments of 0.0050 per each 50 increase in angle A (Fig.

1). The center of rotation (CR) distance is 28.5 mm which is most common in ophthalmic lens prescriptions. At a point of 30 obliquity astigmatism is reduced to nearly "0" (i.e. 0.02) while field curvature (power error) has been reduced to essentially "0" (i.e. sagittal error is -0.00 and tangential error is 0.02). At this 300 position, the refractive index has been increased to 1.5900.

Referring more particularly to Fig. 3, the sagittal (s) and tangential (t) errors have been plotted for the lens used in the example of Table II. This illustrates that by the selection of a concave base curve of -6.25 diopters or slightly less, essentially complete correction of oblique astigmatism and field curvature (power error) can be accomplished for a viewing angle A of 300 For obliquities less or greater than 300, it can be seen from Table II that only slightly less but substantially complete correction of oblique astigmatism and field curvature has occurred.

It should be understood that all examples given hereinabove have employed a stepped gradient of refractive index, i.e. 0.0050 per each 5 changes in angle A. By employing a continuous and/or non-linear gradient of refractive index between the center and edges of a lens, a still further improvement in oblique astigmatism and field curvature correction can be accomplished.

Using the foregoing example of a lens having axial power of 3.00 diopters and a center of rotation (CR) distance of 28.5 mm, Table III which follows illustrates a variation in index of refraction between the lens center and peripheral portions which may be incorporated to accomplish the effect of off-axis correction over the entire lateral field of view, e.g. from 0 to 35 in ophthalmic lens design.

Referring in still more detail to Tables II and III, it can be seen that the gradation of refractive index has very little effect on off-axis astigmatism. Astigmatism remains at zero or very close thereto throughout all lateral viewing angles A, i.e. from 0 to 350. The refractive index gradation, however, has the effect of dramatically reducing field curvature (power error).It can be seen, for example, that at a 35 obliquity (Table II) the error in the sagittal meridian has been reduced to -0.04 diopters, and in the tangential meridian to -0.03 diopters Thus, one may strategically select a base curve to correct for off-axis astigmatism according to TABLE III Index is 1.56 Center of Rotation Distance is 28.5 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism Index 5. 3.00 3.00 -0.00 -0.00 0.00 1.5600 10. 3.00 3.01 0.00 0.01 0.01 1.5630 15. 3.00 3.01 -0.00 0.01 0.01 1.5660 20. 2.99 3.00 -0.01 0.00 0.02 1.5700 25. 2.99 3.01 -0.01 0.01 0.02 1.5785 30. 2.99 3.01 -0.01 0D1 0.02 1.5885 35. 3.00 3.01 -0.00 0.01 0.01 1.6025 conventional practice, and then be able to employ a gradient of refractive index to eliminate or reduce field curvature (power error) to an insignificant value.

In the foregoing examples of the use of a re fractive index gradation the index of refraction has been lowest at the center or axis of the lens and increased in directions outwardly toward the edge of the lens. This, however, is not necessarily the direction of refractive index gradient which should be used for all lenses. The direction of index gradation, i.e. whether decreasing or increasing in directions away from the center of a lens will depend upon the power range in which one is working and just what is being attempted to achieve in terms of off-axis correction. For example, in working with lenses of high plus power such as cataract lenses, a refractive index of highest value at the lens center and dropping off from center to edge may produce the most desirable results.

The following tables IV, V and VI demonstrate that a useful direction of index gradation for a high plus power lens, e.g. a cataract lens, is one which decreases from center of the lens towards its periphery.

Table 1V illustrates what can happen with respect to off-axis aberrations in a lens of constant refractive index value from center to edge when, for example, axial power is 14.00 diopters with a concave curve of -3.50 diopters and center thickness of 11 mm. Sagittal power remains nearly constant from center to edge of the lens, i.e. from 0 to 350 of angle A while the tangential Dower increases in value to a noint where at 350 it is nearly 3.00 diopters stronger than at the centre. The resulting astigmatism at 350 rotation in the eye is 2.72 diopters.

Table V, with the same prescription of 14.00 diopters axial power illustrates the improvement that can be accomplished with an incremented index of refraction. The index incrementation in this example is provided by dropping .005 for each 5 of eye rotation away from the center of the lens Bv such means. it can be seen that while the less important sagittal power errors have increased somewhat, the more important tangential errors have dramatically decreased.

At 350 the tangential error has been reduced to approximately 2.00 diopters with astigmatism accordingly being reduced to about 2.00 diopters By employing a non-linear refractive index gradient as shown in Table VI, a substantially constant tangential power (nearly 0 tangential error) can be accomplished.It should be understood that although tangential error is generally considered more serious than sagittal error, it is not necessary to reduce it to zero at TABLE IV Indexis 1.56 Index Increment is 0.0000 Center of Rotation Distance is 25.0 mm Axial Power is 14.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism 5. 14.00 14.05 -0.00 0.04 0.04 10. 14.00 14.18 -0.00 0.17 0.18 15. 14.00 14.40 -0.00 0.40 0.40 20. 14.00 14.74 -0.01 0.74 0.74 25. 14.00 15.22 -0.00 1.21 1.21 30. 14.02 15.87 0.01 1.86 1.85 35. 14.04 16.77 0.04 2.76 2.72 TABLE V Index is 1.56 Index Increment is -0.0050 Center of Rotation Distance is 25.0 mm Axial Power is 14.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism Index 5. 13.87 13.91 -0.14 -0.10 0.04 1.5550 10. 13.73 13.90 -0.28 -0.11 0.17 1.5500 15. 13.58 13.97 -0.42 -0.04 0.38 1.5450 20. 13.44 14.13 -0.57 0.12 0.69 1.5400 25. 13.29 14.40 -0.71 0.39 1.10 1.5350 30. 13.15 14.80 -0.85 0.79 1.64 1.5300 35. 13.01 15.37 -0.99 1.36 2.35 1.5250 TABLE Index is 1.56 incremented downwardly from center-to-edge Center of Rotation Distance is 25.0 mm Axial Power is 14.00 diopters Angle Sagittal Tangential s t Astig AO Power Power Error Error matism Index 5. 13.96 14.01 -0.04 0.00 0.04 1.5585 10. 13.84 14.01 -0.17 0.01 0.17 1.5541 15. 13.61 13.99 -0.39 -0.01 0.38 1.5460 20. 13.32 14.00 -0.68 -0.01 0.68 1.5358 25. 1296 14.01 -1.04 0.01 1.05 1.5232 30. 12.52 14.02 -1.48 0.01 1.50 1.507 35. 12.01 14.02 -2.00 0.01 2.01 1.4900 the expense of substantial amounts of sagittal error.Accordingly, Table VI is intended mainly to show that additional control in off-axis correction can be achieved by varying the refractive index gradient not only in predetermined increments but rather in a non-linear manner.

While the examples of the present invention in Tables II, III, V and VI have illustrated the invention as applied to conditions using the most common 28.5 mm center of rotation distance (CR) for general purpose ophthalmic lens prescriptions and 25 mm CR for high plus (i.e.

cataract lens) prescriptions, it should be understood that astigmatism and curvature of field (power error) can be essentially eliminated, i.e.

reduced to negligible amounts, for other center of rotation (CR) distances as follows: Tables VII and VIII illustrate a control of astigmatism and power error available by gradation of refractive index in a lens having a front surface curvature of +8.64 diopters, a TABLE VII Index is 1.56 Center of Rotation Distance is 25.0 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig AO Power Power Error Error matism Index 5. 3.00 3.00 --0.00 -0.00 0.00 1.5600 10. 3.00 3.01 0.00 0.01 0.01 1.5630 15. 3.00 3.02 0.00 0.03 0.02 1.5660 20. 3.00 3.04 -0.00 0.04 0.04 1.5700 25. 3.01 3.06 0.01 0.06 0.06 1.5785 30. 3.01 3.08 0.01 0.08 0.07 1.5885 35. 3.02 3.10 0.02 0.10 0.08 1.6025 TABLE VIII Index is 1.56 Center of Rotation Distance is 32.0 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism Index 5. 2.99 3.00 -0.00 -0.00 0.00 1.5600 10. 3.00 3.00 -0.00 -0.00 0.00 1.5630 15. 2.99 2.99 -0.01 -0.01 0.00 1.5660 20. 2.98 2.98 -0.02 -0.02 0.00 1.5700 25. 2.98 2.97 -0.02 -0.03 0.00 1.5785 30. 2.97 2.96 -0.03 -0.04 0.01 1.5885 35. 2.97 2.94 -0.03 -0.06 0.03 1.6025 rear surface curvature of -6.00 diopters, center thickness of 3.84 mm and index of refraction of 1.56 at its axis affording axial power of 3.00 diopters. The refractive index is increased radially from center to edge of the lens.

Referring now to Tables IX and X which follow, the same lens design data but without refractive index gradation is presented to illustrate the respective corrections in curvature of field (power error) which were accomplished with the refractive index gradation of the Tables VII and VIII situations.

Comparing Tables VII and IX it can be seen that curvature of field (sagittal and tangential errors) was considerably reduced by refractive index gradation (Table VII). For the 25 mm center of rotation distance situation, i.e. with constant refractive index (Table Ix) s error equals -0.20 and terror equals -0.14 at 35 while with refractive index gradation (Table VII) s error equals 0.02 and terror equals 0.10.

Similarly, a comparison of Tables VIII and TABLE Index is 1.56 Center of Rotation Distance is 25.0 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig A Power Power Error Error matism 5. 3.00 3.00 -0.00 -0.00 0.00 10. 2.99 3.00 -0.01 -0.00 0.01 15. 2.97 2.99 -0.03 -0.01 0.02 20. 2.94 2.98 -0.06 -0.02 0.04 25. 2.91 2.96 -0.09 -0.04 0.05 30. 2.86 2.92 -0.14 -0.08 0.06 35. 2.80 2.85 -0.20 -0.14 0.05 TABLE X Index is 1.56 Center of Rotation Distance is 32.0 mm Axial Power is 3.00 diopters Angle Sagittal Tangential s t Astig Power Power Power Error Error matism 5. 2.99 3.00 -0.00 -0.00 0.00 10. 2.98 2.98 -0.02 -0.02 0.00 15. 2.96 2.96 -0.04 -0.04 0.00 20. 2.92 2.92 -0.08 -0.08 0.00 25. 2.88 2.87 -0.12 -0.13 0.01 30. 2.82 2.80 -0.18 -0.20 0.02 35. 2.75 2.70 -0.25 -0.30 0.05 X shows a highly significant correction of curvature of field for the 32 mm center of rotation situation. There, for the 45" angle of viewing, s error has been reduced from -0.25 to -0.03 and t error from -0.03 to -0.06.

It should also be understood that while all examples given hereinabove have been based upon the use of an index of refraction of 1.56 at the center of a lens, higher or lower refractive indices may be used in order to avoid undue lowering or raising of refractive index at edges of the lenses.

The foregoing illustrates the advantages of designing ophthalmic lenses with refractive index gradients uniquely employed as "tools" in the correction of off-axis aberrations. Data embodied in the various examples of Tables I-VIII has been arrived at by conventional lens designer's ray tracing calculations and the assistance of programmable electronic computer technology, the latter being dispensable but entremely useful to the designer.While the data of Tables I-VIII has been selected to illustrate principles of the present invention, it is believed to have been made apparent that similar information can be arrived at or adjusted according to the requirements of any one of the virtually unlimited number of ophthalmic lens prescriptions encountered in the art whether these prescriptions are for spherical lenses of the single vision or multifocal types or whether, in either case, they contain a cylinder correction.

Those interested in details of ray tracing as used in ophthalmic lens design work may refer to U.S. Patents Nos. 3,434,781 and 3,169,247 and/or one or more of the above-identified pieces of literature on the subject.

As mentioned earlier in this specification, correcting off-axis aberrations in ophthalmic lenses with a controlled gradation of refractive index can be utilized to obviate a need for aspheric surface design or may be incorporated in and/or with aspheric design for greater "fine tuning", i.e. correction, of oblique aberrations.

As in all cases of lens design including the present concept of using refractive index gradation, base curve is carefully chosen and in using a refractive index gradient to replace or obviate a need for off-axis correction by aspheric curvature, base curve can be so chosen as to minimize astigmatism and the refractive index gradient so applied as to minimize curvature of field (power) error.

In the case of "fine tuning" with aspheric surface design, particular attention may be paid to correction of a third off-axis aberration, distortion whether of the "barrel" or "pin cushion" type. In this case, base curve selection may be made more specifically with reduction of distortion in mind, surface asphericity chosen to minimize astigmatism and a refractive index gradient chosen and utilized to reduce curvature of field (power error), the latter having been demonstrated hereinabove. Those interested in details of manipulating base curves and applying surface asphericity for reduction of off-axis aberrations may refer to U.S. Patent No. 3,960, 442 and/or U.S. Patent No. 3,169,247.

While the embodiments of the present invention described above relate to the use of a refractive index gradient in ophthalmic lenses as a "tool" for correcting off-axis aberrations and is applicable to lens blanks having a gradation of refractive index from center to edge regardless of how the lens blanks may be fabricated, treated or otherwise provided with the refractive index gradation, examples of techniques useful in producing such lens blanks have been illustrated in Figs. 5-8. These exemplary lens blank manufacturing techniques, are illustrative of only some of the schemes available to the artisan for providing lens blanks which are useful in producing lenses as disclosed in the above described embodiments of invention.

Referring to Figs 5 and 6, a billet 10 of lens material, e.g. optical glass, having a diametral dimension equal to or greater than that of the maximum transverse dimension required of an ophthalmic lens to be produced according to the invention, is provided. Billet 10 is immersed in a salt 12 containing alkali metals to cause ionexchange between ions in the glass and those in the salt in amounts gradually penetrating radially into billet 10.

Details of techniques having utility in the manufacture of ion-exchanged billets may be had by reference to U.S. Patents Nos. 3,650,598 and 3,827,785.

With a controlled passage of time in salt 12, ion exchange can be caused to proceed a predetermined distance toward and/or to the axis of billet 10 with refractive index varying according to the gradation of ion exchange having taken place. The resulting substitution of monovalent alkali metal ions of one size in salt 12 for ions of another size in billet 10 progressively radially inwardly thereof produce a variable denseness of the material of billet 10 which results in corresponding refractive index changes. While not shown in Fig. 5, opposite ends of billet 10 are preferably covered with a protective coating or the like which prevents ion exchange in directions axially of billet 10.

Having so treated billet 10 in salt 12 the billet 10 is removed from salt 12 and cleaned to terminate the ion exchange process. The billet 10 is then transversely cut into sections 14 (Fig.

6) of thickness and diameter necessary for the formation of lenses such as lens L in Fig. 4.

The meniscus configuration of lens L is accomplished with conventional grinding and polishing operations.

As illustrated with stippling and arrow 16 in Fig. 4, lens L is provided with a gradation of refractive index from its axis outwardly in the direction of arrow 16. The density or refractive index may be caused to decrease in the direction of arrow 16 or vice versa.

In Fig. 7 an alternative technique for fabricating lens blanks having a gradated refractive index is illustrated. This comprises the fabrication of a billet 10' composed of a central rod 18 having a preselected refractive index and successively surrounding closely interfitted sleeves 20 each of a preselected different refractive index. The assembly may comprise more or less than the five concentrically related components which have shown, i.e. the incremental gradation of refractive index from rod 18 radially outwardly may be of any desired step function either increasing or decreasing in value.

Components 18 and 20 of billet 10' would normally be heated and/or otherwise fused together as a unit and cut transversely to the thickness desired of a lens blank 22, for example.

Fusion of components 18 and 20 together and transverse cutting of billet 10' to form blank 22 can be followed by irradiation and/or other treatment of blank 22 to produce a blending or gradation of index of refraction of one of components 18 and 20 with an adjacent component.

It is also contemplated that components 18 and 20 may be surface treated before assembly by immersion in a diffusant such as heated silver chloride to provide a graduated transition of refractive index therebetween in the final assembly.

Still another technique for producing a gradient refractive index billet 10" from which lens blanks may be cut is illustrated in Fig. 8.

This includes the provision of a multiple chamber glass furnace 24 having a plurality of concentric orifices through each of which a lens material of a preselected refractive index may be directed and caused to form the composite billet 10".

Following the formation of billet 10" and cooling thereof to a solid state, transverse cutting along lines 28 will produce lens blanks 30.

It should be understood that a multiple orifice plastics material (i.e. ophthalmic resin) dispenser may be substituted for furnace 24.

The artisan will readily appreciate that there are various other modifications and adaptations of the precise forms of the invention herein shown which may be made to suit particular requirements.

WHAT WE CLAIM IS: 1. A method of manufacturing a meniscus lens comprising the steps of forming a lens blank of material having a known refractive index value adjacent its axis and a gradation of refractive index radially of the axis, and providing the lens with optically finished con vex and concave opposite faces of preselected curvatures, the lens having a preselected centre thickness, and the geometrical surface con figuration of one of the faces being selected ac cording to desired off-axis correction of at least one opthalmic lens aberration, such as power error, astigmatism or distortion, and the grada tion of refractive index being selected accord ing to desired off-axis correction of power error or astigmatism.

2. A method according to claim 1, wherein the radial gradation of refractive index increases with distance away from the lens axis.

3. A method according to claim 1, wherein the radial gradation of refractive index decreases with distance away from the lens axis.

4. A method according to any of claims 1, 2 or 3, wherein the radial gradation of refractive index is effected in step fashion between the axis and the edge of the lens.

5. A method according to any of claims 1, 2 or 3, wherein the radial gradation of refractive index is in the form of a continuous change of values between the axis and the edge of the lens.

6. A method according to any one of the preceding claims, wherein an aspheric correction is applied to at least one of said opposite faces of the lens.

7. A method of manufacturing a meniscus lens according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

8. A lens having optically finished convex and concave opposite faces of preselected curvatures, a particular centre thickness and refractive index value adjacent its axis and a gradation of refractive index in the material of the lens radially from its axis, the geometrical configuration of one of the lens faces being selected, in conjunction with the centre thickness and refractive index value, and the curvature of the opposite face according to the off-axis correction desired for at least one opthalmic lens aberration, such as power error, astigmatism or distortion and the gradation of refractive index being such as to provide a desired correction of the power error or astigmatism.

9. A corrected ophthalmic lens according to claim 8, wherein the gradated refractive index increased with distance away from the axis of the lens.

10. A lens according to claim 8, wherein said gradated refractive index decreases with distance away from the axis of the lens.

11. A lens according to claim 8, 9 or 10, wherein said radial gradation of refractive index is effected in step fashion between said axis and edge of the lens.

12. A lens according to claim 8, 9 or 10, wherein the gradation of refractive index is in the form of a continuous change of values between the axis and the edge of the lens.

13. A lens substantially as hereinbefore described with reference to Figures 3 to 8 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. index is illustrated. This comprises the fabrication of a billet 10' composed of a central rod 18 having a preselected refractive index and successively surrounding closely interfitted sleeves 20 each of a preselected different refractive index. The assembly may comprise more or less than the five concentrically related components which have shown, i.e. the incremental gradation of refractive index from rod 18 radially outwardly may be of any desired step function either increasing or decreasing in value. Components 18 and 20 of billet 10' would normally be heated and/or otherwise fused together as a unit and cut transversely to the thickness desired of a lens blank 22, for example. Fusion of components 18 and 20 together and transverse cutting of billet 10' to form blank 22 can be followed by irradiation and/or other treatment of blank 22 to produce a blending or gradation of index of refraction of one of components 18 and 20 with an adjacent component. It is also contemplated that components 18 and 20 may be surface treated before assembly by immersion in a diffusant such as heated silver chloride to provide a graduated transition of refractive index therebetween in the final assembly. Still another technique for producing a gradient refractive index billet 10" from which lens blanks may be cut is illustrated in Fig. 8. This includes the provision of a multiple chamber glass furnace 24 having a plurality of concentric orifices through each of which a lens material of a preselected refractive index may be directed and caused to form the composite billet 10". Following the formation of billet 10" and cooling thereof to a solid state, transverse cutting along lines 28 will produce lens blanks 30. It should be understood that a multiple orifice plastics material (i.e. ophthalmic resin) dispenser may be substituted for furnace 24. The artisan will readily appreciate that there are various other modifications and adaptations of the precise forms of the invention herein shown which may be made to suit particular requirements. WHAT WE CLAIM IS:

1. A method of manufacturing a meniscus lens comprising the steps of forming a lens blank of material having a known refractive index value adjacent its axis and a gradation of refractive index radially of the axis, and providing the lens with optically finished con vex and concave opposite faces of preselected curvatures, the lens having a preselected centre thickness, and the geometrical surface con figuration of one of the faces being selected ac cording to desired off-axis correction of at least one opthalmic lens aberration, such as power error, astigmatism or distortion, and the grada tion of refractive index being selected accord ing to desired off-axis correction of power error or astigmatism.

2. A method according to claim 1, wherein the radial gradation of refractive index increases with distance away from the lens axis.

3. A method according to claim 1, wherein the radial gradation of refractive index decreases with distance away from the lens axis.

4. A method according to any of claims 1, 2 or 3, wherein the radial gradation of refractive index is effected in step fashion between the axis and the edge of the lens.

5. A method according to any of claims 1, 2 or 3, wherein the radial gradation of refractive index is in the form of a continuous change of values between the axis and the edge of the lens.

6. A method according to any one of the preceding claims, wherein an aspheric correction is applied to at least one of said opposite faces of the lens.

7. A method of manufacturing a meniscus lens according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

8. A lens having optically finished convex and concave opposite faces of preselected curvatures, a particular centre thickness and refractive index value adjacent its axis and a gradation of refractive index in the material of the lens radially from its axis, the geometrical configuration of one of the lens faces being selected, in conjunction with the centre thickness and refractive index value, and the curvature of the opposite face according to the off-axis correction desired for at least one opthalmic lens aberration, such as power error, astigmatism or distortion and the gradation of refractive index being such as to provide a desired correction of the power error or astigmatism.

9. A corrected ophthalmic lens according to claim 8, wherein the gradated refractive index increased with distance away from the axis of the lens.

10. A lens according to claim 8, wherein said gradated refractive index decreases with distance away from the axis of the lens.

11. A lens according to claim 8, 9 or 10, wherein said radial gradation of refractive index is effected in step fashion between said axis and edge of the lens.

12. A lens according to claim 8, 9 or 10, wherein the gradation of refractive index is in the form of a continuous change of values between the axis and the edge of the lens.

13. A lens substantially as hereinbefore described with reference to Figures 3 to 8 of the accompanying drawings.