CA2150478A1

CA2150478A1 - Multifocal contact lens

Info

Publication number: CA2150478A1
Application number: CA002150478A
Authority: CA
Inventors: Michael H. Freeman
Original assignee: Individual
Current assignee: Novartis AG
Priority date: 1993-01-27
Filing date: 1994-01-25
Publication date: 1994-08-04
Also published as: AU6095994A; EP0681711A1; JPH08507158A; GB9301614D0; WO1994017435A1; EP0681711A4

Abstract

A multifocal contact lens has diffractive power arising from a series of concentric zones each providing an asymmetric retardation of light across the zone width to direct light predominantly into a required order and sign of diffraction at least some of the concentric zones are shaped so that the step height varies so that it is different in one region (A) of the lens from that in another region (B), whereby the intensity of light associated with an image observed by diffraction at one order in that region (A) of the lens is greater than the light intensity associated with the same image when observed at the same order through another region (B) of the lens.

Description

Wo 94/17435 ~ l S 0 ~ 7 ~ PCT/US94/00918 MULTIFOCAL CONTACT LENS
This invention relates to multifocal (including bifocal) contact lenses, and more particularly to such lenses having t1iffr~ctive power.
US-A-4637697 describes a bifocal contact lens in which at least a portion of the light passing through the lens is focussed by asymmPtric zone plate snrf~res. Such zone plate surfaces comprise a plurality of concçntric zones arranged so as to cause diffraction of light tr~n~mitt~d through the lens, each zone providing an asymmetric l~rdalion of light across the zone width. In one form of that invention, the zones are defined and the asymmPtric retardation is provided by the surface con~ouL of the lens.
In particular, the zones may be defined by steps in the lens surface. The lenses of US-A-4637697 are ~çsignP~ to operate as bifocal lenses and in one form the concPntric zones are arranged so that the lens surface forming the zone is in the form of a slope whose profile is u-~iro~ll all round the conrPntric zone. Zones of this form may be shaped dir~clly by means of laser m~r~linin~
using a suitably shaped mask, or by dirælly cutting with a diamond tool. This form of zone plate commonly results in most of the light in the visible sl)ecL,ul~l being directed into a zero power and one positive power image. In the type of lens where there is a need for some refractive power to correct far rli~t~nce vision, the refractive power will be lln~ tllrbed by the zero-order image of the diffractive power. The power for the near image is provided by the diffractive effect, and the added power, which is the difference between the overall power ~or distance viewing and the overall power for near viewing, is provided entirely by the liffr~ctive Wo 94/17435 PCT/US94/00918 215Q ~ ~

effect. In this type of lens there is no need for orientation on the eye as the bifocal lens operates in the so-called cimlllt~n.-,ous vision mode with both near and ~lict~nce viewing being available to the eye at all points on the optical portion of the lens.
Multifocal contact lenses are also known which do not rotate on the cornea and may be stabilized in position by several methods.
The most common method is one involving some form of b~ cting i.e. ch~ping the lens so that it is thicker and thus heavier at one portion of the edge. Lenses can be produced with a wedge or prism shape with the thicker portion at the bottom. Lenses can also be trnnc~tffi or cut off so that a lower portion is wider and heavier than the rest of the lens and thus able to m~int~in a particular c rient~tion on the eye. These lenses contain se~ te areas on the lens for ~lict~nce~ near and where nPcesc~ry interm~Ai~t~ vision.
The b~ cting causes the lens to return to a stable lower position on the eye after blinking when the wearer is looking straight ahead. In this position, the rlict~nce portion of the lens is located so that it is aligned in front of the pupil. Moving the eye down to read causes the lens to be pushed up as it is cont~ted by the lower lid so that the pupil becomes aligned with the portion of the lens that contains the near viewing portion. Such lenses can suffer from jump i.e.
the problem of a f~ red image that occurs as the eye shifts from ~lict~nce to near and passes the boundary between the near and the ~lict~nce portion. Such jump can be elimin~ted by using so-called mono-centric bifocals but many p~tientc nee~ling bifocals may find problems in being fitted with such lenses.
Contact lenses have been made available in the m~rketrlace which utilize the ~liffr~rtive effect for focussing the near image.

WO 94/17435 21~ 7 8 PCTtUS94tOo918 Such lenses, unlike the b~ ted multifocal lenses referred to above, do not need to be ori~nted on the eye but nevertheless are subject to some movement on the eye particularly when the eye moves down to read. The users of the multifocal lenses in general 5 have reached an age where the amount of light needed for reading and close work is subst~nti~lly greater than that needed by e.g. a twenty year old. The ability to read and do close work is therefore inflll~nced by the light intensity of the image. In the case of a lens which uses asymm~tric zone plate sl-rf~ce~, as described in US-A-4637697, when a zone plate is arranged with the step height of the individual concentric zones unirorlll, the light inten~ity of a near image is virtually the same whichever part of the lens is being used.
It has now been found that the pc;~ro~ ance of a multifocal 15 contact lens which uses the liffr~ctive effect for rOc~ g at least one image can be improved by increasing the light inlellsily of an image at one order as seen in one part of the lens from the same image at the same order in another part of the lens. This invention is based on achieving this change in light intensity by selecting an 20 area of the lens in which a change in light intensity is desired and within that area ch~nging the ~liffr~ctive effect, e.g. by ch~nging step height of each individual zone when it becomes a part of that area.
It has been proposed to use bifocal lens designs having 25 concen ric zones which in the outer circumferential area of the lens differ i,~ step height from zones in the inner portion of the lens see, e.g. US-A-4881805 and EP-A-0343067. In so far as these wo 94/17435 2 ~ 8 PCT/US94/00918 specifications are understood, a lens according to these prior specific~tions is deci~ned to direct light into a dirrelel~t diffractive t order, in dirrelellt concçntric regions. These designs are in essence symm~tric~l in the sense that in any such region, all zones are at 5 the same step height at all points on their circumference, and may be said to be circularly symmetrical.
According to one aspect of the present invention, there is provided a multifocal contact lens in which at least one image at one order (preferably selected from +l and -1) is produced by 10 ~liffr~tiQn and that image as seen by the eye through one part of the lens is dirrelæ~ t~A from the same image at the same order as seen by the eye through another part of the lens by having a different intensity of light.
In order to achieve the change of light intensity associated 15 with a focussed image at one order in one part of a lens from that in another part of the lens, which uses concçntric asymmetric zone plate surfaces, it is neceSc~ry to change the step height of the zones in an oriented manner. Instead of any one zone having the same step height all round the concçntric zone, the height is changed 20 relative to another value as one moves into a predetermined region of the lens and back to the origin~l height as the region is left.
This may be done for all zones passing through the region so that the whole of the pre~etermin~d region is at the same step height which differs from the step height in the rest of the lens. This 25 change in step height is not associated with any change in zone width. It is ~l~relled that an abrupt change in step height be avoided and to move in a smooth manner from one step height to WO 94117435 2 1 5 0 4 7 8 PCr/US94/00918 the other so as to blend the region of one step height into the region of the other step height.
The invention further provides a multifocal contact lens having ~liffr~t~tive power, comprising a plurality of conc~ntric zones S arranged so as to cause liffr~ti~n of light tr~n~mitt~d through the lens, each zone providing an asymmetric retardation of light across the zone width in a manner which directs light of a design wavelength predominantly into one required order and sign, (preferably chosen from + 1 to-l), at least a number of the concentric zones being shaped so that the step height of each zone changes so that it is different in one region of the lens from that in another region of the lens, whereby the intensity of light associated with an image observed by diffraction at one order in that region of the lens is greater than the light intensity associated with that same image when observed at the same order through any other portion of that lens.
The manufacture of such a surface contour can be achieved by laser ablation or by cutting with a suitable tool, e.g. on a co,l,~ulel-controlled lathe. In the case of laser ablation, the laser beam is m~ked in such a way that the energy tr~n~mitt~d is varied by using a mask or combination of masks of varying tr~n~mi~ion.
The use of such a mask or masks pelllliL~ tr~n~mi~ion of an amount of energy corresponding to the amount of m~teri~l it is necess~.y to remove to achieve a particular contour.
The use of laser ablation to form the surface contour is prert;lled and the present invention includes a method of m~nnf~cturing a multifocal contact lens having diffractive power wo 94/17435 2 ~ 5 ~ 4 7 8 PCT/USg4/00918 which compri~es inlel~osing a mask shaped to provide a zone plate pattern on the fini~hed lens between a laser source and the lens blank and in ~ lition~ providing means to vary the ablative effect of said laser over the area of the mask, whereby a zonal plate 5 pattern is formed by ablation on the surface of the lens blank.
In general, the varying means is arranged to vary the ~iffra~tive power over the lens surface in such a way that the intensity of the image focussed by the diffractive power in one area of the lens at one order is dirrelent from the intensity of the image 10 at the same order as seen through another area of the lens.
In one man-lfar-t-lring scheme, the varying means comprises a second mask whose tran~p~rency towards the light emitted by the laser varies across its ~llrfat e, thereby varying the ablative effect of the laser beam.
Thus, the invention further particularly provides a method of producing a contact lens with diffractive power comprising the step of ablating a lens surface with a laser beam which has passed through two masks, one of which has pattern defining zones with a density grading across each zone width effective to produce 20 diffractive zones on the lens surface and the other of which has a density grading effective to modify the intensity of ablation across the visually used area of the lens surface so that different parts of the lens surface give different intensity fliffr~ctive effects.
Preferably, said one mask is such that it would produce diffractive 25 zones of a ullirOllll step height and the other mask is effective to vary that step height across the lens or at least part of the lens.

Wo 94/1743~ Q 4 7 8 PCT/US94/00918 It will be appreciated that the two masks can be combined to form a single mask pelro~ ing the two functions described above.
The ability to manufacture a lens according to the invention by laser ablation means that the lens can be fitted on an individual 5 basis tailored to the particular needs of a patient. A b~ ted blank lens (or other blank lens having means to m~int~in a particular orient~tion on the eye) with no means to focus an image by diffraction can be orientP~ on the eye, and then removed and ablated to provide diffractive power on all or a part of the lens, the 10 ablation also being controlled so that the step height of each zone is varied in a manner which results in separate regions being formed whereby the ratio of the light intensity associated with an image at one first order in one part of the lens to the light intensity associated with the same image at the same order in another part of 15 the lens is chosen to meet the particular needs of the wearer.
Although, in ~lcr~llcd embo~limPnt~ the zones have been described as concentric zones, the lens as formed can be made such that it encomp~es only a portion of the concentric zone system, and that portion can be further sub-divided by varying the step 20 height of each zone so as to provide regions where the light intensity associated with an image in one region of the lens varies from the light intensity associated with the same image in another region of the lens.
A lens ~ ren~Pr (ophth~lmic optician) can, by virtue of the 25 present invention, then be provided with a means of adding a further variable to what c~n be achieved with lens fifflng thus increasing the ability to satisfy a patient's needs. The actual lens wo 94/17435 PCTIUSg4/00918 2150~7~

to be ablated does not necç~ rily need to be used in determining fit, as the dispenser can have a fitting set and order a lens from a central source based on the use of the fitting set.
The production of lenses in accoldance with the invention is described below with reference to the acco-"panying drawings, in which:-Figure 1 is a m~gnifi~d section of the graded pattern suitable for a mask produced by a photo-typesetter.
Figure 2 is a m~gnifiPcl reproduction of another graded pattern of spots which changes in density in a uniform manner.
Figure 3 is a diagr~mm~tic view of how a pair of masks, one based on the graded pattern of Figure 1, and the other on the pattern of Figure 2 can be placed between the lens to be ablated and a laser beam.
Figure 3A graphically depicts in cross-section the change in step height for an upper portion of one of the concP-ntric zones of the ablated lens.
Figure 4 is a diagr~mm~tic represçnt~tion of polar coordinates used to define position on the lens surface.
Figure 5 is a diagram showing step height against polar coordinate angle for one embodiment of the lens.
Figure 6 illustrates diagramm~tic~lly a lens, with two pupil positions for the eye idPntifi~d by circles A and B.
Figure 7 is a sçhPm~tic ~lcsç~ on of a series of "image rays" passing through a lens.

2 15 ~ ~ 7 8 PCT/US94/00918 g Figure 8 is a diagram of a lens with a zone plate pattern applied thereto, and a change in step height having an effect within a "D" shaped segment on the lens surface.
Figure 9 is a ~ gr~m showing step height against polar 5 coordinale angle with a smooth change to and from a maximum step height in a prererfed area in another embodiment of the lens.
Lenses may be ablated in the case of soft lenses in both the hydrated and xerogel state. Co-pending British application 9008580.4 (GB-A-2243100) describes a convenient system for the 10 ablation of contact lenses to provide diffractive power.
Lenses may also be ablated using an excimer laser where the beam profile is modified in the first in~t~nre so as to create a series of (1iffr~cting zones of Imifol-ll step height and in the second in~t~nce to create a smoothly varying intensity over the optical area 15 of the lens, both these m~-1ific~tions being imposed on the same beam profile before it is incident on the surface to be ablated.
Figure 1 shows a pattern of spots of varying density produced by a col..puler-controlled photo-typesetter output which may be reproduced on a light tr~n~mitting substrate in the form of a 20 coating of metal refl~ctinp: spots. The pattern defines a series of concentric zones, only part of which is shown in Figure 1, with a graded density across each zone width. The superimposed rulers simply in~ te scale with the upper number representing inches the lower numbers repfese~ing centim~tp-rs. This pattern may then be 25 imaged with reduction onto the surface to be ablated using an optical system which does not resolve (reproduce) the individual - spots and so creates a smoothly varying effect within each zone.

wo 94/17435 PCT/US94/00918 47~

At the same time, another transparent substrate is introduced into the beam which has, for eY~mple, a slowly varying density of spots from, say top to bottom, as shown in Figure 2. Figure 3 shows a first mask 1 having a series of concP-ntric zones as partly shown in 5 Figure 1 and a second mask 2 having a general even gr~tion as shown in Figure 2 mounted in the light path of an ablating laser beam L from a laser source 3 to a surface to be ablated of a lens 4.
With both masks 1 and 2 siml-lt~neously in the beam path, the ablated surface is influenceci by both mask profiles, the grading 10 infl~lPnce being within the zones by mask 1 and overall by mask 2.
The pattern of concçntric zones on mask 1 is cle~ign~l in the manner described in GB-A-2243100 in order to produce a lens having a lenticular surface formed with a series of concP-ntric zones as described in US-A-4637697. Preferably, the conce-ntric zones 15 are forrned on the concave back surface of the lens, although it is possible to provide some or all of the ~liffr~ctive power on the concave front surface. The effect of the second mask 2 is to modify the step height of the zones in the manner in(lic~te~ below.
Figure 3A shows a cross-sectional view of an upper portion 20 of one of the series of concpntric zones of the ablated lens 4 shown in Figure 3. The step sizes of the diffraction grating pr~gressi~rely increase from the top of the lens to the bottom, thereby ~h~nging the blaze angle ~y, and therefore, the focal point of an order of a principle ~liffr~ction maxima m, in accordallce with the general5 equation asin(-2 y) = m~O

Wo 94/17435 ~ 7 8 PCT/US94/00918 wherein ~O is the chosen wavelength and a is the length of the step.
The optical action of a profile step is normally expressed in terms of the wavelength (~) of some chosen color of light; for vision purposes this could be green. The action required for a S bifocal effect would be in the region of 0.5 ~0 although this can also be expressed as 1.0 ~d where ~\d iS a 'design' wavelength rather than the utilized wavelength.
Using ~O in this in~t~nce it can be seen that if the uniform step height ablated if the mask of Figure 1 were used on its own 10 was 1.0 ~O~ then the inflllçnce of Figure 2 is to reduce this step height to a fractional value which changes for different regions of the optical area.
This action is also dependant on the refractive indices of the ççnt m~ttori~l~ but for simplicity, 'step height' is here taken to 15 mean the optical action of the step so that a 'step height' of ~0 has a nominal full diffraction çfficiçncy.
For eY~mple, the u~-ifollll change of Figure 2 could give a zero step height at the top and 1.0 ~0 step height at the bottom.
Around each zone the step height would change in a cyclic f~chion.
20 Taking Figure 4 as ~e,fining the region of the lens in terms of polar coor~ es, i.e. in~ ting a particular point by radius 'r' and angle ~e~ be~wæn O degrees and 360 degrees, the effect of the wedge filter described in Figure 2 would be a step height for the outer zones (r large) which varies from 0 to 1 )~O while the step height for 25 the inner zones (r small) would vary about the same mean value but by a smaller amount. Figure 5 shows the general effect with the Wo 94/17435 21~ 0 ~ ~ ~ PCT/uSs4/009l8 full line representing the outer zones and the broken line the inner zones, the maximum step height being at 270 degrees, i.e. towards the bottom of the lens, and the minimum step height being at 90 degrees, i.e. towards the top of the lens. The mathematical 5 description could be:

h = r (1 + sin e) where h is the height of the step, r is the zone radius, R is the maximum zone radius and e the orient~tion angle as defined in 10 Figure 4.
Such a smoothly generated wedge effect is shown diagr~mm~ti~lly in Figure 6 where the circles in~ ting the zone edges have been thick~ned in the region where the step height is greater. Figure 6 shows that a lens placed on the eye so that the 15 pupil is in position A will view the outside world via mainly low step height zones and will see a strong in-focus image for distant objects. If the lens is repositioned on the eye (by the eye looking downwards, for instance), the pupil has an effective position given by B in Figure 6. It is now viewing via a region of the lens where 20 the step height is large and will see a strong in-focus image for nearer objects.
Figure 7 gives an in~ic~tion of the strength of the image light in terms of rays but this is a purely s~Pm~tic diagram as the ~liffr~tive effects, particularly in the central region with a more 25 even division of the images, cannot be e,~lessed in terms of rays.

Wo 94/17435 PCT/US94/00918 - However, for appreciable pupil sizes covering 3 to 4 zones of the diffractive pattern, these rays give a r~rese~-t~tive in~lplelation.
Figure 7 is an effective vertical section through a lens as illll~tr~t~d in Figure 6, i.e. having greater step heights towards the bottom of S the lens and smaller step heights towards the top. The rliffr~ted 'rays' passing through the bottom par~ of the lens are therefore of greater intensity than the non-diffracted (or zero order) rays passing through that part. Looking through the bottom part of the lens therefore gives a near image N (produced by diffraction) of greater 10 intensity than the zero order image F. Conversely, the non--liffr~cted (or zero order) rays passing through the upper part of the lens are of greater intensity than the diffr~cted 'rays' passing through the upper part and therefore looking through the upper part of the lens gives a far image F (to which the non-diffracted rays are 15 refr~t~d) of greater intensity than the near image N.
Figure 8 sçht-m~tically shows a lens having concçntric zones providing a diffractive effect additional to any refractive effect of the lens. The step height of the zones is ullirolln (but relatively low) where the zones are in~ tyl by broken line but in a 'D' 20 shaped segmPnt C the step height is graded as previously discussed so that it increases gradually from the top of the segment (which is subst~nti~lly hori70nt~l) to the bottom. Hence, as previously eYrl~ined, a greater intensity near image is seen by looking through the lower part of the segment C than through its upper part of 25 looking through other regions of the lens where the far image has greater intensity.

Wo 94/17435 PCT/US94/00918 2l5~78 Usually, the multifocal lenses produced in accordance with the invention will have a refractive power attributable to the general curvature of the front and back surfaces and the refractive index of the lens m~tt-.ri~l The concentric zones provide 'add-on' or S 'subtractive' power compa~ed with the refractive power of the lens.
In order to operate satisfactorily, a lens such as shown in Figures 6 or 8 will need to be t)rient~ted in the appl~liate way on the cornea. This can be achieved, e.g. by providing a ballast on one side of the lens, to ensure that the desired area for near vision comes to rest p,eferenlially on the lower part of the cornea. A
b~ cted lens blank (or a lens blank having other orient~tion means) and having the desired refractive power for distant vision for a particular patient is conveniently used as the lens 4 (see Figure 3) in a laser ablative method of forming a lens with non-unifoln diffractive power in accordallce with this invention.
Methods of pl~aling b~ cted lenses are well known in the art. For example, they are described in the book by Stein et al entitled "Fitting Guide for Rigid and Soft Contact Lenses, published by The C.V. Mosby Company, St. Louis, Missouri (1990), pages 319 et seq. In addition, reference may be made to the following US patents for details of m~nuf~ctme of such lenses, viz: US Patent No. 4,407,766; US Patent No. 4,642,112; US Patent No.
5,009,497; US Patent No. 5,009,497; US Patent No. 5,100,226 and US Patent No. 5,198,844.
In an alternative version of a lens of the type shown in Figure 8, the step height may be unifo~ln throughout the segment C
but higher than the uniform step height over the rem~in~ler of the 215047~

- lens. With this arrangement looking anywhere through the segmt-nt C would give a near image of greater intensity than looking anywhere else through the lens to give a far image of greater intensity.
However, sudden changes in step height may be undesirable and it may be preferable to give a progressive change. Figure 9 shows a localized step height variation depicted on the polar coordinate basis previously mentioned. For the outer zones (indir~ted by full line) of the lens the step height is very low over the O degrees and 180 degrees region but gradually increases after 180 degrees to a maximum fl~ttened peak sp~nning the 270 degrees area (i.e. the bottom part of the lens) and then gradually decreases back to the very low value at 360 degrees/O degrees. The inner zones (indicated by broken line) of the lens have a step height which follows a similar, but less pronounced, ~r~g.e~ e gr~d~tion so that their maximum step height at 270 degrees (i.e. towards the bottom of the lens) is less tnan that of the outer zones and their minimum step height from about O degrees to 180 degrees (i.e. in the upper part of the lens is greater than that of the outer zones).
It will be appreciated that in Figure 9, and also in Figure 5, for ease of illustration the outer zones' step height is represen~ed by a single full line and the inner zones' step height is represented by a single broken line. In pr~ti~e, of course, there may be a progressive change in step height from the innermost zone to the outermost zone so that Figures 9 and 5 would, if properly r~,ese.~ting the full situation, have a number of step height lines corresponding to the number of zones. For convenience, however, .

Wo 94/17435 - PCT/USg4/00918 ~lso~8 the full line and broken line shown can be considered as represt~nting the outermost and innermost zone step height respectively.
In the particular embodiment~ and examples specifically 5 described above it is generally envisaged that the used order of iffracti~n is first order and the diffractive power is positive, i.e.
the + 1 order. It will be understood however, that the diffractive power could be negative, e.g. the -1 order could be used, or other orders, whether positive or negative could be used. Negative 10 diffractive power could effectively subtract from positive refractive power of the basic lens so that the refractive power gives a near image and rliffr~ctinn provides a far image. Usually, the described wedge effect would then be reversed so that the more intense diffraction occurs towards the top of the lens to give a strong image 15 of near objects being given through the lower part of the lens by more intense refraction.
It will further be understood that while the diffractive action is preferably achieved by the use of concçntric zones having a~lu~.iate surface relief step heights giving the required different 20 intçn~iti~s or efficienci~s of r~iffr~tion, it could ~lt~ tively be achieved by the use of refractive index variations in the m~teri~l of the lens which give the required image differentiation when viewed through dirrel~llt parts of the lens. This can be achieved by conventional means such as varying the monomer composition 25 through a layer and polymeri7ing before diffusion effects offset the variable composition. As is also conventionally known, a more viscous composition may be helpful in reducing diffusion.

Wo 94/17435 pcrlus94/00918 :
21 ~478 Furthermore, reference can be made to Summerville, Plastic Contact Lens, Noyes Data Corporation, Park Ridge, New Jersey (1972), which discloses at pages 69-71 the fusion of m~ttori~ of different refractive index.
Although lenses in acco~ance with the invention are conveniently pl~a,~d by the laser ablative method described above, an alternative method of production involves the use of a co,,,pu~er-controlled lathe. Such a lathe may operate to position a cutter in accordance with signals derived in the manner that Figures 5 and 9 have been derived from a co",l,u~ stored analogue of the masks 1 and 2, shown in Figures 1, 2 and 3.
The lenses of the present invention may be hard (e.g. gas permeable lenses) or soft, e.g. hydrogel lenses, the ch~o-mic~
constitution of which is well known in the art.

Claims

CLAIMS:

1. A multifocal contact lens having diffractive power in which at least one image at one order is produced by diffraction and that image as seen by the eye through one part of the lens is differentiated from the same image at the same order as seen by the lens through another part of the lens by having a different intensity of light.

2. A multifocal contact lens having both refractive and diffractive power, the diffractive power being provided by asymmetric zone plate surfaces and being distributed over a viewing area of the lens in such a way that the intensity of the image focussed by the diffractive power in one part of said area is greater than that in another part.

3. A multifocal contact lens having diffractive power which comprises a plurality of concentric zones arranged so as to cause diffraction of light transmitted through the lens, each zone providing an asymmetric retardation of light across the zone width in a manner which directs light of a design wavelength predominantly into one required order and sign, at least some of the concentric zones being shaped so that the step height of each zone changes so that it is different in one region of the lens from that in another region of the lens, whereby the intensity of light associated with an image observed by diffraction at one order in that region of the lens is greater than the light intensity associated with the same image when observed at the same order through any other portion of that lens.

4. A multifocal contact lens as claimed in claim 3 wherein each zone directs light predominantly into the + 1 or -1 order.

5. A multifocal contact lens as claimed in claim 3 or 4 wherein the diffractive power is additional to the refractive power provided by the material of the lens and the basic curvature of its lenticular surfaces.

6. A multifocal contact lens as claimed in any one of claims 3 to 5 wherein some of said concentric zones have a step height which is non-uniform across the zone width circumferentially of said zone.

7. A multifocal contact lens as claimed in claim 5 which is ballasted so that the region of the lens which provides the higher intensity of light associated with add-on diffractive power is oriented to rest downwardly on the cornea.

8. A multifocal contact lens as claimed in any one of claims 3 to 7 in which the concentric zones are formed by shaping the surface contour of lens surfaces.

9. A multifocal contact lens as claimed in claim 8 in which said shaping is effected by cutting with a computer-controlled lathe.

10. A multifocal contact lens as claimed in claim 8 in which said shaping is effected by ablation with a laser.

11. A multifocal contact lens as claimed in any one of claims 1 to 7 in which the diffractive power is provided by variation in the refractive index of the lens material.

12. A method of manufacturing a multifocal contact lens having diffractive power which comprises interposing a mask shaped to provide a zone plate pattern on the finished lens between a laser source and the lens blank and in addition, providing means to modulate the ablative effect of said laser over the area of the mask, whereby a zonal plate pattern is formed by ablation on the surface of the lens blank.

13. A method as claimed in claim 12 wherein the effect of said modulating means is to produce a varying diffractive power over the lens surface.

14. A method as claimed in claim 12 or 13 wherein the modulating means comprises a second mask whose transparency towards the light emitted by the laser varies across its surface thereby modulating the ablative effect of said laser.

15. A method as claimed in claim 14 wherein said modulating means comprises a pattern of microscopic spots, the density of which varies across the surface of the mask.

16. A method as claimed in claim 14 or 15 in which the modulating means is combined with said first mask.