GB1569766A - Multifocal ophthalmic lens - Google Patents

Description

(54) MULTIFOCAL OPHTHALMIC LENS (71) We, AMERICAN OPTICAL 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 ophthalmic lenses in general and is more particularly concerned with novel multifocal ophthalmic lenses in which the distortion is so controlled that a wearer perceives vertical lines as substantially vertical throughout most of the viewing area of the ophthalmic lens.

The human eye is a sensitive yet relatively simple organ. It contains a lens on the outer surface for receiving light from various objects in the field of view of the eye. A retina is positioned behind the lens to serve as a viewing screen for those rays focused by the lens onto the retina A series of muscles surround the lens and act upon the lens to increase or decrease its curvature and focal length in order to focus upon objects which are either near to the eye or at a distance. When the normal eye views relatively distant objects, the lens and the muscles are in a relaxed position. In this position, the ideal lens has the proper curvature on its surface to focus the distant object on the retina. Upon the observance of objects at close range, the eye muscles act on the lens to increase its curvature and decrease the focal length of the lens sufficiently to focus the image of the near object onto the retina. This ability of the eye to adjust itself for varying object distances is commonly known as "accommodation". As the age of a human being increases, his power of accommodation generally decreases. This results from the fact that the eye muscles become stiff and weak. For example, a child can normally change the focal power of his eye by at least 14 diopters. In a middle age person, the power of accommodation is often reduced to about 3 diopters, and in old age the power of accommodation may disappear entirely.

For a long time, scientists and optical engineers have attempted to find solutions to this problem of decreasing accommodation with age. Probably the most common means which has been devised for treating this condition is to construct the corrective ophthalmic lens utilized by the person with decreased accommodation with a plurality of spherical surfaces. These are commonly known as bifocal and trifocal lenses depending upon whether the lens in question contains two or three spherical portions. In the bifocal lens, two separate segments of different dioptric focal powers are provided. The power of one segment is such that vision through it permits focusing on nearby objects such as reading matter while the other segment corrects the vision for viewing disant objects. In a trifocal ophthalmic lens a third spherical segment is interposed between the previously mentioned two segments to provide a measure of clear vision to the wearer intermediate between the dioptric focal powers of the distance and reading segments of the lens. The other surface of the multifocal ophthalmic lens is then provided with either a spherical or toric surface designed specifically to adapt the multifocal lens to the particular ophthalmic prescription of the wearer.

Certain major difficulties are, however, encountered by the users of multifocal ophthalmic lenses. Firstly, there is a line of sharp demarcation optically between the various segments of the multifocal lens. When the line of sight scans across this dividing line, a "jump" usually occurs in the image perceived by the wearer. It is difficult for the wearer to become accustomed to this sensation and to make allowances for it in normal life. Secondly, persons having severely reduced accommodation are unable to focus clearly on objects lying at distances between those at which the various segments are designed to focus. Thirdly, particularly in younger people having reduced accommodation powers, it is often difficult to convince some individuals that they require multifocal ophthalmic lenses for vision purposes. This is generally attributed to the fact that decreased accommodation is associated with oncoming age. The standard multifocal lens has a distinct line of demarcation between the various segments which is readily apparent to people in the vicinity of the wearer. Therefore, as well as the optical problems which exist with multifocal ophthalmic lenses, also certain cosmetic problems exist.

The obvious general solution to these problems is to place an intermediate viewing zone between the long distance viewing zone and the near viewing, or reading zone, which progresses in dioptric focal power from that of the long distance viewing portion to that of the reading portion. By attempting this solution, an ophthalmic lens is provided in which both the optical and cosmetic problems may be solved in that there are neither lines of optical jump between distinct segments nor are there cosmetically obvious lines between the various segments.

Lenses of this type, known commonly as "progressive power" lenses, form the subject matter of our British Patent No. (Application No. 45601/76 Serial No.

1,569,764) and our British Patent No. (Application No. 45602/76 Serial No.

1,569,765) to which attention is directed for a full discussion of this subject.

One of the problems of progressive power opthalmic lenses, however, is that of distortion, particularly skew distortion which, although its effects can be minimised and brought down to tolerable levels, for example using the techniques and principles discussed in the above mentioned patents, cannot be eliminated entirely due to the essential nature of the progressive power portions of a progressive power lens. Such skew distortion causes a swimming or rocking effect when the wearer's head is moved within the visual environment. This effect has served to cause many wearers of progressive power ophthalmic lenses to become nauseated and has definitely prevented the wide acceptance of this type of lens. Furthermore, these lenses are subject to astigmatism which causes blurring of vision through the affected areas of the lens. This effect is, of course, objectionable as well.

Distortion occurs whenever astigmatism is present in the refracting surface of a lens. As mentioned above distortion and astigmatism are, in fact, inevitable consequences of progressive power refractive ophthalmic surfaces. Many attempts have been made in the prior art to minimize the effect of the astigmatism.

Generally, these attempts have concentrated on the manner in which the refractive surface having the aspherical curvature is generated. The more successful attempts have resulted in spreading the astigmatism over a large portion of the refractive surface. This, however, has the unfortunate consequence of reducing the size of the reading zone below that which allows the wearer to read standard material without turning his head. The present invention seeks to provide an alternative solution to the problems of multifocal ophthalmic lenses, without resorting to the use of progressive power refracting surfaces, and in which the use of progressive power refracting surfaces, and in which the boundaries between adjacent zones of different dioptric focal power in an ophthalmic lens can be rendered, at best, effectively invisible, without the lens suffering from the disadvantageous effects of skew distortion which results from the principal axes of astigmatism (as defined hereinafter) being inclined to the vertical and horizontal when the lens is in a normal upright orientation of use. Our British Patent Specification No. 1484196 (Application No. 36171/74) describes a multifocal ophthalmic lens in which different viewing zones are each divided into at least three areas the outer areas of which are corrected for skew distortion. The present invention provides such a lens in which the correction for skew distortion is effected using different optical parameters from those described in the above mentioned British Patent Specification.

Accordingly, the invention in its broadest aspect comprises a multifocal ophthalmic lens having a plurality of viewing zones, comprising a lens body having a refractive surface with a smooth, unbroken principal meridional curve which, with the lens in an intended position of use, lies along the refractive surface in a generally vertical or near-vertical direction dividing the refractive surface into two similar lateral portions, the curvature of the principal meridional curve varying to provide a predetermined plurality of levels of constant dioptric focal power according to a predetermined law thereby dividing the refractive surface into a like plurality of viewing zones in a direction parallel to the principal meridional curve, each pair of juxtaposed viewing zones having a boundary therebetween, the uppermost viewing zone having a first constant dioptric focal power and being adapted for viewing distant objects over the full width of the multifocal ophthalmic lens, and each of the remaining viewing zones being divided into at least three areas transversely of the principal meridional curve, a central area in each of the remaining viewing zones being of a constant dioptric focal power higher than the constant dioptric focal power at the principal meridional curve in the viewing zone juxtaposed thereabove, and the two outer areas in each of the remaining viewing zones being of such aspherical curvature that at a boundary with an adjacent area of different dioptric focal power the relative heights of the viewing zones forming the boundary are constant along the boundary throughout the width of each outermost area and each have a surface curvature substantially in accordance with the equation a2z =0 ax ay where y and x represent horizontal and vertical directions respectively defining a plane, and z is the height of the curved surface from this plane, the surface comprising a portion of a surface of revolution whose axis of revolution is vertical and lies in a common plane with the optical axis of the lens whereby optically to compensate for skew distortion so that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes to permit a wearer of the lens to perceive horizontal and vertical lines in the visual environment as being horizontal and vertical, and the boundaries between the viewing zones being blended so that the boundaries are substantially invisible.

Embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a lens according to the invention; Figure 2 is a front elevation view of an ophthalmic lens whose intermediate and near vision portions are divided laterally into a plurality of areas, the outermost of which are totally corrected for skew distortion; Figure 3 is a schematic diagram of a symmetrical lens according to the invention which has been rotated 10 degrees from the vertical to accommodate for decreasing interpupilary spacing when viewing closer objects; Figure 4 is a schematic diagram of a matched pair of lenses which compensate for the 10 rotation required; Figure 5 is a front elevation view of a multifocal ophthalmic lens according to the present invention whose intermediate and near vision segments are divided laterally into a plurality of areas, the outermost of which exhibits only normal distortion, the lens being susceptible to a manufacturing process for blending out the segment dividing lines; Figure 6 is an illustrative diagram of the image of a square grid as viewed through a multifocal lens of the present invention Figure 7 is a front elevational view of the multifocal lens used for Figure 6 in which the surface discontinuities at the boundaries between segments are blended to render them invisible.

Figure 8 is a perspective elevation view of ophthalmic lens on which the surfaces of the lateral peripheries are illustrated as being constructed in accordance with portions of a figure or surface of revolution; Figure 9 is an illustration of a rectangular coordinate system which may be used to produce a ceramic block slumping surface for making glass molds useful in the casting of lenses according to the present invention; and Figure 10 is an exemplary embodiment of a surface shape which may be provided upon a slumping block used to produce a mold for casting a lens of the type depicted by the coordinate system of Figure 9.

In referring to the various figures of the drawing herebelow, like reference numerals will be utilized to refer to identical parts.

Referring initially to Figure 1, there is shown a multiple focus ophthalmic lens 10. The lens is constructed of an optical material having a uniform refractive index such as optical quality glass or one of the well-known optical quality plastic materials such as CR-39 (allyl diglycol carbonate), Lexan (Registered Trade Mark) (polycarbonate), or methyl methacrylate. Multiple focus is accomplished in the lens 10 by forming one of the two surfaces into an appropriate aspherical form.

Generally, the surface utilized for forming the aspherical surface is the front surface of the lens, i.e., that surface of the lens which is of convex form. However, the principal reason for this choice is that conventional grinding and polishing machinery located at various dispensing branches is configured to apply the spherical or toroidal surface dictated by the intended wearer's particular ophthalmic prescription on the rear surface of the ophthalmic lens. Therefore, in this introductory portion as well as the remainder of the following description of the invention, the aspherical surface will be shown and described as being present as the front surface of the ophthalmic lens. Although it is not intended that the invention be so limited.

For purposes of this description, the multifocal ophthalmic lens 10 is illustrated in approximately the orientation in which it is to be worn by the patient.

More particularly, as shown in Figure 1, the lens 10 is oriented such that the aspherical surface is tangent to a first vertically oriented plane 12 at the geometrical center 14 of the lens blank 10. A second vertically oriented plane 16, perpendicular to the first vertically oriented plane 12, also intersects the ophthalmic lens 10 at point 14 and divides the lens 10 into two symmetrical halves.

The plane 16 is generally referred to as the principal vertical meridional plane. The principal vertical meridional plane 16 intersects the aspherical surface of the lens 10 in a plane curve 18 called the principal meridional line or curve.

In general, it is preferred that astigmatism along the principal meridional line 18 be essentially zero. Astigmatism is generally defined with respect to a point on a refractive surface and two perpendicularly disposed planes intersecting thereat and passing through the normal to the refractive surface at the point, the first or sagittal plane established by the minimum radius of curvature of the refractive surface and the second or meridional plane established by the maximum radius of curvature of the refractive surface at the point, then the magnitude of the astigmatism is taken as the difference between the dioptric focal power of the refractive surface in the first plane and the dioptric focal power of the lens in the second plane. The amount of astigmatism at any point on the refractive surface of the lens is measured by the difference in the dioptric focal power between the sagittal plane and the meridional plane of the point. The curvatures of the refractive surface at any point in the sagittal and meridional planes are commonly known as the principal curvatures at that point. This astigmatism could be called intrinsic in order to distinguish it from that astigmatism that arises when a spherical surface is illustrated by rays of light striking the surface at oblique angles of incidence.

Along the principal meridional curve 18, the refractive surface is umbilic, i.e.

there is only a single radius of curvature at any given point. If r is the radius of curvature of the principal meridional line at Q, and n is the index of refraction of the lens material, then, if the principal curvatures of the surface at Q are equal, the dioptric focal power P0 at this point is given by n-l Pro = r It is convenient to describe the distortive properties of a refractive surface such as those associated with the present invention in terms of the image of a square grid as seen through the lens. While not totally accurate, this test is a reasonable approximation of the visual affect gained by wearer of the resulting ophthalmic lens.

Two general types of distortion can be distinguished, normal and skew.

Normal distortion refers to the unequal image magnification in the two orthogonal directions parallel to the lines of the grid. Skew distortion refers to a departure from the orthogonality of the original grid lines. Suppose that a single square of such a grid is viewed through a small area of a given ophthalmic lens. If the principal axes of astigmatism in that area of the lens are parallel to the lines of the grid being viewed, then the image perceived shows pure normal distortion, i.e., the image of the grid square is a rectangle whose sides are parallel to those of the square. If the principal axes of astigmatism in that area of the lens bisect the right angle between the respective orthogonal grid lines, then the image shows pure skew distortion, i.e., the image of the square is an equilateral parallelogram. In the general case, where the principal axes of astigmatism have arbitrary orientation with respect to the lines of the object grid, the image of the square perceived will exhibit a combination of normal and skew distortion, i.e., the image will be a nonequilateral parallelogram.

Of the foregoing two specific types of distortion, skew distortion is by far the more objectionable in ophthalmic applications. In an ophthalmic lens, skew distortion produces a sensation of rocking and swaying with respect to the environment. In most instances, this rocking and swaying effect results in disorientation and nausea on the part of the wearer.

The astigmatism present in a refractive surface varies laterally at a rate dependent on the rate of addition of focal power along the principal meridional curve. Therefore unless correction or compensation for distortion is undertaken in the peripheral areas of the multifocal ophthalmic lens, considerable distortion is necessarily present in the surface.

It is not possible to eliminate distortion in the surface. It has been discovered, however, that it is entirely possible to construct an ophthalmic surface which in the peripheral areas is totally corrected for skew distortion. That is to say, the principal axes of astigmatism in the peripheral areas may be caused to lie in horizontal and vertical planes with respect to the visual environment such that only normal distortion occurs in these peripheral zones. This normal distortion is far less objectionable than the skew distortion and the incorporation of this aspect into a multifocus lens forms one of the principal features of the present invention.

This corrected condition of the refractive surface in the peripheral areas for skew distortion can be most easily expressed mathematically by letting the lens surface be tangent to the x-y plane at the origin of the coordinate system, where the x-axis points downward in the direction of increasing optical power, and by assuming that the surface is represented by the following expression: z = f(x,y) (I) when y and x are the horizontal and vertical directions respectively and Z is the height of the surface from the xy plane the surface has the condition that the directions of principal curvatures, the principal axis of astigmatism, at all points lie in planes which are parallel to the x and y axes; this implies that a2f =0 =0 (2) axay (2) When this expression is satisfied for all points in a given area, only pure normal distortion may be perceived through that area of the lens.

This partial derivative expression can be alternatively expressed as

The two above partial derivative operators can be shown, by mathematical tensor analysis which need not be repeated here to fully comprehend the present invention, to be substantially proportional to the cosines of angles between surface grid lines. An angle for which the cosine is zero, as required by equation (2), is 90 degrees, the orthogonality angle. Thus, when partial derivative expression (2) is satisfied, orthogonality is provided and skew distortion is virtually eliminated; vertical and horizontal lines in the visual environment appear to the wearer of the lens having this correction as vertical and horizontal respectively.

Yet another way of viewing the substantial significance of equations (1) and (2) is to appreciate first that in general cylindrical lens surfaces with vertical and horizontal mutually orthogonal axes provide no skew distortion, and second that for these orthogonally-oriented cylinders with their axes co-linear with the Y and X coordinate axes respectively, equation (I) can be separated into: Z = f(x,y) = f,(x) + f2(y) (4) where f,(x) is a function onlv of the x variable and (y) is a different function only of the y variable. By operating on equation (8) with the operators of equation (7) one necessarily obtains zero, since: af2(y) af, (x) =0 and =0 ax ay Thus, the zero condition of equation (2) implies this cylindrical axial orthogonality condition which provides zero skew distortion.

Referring now to Figure 2 of the drawing, there is shown a multifocus ophthalmic lens 50 according to the present invention. A principal meridional line 52 bisects the lens in a generally vertical direction. The lens 50 is divided into three juxtaposed viewing zones 54, 58, and 56 respectively one above the other. The.

uppermost viewing zone 54 is formed with a refractive surface having a constant dioptric focal power to accommodate for distant vision. The lower viewing zone 56, in the central regions thereof, is formed with a second higher constant dioptric focal power surface adapted for near vision. The intermediate zone 58 which is disposed between the near and far viewing zones 54 and 56, respectively, provides a constant dioptric focal power intermediate between that of the near and far viewing zones 54 and 56. As described thus far, the lens of the invention is not appreciably different from those of the prior art. However, in the intermediate and near vision zones 58 and 56, the refractive surface of the progressive power lens 50 is further subdivided laterally into five areas. The dividing lines between these areas AB, CD, A'B', and C'D', are chosen arbitrarily with respect to shape and position.

Although illustrated as being symmetrical with respect to the principal meridional curve 52, this is not a condition of the invention. In the present embodiment, the dividing lines A'F'B' and C'D' may be mirror images of AFB and CD with respect to the principal meridional line 52. The central area A'AFF' is formed in accordance with any chosen multifocal ophthalmic lens design.

The peripheral areas of the refractive surface CDE and C'D'E' are constructed to connect smoothly to the far vision viewing zone 54 along the lines CE and C'E'. The smooth optical connection is achieved by having a smooth unbroken surface over the entire lens. At each point in areas CDE and C'D'E', the principal axes of astigmatism lie in horizontal and vertical planes according to the foregoing expression. It follows that, when viewed through these peripheral areas of the multifocal ophthalmic lens, the horizontal and vertical lines within the visual environment are not subjected to skew distortion. Furthermore, when viewed through the periphery of the lens a vertical line will remain vertical and unbroken throughout the total height of the periphery of the lens. In other words, a line which is viewed as vertical in the peripheral portions of the far vision viewing zone 54 continues vertical and unbroken in the intermediate viewing zone 58 and the near vision viewing zone 56.

The intermediate regions ABDC and A'B'D'C' are areas of optical blending between the central portion and the skew distortion corrected peripheral portions of the opthalmic lens 50. The purpose of these areas is to provide a smooth optical connection between these areas of diverse optical functions. Once again, these areas also connect smoothly to the far vision area 54. The precise choice of refractive surface configuration within these areas of blend depends on a great number of factors. These include the amount of add present in the fends, the overaff width of the lens, and the height of the intermediate viewing zone 58.

In the foregoing discussion consideration has only been given to lenses that are symmetrical about a vertical principal meridional line, that is lenses which, when in a normal or intended position of use have their principal meridional line in fact vertical on the surface of the ophthalmic lens and precisely dividing the lens in symmetrical lateral portions. From the point of view of product inventory, such perfectly symmetrical lenses are extremely advantageous. With proper marking applied, a symmetrical lens blank can then be used for either a left or right eye lens.

Functionally, however, it is preferable to design multifocal ophthalmic lenses separately for the left and right eyes respectively. The resulting lenses are asymmetrical since the interpupillary spacing of human beings decreases as their focus changes from distant objects to nearby objects. Therefore, in fitting the symmetrical lens to the patient, the principal meridional line of symmetry should be inclined approximately 10 from the vertical to provide an effective inset of the near vision viewing zone. This 10 rotation of the lens about its central point ensures that the line of sight can pass along the principal vertical meridional line for clear vision at all distances.

However, if a lens such as has been described above were to be thus rotated, the principal axes of astigmatism would be no longer aligned with the horizontal and vertical elements of the visual environment. And, particularly in the case of those lenses with higher adds, this misalignment may result in noticeable incorporation of skew distortion throughout the peripheral areas of the ophthalmic lens. This, of course, would be objectionable for the identical reasons that those lenses of the prior art were objectionable to many wearers. Such a lens is shown in Figure 3, where the orientation of a principal axis of astigmatism is shown at points in the various areas when the lens is rotated 10 .

It is, therefore, included within the purview of the present invention to correct this situation by modification of the foregoing symmetrical design. In the modified versions, the far vision viewing zone and the central portions of the intermediate and near vision areas remain unchanged from the foregoing symmetrical design.

However, the peripheral areas of the intermediate and near vision viewing zones are modified such that when the principal meridional line is inclined approximately 10 with respect to the vertical, the principal axes of astigmatism in these peripheral zones again are aligned with the horizontal and vertical elements in the visual environment. The blending areas are appropriately modified as well in order to provide a smooth optical correction between the central portions and the peripheral areas. Figure 4 illustrates the orientation of the principal axes of the astigmatism at various locations for both a right and a left lens modified to compensate for the decreasing interpupilary spacing at near vision.

As was explained hereinabove, the prior art agrees quite generally on the conditions which exists along the principal vertical meridional line. The prior art consists largely of a series of attempts to acquire a means for generating the resulting aspherical surface. This generation problem caused certain limitations to exist in the design of the lens. The present lens has avoided this situation by discarding the requirement for ascertaining a precise means of generating directly a finished ophthalmic lens. the lenses according to the present invention may be formed directly or as cast lenses. The lenses are formed by programming initially a machine such as a numerically controlled milling machine to produce essentially the complement of the refractive surface in a porous ceramic block. After the complementary surface is formed, a vacuum is applied to the back surface of the ceramic block and a sheet of highly polished glass is heated and slumped into the cavity formed in the ceramic block. This glass sheet may then be polished to form directly the refractive surface on a blank. Alternatively, the opposite side of t of manufacture including the sagging block method described hereinabove can effectively hide such a ledge.

However, in a lens made according to the present invention the theoretical ledge height can be restricted to some minimum value which can be effectively blended out through the sagging process of manufacture.

Such a multifocal lens is shown in Figure 5 of the drawing. The meridional power law has the stepwise form of the ordinary trifocal and is shown at the right of Figure 5. If b1 and b2 are the values of the power discontinuity steps and B is the total add, then B =b1 + b2.

In the case where either b1 or b2 equals zero, the lens simply becomes a bifocal ophthalmic lens. The multifocal lens 100 shown in Figure 5 is formed of a constant dioptric focal power distance viewing zone 104, a second viewing zone 106 disposed directly below the distance viewing zone 104, and a third viewing zone 108 at the bottom for viewing nearby objects. The intermediate and nearer viewing portions are divided laterally into at least three, and preferably (as shown in the drawing) five areas. The central area ABB'A' is centered on the principal meridional line 102 and is comprised of two constant dioptric focal power areas obeying the power law shown at the right of Figure 5. Adjacent to the central zone are blend areas ACDB and A'C'D'B'. These areas of blend perform the same function as those areas of blend described in relation to Figure 2. Similarly, peripheral zones CDE and C'D'E' also are again corrected for skew distortion in the manner described hereinabove. In these peripheral areas of the intermediate and near vision zones, a vertical line of the environment is viewed through the lens as an unbroken vertical line from the top to the bottom of the lens. In other words, along any vertical line drawn through the periphery of the lens, the amount of horizontal prism is constant.

The condition of verticality of lines used for the periphery of the lens is equivalent to the correction of skew distortion, which in the case of ordinary solid type trifocals is concentrated at the horizontal ledges that are associated with the various power steps.

The principal advantage arising from the correction of skew distortion is the reduction of the height of the horizontal ledges. The ledges are not removed entirely but the height that remains can be rendered cosmetically invisible by use of the sagging method of manufacture. For example, consider the type of distortion correction in which the width of the blending areas ABDC and A'B'D'C' are reduced to zero, i.e., the blending is done abruptly. The distortion of a squared grid as seen through such a multifocal lens is illustrated in Figure 6 of the drawing.

Suppose that the width of the central region A' AB B' is taken as 24 mm. If the change in dioptric focal power at the boundaries of the intermediate zone is taken as 1.00 D, then, the ledge height L where y equals 12 mm, is 0.14 mm. The ledge height L where ly is greater than 12 mm remains constant and equal to 0.14 mm.

Although on a generated lens, a ledge of this height would still be easily visible, the height L is not so great that it cannot be smoothed out and made cosmetically invisible in the foregoing sagging method of manufacture. The area of blend produced by this process is indicated at 110 in Figure 7 of the drawing.

The type of multifocal lens thus produced compares favorably to the segment type multifocal currently available. However, the dividing lines between the various portions of the lens are not visible in the lens according to the present invention as they are in the segment type multifocal lens.

Throughout the foregoing discussion of the various features of the invention, the various features of the lenses according to the invention have been discussed independently. That is, the vertical discontinuities concept has been discussed separately from the feature of dividing the lens horizontally into areas which are treated separately in order to correct for skew distortion. It is, however, included within the purview of the invention to combine these various features in various combinations in order to provide optimal performance of a multifocal ophthalmic requirements.

To correct for skew distortion in peripheral areas of a multifocal lens, one requires that vertical and horizontal lines in the environment are perceived by the wearer of a skew-distortion corrected lens as being respectively vertical and horizontal. This correction can be achieved by requiring that these peripheral areas comprise portions of a surface of revolution. The axis of the surface of revolution is vertical, and lies in the vertical meridional plane. By symmetry it should be clear that the principal axes of astigmatism at every point of such a surface of revolution lie in horizontal and vertical planes. Therefore, when the peripheral areas of the lens have this form, only pure normal distortion may be perceived through those areas.

Referring to Fig. 2 and Fig. 8, the peripheral areas of the refractive surface CDE and C'D'E' are portions of a surface of revolution whose axis LL' is vertical and lies in the vertical meridional plane. For the situation where the entire distance portion, comprising the upper half of the lens, is spherical, the axis of revolution for peripheral regions CDE and C'D'E' passes through the center of curvature of the distance viewing zone. If the axis were to pass through any other point of the evolute 22, it would not be possible smoothly to connect the lower half of the lens with the spherical upper half. In other words, because the equatorial dividing line EE' is circular, the axis of revolution for the peripheries of the lower half of the lens must pass through the center of that circle.

Further in connection with Fig. 8, in designing such a lens the locations of the lateral dividing lines AB, CD, A'B', and C'D' are first decided upon. Then with the knowledge that peripheral areas are to be portions of a figure or surface of revolution, the lateral blending zones are designed in such a way that the smoothest possible connection is achieved between the central and peripheral portions of the lens. The peripheral regions are then formed by rotating the boundary curves CD and C'D' about the rotation axis LL' of Fig. 22. Equations applicable to a lens, or to a slumping block that may be used to make such a lens, will now be given. These equations express the elevation z = f(x,y) of the surface above the xy plane. They give explicitly the form of the rotationally symmetric peripheral zones.

For the spherical distance zone, z = rD(rD2X2~y2)1/2 For the central portion of the lower half of the lens or block, including both the intermediate and reading levels z = f01(x,y),

where u = Qr,

For the rotationally symmetric peripheral areas of the lower half of the lens or block, z=f23(x,y)

where

For the blending areas between the central and peripheral areas, z = f12 = A fo1 +Bf23.

A = a(y2-y)3 + b(y2-y)4 + c(y2-y)5, B = a(y-y1)3 + b(y-y1)4 + c(y-y1)5, 10 -15 6 and a= 3, b= 91, c= 5.

(y2-y1) (y2-y1) (y2-y1) These equations, while providing an exact description of the geometrical properties required of the lens or block, and which are mathematically descriptive of the portions of the surface of revolution earlier referred to, are nevertheless quite complicated and are resistant to ready numerical evaluation.

Referring more particularly to the type of lens depicted in Figure 5, such a lens is distinguished from prior art lenses that involve blending in that there is no prism jump in the image as the line of sight moves along the principal vertical meridian.

Let it now be required to manufacture a blended multifocal of this type. The lens is to be CR-39, cast in a glass mold which is made by slumping into a forming block of ceramic material. A general mathematical description of the surface contour of the ceramic slumping block may be given as follows wherein the rectangular coordinate system used is shown in Figure 9: This view shows the concave surface of the block. The surface is tangent to the xy plane at the origin 0. That portion of the block corresponding to the distance portion of the mold and lens lies above the yz plane. Those portions that correspond to the intermediate and reading areas lie below that plane. The intermediate area is of height h. The block is symmetrical about the xz or merdional plane. On the lower half of the block, the lateral blending zone on the right-hand side is bounded on the left and right by the curves y = y,(x) and y = y2(x).

The glass employed is an ophthalmic crown glass, which is placed on this ceramic block defined by the equations presented and the combination is inserted into an oven which is heated as follows. The temperature is raised in the oven to a maximum temperature of approximately 1210 degrees fahrenheit over a period of time of approximately four hours. The temperature in the oven is contained at this value for approximately one hour. Then, approximately eight hours are utilised to reduce the temperature in the oven from this value downward. The radii of curvature of the distance, intermediate and reading areas are rD, r, and rR respectively. The form of the block surface is expressed as the height Z above the x-y plane.

For the spherical distance zone, which occupies the upper half of the block and lens, Z = r0r02-x2-y2)112 For the central region of the intermediate level z= f01(I) (x) = r1r,2-x2-y2)l'2 For the peripheral region of the intermediate level Z = f,,'" (x)

where

For the lateral blending areas of the intermediate level Z = f12(1)(X,Y) (x,y) =Af01(1) + + Bf23('? where A and B are the astigmatism and the total add respectively.

For the spherical central area of the reading level, where h is the height of the intermediate area:- z = f01 R?(x,y) = r1-(1-rR) (r2-h2)l'2- r1

For the peripheral region of the reading level Z = f23(R) (x,y) where

For the lateral blending area of the reading level Z = f12(R)(x,y) = Af10R} + Bf23(R) where A and B are as defined above.

A less exact but simpler set of equations for the surface is as follows.

The equation of the spherical distance remains the same as above.

For the lower half of the block Z = f(x,y) = W-[r2-(x-u)2-(y-v)2]112 where, For the intermediate level, r = r, u=O (y-v)2 = y2 central region = y2-(l- r1)l'(y-y1)2 lateral blending zone r0 = y2-(1-r1) (y2-n') peripheral zone rD W=r, And for the reading level, r = u = h(1-rR) r1 (y-v)2 = y2 central zone = y-(1-rR)1' (y-y1)2 lateral blending zone rD - y2-(l-r,) (y2-n') peripheral zone rD In these equations h = height of intermediate area = = distance radius r, = intermediate radius = = reading radius I' = y2/(y2-y1) n' = y1y2 y, =y1(x) y2=y2(x) A numerical example will now be given. The lens is to be a trifocal with a spherical distance portion of radius 83.33 nm and with a reading addition of 2.00 diopters. Using an ophthalmic crown glass for the slumping blank the following values of the block radii are required to produce a CR-39 lens with the required optical characteristics: rD = 88.113 mm r, = 68.440 mm = = 68.440 mm The lines y1 and y2 between which lateral blending occurs are shown in Figure 9. These lines, being composed of sections of straight lines are defined by the following coordinate points: x(mm) y1(mm) y2(mm) 0 0 0 6.8 14.0 14.0 13.6 17.5 28.0 The equations of the lines y1 and y2 are y1=2.06x (mm) 0 < x < 6.8 = 0.51x + 10.53 (mm) x > 6.8 y2 = 2.06x (mm) all x The height h is taken to be 6.8 mm.

These numerical values of rD, r1, rR, h, y1 and y2 are put into an electronic computer which, with the simplified set of equations defined above, has been programmed to calculate the surface elevation Z of the ceramic block at 4 mm intervals over the area of a block 86 mm in diameter. The results of the computation are shown in Figure 10.

WHAT WE CLAIM IS: 1 A multifocal ophthalmic lens having, a plurality of viewing zones comprising a lens body having a refractive surface with a smooth, unbroken principal meridional curve which, with the lens in an intended position of use, lies along the refractive surface in a generally vertical or near-vertical direction dividing the refractive surface into two similar lateral portions, the curvature of the principal meridional curve varying to provide a predetermined plurality of levels of constant dioptric focal power according to a predetermined law thereby dividing the refractive surface into a like plurality of viewing zones in a direction parallel to the principal meridional curve, each pair of juxtaposed viewing zones having a boundary therebetween, the uppermost viewing zone having a first constant dioptric focal power and being adapted for viewing distant objects over the full width of the multifocal ophthalmic lens, and each of the remaining viewing zones being divided into at least three areas transversely of the principal meridional curve, a central area in each of the remaining viewing zones being of a constant dioptric focal power higher than the constant dioptric focal power at the principal meridional curve in the viewing zone juxtaposed thereabove, and the two outer areas in each of the remaining viewing zones being of such aspherical curvature that at a boundary with an adjacent area of different dioptric focal power the relative heights of the viewing zones forming the boundary are constant along the boundary throughout the width of each outermost area and each have a surface curvature substantially in accordance with the equation a2z =0 ax ay where y and x represent horizontal and vertical directions respectively defining a plane, and Z is the height of the curved surface from this plane, the surface comprising a portion of a surface of revolution whose axis of revolution is vertical and lies in a common plane with the optical axis of the lens, whereby optically to compensate for skew distortion so that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes to permit a wearer of the lens to perceive horizontal and vertical lines in the visual environment as being horizontal and vertical, and the boundaries between the viewing zones being blended so that the boundaries are substantially invisible.

2. A multifocal ophthalmic lens as claimed in Claim 1, in which the said predetermined law defines a vertical magnification at each point along the principal meridional curve, the vertical magnification being substantially constant throughout the entire width of the lens along lines perpendicular to said principal meridional curve.

3. A multifocal ophthalmic lens as claimed in Claim 1 or Claim 2, in which the

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

Claims (6)

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

   x(mm) y1(mm) y2(mm) 0 0 0 6.8 14.0 14.0 13.6 17.5 28.0 The equations of the lines y1 and y2 are y1=2.06x (mm) 0 < x < 6.8 = 0.51x + 10.53 (mm) x > 6.8 y2 = 2.06x (mm) all x The height h is taken to be 6.8 mm.

   These numerical values of rD, r1, rR, h, y1 and y2 are put into an electronic computer which, with the simplified set of equations defined above, has been programmed to calculate the surface elevation Z of the ceramic block at 4 mm intervals over the area of a block 86 mm in diameter. The results of the computation are shown in Figure 10.

   WHAT WE CLAIM IS: 1 A multifocal ophthalmic lens having, a plurality of viewing zones comprising a lens body having a refractive surface with a smooth, unbroken principal meridional curve which, with the lens in an intended position of use, lies along the refractive surface in a generally vertical or near-vertical direction dividing the refractive surface into two similar lateral portions, the curvature of the principal meridional curve varying to provide a predetermined plurality of levels of constant dioptric focal power according to a predetermined law thereby dividing the refractive surface into a like plurality of viewing zones in a direction parallel to the principal meridional curve, each pair of juxtaposed viewing zones having a boundary therebetween, the uppermost viewing zone having a first constant dioptric focal power and being adapted for viewing distant objects over the full width of the multifocal ophthalmic lens, and each of the remaining viewing zones being divided into at least three areas transversely of the principal meridional curve, a central area in each of the remaining viewing zones being of a constant dioptric focal power higher than the constant dioptric focal power at the principal meridional curve in the viewing zone juxtaposed thereabove, and the two outer areas in each of the remaining viewing zones being of such aspherical curvature that at a boundary with an adjacent area of different dioptric focal power the relative heights of the viewing zones forming the boundary are constant along the boundary throughout the width of each outermost area and each have a surface curvature substantially in accordance with the equation a2z =0 ax ay where y and x represent horizontal and vertical directions respectively defining a plane, and Z is the height of the curved surface from this plane, the surface comprising a portion of a surface of revolution whose axis of revolution is vertical and lies in a common plane with the optical axis of the lens, whereby optically to compensate for skew distortion so that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes to permit a wearer of the lens to perceive horizontal and vertical lines in the visual environment as being horizontal and vertical, and the boundaries between the viewing zones being blended so that the boundaries are substantially invisible.
2. 2. A multifocal ophthalmic lens as claimed in Claim 1, in which the said predetermined law defines a vertical magnification at each point along the principal meridional curve, the vertical magnification being substantially constant throughout the entire width of the lens along lines perpendicular to said principal meridional curve.
3. 3. A multifocal ophthalmic lens as claimed in Claim 1 or Claim 2, in which the

   magnification in the two outermost of the three areas is equal to the magnification of the uppermost viewing zone.
4. 4. A multifocal ophthalmic lens as claimed in any of Claims 1 to 3, in which there are three viewing zones.
5. 5. A multifocal ophthalmic lens as claimed in any preceding Claim, in which an additional area is interposed, in each of the said remaining viewing zones, between the central area and each of the two outermost areas, the refractive surface in the additional areas being aspherical and providing optical blending between the central area and each of the two outermost areas whereby the wearer of the lens perceives a smooth transition when scanning his line of sight laterally from the central area toward one of the outermost areas.
6. 6. A multifocal ophthalmic lens as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.