GB1569765A

GB1569765A - Progressive power ophthalmic lens

Info

Publication number: GB1569765A
Application number: GB4560276A
Authority: GB
Original assignee: American Optical Corp
Current assignee: American Optical Corp
Priority date: 1975-12-08
Filing date: 1976-11-02
Publication date: 1980-06-18
Also published as: HK22481A

Description

(54) PROGRESSIVE POWER OPHTHALMIC LENS (71) We, AMERICAN OPTICAL CORPORATION, a corpdration 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 is related to ophthalmic lenses in general and is more particularly concerned with novel progressive power and multifocal ophthalmic lenses in which the distortion is either so controlled that a wearer perceives vertical lines as vertical throughout most of the viewing area of the ophthalmic lens or the degree of distortion is severely diminished.

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 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 accomodation 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 distant 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 distances 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 accommbdation 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 distance viewing zone and the reading zone which progresses in dioptric focal power from that of the 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. Furthermore, all intermediate focal powers between the distance and reading portions are provided such that the wearer is able to perceive objects at any distance clearly through a portion of this intermediate zone. Such a lens is known commonly as a progressive power ophthalmic lens. An excellent survey of such lenses was provided by A. G. Bennett in the October and November 1970, and February and March 1971 issues of The Optician. In this work, the various attempts are discussed which have been made to provide such progressive power ophthalmic lenses by various scientists and optical engineers over approximately the last 70 years.

All progressive power lenses of the prior art have suffered from at least one common failing. As a necessary concomitant of an aspherical surface such as is found in the progressive power lenses, a certain amount of astigmatism and distortion is inherently found in the refractive surface, particularly in the peripheral portions of the transitional zone. The 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 such ophthalmic lenses to become nauseated and has definitely prevented the wide acceptance of this type of eyewear.

Furthermore, the astigmatism 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.

Thus distortion, like astigmatism, is an inevitable consequence 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 centered on the scheme by 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, at a minimum, reduced the size of the reading zone below that which allows the wearer to read standard material without turning his head.

In our British Patent No. 1,484,383 (Application No. 35984/74) there is described a progressive power lens in which a first viewing zone (preferably a central band across the lens) is separated into three laterally disposed areas a central one of which has a progressive power from top to bottom and the two outer areas of which have a surface curved so that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes so that a wearer of the lens perceives horizontal and vertical lines through these areas as being horizontal and vertical. The present invention constitutes a modification of the above, in which different optical parameters are used t9 obtain the same effect.

According to the present invention there is provided an ophthalmic lens comprising a lens body having a first viewing zone with a smooth, unbroken principal meridional curve of continuously varying slope which, with the lens in an intended orientation of use extends along the first viewing zone in a generally vertical direction separating the first viewing zone into two similar lateral portions, the curvature of the principal meridional curve varying progressively from point to point therealong to provide a predetermined dioptric focal power at each such point according to a predetermined law, the dioptric focal power increasing generally from top to bottom of the first viewing zone along the principal meridional curve, and the curvatures of cross curves defined on the first viewing zone by planes perpendicular to the principal meridional curve, at their points of intersection with the principal meridional curve, being in each case equal to the curvature of the meridional curve at the respective point of intersection, the first viewing zone being defined by a power range varying from a first dioptric focal power at the top of the first viewing zone to a second, higher dioptric focal power at the bottom thereof, the first viewing zone being divided into at least three laterally disposed areas a first of which is centrally disposed in the viewing zone and the other two of which, the two outer areas, are disposed at the lateral peripheries of the first viewing zone, each having a surface which comprises a portion of a surface of revolution the axis of revolution of which is vertical and lies in the meridional plane whereby the surface at the lateral periphery of the lens is so curved, to compensate optically for skew distortion, that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes whereby to permit a wearer of the lens to perceive horizontal and vertical lines in the visual environment as being horizontal and vertical, and a second viewing zone in vertical juxtaposition to the first viewing zone, the second viewing zone having a constant dioptric focal power throughout, there being a downwardly positive discontinuity in dioptric focal power (as hereinafter defined) of less than 0.5 diopters at the boundary between the two viewing zones.

A downwardly positive discontinuity means a discontinuity which is in a positive sense as one moves downwardly along the principal meridional curve.

One embodiment provides a progressive power ophthalmic lens as above but including a third viewing zone in juxtaposition to the second viewing zone and on the side thereof remote from the first viewing zone, the third viewing zone having a substantially constant dioptric power no greater than that of said second viewing zone and all adjoining boundaries of said first, second and third viewing zones and said viewing areas of said first viewing zone being smoothly surface blended.

Thus, in accordance with the present invention the two outer of the three areas of the first viewing zone of a lens such as that defined in our British Patent Specification No. 1,484,383 (Application No. 35984/74) are corrected for skew distortion by each having a surface which comprises a portion of a surface of revolution the axis of revolution of which is vertical and lies in the meridional plane of the lens. It is an advantage of lenses made according to the present invention that the swimming or rocking of images viewed through the peripheral portions of the lens is eliminated.

Another advantage of embodiments of the invention is that the reading portion of the lens is conveniently large so that normal reading habits may be maintained by wearers of the lenses.

Lenses made as embodiments of the invention have a high degree of freedom in design parameters such that the design may be adapted to a wide variety of different specific ophthalmic configurations. Moreover, ophthalmic lenses according to the invention are relatively simple in construction and capable of large quantity manufacture.

Further objects, advantages, and preferred features of the invention will be apparent in the arrangement and construction of the constituent parts, as described in the following detailed description with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a prior art progressive power ophthalmic lens; Figure 2 is a vertical sectional view of the progressive power ophthalmic lens of Figure 1 taken along the principal vertical meridional curve; Figure 3 is a front elevation view of a progressive power ophthalmic lens showing the various viewing zones and the associated power law; Figure 4 is a schematic illustration of the technique for generating the progressive power surface of Figure 3; Figure 5 is a perspective view of a progressive power ophthalmic lens the intermediate and near vision portions of which are divided laterally into a plurality of areas in the same way as the lens of British Patent Specification No. 1,484,383 (Application No. 35984/74), the outermost of which areas are totally corrected for skew distortion in accordance with the present invention Figure 6 is a schematic diagram of a symmetrical progressive power lens which has been rotated 10 degrees from the vertical to accommodate for decreasing interpupilary spacing when viewing closer objects; Figure 7 is a schematic diagram of a matched set of progressive power lenses which compensate for the 10 rotation required; Figures 8A, 8B and 8C are exemplary power law diagrams illustrating the use of discontinuities; Figure 9 is a table illustrating the effect of power law discontinuities; Figure 10 is an illustrative diagram of the image of a square grid as viewed through a prior art progressive power lens having no finite power discontinuities at the boundaries of the intermediate viewing zone.

Figure 11 is an illustrative diagram of the image of the same square grid as viewed through a progressive power lens according to the invention having finite power discontinuities at the boundaries of the intermediate viewing zone; Figure 12 is a front elevation view of a progressive power ophthalmic lens in which finite power discontinuities at the boundaries of the intermediate viewing zone are blended to render them invisible.

Figure 13 is a front elevation view of a lens mold or block showing the concave surface of the mold or block; Figure 14 is a diagram of a square grid as viewed through a lens of the present invention which, when compared with Fig. 11, illustrates a correction of skew distortion and smoothing according to the invention; and Figure 15 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 in Fig. 14.

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 prior art progressive power 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. Progressive power 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 progressive power ophthalmic lens 10 is shown fixed in space 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 centre 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.

If the progressive power ophthalmic lens 10 is to work properly, the principal meridional line 18 must be continuous and it must have continuously varying slope.

The first condition ensures that there will be no visible discontinuity in the surface of the lens at the principal meridional line. The second condition ensures that there will be no image jump as the wearer's line of sight moves vertically along the principal meridional line. In order to provide for progressive accommodation in the lens, the curvature of the principal meridional line 18 increases in a downwardly positive manner from a far vision value near the top of the lens to a near vision value near the bottom. Depending upon the requirements of the particular design, the amount of dioptric focal power addition between the upper and lower limits (commonly known as "add") may vary appreciably. The absolute amount of dioptric focal power addition is variable and is dependent upon the retained powers of accommodation on the part of the wearer. The rate of addition along the principal meridional line 18 is also variable. In other words, the transitional dioptric focal power may be introduced over a very short portion of the principal meridional line or it may be introduced over essentially the entire length of the principal meridional line.

In general, it is preferred that the 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 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 illuminated by rays of light striking the surface at oblique angles of incidence.

Along the vertical 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 Po = r Referring now to Figure 2, there is shown a sectional view of the prior art lens 10 taken along the principal vertical meridional plane 16. The locus of the centers of curvature of the principal meridional line 18 comprises a continuous plane curve 22 called the evolute of the principal meridional line which is also located within the principal meridional plane. To each point Q of the principal meridional line 18 there corresponds a point q on the evolute. The radius vector 20 connecting any two such points is perpendicular to the principal meridional line 18 at Q and tangent to the evolute curve 22 at q.

A typical and particularly useful known form of progressive power ophthalmic lens incorporating the foregoing principles is shown in Figure 3. The lens 30 consists of three vertically disposed viewing zones 32, 36, and 34 respectively. Here again, a principal meridional line 18 bisects the lens in a generally vertical direction. The uppermost viewing zone of the lens 32 is formed with a constant dioptric focal power which accommodates vision to distant objects, i.e., the surface in viewing zone 32 is spherical. The lowermost viewing zone 34 of the lens is again of constant dioptric focal power and accommodates the vision to nearby objects.

Interposed between viewing zones 32 and 34 is an intermediate viewing zone 36 having progressive power which provides a gradual optical transition between viewing zones 32 and 34. In other words, the dioptric focal power varies continuously over a range from a first dioptric focal power at the top of the intermediate viewing zone to a second, higher dioptric focal power at the bottom of the zone. This is consistent with the requirement that the dioptric focal power increase generally from top to bottom of the progressive power ophthalmic lens along the principal meridional curve.

The height of the intermediate portion of the lens along the meridional curve is identified as h. The graph at the right of Figure 3 is known as the "power law" of the lens 30. The power law in this instance consists of three linear portions 38, 40, and 42 which are respectively associated with the lens viewing zones 32, 34, and 36 respectively along the principal meridional line 18. The portion 38 represents the constant dioptric focal power in the viewing zone 32 and the portion 40 represents the constant dioptric focal power in the viewing zone 34, the constant dioptric focal power in the portion 40 being of a greater magnitude than that in the portion 32.

The sloping portion 42 of the power law defines that the dioptric focal power through the intermediate area 36 changes at a constant rate. This is a typical type of power law often utilized in prior art progressive power ophthalmic lenses. Of course, the height h is variable and may be increased to the full height of the lens.

The power law shown in Figure 3 is linear through the progressive power viewing zone. The power law need not be linear and may be of any aribitrary character as required by a particular application. It should, however, be a continuous curve through the progressive power viewing zone.

The basic construction of the known progressive power surface of Figure 3 is shown in Figure 4. This construction does not incorporate the novel features of the present invention which will be described herebelow. The progressive power refractive surface is generated by a circular arc C of variable radius and constant inclination which passes successively through all points Q of the principal meridional line. The axis aa' of the generating circle lies in the principal meridional plane, and makes a constant angle with the vertical. The radius vector Qq defines point q on the evolute for a given point Q of the principal meridional line. The radius QR of the generating circle passing through a given point Q is determined by the condition that its axis aa' pass through the corresponding point q of the evolute 22. The radius of the generating circle equals the length of the line segment QR of Figure 4.

It can be shown that, as a consequence of this construction, the principal curvatures at each point of the principal meridional line are equal. In other words, the surface is umbilic (free of astigmatism at the principal meridional line).

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. Supppose 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 non-equilateral 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 prior art progressive power and variable ophthalmic lenses were either totally uncorrected for skew distortion, resulted in only a partial correction for skew distortion, or resulted in a reading area too small for general use.

The astigmatism present in a refractive surface, for a linear power law, varies laterally at twice 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 ophthalmic lens utilizing progressive power, considerable distortion is necessarily present in the surface. For example, in the form of progressive lens shown in Figure 3 of the drawing, the principal axes of astigmatism form a 45" angle with respect to the horizontal and vertical lines of the visual environment throughout the intermediate area 36. Therefore, these lenses give rise to substantial amounts of skew and normal distortion in the peripheral areas of the intermediate area 36.

As has been stated above, and as is the case with astigmatism in such progressive power refractive surfaces, 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 progressive power lens forms one of the principal features of the present invention.

Referring now to Figure 5 of the drawing, there is shown a progressive power ophthalmic lens 50 having viewing areas divided up in the same way as in the lens described in British Patent Specification No. 1,484,383 (Application No. 35984/74) and which is corrected for skew distortion in accordance with 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 zone 58 which is disposed between the near and far viewing zones 54 and 56, respectively, provides progressive transitional dioptric focal power therebetween.

For the purposes of the present specification the lens 50 will first be described, incorporating some of the features of the invention, but as it would be if the dioptric focal power were continuous across the boundary between the upper, far viewing zone, and the intermediate zone 58, and across the boundary between the intermediate zone 58 and the lower, near viewing, zone 56. The beneficial effects due to the provision of discontinuities in the dioptric focal power will axes of astigmatism in the peripheral areas CDE and C'D'E' are in vertical and horizontal planes. In other words, the distortion in the centre is pure skew distortion and in the periphery, pure normal distortion. The areas of blending ABCD and A'B'C'D' have aspherical surfaces which serve to transform the orientation of the principal axes of astigmatism between the other areas smoothly so that discontinuities are not introduced into the surface or image. The width of the areas of blend is, however, variable and may as a limiting case be reduced to zero. In other words, the purview of the invention extends to those cases where there are only the central and peripheral areas present in the refractive surface.

It is also within the purview of the invention that the peripheral areas CDE and C'D'E' may actually fall outside the area of the lens, in which case the blending areas ABCD and A'B'C'D' become, in effect, the peripheries of the lens. The areas CDE and C'D'E' serve the purpose of providing a basis for defining the form of the areas ABDC and A'B'D'C'. The lens having this form does not completely correct for skew distortion in the peripheral areas of the lens. The effect of skew distortion is, however, softened in comparison with that found in the lens of Fig. 3. The advantage of such a lens is that it can be made to have only monotonically changing curvatures. This means that, although skew distortion is not entirely corrected, the lens nevertheless shows a smooth lateral optical effect The 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) (1) when y and x are the horizontal and vertical directions respectively and Z is the height of the surface from the xy plane, i.e., Z can be represented by: Z = f(x,y) = g - (r2-u2-v2)1'2 (3) where:

u=rQ (5) xdx Q= I (6) o r r0 = distance viewing zone meridional radius, r = meridional radius of curvature, and v = various mathematical expressions dependent upon the portion or area of the lens surface being described.

The lens surface of the present invention is divided into viewing zones and areas within some of those zones. In the upper half circle of the lens body, the farvision or distance zone is provided. In accordance with the mathematics of the specification where r is the meridional radius of curvature, in this zone r = rD and v = y.

In the intermediate zone of height h (the progressively-varying optical power zone),

(r= rD + bx) where

b=(l)(l il and where rR is the reading or near vision meridional radius. Thus, in the near vision zone, r = rR By way of further explanation, in the intermediate and near vision zones, there are three mathematical expressions for v, depending on the area. In the area bisected by the vertical meridional line, v=y. In the outermost areas from the vertical meridional line, v is expressed as:

And in the two "blending" areas that lie between the outermost areas and the central area, v is expressed as:

where: 2 (y2 y1) 3 (Y2+Y1) 3 (y2-y1) ~ 1 (v2+yi) m=- (Y2 - (y2 + i'y1y2+y22) n = If the surface has the condition that the directions of principal curvatures, the principal axes of astigmatism, at all points lie in planes which are parallel to the x and y axes, then 92f =0. (2) 0. (2) axay When this expression is satisfied for all points in a given area, only pure normal distortion may be perceived through the lens.

This partial derivative expression can be alternatively viewed as ( Fx) (y) f(x1y) (7) 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 equatioin (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 coordinates axes respectively, equation (1) can be separated into: Z = f(x,y) = ft(x) + f2(y) (8) where ft(x) is a function only of the x variable and f2(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 =0 and af, (x) = 0 and = 0 (9) ax ay Thus the zero condition of equation (2) implies this cylindrical axial orthogonality condition which provides zero skew distortion.

Thus far, the foregoing discussion has considered only those lenses that are symmetrical about a vertical principal meridional line, that is lenses in which the principal vertical meridional line is in fact vertical on the surface of the ophthalmic lens when the lens is in the normal or intended orientation of use, and divides 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 the progressive power ophthalmic lenses separately for the left and right eyes respectively. The resulting lenses are asymmetrical since the interpupilary 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, once the lens has been rotated accordingly, the principal axes of astigmatism in the periphery of the symmetrical, skew distortion corrected lens described hereinabove are no longer aligned with the horizontal and vertical elements of the visual environment. Therefore, 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 6, 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 7 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, however, 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 the glass plate from that which comes into contact with the ceramic block may be used to form a mold surface for casting plastic lenses according to the present invention.

This casting technique has numerous advantages, not the least of which is that a lens comparable in price to present glass ophthalmic lenses may be produced.

However, additional advantages inhere in the process due to the fact that the glass sheet slumped into the ceramic block has finite thickness. The finite thickness tends to blend any local discontinuities which may exist in the surface such as are produced between adjacent cuts of the grinding tool in the generating machine.

The resulting lens has a smooth optical quality refractive surface thereon.

It can be shown that, when the power law within the intermediate progressive power viewing zone is linear, i.e., a constant rate of addition, as is the case for the power law of the progressive power lens shown in Figure 3 of the drawing, the astigmatism increases with perpendicular distance from the principal vertical meridional line at twice the rate of add of dioptric power along the principal vertical meridional curve. Thus, if the add is B and the intermediate area is of height h, then the astigmatism A at a distance lyl from the meridional line is given by B A=2-- IYI h The "corridor of clear vision" is defined as that region of the intermediate viewing zone bounded on the right and left sides by lines having one diopter of astigmatism.

(It is known that 1.0 diopters of astigmatism reduces visual acuity by approximately one-half). For example, if B equals 2.OD and h equals lOmm, then from the foregoing equation, the width w of the corridor of clear vision is 5.0mm. From this typical example, it is clear that a considerable price is paid for the feature of progressive power in the intermediate viewing zone. That is, the visual acuity in the intermediate viewing zone is very poor everywhere except through a narrow central corridor, the width of which is controlled largely by the height of the intermediate viewing zone and the rate of add.

The difficulty of having a narrow central corridor of clear vision can be at least partially relieved by combining the progressive power variation with finite power discontinuities at either or both of the boundaries separating the intermediate viewing zone from the far vision and near vision viewing zones. The solid curves shown in Figures 8A, 8B and 8C represent alternative progressive power laws incorporating such discontinuities. Figure 8A shows the meridional power law of a progressive power ophthalmic lens according to the invention which has a power discontinuity, i.e., ajump, at the upper boundary of the intermediate viewing zone, but no such discontinuity at the lower boundary. In Figure 8B, the situation is reversed with the power discontinuity occurring only at the lower boundary of the intermediate viewing zone. Figure 8C shows the meridional progression of power in a lens having finite discontinuities at both the upper and lower boundaries of the intermediate viewing zone. In each of these examples, the dotted line superimposed on the power law diagram corresponds to the power law of a progressive power lens having no power discontinuities at the boundaries of the intermediate viewing zone.

A simple inspection of Figures 8A-8C provides a clear indication that the power law discontinuities have the effect of reducing the rate of addition of dioptric focal power across the intermediate viewing zone. Therefore, by the foregoing relationship, the corridor of clear vision is thus appreciably widened. If the power discontinuities have magnitudes b, and b2, then, from the foregoing equation, the astigmatism inside the intermediate area will be given by (B-b1-b2) A = 2 try h Suppose that B once again equals 2.OD, h equals lOmm, and b, and b2 equal 0.5D each. Now the width w of the corridor of vision becomes l0mm. This is a 100 percent improvement in width over the case of a continuous power law previously described, i.e., where b,=b2=0. The table of Figure 9 gives the width of a corridor of clear vision for various total additions B and total discontinuous power variations (b+b2) as the top number in each block. The lower number is the percentage increase of w over the width associated with a continuous power law.

The magnitude of an individual power discontinuity should not be so great as to destroy the wearer's sense of visual continuity intended by the concept of progressive power. This criteria limits such individual discontinuities to less than 0.5 diopters.

The use of power discontinuities also helps to reduce the distortion through the intermediate viewing zone. Figure 10 shows the distortion of a square grid as viewed through a progressive power ophthalmic lens having a continuous power law. This image is similar to the grid which would be observed through a progressive power lens such as shown in Figure 3. On the other hand, Figure 11 shows the distortion of the same grid when viewed through a progressive power lens having finite power discontinuities at both the upper and lower boundaries of the intermediate progressive power zone. In both cases the power law within the intermediate viewing zone is linearly increasing and the total a-d is equal.

Obviously, the distortion of the grid within the intermediate viewing zone in the lens shown in Figure 11 is markedly less than that of the lens shown in Figure 10.

If a progressive power lens as defined in the prior art were to have incorporated therein finite power discontinuities, the price paid for such discontinuities would be the appearance of a ledge, i.e., a distinct break in the surface continuity extending across the surface of the lens at the level of the power discontinuity. If, for example, the axis of the generating circle as defined in Figure 4 is vertical, the ledge associated with a power discontinuity would make a horizontal line across the lens. The height L of this ledge grows approximately quadratically with distance y from the meridional line according to the following relationship I b L=~ y2 2 n-l For example, if b equals 0.5D and n equals 1.5, then at I y I equal to 25mm, L equals 0.62mm. If the cosmetic advantages of progressive power lenses are to be maintained, this ledge would then have to be blended into the viewing zones which it separates. This would be extremely difficult in the prior art progressive power lenses. However, with the process of sagging mold surfaces utilized in the present invention, the blending occurs automatically by virtue of the finite thickness of the sheet of glass utilized to form the mold surface. The resulting lens then contains blended areas 60 of rapidly changing power as indicated in Figure 12. These areas are not likely to be troublesome visually since they are adjacent to those portions of the intermediate viewing zone where the visual acuity is already severely diminished by surface astigmatism.

As previously stated, two types of advantages are obtained with lenses formed as embodiments of the present invention. These are continuous accommodations throughout the height of the lens, an optical advantage, and invisible dividing lines between the various viewing zones of the lens, a cosmetic advantage. It has been shown that a price is paid for the advantage of progressive accommodation along the principal vertical meridional curve, that is, in the intermediate viewing zone visual acuity is diminished severely except along a relatively narrow corridor of clear vision centred on the principal meridional line. If, however, the wearer is willing two sacrifice some of the advantage of progressive accommodation, it becomes possible utilising power discontinuities as discussed above substantially to widen the corridor of clear vision and still retain the cosmetic advantage which adheres to progressive power ophthalmic lenses.

Throughout the foregoing discussion 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, to be understood that in embodiments of the invention these various features are combined in order to provide optimal performance of a progressive power ophthalmic lens for particular ophthalmic requirements.

A detailied description of the design and manufacture of a progressive power lens having a grid distortion pattern and discontinuities at the viewing zone boundaries will now be given. The lens is to be made of plastics material cast in a mold made by the slumping process described above.

Therefore, in the manner of design, what is needed is a detailed description of the surface of the ceramic block on which the glass mold is to be slumped. The curvatures of the block must be such that the resultant mold produces the required lens. The rectangular coordinate system used is shown in Fig. 13. This view shows the concave surface of the block. The surface is tangent to the xy plane at the origin O. 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 meridional 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), and the lateral blending zone on the left hand side is bounded on the right and left by the curves y=y1(x) and y=y2(x). The radius of curvature of the spherical distance portion of the block is rD, that of the spherical reading portion rR. In general, the radius of curvature at a point x on the vertical meridian is given by r=r(x). The form of the surface of the block, expressed as an elevation z=f(x,y) above the xy plane, is given by the set of equations presented earlier, viz. Equation (3) and subsequent related expressions.

Restating these equations employing terms previously utilized and employing certain terms which further simplify the mathematics, we obtain:

where the functions Q, r and u have their previous meanings, and K=O in the central portion of the slumping block, K=y2-n in the peripheral portions of the slumping block, K=l(y-y,)3-m(y-y,)4 in the blend zones of the block where 1, m, and n are identical to that which was given earlier.

As noted in the foregoing paragraph these equations define the surface of the ceramic block against which the mold is slumped employing a process previously described and to be further detailed. 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 utilized to reduce the temperature in the oven from this value downward.

Another mathematical formulation based on surfaces of revolution can be employed to help conceptualise or understand the principles involved in the present invention. However, since these other mathematical formulations may not lend themselves readily to computer analysis, they are presented merely as a tool of insight and to provide another point of view.

To begin with, to correct for skew distortion in peripheral areas of the progressive power zone, 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.

The general set equations applicable to a required 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-(rD2-X2-y2)1/2.

For the central portion of the lower half of the lens or block, including both the intermediate and reading levels z = fo1(x,y),

where u = Qr, and

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

Z2 = toi(x1Y1)+ (rD2-y12)-(rD2-y22) (rD Y2 +0/2 (r rD)Yl(Y2 where For the blending areas between the central and peripheral areas, z=fg2 =Af01 +Bf23.

A = a(y2-y)3 + b(y2y)4 + c(y2-y)5, B = a(y-y,)3 + b(y-y,)4 + c(y-y1)5, 10 -15 6 and a= (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 back to Fig. I 1 in particular, it will be noted that, although the introduction of discontinuities in the meridional power law has reduced the level of aberrations in the intermediate area as compared with the aberrations illustrated in Fig. 10, there is still present a certain amount of skew distortion in the intermediate viewing zone. The skew distortion in the peripheries of the intermediate viewing zone can be turned into the less undesirable normal distortion in exactly the same way as it was accomplished for the progressive power lens with continuous meridional power law. The intermediate and reading levels of the surface of the lens of Fig. 11 are divided laterally into five contiguous areas just as shown in Fig. 5.

The central region remains as shown in the corresponding area of Fig. 11. The peripheral areas, however, are now formed in accord with the construction of Fig.

5, i.e., these areas comprise portions of a surface of revolution whose axis is vertical and passes through the center of curvature of the distance portion. Because of this construction, the principal axes of astigmatism in the peripheral zones lie in horizontal and vertical planes, thus ensuring that horizontal and vertical lines of the visual field will be perceived through those areas as being horizontal and vertical.

The areas between the central and peripheral areas serve to blend the latter areas smoothly into each other. Figure 14, which may be compared with Figure 11, illustrates this correction of skew distortion in the peripheries of the lens, and also the smoothing provided by the slumping process of the surface discontinuities at the upper and lower boundaries of the intermediate viewing zone.

An example of the design and manufacture of a lens of the type depicted in Figure 14 will now be given. The lens illustrated here is to be made of plastic, cast in a mold made by the slumping method. Therefore, a detailed description of the curvature of the ceramic slumping block is required. The geometrical layout of the various viewing and blending zones is as described above in relation to Figure 13.

The general equations for a lens with a discontinuous power law are as follows: 1 1 - = - distance zone r rD 1 1 1 1 6K1 (1 ? 6K1 6KR r0 +lr rB intermediate zone =-- reading zone rD where rD = radius of curvature of spherical distance zone = = radius of curvature of spherical reading zone K, = curvature jump at DP-IP boundary K2 = curvature jump at IP-RP boundary h = height of intermediate area, and it has been assumed that the meridional law of curvature is linear within the intermediate zone.

It is required to manufacture a CR-39 lens with distance portion with convex radius of curvature 83.33mm and with reading addition 2.00 diopters. Power jumps of 0.5 diopters will be introduced at the upper and lower boundaries of the intermediate area.

The mold will be slumped starting with a meniscus glass blank having surface powers of +6.00 and -6.37 diopters and having a centre thickness of 4.0 mm. It has been determined that in order to obtain a lens with the stated refractive characteristics and using a glass slumping blank of the above description, the starting parameters will have to be: rD = 88.113 mm = = 68.440 mm AK, = (0.815) 10-3 mm-" AK2 = (0.815) 10-3 mm-l h = 10 mm Thus, the meridional curvature law becomes 1 = 11.3491 x 10-3 (mm~') distance zone r =(12.1641 + 0.1632x) 10-3(mm-1) intermediate zone = (14.6113) 10-3 (mm-1) reading zone The lines y, and y2 are chosen to be straight lines having the following equations: y, = 7.00 + 0.21 x (mm) Y2 = 14.0 + 0.42 x (mm) These values of rD, rR, r, y, and y2 are now provided to a computer which has been programmed to compute the surface elevation of the ceramic block at 4 mm intervals over the area of a block .86 in diameter. The computations are performed using the simplified set of equations defined previously. Figure 15 shows the results of the computation.

Similar ophthalmic lenses are described and claimed in our U.K. Patent Specification No. 1,484,382 to which attention is directed.

WHAT WE CLAIM IS: 1. An ophthalmic lens comprising a lens body having a first viewing zone with a smooth, unbroken principal meridional curve of continuously varying slope which, with the lens in an intended orientation of use extends along the first viewing zone in a generally vertical direction separating the first viewing zone into two similar lateral portions, the curvature of the principal meridional curve varying progressively from point to point therealong to provide a predetermined dioptric focal power at each such point according to a predetermined law, the dioptric focal power increasing generally from top to bottom of the first viewing zone along the principal meridional curve, and the curvatures of cross curves defined on the first viewing zone by planes perpendicular to the principal meridional curve, at their points of intersection with the principal meridional curve, being in each case equal to the curvature of the meridional curve at the respective point of intersection, the first viewing zone being defined by a power range varying from a first dioptric focal power at the top of the first viewing zone to a second, higher dioptric focal power at the bottom thereof, the first viewing zone being divided into at least three laterally disposed areas a first of which is centrally disposed in the viewing zone and the other two of which, the two outer areas, are disposed at the lateral peripheries of the first viewing zone, each having a surface which comprises a portion of a surface of revolution the axis of revolution of which is vertical and lies in the meridional plane whereby the surface at the lateral periphery of the lens is so curved, to compensate optically for skew distortion, that at all points thereon the principal axes of astigmatism lie in vertical and horizontal planes whereby to permit a wearer of the lens to perceive horizontal and vertical lines in the visual environment as being horizontal and vertical, and a second viewing zone in vertical juxtaposition to the first viewing zone, the second viewing zone having a constant dioptric focal power throughout, there being a downwardly positive discontinuity in dioptric focal power (as hereinbefore defined) of less tha 0.5 diopters at the boundary between the two viewing zones.

2. An ophthalmic lens as claimed in Claim 1, in which the lens body has a third viewing zone, in opposite vertical juxtaposition to the first viewing zone from the second viewing zone, and having a constant dioptric focal power throughout which is equal to the dioptric focal power at the adjacent end of the range of dioptric focal power in the first viewing zone.

3. An ophthalmic lens as claimed in Claim 1, in which the lens body has a third viewing zone in opposite vertical juxtaposition to the first viewing zone from the second viewing zone, the third viewing zone having a constant dioptric focal power throughout, there being a second downwardly positive discontinuity in dioptric focal power of less than 0.5 diopters at the boundary between the first and third viewing zones.

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

Claims

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

1 = 11.3491 x 10-3 (mm~') distance zone r =(12.1641 + 0.1632x) 10-3(mm-1) intermediate zone = (14.6113) 10-3 (mm-1) reading zone The lines y, and y2 are chosen to be straight lines having the following equations: y, = 7.00 + 0.21 x (mm) Y2 = 14.0 + 0.42 x (mm) These values of rD, rR, r, y, and y2 are now provided to a computer which has been programmed to compute the surface elevation of the ceramic block at 4 mm intervals over the area of a block .86 in diameter. The computations are performed using the simplified set of equations defined previously. Figure 15 shows the results of the computation.

4. A progressive power ophthalmic lens as claimed in any of Claims 1 to 3, in

which the predetermined law defines a constant rate of change of dioptric focal power through the first viewing zone along the principal meridional curve.

5. A progressive power ophthalmic lens as claimed in any preceding Claim, in which the lens is made from a single piece of lens material of a preselected index of refraction and the said second viewing zone has a substantially constant predetermined dioptric focal power throughout and a surface so curved that the principal axes of astigmatism at all points thereon lie substantially in vertical and horizontal planes to permit a user of the lens to perceive horizontal and vertical lines in the visual environment as being respectively vertical and horizontal as viewed through the second zone.

6. A progressive power ophthalmic lens as claimed in any preceding Claim, in which all adjoining boundaries of the said first and second viewing zones and the said viewing areas of the first viewing zone are smoothly surface blended.

7. A progressive power ophthalmic lens as claimed in Claim 1 including a third viewing zone in juxtaposition to the second viewing zone and on the side thereof remote from the first viewing zone, the third viewing zone having a substantially constant dioptric power no greater than that of said second viewing zone and all adjoining boundaries of said first, second and third viewing zones and seid viewing areas of said first viewing zone being smoothly surface blended.

8. A progressive power ophthalmic lens as claimed in Claim 1 and substantially as hereinbefore described with reference to Figures 5 to 15 of the accompanying drawings.