Features

Progressive power lenses - part 8

In the eighth part of our comprehensive series discussing progressive spectacle lens design, Professor Mo Jalie looks at some of the less common designs to be patented, some of which may well appear in the future

Patents have been taken out in recent years by several major lens manufacturers disclosing progressive designs of unusual configuration. Some simply embody improvements in the design and manufacture of the lenses. For example, US Patent 8550622:20131 assigned to Hoya Lens Manufacturing in the Philippines, which allows for input of the intermediate centration distance (CD) for a given working distance, obtained either by means of optometric instrumentation such as the Nidek Accommodator AA-2000, or the Grand Seiko Autorefractor GR-2100/GR-3100K. In the absence of such instrumentation it is suggested that the intermediate CD be calculated from a knowledge of the distance CD and the intermediate working distance.

Other recent patents disclose novel features relating to the purpose of the lens and it is these which are described in more detail in this part of the series. Some of these designs are not yet in production but may appear in the near future if there is a demand from the marketplace.

Prism controlled progressive lenses

A patent for a prism controlled progressive lens2 in which horizontal prism of ever increasing strength, either base in or base out, can be obtained between the distance and near portions. The object of the patent is to provide horizontal prismatic effect of at least 2Δ in strength, the prism being defined as the difference between the horizontal prismatic effect, if any, at the distance reference point, D, and the horizontal prismatic effect at the near reference point, N. The patent mentions that the prism power should lie within the range 2Δ to 5Δ and preferably be of the order of 2.5Δ to 3.5Δ. This amount of prism, the patent suggests, if ordered base in would be helpful to subjects who exhibit symptoms of exophoria at near, whereas, in the case of subjects who exhibit esophoria at near, base out prism would be helpful to relieve symptoms.

The patent suggests that the invention might also be useful for myopia control in young people. A result of studies undertaken by the applicants on slowing down progress in myopia has shown that prescribing only an addition of power in the correcting lens is not effective for all subjects with myopia. They point out that in near vision both accommodation and the convergence may be overworked. One aim of the invention is to provide a mechanism whereby both accommodation and convergence can be balanced simultaneously in order to prevent or, at least slow down the onset of myopia. The patent also suggests that in cases of presbyopia, the addition of base in prism to the near portion may offer relief from convergence insufficiency.

Figure 1 illustrates cross-sectional views through the horizontal meridian of a prism controlled progressive lens which incorporates base in prism for the right eye. For simplicity, the cross-sections through the lens shown on the right-hand side assume the front and back surfaces in the distance portion are plane and viewed in plan elevation when the gradual increase in thickness is seen to occur on the nasal side. It can be seen that the eye would encounter a gradual increase in thickness as it rotates downwards through the intermediate portion until it reaches the near zone of the lens.

Figure 1: Horizontal sections to show progressive addition of horizontal prism in the lower portion of a PPL

Figure 2 shows how the prism increases from the distance reference point, D, to the near reference point, N, as the eye moves down the lens. The prism power profile indicates that there is no prismatic effect in the distance portion, the prism power increasing to 3.5Δ in the near portion. The power profile down the lens, not shown, would of course, depend upon the near addition.

Figure 2: Progressive addition of horizontal prism in the intermediate and near zones of a PPL (Essilor)

Progressive lenses designed to reduce the risk of falls

Several papers have appeared in recent years which discuss the increased risk of falling by those who wear bifocal and progressive power spectacles.3 In the main, the authors point to the lack of clear vision in the lower portion of the lens resulting in subjects stumbling on stairs or other obstacles at floor level. Indeed, in the reference quoted, it was concluded that less risk of falls would be secured if subjects were prescribed multifocals with just an intermediate addition. This would provide the ability to see both the ground and occasional, if short-term, near vision. It will be recalled that, in the past, one advantage claimed for ribbon bifocal segments was that they provided clear distance vision beneath the segment for ease of negotiating stairs.

A patent for a progressive lens in which the power increases to provide the full near addition at a point some 12mm below the fitting cross position, and then decreases, to provide a zone beneath the near portion which allows clear vision at intermediate distances between one and one-and-a-half metres.4 The purpose of the design is claimed in the patent to assist with negotiating stairs and to reduce the risks of falls. Figure 3 illustrates the four different zones of the lens and the power profile for the design for a lens of power plano, add +2.00 D. It is seen that the power increases from the distance portion, through the first progressive zone to the near portion which lies some 12mm below the fitting cross. The fitting cross itself lies some 4mm above the geometric centre of the lens. At the near reference point the lens provides the full near addition of +2.00 D. On moving further down the lens, the power then degresses to a value of about 50% of the full near addition, +1.00 D in this example, to provide an intermediate zone at the bottom of the lens through which subjects should be able to see obstacles on the ground at their feet thus avoiding trips and falls.

Figure 3: Progressive lens with intermediate zone beneath near portion (Essilor)

Figure 4 illustrates the power profile for the lens Plano Add +2.00 and indicates how the addition power varies as the eye rotates along the meridian line. In the distance zone of the lens the mean addition power remains constant until about 8mm above the geometric centre (the prism reference point) when it starts to increase. At the position of the fitting cross, marked FC in figure 4, the addition has increased to about +0.25 D and at the prism reference point, G, to a value equal to

(A - 0.50) / 2

where A is the full near addition. For the design illustrated in figure 4 where the full near addition is +2.00 D, the addition at the prism reference point is +0.75 D.

Figure 4: Power profile for progressive lens with intermediate zone beneath near portion (Essilor)

Below the geometric centre of the lens, the addition increases to its full value at the near reference point, N, some 8mm below the point G. The near portion where the addition is within 0.25 D of the prescribed addition is thus a narrow strip some 6mm in depth centred on the point N. Below N, the addition power decreases by about one-quarter of its full magnitude to a point E which lies 5mm below N, where the addition indicated in figure 4 has reduced to +1.50 D.

Eventually, at a point some 15mm below the prism reference point the addition power has reduced to about 50% of its full value, here to +1.00 D.

The patent suggests that progressive lenses which have the feature of an intermediate zone beneath the near portion are only necessary for the addition range starting at +2.00 D, eg adds +2.00 to +4.00 D.

Progressive lenses which fully correct astigmatism in near vision

It is a well-known fact that except in cases where the eye cannot accommodate, eg aphakia, the cylinder which corrects an eye for distance vision does not correct the eye for near vision. Owing to the forward position of the spectacle lens in front of the eye, the astigmatism in the pencil leaving the lens is not the same as the astigmatism in the pencil reckoned at the eye’s first principal point.

For example, in the case of the spectacle prescription +5.00/+2.00 x 90 mounted 15mm in front of the eye’s first principal point, where the astigmatism measured in the spectacle plane is +2.00 DC x 90, the astigmatism in the pencil arriving at the eye is +2.41 DC x 90, which represents the actual astigmatism possessed by the eye (figure 5). Tracing a paraxial pencil from the near point at 33.33cm in front of the lens (which is assumed to be thin), the astigmatism in the pencil arriving at the eye is +2.20 DC x 90, an under correction of almost +0.25 DC x 90.

Figure 5: Ocular astigmatism in DV and NV for +5.00/+2.00 x 90

In order to fully correct the ocular astigmatism, the astigmatism in the pencil arriving at the eye should be +2.41 DC x 90. Assuming that the eye can cope with the minimum accommodative demand which arises in the vertical meridian, the vergence arriving in the horizontal meridian should be (+2.06 + +2.41) = +4.47 DC x 90. This represents a vergence leaving the lens along the horizontal meridian of +4.19 D, so the power of the lens which fully corrects the ocular astigmatism for near vision at -⅓ metre is +5.00/+2.19 x 90. It can be seen that the distance vision cylinder must be increased to fully correct the ocular astigmatism in near vision.

The difference between the cylinder power for distance and near vision can be found from the simple approximate relationship5

Increase DV cylinder by -2d(L + A)/10%

where d is the distance from the lens to the eye’s first principal point, L is the dioptral distance of the near point from the lens and A is any prescribed near addition. In the example illustrated in figure 5, d = 15mm, L = -3.0 D and A = 0, since no near addition has been given. Thus the increase required for the near vision cylinder is -2 x 15(-3.0) /10 = 9%. Nine percent of +2.00 D is +0.18 D which is very close to the result obtained by ray tracing.

Clearly, a change in cylinder power for near is usually only necessary for medium to high power cylinders. Even so, the changes required are not dissimilar to those required for a compensated prescription, one whose verification power differs from the ordered power owing to changes in the as-worn pantoscopic and/or face form angles. Since free form manufacturing methods enable surface powers to be produced in one-hundredth dioptre intervals when necessary, it is easily possible to adjust the near vision power on a free form progressive surface. There may also be a need to alter the axis direction of the near vision cylinder some eyes exhibiting a measurable cyclorotation upon depression and convergence of the visual axes in near vision.

Some major lens manufacturers now offer to provide spectacle lenses in which the cylinder power and/or the axis direction in the near portion is different from that in the distance portion. Although this specialised form of correction has been possible for many years with bifocal lenses, free form production now makes it possible to provide different cylinder powers or axis directions for distance or near in progressive lens forms. A patent for the determination of an ophthalmic lens for a wearer, for whom near vision and far vision prescriptions have been made out, the near vision astigmatism being different from the distance vision astigmatism was assigned to Essilor as long ago as 2007.6

Progressive lenses with prioritised zones

Several modern progressive designs are offered in different forms which offer wider fields of view in particular zones of the lens. For example, progressive lenses whose progressive surface is ‘hard’ in design, generally with shorter corridor lengths, will offer a wider clear distance zone with less Minkwitz astigmatism than ‘soft’ designs, which generally possess longer corridor lengths and which aim to increase the intermediate and/or near zones of vision. Examples of the first are marketed under trade descriptions such as ‘Driver’, ‘Outdoor’, ‘Road’, ‘Clear’, ‘Freeway’, etc.

Typical differences in the distribution of Minkwitz astigmatism for a progressive design suitable for driving (figure 6a) compared to a design for more general use (figure 6b) are illustrated in figure 6.

Figure 6: Minkwitz astigmatism for modern progressive designs

The US 66853167 patent, assigned to Rodenstock which discusses prioritised zones also discloses a cosmetic improvement of the appearance of the lens by employing a toroidal surface with the steeper curve along the longest meridian of the lens shape. For example, most modern spectacle frames have a greater A-dimension (horizontal box dimension) than the B-dimension of the shape (vertical box dimension) as depicted in figure 7a.

Figure 7b illustrates a progressive lens whose nominal DP curves are shown where the shapes of the curves in the upper portion of the lens are essentially spherical. The power of the distance portion is -2.00 D and the front curve in the DP is +4.00 D. In Figure 7c the shape of the front surface in the upper portion is toroidal with curves +4.00 DC x 180/+6.00 DC x 90, the steeper +6.00 D curve running in the horizontal meridian of the lens. This means it will reduce the overhang of the front surface from the front rim of the frame when compared with the horizontal spherical curve which is only +4.00 D. Naturally, the astigmatism introduced by the convex toroidal surface has to be neutralised by a concave toroidal surface worked on the back whose nominal curves are seen in figure 7c to be -6.00 DC x 180/ -8.00 DC x 90. The DP power of the lens remains -2.00 DS.

Figure 7: Use of a combined atoroidal/progressive surface to equalise edge thickness in a modern frame

Needless to say, if the progressive surface is to be combined with the toroidal surface on the back of the lens, the concave surface would become a free form atoroidal, progressive surface. Figure 8 taken from the patent compares the iso-cylinder and mean power plots for the two designs, one with a nominally spherical front surface and one with a nominally toroidal front surface and it is seen that there is no discernible difference in the optical performance of each design.

Figure 8: Iso-cylinder and mean power plots for progressive lenses from US Patent 6 685 316 (2004)

Progressive designs for distance and intermediate use

A further patent assigned to Rodenstock8 discloses a progressive lens designed for distance and intermediate vision but without an area for a full near vision addition being provided. In principle its modus operandi is similar to that of a degressive power lens described in part six of this series except that particular attention is paid to the requirement that the upper portion of the lens provides a wide area for distance vision and the intermediate zone provides a wide zone of clear vision down to about one metre from the subject. There is no near portion on the lens.

Consider the prescription +0.50 add +1.50 for near which is to be dispensed using lenses of this invention. The power of the upper distance portion of the lens is +0.50 D and the power of the lower intermediate portion is +1.25 D, it being assumed that an intermediate addition of 50% of the full near addition has been supplied. Figure 9a shows an iso-cylinder plot for a progressive lens made to the prescription +0.50 add +0.75 given in the patent with the astigmatism exhibited by the lens plotted in 0.25 D intervals.

Figure 9: Comparison of iso-cylinder plots for progressive lenses from US Patent 7033022 B2 (2006)

The iso-cylinder plots are given for the through power, eye plus lens, rather than just the surface astigmatism. Figure 9b shows an iso-cylinder plot for a progressive lens made to the same specification +0.50 add +0.75 according to the patent. It can be seen that the new design has a wider distance field where the Minkwitz astigmatism is less than 0.25 D and narrows down to a funnel shape in the intermediate zone, rather than the hourglass shape of a typical progressive lens.

The uncut diameter shown in figure 9 is 60mm and a spectacle lens with a typical shape and size has been superimposed upon the uncut. It can be seen that the astigmatism within the cut lens shape is very small only reaching 0.75 DC in the lower nasal side of the lens, which is unlikely ever to be used by the wearer.

Whether any of the designs described above will ever be brought into production will depend upon the manufacturer’s perceived demand from the marketplace. It is certain that the development of new progressive lens designs will continue, with the best features which emerge with each new generation being combined into the latest state-of-the-art designs.

Professor Mo Jalie is a visiting professor at Ulster University and author of the new edition of Principles of Ophthalmic Lenses

References

1 Mori T, et al, (2013) US Patent 8550622 B2 Progressive power lens and progressive power lens design method.

2 Poulain I, et al, (2007) US Patent 7216977 Ophthalmic lens with progressive addition of power and prism.

3 Elliott DB, et al, Intermediate addition multifocals provide safe stair ambulation with adequate short term reading, Ophthalmic Physiol. Opt. 2016 Jan; 36(1) 60-8, Epub 2015 Aug. 25.

4 Giraudet G, et al, (2011), US Patent 8061838 Progressive lens for ophthalmic spectacles having an additional zone for intermediate vision.

5 Jalie M. (2016) Principles of Ophthalmic Lenses, 5th Edition, ABDO, Godmersham.

6 Donetti B. et al, (2007), US Patent 7249850, Method for determination of an ophthalmic lens using an astigmatism prescription for far sight and for near sight.

7 Baumbach P, et al, (2004), US Patent 6685316 Method of Manufacturing Progressive Ophthalmic Lenses.

8 Dorsch R, et al, (2006), US Patent 7033022 B2, Progressive spectacle lens for seeing objects at a large or average distance.