50 years of developments in ophthalmic lenses

Without doubt, the most significant development in spectacle lens design is the change in the manufacturing process from lapping to computer numerically controlled milling methods usually referred to, simply, as freeform or digital surfacing. In its simplest description freeform surfacing refers to describing the surface by its sag heights which can vary at random but continuously across its entire surface and is produced by a milling process, the cutting tool being under the strict control of a computer. The first part of this paper is designed to remind you of the principles of design which have led to the modern personalized and optimized progressive power lens.

The greatest enemy in the design of a progressive lens is the presence of Minkwitz astigmatism, which, in brief, states that if a surface changes power between two points on the surface, the change in power is accompanied by surface astigmatism at right angles to the direction in the change in power, the astigmatism increasing at twice the rate of the change in power.

All lens designers are aware of this aberration, as shown by these pages illustrated in figure 1, taken from various patents which have appeared over the years.

The source of the astigmatism is illustrated in figure 2 which shows an eye using a zone in the intermediate portion of the lens having rotated x mm horizontally from the meridian line.

Due to the ever-increasing power of the progressive surface as the eye rotates downwards through the intermediate zone, it can be seen that the instantaneous power of the zone in use, F, at the top of the refracted pencil as it leaves the lens, differs from the power halfway down the zone which has increased by the increment, δF, which represents the amount of Minkwitz astigmatism. 

It can be shown¹ that the amount of Minkwitz astigmatism, δF, is given by 2 x A / h, where A is the full near addition and h is the length of the corridor from the distance reference point to the near reference point.

The first generation of progressive lenses suffered from enormous amounts of Minkwitz astigmatism due to the elephant’s trunk construction of the intermediate and near portions of the progressive surface as shown for the earliest commercially successful progressive lens, the Varilux design (1959) invented by Bernard Maitenaz^2,3.

First attempts to reduce effects of Minkwitz astigmatism

It was realized that by employing aspheric sections, rather than the circular sections which formed the intermediate and near portions of the surface, the effects of Minkwitz astigmatism could be massively reduced.

Effectively, the flattening of the sections reduced the near addition in the periphery of the intermediate and near portions a feat made possible by the use of varying conic sections across the lens.

Wearers could adapt to these second-generation progressive lenses, Varilux 2 (1972), far more easily than the first, and subsequently, progressive lens wear steadily became the norm for the correction of presbyopia (Fig. 3).

Another method of reducing Minkwitz astigmatism was employed in the early days of progressive lens design. It will be apparent from the Minkwitz rule that the longer the corridor, represented by the symbol h in the formula, the smaller will be the degree of astigmatism.

This fact was capitalized upon by a design from American Optical in 1989 who introduced a lens called the Truvision OMNI. The principle of the power change across the progressive surface is shown in the diagram of the front page of the patent 4 (Fig. 4).

As described in the patent information, the corridor length was increased to 38 mm from the more typical value of 16 mm, the effect being to more than halve the amount of Minkwitz astigmatism as shown by the 3D comparisons of the surface astigmatism with AO’s original progressive design.

This lens comparison was for a design of power Plano Add 2.00 and the astigmatism is seen to reduce from one-quarter of a diopter per horizontal movement of the eye from the center of the corridor to one-ninth of a diopter for each mm of movement (Fig. 5).

Most of the major lens manufacturers began to introduce their own progressive lens designs in the 1980s, for example, Zeiss⁵, Rodenstock^6,10, Seiko⁸ and Sola⁹.

Hard designs and soft designs

It was later realized that by allowing the astigmatism to extend into the distance portion it would widen the corridor and enlarge the near portion. This was the origin of the terms “hard design” and “soft design” which have been used to describe the design of a progressive lens (Fig. 6). Notice that the harder the design, the greater becomes the Minkwitz astigmatism.

Another improvement which has come with modern designs is that the power in the progression and near portions reduces as the eye rotates horizontally from the center to the edges of the lens.

This softens the design considerably, as is shown by the Iso-mean power plot shown in figure 6 on the right.

Note that the full addition of +2.00 D lies in the center of the near zone, reducing to just +0.50 at the edge of the zone (Fig. 7). Several major ophthalmic lens companies introduced designs incorporating this feature, notably Essilor International with their popular Varilux Comfort design^11,12.

Freeform surfacing – the next revolution

The next major breakthrough in progressive lens design, occurred towards the end of the 20^th Century with the introduction of freeform surfacing techniques. By the 1990s, the progression was worked on the convex surface and the final prescription obtained by incorporating the prescription on the concave surface of the lens. 

The 1995 patent granted to Kelch et al¹³ (Fig. 8) pointed out that if the prescription was the same, or very close to, the prescription for which the surface was designed, then the optical performance of the final lens would match the typical appearance of a well-designed progressive lens.

Suppose that the progressive surface of this design was optimized for the prescription +5.00 Add +2.50. Provided that the ordered prescription was exactly +5.00 Add +2.50, the Minkwitz astigmatism would be as shown in the patent (Fig. 9).

However, in actual practice, the base curve and addition worked on the convex surface of the lens, needed to be used for a range of powers, typically one diopter either side of the design value, and for any prescribed cylinder. 

So, what happens with a design near the edge of the intended range which incorporates a strong cylinder with an oblique axis direction, such as this prescription +6.00/-4.00 x 150 with an addition of +2.50 Diopters for near?

When this prescription is incorporated using traditional rotating surfacing methods as a simple concave toroidal surface, the iso-astigmatism pattern changes to that shown in figure 10. Such a design would take a subject a long adaptation period before they adjusted to the performance – if they ever got used to the lenses!

The Kelch Patent pointed out that if a properly designed freeform atoroidal surface is used, where the Minkwitz astigmatism can be controlled, the original design specification can be restored to the lens. A similar Patent was granted to Mukaiyama H. et al,14 which was assigned to Seiko-Epson.

Again, all these achievements were only made possible by the new method of surfacing which began the modern production methods we know as freeform surfacing, and that computers had become powerful enough to perform the necessary computations in real time.

Freeform becomes a standard

Some five years later, in the year 2000, Hof et al¹⁵ pointed out that freeform technology would allow semi-finished blanks with spherical (or aspherical) convex surfaces to become optimized progressive lenses where any prescription could be produced with iso-astigmatism lines lying in the same position as that which the designer intended (Fig. 11). This is the most widely used form of the modern freeform progressive lens.

In the year 2000 a patent was assigned to Johnson & Johnson¹⁶ for a progressive design which had the progression in power divided equally between the two surfaces.  The claim was that “The invention provides progressive addition lenses in which unwanted astigmatism is reduced and channel width through the intermediate and near vision zones is increased compared to conventional progressive addition lenses.”

The principle was simply that the Minkwitz astigmatism for a 1.00 D addition is half that for a 2.00 D addition and by splitting the addition between the two surfaces should provide a wider corridor and near vision area. The lens was marketed as the Johnson & Johnson Definity in the United States, and some five years later the patent rights were transferred to Essilor International (Fig. 12).

In 2005, a second patent for a double surface progressive lens was assigned to Hoya¹⁷ (Hoyalux iD Integrated double surface progressive design) whose object was “to provide a bi-aspherical type progressive power lens which provides an excellent visual acuity correction for prescription values, and a wide effective visual field with less distortion in wearing.” In this design the spherical addition was divided into two atoroidal surfaces, the front surface incorporating the addition in the vertical meridian only and the back surface incorporating the addition in the horizontal meridian only (Fig. 13).  Of course, two equal cylindrical components combined at right angles will produce a sphere.

Dr. Mo Jalie SMSA, FBDO (Hons), Hon FCGI, Hon FCOptom, MCMI, is Emeritus Professor at Ulster University and works as a consultant to the ophthalmic industry. He was the Head of Department of Applied Optics at City & Islington since 1968 to 1995. He is recognised as an international authority on the design of spectacle lenses and has written several books. Furthermore he is the author of some 200 papers on ophthalmic lenses, contact lenses, intra-ocular lenses and dispensing – and a consultant editor to the Optician magazine.

References

1. Jalie M. (2021), Principles of Ophthalmic Lenses (6th Ed.) ABDO, Godmersham.  
2. Fr. Patent 1159286 (1958), Manufactures des Glaces et Produits Chimique de Saint-Gobain, Chauny & Cirey, Système optique.
3. US Patent 2869422 (1959) (original Fr. patent applied for in 1953), Bernard Cretin-Maitenaz, Multifocal lens having a locally variable power.
4. US Patent 4861153 (1989), Winthrop J.T., Progressive Addition Spectacle Lens.
5. US Patent 4606622 (1986) Fuërter G., Lahres H., Multifocal spectacle lens with a dioptric power varying progressively between different zones of vision. (German application 3016935, dated 1980.) 
6. US Patent 4240719 (1980) Guilino G., Barth R. Progressive Ophthalmic Lens. 
7. US Patent 4274717 (1981) Davenport L.J., Ophthalmic Progressive Power Lens and Method of Making Same
8. US Patent 4580883 (1986), Shinohara T., Progressive Multifocal Ophthalmic Lenses    
9. US Patent 4676610 (1987), Barkan E.F.,Sklar D.H., Method of making Progressive lens and Resulting Article
10. US Patent 4946270 (1990), Guilino G. & Barth R. Progressive Power Ophthalmic Lens
11. US Patent 5270745 (1993) Pedrono C., Progressive Multifocal Ophthalmic Lens. 
12. US Patent 5488442 (1996) Harsigny et al, Progressive Multifocal Ophthalmic Lens
13. US Patent 544503 (1995), Kelch G., Lahres H., Wietschorke H, Spectacle Lens.
14. US Patent 6019470 (2000), Mukaiyama H., & Kato K., Patent assigned to Seiko-Epson. Progressive Multifocal Lens  Manufacturing Method of Eyeglass Lens and Progressive Multifocal Lens
15. US Patent 6089713 (2000) Hof A., Hanssen A. Spectacle Lens with Spherical Front Side and Multifocal Back Side and Process for its Production 
16. US Patent 6106118 (2000) Menezes E.V., Gupta A., Kokonaski W. Progressive Addition Lenses 
17. US Patent 6935744 (2005), Kitani A., Kikuchi Y., Bi-aspherical Type Progressive Power Lens.