The History of Progressive Lenses

Europe's rise to scientific authority starting from the 13th century is mainly thanks to the invention of the corrective lens, which is therefore also one of the ten most significant inventions in human history after the wheel and fire. People who have corrected long-sightedness can read, study, write and share their knowledge for longer. Therefore, not only do they enjoy enhanced quality of life. They also contribute far more significantly to the development of science and culture than people without reading glasses could in days gone by. As the remedial means of choice for treating visual impairments in the elderly, modern progressive lenses are the result of centuries of research, development and testing of a wide range of solutions. This three parted series gives you insights into the development of progressive lenses. 

An Idea Becomes Reality

The competition for the first commercial progressive lenses

The first part provided an outline of how long the idea of an eyeglass lens with a "progressive diopter number" had already been circulating, but also that bifocal lenses remained the means of choice for long-sighted people until well into the 20th century. After all, for tolerable progressive lenses, innovations were and are required in three areas: optical design, production and acceptance among eyeglass wearers. The race for the first progressive lens starts again in the 1950s – this time between Paris and East Berlin.

The year 2019 marks 60 years of "Varilux" – the first commercially successful progressive lens. Originally an engineer, Bernhard Maitenaz (3) had rightly taken the production process into consideration when he developed his idea of a lens that enabled progressive vision at all distances. Thus, the 1950s and 60s are the time in which the design, calculation and manufacture of progressive lenses became possible, although their commercial value had still been underestimated at the outset. The developments up until the 1980s were characterized by continuous improvements and breakthroughs such as horizontal symmetry for significantly improved tolerance.

Maitenaz' motivation to come up with a superior alternative to bifocals was also shared by scientists in East Berlin. Thus, an engineer on the Seine and physicists and mathematicians on the Spree work on finding the solution of the three problem areas associated with progressive lenses. A typical example from the 1950s may provide an insight into why it was not an optician such as Owen Aves (1907) or an optical designer such as Estelle Glancy (1924) who ultimately made the breakthrough.

Progressive lens and Minkwitz theorem

At the 1956 Congress of the German Society of Ophthalmology, an ophthalmologist from Leipzig contacted Zeiss in Jena and in Oberkochen as well as Rodenstock in Munich with his idea for "eyeglasses with a progressive diopter number". He rightly recognized that his lens featuring a lower spherical near vision zone and progressive radii to the upper aspherical distance vision zone could fail due to "insufficient means of production". What is more interesting are the reasons for which the German lens manufacturer rejected his idea. They were sure that the "basic disadvantages" of such lenses, i.e. astigmatism, could not be rectified. Above all, however, the benefit for eyeglass wearers was, in their words "more than doubtful". In this regard, the uninterrupted success of progressive lenses since the introduction of Varilux would show all manufacturers one better with regard to the customer requirement.

The question of astigmatism was solved by a group from East Berlin, which subsequently failed also due to the "insufficient means of production" of the state-owned industry of East Germany.

Patented in 1954: Diagram of the "Eyeglass with progressive diopter number" by Ernst Lau and Rolf Riekher. Both also coined the term progressive lens.

Dissatisfied with bifocal lenses, Ernst Lau and Rolf Riekher at the Institute for Optics and Spectroscopy at the German Academy of Sciences began work on "eyeglass lenses with a progressive diopter number" in 1953. They managed to convince test subjects using rotation-symmetrical aspherical lenses manufactured using a thermal process. Nevertheless, their manufacture in Jena was later abandoned. Their 1959 patent used the term "progressive diopter numbers" for the first time instead of multifocals to describe this new type of lens.

Günter Minkwitz joined this work group in the same year. His "Minkwitz theorem" (this name incidentally coined in 1980 in an article in DOZ - 4) is considered the basis for the understanding and design of progressive lenses to this day. Indeed, it is no coincidence that Minkwitz was not a physician or ophthalmologist, rather he was a mathematician and grappled with the astigmatism problem of these lenses from a fresh perspective. Although Minkwitz' findings on the mathematical bases of "spherical, optical surfaces" and the term "progressive lens" coined by Ernst Lau did not assure the East Berlin natives commercial success, it did guarantee them a place in the history of progressive lenses.

One consequence of the "Minkwitz theorem" is that the maximum astigmatism in the lens increases proportionately to the addition. The practical consequence of setting the addition as low as possible is known to every ophthalmologist.
Minkwitz' second conclusion is that shorter progression corridors require greater astigmatism or smaller visual zones. Shorter progression lengths – indeed for narrower eyeglass frames were to present optical designers with further challenges. These astigmatism drawings were incidentally used in the USA for the first time in the 1970s, since comparative advertising was allowed there and new designs were therefore promoted in comparison with competing products.

Until the start of the 1980s, the proportion of Varilux in the multifocal lenses sold increased significantly in Geermany also. Rodenstock and ZEISS placed their own progressive lenses on the market; the misgivings regarding consumer acceptance were dispelled. The construction and manufacture availed of the simple principle of increasing surface curvature from the top down. This led to massive surface astigmatism in the progression zone and thus to high intolerance rates of all available progressive lenses.


A distinction between "soft" and "hard" surface designs, which however hardly fits the characterization of progressive lens designs anymore, originates from this time, the 1970s. The terms referred to the distribution of astigmatism. In principle, this could not be avoided, but it was possible to reduce it and above all to spread it differently. In "soft" designs, the astigmatism is pulled into the near and distance vision zones. This avoids relatively sudden blurring for the moving eye in side glances. In "hard" designs, the visual zones are expanded, thus the astigmatism increases more significantly at the edge.

Symmetry? But horizontal please!

A design principle from progressive lenses of the first decades particularly restricted the eyeglass wearer's sense of wellbeing: the symmetry of the right and left lens. This means that both lenses were produced symmetrically. During grinding, they were then rotated nine to ten degrees in order to bring the near zones into line with vergence during reading. Also, the fact that addition was arranged in a straight line, viewed from the distance reference point ("umbilical point line") meant restricted binocular vision in the side view.

Symmetric vs. asymmetric progressive lenses

The Gradal HS lenses introduced by ZEISS in 1983 offered "horizontal symmetry" for the first time. The required asymetric procedure was applied in their calculation and production. The right and left lenses were calculated separately and thus differently for the first time, in order to offer horizontal symmetry in the visual zones for binocular vision. This optimized correction for both eyes in all viewing directions means significantly increased tolerance.

Gradal HS advertising from ZEISS from the 1980s. Multifocal or varifocal lens? Progressive lens was chosen.

The history of innovation in progressive lenses had just begun

By solving the basic challenges in design and manufacture as well as in the popularization of progressive lenses as a visual correction tool of choice for people with long-sighted vision, an "old complaint" had found a contemporary solution. The ophthalmic industry had impressively disproved yet again a preconception that had been cherished since the 1920s: that eyeglasses were "fully developed" and groundbreaking progress was no longer to be expected.

The use of computers, the digitization of value creation chains, especially in the production with the advent of freeform technology should lead to countless innovations in progressive lenses also over the coming decades. Especially, individualized lenses, functional added benefits and a wealth of options will shape the progressive lens market starting from the end of the 1990s. These are accompanied by benefits for eyeglass wearers which were not obtainable for Maitenaz, Lau and other designers of the 1950s to 1980s. And: innovation cycles will become drastically shorter as digitization unfolds. If the first 90 years of the history of precision and progressive lenses can be told in ten-year steps, after 2000, the innovations, inventions and breakthroughs occur at far shorter intervals.

 

Lots of possibilities

During the first and second part of this series we looked at the centuries long search for ophthalmic lenses that are convenient for their optical and manufacturing characteristics for presbyopic patients - to the breakthrough with Varilux, and the advances for higher comfort and simple adaptation and the commercial preeminence of varifocal lenses on the market for glasses for presbyopic patients. The triad of basic challenges in this segment also marks the development in the 21st century: successful varifocal lenses can only be created with a new optical design, groundbreaking manufacturing procedures and innovations for superior consumer benefits.

Technical innovations are consistently being presented after introducing horizontal symmetry in the design of varifocal lenses. Since the late 1980s, designs with shorter progression lengths have been commonly used as a reaction to the preferences in eyeglass frames.

The new millennia brings a new era in the calculation and manufacturing of prescription lenses: after 200 years, a new manufacturing technology takes the place of the conventional manufacturing techniques that still employed grinding spindles and polishing dishes. The fundamental innovation that permanently changed specifically the development of varifocal lenses is the introduction of the freeform technology in 2000. With this came gains in efficiency during manufacturing, however, most of all it permitted the opportunity of individualization of the glass design and thus the ophthalmic lenses. Ophthalmic opticians can now offer more choices for a natural, individually optimal vision to their customers and no longer merely recommend a fitting glass based on the objective and subjective refraction values as well as the application. Thanks to the freeform technology, single vision lenses and varifocal lenses can be customized. This has an impact on all relevant aspects of the ocular optics praxis, refraction, consultation, tolerance, consumer benefit - and offers diverse possibilities.

More flexible, individual, stylish thanks to freeform

To evaluate this technological revolution it should be noted that it is connected to a paradigm shift. Manufacturers intended with all varifocal designs to perfect the glass optically, reduce astigmatism, widen the field of vision and adapt to applications and/or eyeglass frame trends.

With freeform, this process is turned upside down. The design is no longer the only factor, but also to implement point by point the eyeglass wearer's requirements for correction. It all starts now with the individual prescription which is combined with the glass design. The design is individually adapted, the data required for manufacturing are calculated per lens. If production batch 1 is today calculated for manufacturing of "Industry 4.0", this has already been applied in modern prescription production for over twenty years.

Schematic representation of the "point by point" calculation of modern freeform varifocal lenses, including the semi-finished product of the already specified base curve (Daryll Meister, ZEISS, 2006).

The availability of sanding and polishing tools had been a limiting factor for the spectrum of selection of glass designs since Johann Heinrich August Duncker's invention of the multispindle machine in 1801. Not just hundreds but thousands of lenses had to be calculated for a new glass portfolio and additionally the tools had to be created, which could amount easily to tens of thousands of new tools. Accordingly, a lot of time was required for the preparation, the design and tool creation of new glass types. Optical designers, technologists and IT experts are now presented with the challenge of transposing this innovation into the production and manufacturing processes. The freeform technology thus significantly accelerates innovation cycles - a fact which can be witnessed for example in the periodical announcement of new varifocal lenses by manufacturers.

A significant increase in the possibilities of correction is linked to the development of glass forms from spherical to freeform (Schneider, 2002).

Over the past ten years, refraction techniques, manufacture processes, optical designs but also consumers' requirements for the performance of eyeglasses have significantly changed. The flexibility, individuality and spontaneous tolerability of eyeglasses are today bigger than ever before. The development is fueled by technology and personal requirements, visual comfort and preferences, style trends and digitization.

Discovering potentials

The freeform technology has been employed since the year 2000 for the precisely, flexibly and individually calculated manipulation of optical surfaces in eyeglass manufacturing. The ZEISS patent has caught on predominantly amongst the basic types - with a combination of prefabricated progression of the basic curve on the front side, spheric or aspheric back side with toric or atoric form. A spheric front side as the basic curve and a freeform surface on the back side. The advantages can be found in the manufacturing - glass is only processed on one side, which results in shorter process times and a lower error rate. The advantage, however, is predominantly in the vision correction and esthetic of the eyeglasses. The optical quality is optimized, the symmetrical front side is esthetically pleasing.

New parameters were taken into account, the tolerability was significantly improved. Next to technological possibilities, stylistic trend requirements also play an important role. At Gradal Short I (2003), the progression zone is shortened by 20 percent compared to Gradal Individual and/or by 40 percent compared to conventional varifocal lenses. The optimal solution for the back then frequently requested frames with a mere 16 millimeter fitting height.

The centring data for fitting to a personal visual profile and the eye are the position of the lenses within the frame and in relation to the physiognomy of the eye as well as the precise consideration of distances of objects for the close-up and long distance zone. Video supported centring data collection has already been introduced in 1992. Today digital 3D centring data collection is possible, which provides all necessary data for an individualized adjustment of eyeglass frames.

These advances are combined with the eyeglass customization through innovation of objective and subjective refraction. Measurement steps of 0.25 diopters are set for the subjective refraction depending on the process. The determination of the starting point of visual acuity determination depends very much on the experience of the optometrist and the consumer's the form of the day. The objective method of measurement solves this problem. The i.Profiler by ZEISS for example provides most of all a variety of individual data. The wavefront technology considers up to 1.500 measurement points per eye. The customization of eyeglasses to visual fields of a higher order, the measurement for differing pupil widths to simulate the vision during dawn/dusk and night result in significantly more complex, but also optically optimized eyeglasses that offer a better tolerance and can be adapted much more precisely to personal preferences and needs and visual profiles. Since introduction of the horizontal symmetry in 1983 and the individual varifocal lenses in 2000, a new step in the development has been reached.

Carl Zeiß and Ernst Abbe were only able to perfect their revolutionary optical instruments, once Otto Schott developed optical lenses with characteristics that could be influenced directly. This basic principle also defines the eyeglass industry. New plastic materials with improved material characteristics have an important role and deserve their own chapter. As an example should suffice here the implementation of plastic materials with a refractive index of 1.74.

Customizations for eye, physiognomy, frames, lifestyle ...

Perfecting the customization of eyeglasses to the human eye and visual acuity was made possible with the objective refraction and new varifocal lens designs. The next step concerns the improvement of the synergy between frame-eyeglass-eye. In reality, this is also an old problem, which was first solved in 1933 with Perivist frames by ZEISS, which made possible an anti-slip position of the glasses with the customization of the frame to the human anatomy.

In 2006, ZEISS offered for the first time flexible progression lengths. The progression channel can be selected variably in steps of a tenth of a millimeter. Up until then the selection of the progression length was a compromise based on a few options and frame size, now the varifocal lense can be customized almost steplessly and precisely in all common frame sizes.

Following the freedom to create fashionable glasses came a variety of options for selecting varifocal lenses to fit individual lifestyles and work styles over the following years. Specific designs for computer and office work answered requirements for the human vision. The preferred personal visual distances were for example measured exactly down to the centimeter for screen, office and medium distances and taken into consideration in the eyeglass when optimizing optical characteristics.

The consideration of imaging errors of a higher order, the individual physiognomy, age, lifestyle-related factors such as driving a car, office work or Smart Phone use, frame preferences are by far not all factors used today for selecting individual varifocal lenses.
This development is not just interesting for reasons of ocular optics or design technology. Health factors are increasingly playing a major role. The requirement for a relaxed and fatigue-free eyesight has now become a matter of course. Health relevant requirements for the performance of modern precision eyewear lenses are becoming increasingly important to the development the more important health awareness is to consumers most of all in established markets. The added value offered by a varifocal lense is thus also measured among the factors which were never before considered in the classical optical design.

Finish for durability and style

No eyeglass lens without high tech finish: New technologies are present even on top of the optical surface. Scratch resistance, clarity, easy cleaning - a standard for modern eyeglass finish, which consisted up until today of up to nine extremely thin layers of evaporated metal oxides.

The process, that was first used in 1935 for binoculars and has been available since 1959 for eyeglass lenses, remains basically unchanged. New are the performance features which represent most of all convenience for the consumer, additional functionality and durability of the product. Because modern varifocals today are not just a product - the variety of possibilities exceeds mere vision correction. When describing the fashion choices such as health consciousness as the driving force for development of glass innovation, this of course also applies to finishes, mirroring or reflex colors. They fulfill esthetical requirements but also increasingly add significantly to the added value of varifocal lenses for consumers.

Finishes, such as for example to aid in the reduction of the parts of blue light of superficial sources that are presumably damaging to the eye, respond to health relevant requirements. However one estimates the risks, such features belong today indispensably to the "varifocal lens kit". In "eyeglasses for car drivers" the reduction of the blue light peaks typical in modern headlights contributes with a finish for less psychological glare from oncoming vehicles - an important product even for presbyopic patients.

Varifocal lenses for the modern world

Devices are indispensable in today's everyday world with computer screens, smart phones and eReaders and this means most of all one thing for human vision: stress, potential health risks and unusual visual conditions. Adapting to different reading distances, having difficulty with small fonts, the need to switch quickly and frequently between one vision range and another, the impact of unnatural light compositions on our sleep-wake rhythm and hormone levels – like other manufacturers, ZEISS is also addressing these changes with new lens designs for the digital world.

Unlike some technologies in the past, the adaptation of progressive lenses to the special and obvious challenges posed by the intensive use of digital devices is not an option that the wearer selects or not. Independent of age, starting in their mid thirties, every person is exposed to these strains and features for seeing in the digital world become the standard, such as finishes or individual progression lengths.

Varifocal lenses are increasingly used even for pre-presbyopic patients which have not yet reached the typical age for varifocal lenses, but experience vision especially with intense use of smart phones with a smaller addition starting at 0.5 diopters in the eyeglass lens as relaxing, helpful and good for their health. Popular "felt single vision lenses" are for example ZEISS Digital eyeglasses for patients aged 35-45. They can be worn without break-in period, offer near vision support, especially in the typical 30-35 centimeter zone of the generation Smart Phone and are a good way to get used to the later transition to varifocal lenses wtih an addition of over 1-1.5 diopters over the years.

Increasing requirements for optometrists

Only for the sake of completeness it should be mentioned, but will not be appropriately described to keeping things short: The success of modern freeform varifocal lenses depends significantly from the professionality of optometrists. The optometrist's service is indispensable for well-fitted and individually optimal varifocal lenses.

Without precise centering, varifocal lenses do not offer their full spectrum of performance.

It should be mentioned as an example: data collection via anamnesis, objective and subjective refraction offers the basis for the individually calculated lenses. Since freeform calculations without parameters for position of the lens and the frame are impossible, the digital centering data collection is today's gold standard. And of course special care is taken during frame consultation with customers as well as during the grinding process.

More options or new clarity?

All leading manufacturers use freeform technology and have devised their own design philosophies in the past 20 to 25 years. The production technology may be the same, and the number of materials limited, but the designs and options available for branded lenses differ greatly. ZEISS opts in the design for a clear, dynamic and thin look and the requirements for vision are even considered in the design with Digital InsideTechnology for presbyopic patients in the digital world.

A look at consumer needs, the competition and standard market offerings leads to the question: what trends will set the pace in the future? Progressive lenses or varifocal lenses for left-handed wearers, drivers, smartphone users, frequent readers, people with colorblindness, office-workers, golf-players - the possibilities are practically endless. The sheer wealth of advertised technologies is a further indicator that the possibilities of freeform technology and digital lens design are far from exhausted. But will even more technologies, even more options and even more features actually help the eye care professional and consumer? Or is it becoming increasingly difficult to gain an overview of the progressive lens portfolio? And is it therefore becoming more and more difficult for eye care professionals to give good advises and provide clear and personalized recommendations. And it is unnecessarily more difficult for the consumer to make an informed decision for his/her best lenses. Should everything that is possible be actually implemented in the progressive lens design simply because it is technically feasible?

ZEISS has decided to structure the entire progressive lens portfolio in line with actual demand and requirements. Technologies and innovative features are integrated into the lens design if they offer clear benefits to the wearer, support eye care professionals in their consultations and sales activities, and if there is market potential for them. Whatever the innovation policies of the individual manufacturer, there are basic trends that will remain intact and dominate in the future: individualization, digitization, the adaptation of lens designs to fashion and health aspects and the need to offer tailored progressive lenses for individual lifestyles and occupations.

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Joachim Kuss
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