A number of technical innovations have recently impacted the field of progressive addition lenses (PALs). They show promise of changing what we have come to expect when fitting these popular lenses.

The market share for progressives in the United States (40 percent) still lags somewhat behind other countries, but recent surveys have shown that the majority of beginning presbyopes in this country are now ordering PALs for their first multifocal. Deciding on which one can be a real challenge, especially since a recent count of the number of PAL designs and materials currently available in this country showed 136 variations.

Eyecare professionals are sometimes heard to comment that progressive lenses are as close to perfection as they are likely going to get. But is this true? This Eyecare Business Technical Report reviews the progression of design concepts that have appeared during the relatively short life span of progressive addition lenses.

Design improvements were frequent during the early years of progressives and became a major influence in convincing practitioners to offer these sophisticated lenses to their patients. These ongoing improvements also influenced the fitting success ratio for PALs.

At the current stage of development, manufacturers are still refining their progressives, but improvements in designs have become more subtle than the earlier advances. Successful fitting of progressives is pretty much taken for granted today, and the quest now is to improve visual comfort for wearers.

The Bifocal Days

What follows is a review of the major changes taking place in progressive evolution, including some that have just recently become available to the eyecare professions.

Historically, the initial motivation for an improved multifocal was to produce a multifocal with no telltale lines to give away the age of the wearer.

The first progressive patent was issued in 1907, but the lens was never produced. The first useable progressive addition lens was Varilux I, introduced in France in 1959. PAL designs have changed considerably since the first progressive became available to eyecare providers in the United States. Though Varilux I was available soon thereafter, the first lens sold in the United States was American Optical's UltraVue. A series of national seminars conducted by American Optical became the eyecare profession's introduction to the concept of a variable focus multifocal.

Design Enhancements

Symmetric Design. Compared to current progressives, UltraVue was a primitive, "symmetric" design. This term refers to the fact that right and left lenses are identical in design. To inset the reading portion of the lens, labs merely rotated two identical lens blanks to create the required inset for the patient's near PD. Labs and manufacturers liked this design style since it was easy to maintain a balanced inventory of lefts and rights.

Asymmetric Progressives. Eventually, lens scientists recognized that the design of a left progressive should be different than one used for a right lens. Manufacturers gradually shifted to "asymmetric" designs in which the design for the left lens was a mirror copy of the right lens.

All modern progressives are asymmetric in design. In asymmetric designs, more of the unwanted distortion in the reading area is pushed toward the nasal side of the progressive channel.

Mono/Multi Designs. All earlier progressives used the same design for a +1.50 add as for a +3.00 add. In general terms, as add power increases, the width of the progressive channel and diameter of the reading area gradually shrink. It's logical then that the next major advancement for progressives came with recognition that the design should change as the add powers change.

It was found that modifying the design for each add power made it possible to maximize the width of the progressive channel and the reading area for each add power. PALs without this modification for add power were called "mono" designs. Lenses that changed design depending on add powers were considered "multi" design.

Prescription-Based Designs. The next advancement came when designers realized that a progressive worn by a myope should differ from the same lens when worn by an emmetrope or hyperope. This makes sense when you consider that the progressive curves, always found on the front surface of the lens, are positioned further from the eye on a plus lens than they are when the prescription is plano or minus.

This led to modifications of the progressive design as base curves changed. The design for an eight-base blank would differ from the design for a six-base blank and would be different again when used with a four-base blank. This concept attempted to equalize the field of vision for near and distance as much as possible, regardless of power.

All-Aspheric Designs. Every progressive lens utilizes aspheric curves to constantly change power as the wearer's eye travels from distance to near. Earlier progressives featured spherical curves in the upper portion of the front surface. In recent years some progressives have been positioned as "all-aspheric," utilizing conventional aspheric type curves for the top front surface of the lens.

Aspheric curves used for the top front surface of a progressive differ from the flattened curves used for single vision aspheric lenses. Since it's not possible to fully aspherize a progressive lens as much as a single vision lens, that created some confusion over the term "all-aspheric" when applied to progressives. Generally speaking, even though an all-aspheric progressive isn't aspherized as much as a single vision lens, an aspheric top does enhance visual performance of the lens.

Shorter Progressive Channels. Two relatively recent events combined to create problems for progressive fitters. The first event was the growing acceptance of PALs, and the second came from the gradual shrinking of frame sizes.

As small "retro look" frames grew in popularity, doctors and dispensers found that many vertically narrow frames simply didn't provide enough vertical dimensions for progressive lenses. Many PALs require a minimum fitting height of 20 to 24mm and providing this minimum height was seldom possible with the more popular frame styles.

To solve this dilemma, one manufacturer, American Optical, came up with Compact, a progressive that features a 17mm minimum fitting height, producing an immediate impact on the world of progressives. During the last year and a half, other lens producers have introduced progressives that can be fit lower than 20mm.

In the fashion world of frames, minimum fitting height has become an important component of modern progressives.

Harder vs. Softer Designs. There is another category that helps differentiate progressives from one another. This category refers to how the design handles the unwanted power changes that are pushed to either side of the progressive channel and reading area. Generally speaking, when the quest is for a wide progressive channel and reading zone, changes in unwanted power must be compressed more rapidly on either side-and this produces what has come to be called a "hard" design. In this type of lens, wearers become aware of the unwanted astigmatism more rapidly, helping them contain their gaze within the useable channel.

With softer designs, power changes are more gradual and spread over a wider area. Some fitters consider this a more forgiving lens because as wearers begin to experience the more gentle changes of power on either side, they are able to utilize a wider area because the power changes are milder and easier to accept.

In general, softer designs have narrower distance zones, narrower reading zones, and longer progressive corridors with slowly increasing blur areas to either side. Progressives with harder designs have shorter corridors.

Increasing power changes in a progressive channel more rapidly means there must be increased distortion on either side. Using a harder design usually provides a wider field of view and requires less head or eye movements on the part of the wearer. Softer designs have reduced areas of distortion or "swim" to the side, but limit the size of clear vision zones, requiring more head and eye movement.

Some progressive lens manufacturers object to the term "hard" or "soft," suggesting the terms provide a false impression of how the lens works. They may be right about this but, to date, no one has provided better terms for this variance in design.

Many modern progressives are difficult to pin down as "hard" or "soft" because they were designed to fall somewhere in between those categories. These are usually called "medium" or "moderate" design.

Atoric Progressives

As progressive lens designs matured, one major objective has always been to increase or widen the wearer's field of vision, not just for distance but for intermediate and near as well. This was recently accomplished through means of aspherizing the back surface of the lens instead of the normal front surface.

The progressive addition curves are still placed on the front side of the lens. By making the back surface aspheric (or atoric when cylinder power is required), these new progressives enhance acuity for the wearer while enlarging the field of view for distance, near, and intermediate. Wearers usually notice improved vision immediately.

These enhanced PALs have been successfully launched in Europe and the United States by two manufacturers, Carl Zeiss and Rodenstock. Because of their sophisticated atoric surfaces, the lenses can only be processed by the manufacturers in their European factory laboratories. The lenses retail for about twice the normal fee for progressives but initial reports have been positive. The lenses are air expressed uncut to U.S. laboratories with a 10- to 14-day turnaround.

Compensated Prescriptions

As progressive designs came closer to perfection, some scientists began to consider the way progressive lenses are worn and quickly realized that this was an important component that had been largely overlooked in the quest for the perfect progressive.

Consider what happens when the wearer reads through a progressive. Their line of sight is downward at a rather severe oblique angle through a curved lens that is tilted about 10 degrees (pantoscopic tilt). The correction determined by the refractionist, on the other hand, is accomplished with the patient gazing directly through the optical center of flat trial or phoropter lenses. Finished lenses returned by the lab are verified in a lensometer, again by checking perpendicular through the distance and reading portion of the lenses-considerably different than the way the patient ends up wearing them.

Before the computer age, it would have been impossible for labs to calculate how each prescription should be changed to accommodate errors induced when lenses are in the wearing position. With the advent of high-speed computers, however, this became possible, and four companies currently incorporate this concept in their newest progressives: Rodenstock, and its affiliate 2C Optics; Pentax Vision; and Carl Zeiss.

Each has coined its own name for the concept and each follows a slightly different way of explaining it. They all, however, follow a basic format which is that the doctor's prescription is computed and modified to compensate for optical errors introduced when lenses are curved and tilted the way they will be worn in eyeglasses. The original prescription is also modified to compensate for the oblique angle created when patients gaze downward through the lens for reading.

To determine how the Rx should be modified, the computer must calculate and compensate each component of the prescription to provide the identical correction when the lenses are actually worn in real life.

When this type of lens is returned from the lab, the envelope indicates two values: the correction ordered by the doctor and the modified correction as it will read in a lensometer. These changes can affect sphere and cylinder power, axis and prism.

Rodenstock is now incorporating what it calls Position of Wear(tm) in four of their progressives. Pentax Vision calls its design innovation Retina Forward Design(tm). It's used in all Pentax AF Progressives. Zeiss calls their modification the Zeiss Measurement Value System, stating that it ensures the wearer experiences exactly the power prescribed by the doctor. Zeiss uses the Measurement Value System in all Gradal(r) progressive lenses. 2C lenses are based on Rodenstock designs, so they call their compensation by the Rodenstock term Position of Wear. Regardless of company, all calculations are done at the factory level.

Latest Launch

As for the top progressives in the Essilor/Varilux line, Comfort(r) features a design concept Essilor calls IN-FOS or Instant Focus, covered by three international patents. And, at press time, the company had just released its new Varilux Panamic(tm) lens which boasts applications for five additional patents. The lens became available March 1 through all Varilux authorized labs.

Panamic's greatest benefits, the company states, are faster adaptation, enlarged fields of view, and visual freedom that helps wearers feel close to their pre-presbyopic vision. Essilor credits these benefits on two new technologies-Global Design Management(tm), which "requires the entire surface to be balanced similiar to a single vision lens, with smooth peripheral optics"-and a new "deflectometry" quality measurement process called Design Ray Control(tm) that uses light beams to check across the entire surface of the mold not just at specified points.

As one might imagine, the computer software required for performing all the multiple calculations-whether for determining how to compensate for wearing position or how to achieve enlarged fields of view and easier adaptation-are proprietary to each company. And, as evidenced by this month's launch by Essilor, the quest for improved design is unending.

So, what is the pinnacle of progressive design? It's questionable whether anyone can answer that question. One thing we do know, however, is that progressives represent the most advanced lenses for correcting presbyopia. They also represent the most profitable lens product-for manufacturers, laboratories, and practitioners.