Progressive addition lenses have continued to improve in quality and optical performance since the first generation designs of the 1960s. Many modern progressive lenses have reduced astigmatism to its mathematical limits, and are preferred to conventional bifocal lenses by most wearers. Nevertheless, since mass-produced progressive lenses are typically designed to appeal to the “average” wearer in order to maximize commercial success, many presbyopes may tolerate suboptimal performance with progressive lens corrections.

Now more than ever, customization plays a vital role in the evolution of progressive lens design. While lens designs still vary, modern progressives will also be differentiated by their degree of customization. Better and more meaningful forms of customization allow lens manufacturers to further differentiate their products, as well as offer wearers truly unique optical solutions. Mass lens production, however, places significant restrictions upon the degree of customization that individual progressive lens wearers can enjoy.

Traditionally, progressive lens manufacturers have supplied processing labs with mass-produced, semi-finished lens blanks. The front surface of these blanks is fabricated by the manufacturer with the appropriate progressive lens design, including the intended base curve and add power, while the back surface of each lens blank is surfaced to the desired thickness and prescription by the laboratory. In this traditional model of production and delivery, progressive lens designs can only be minimally customized by base curve and add power. This means that all wearers with the same base curve and add power requirements receive the exact same design. Moreover, due to the number of separate lens blanks required for each lens design (up to 144 in each material), the product development and inventory costs associated with offering lens designs customized for anything other than base curve and add power is extremely prohibitive.

MODERN SEMI-FINISHED CUSTOMIZATION
The simplest progressive lenses use the same basic lens design for all powers, scaled for each base curve and add power (i.e., mono designs). Many modern progressive lenses change the basic lens design by add power (i.e., multi designs) and/or base curve (i.e., design by Rx designs). The distribution of optics in the basic lens design can be tailored for the optical effects specific to the wearer’s range of presbyopia and/or ametropia.

LIMITS OF SEMI-FINISHED CUSTOMIZATION
Basic progressive lens designs can be varied slightly for each base and add combination in order to tailor the performance of the design for only broad categories of wearers. Since a single base curve or lens design can only deliver optimum optical performance with a single prescription power—typically a sphere power located near the middle of the prescription range—even the most advanced semi-finished lens designs can only be customized for broad prescription ranges (and generally no cylinder powers). Consequently, factors specific to the individual wearer—such as his/her specific prescription, frame attributes or unique visual demands are neglected.

THE EVOLUTION
Semi-finished lens designs have pushed the envelope on customization for the average wearer. Fortunately, since newer free-form (or direct surfacing) manufacturing technologies are not subject to the product development and inventory costs associated with mass (semi-finished) lens production, these manufacturing platforms now make possible full customization of the lens design for each individual wearer. With the appropriate optimization strategy and software, a progressive lens can be fully customized for a number of factors specific to a given wearer. Free-form technology virtually closes the gap that results in the traditional delivery model between lens design technology and the actual wearer.

New delivery and production models now allow the customization of the basic lens design as much as desired for the individual wearer. The process requires a computer-numerically-controlled (CNC) free-form generator, which can be directed to create a truly unique and completely customized progressive lens design directly onto a lens blank, along with the desired prescription (sphere and cylinder) curves. This enables the laboratory to deliver progressive lenses specifically tailored to individual wearers.

It’s important to note here there is little visual benefit to free-form manufacturing for the wearer, without the application of a customized lens design. This means that the use of free-form manufacturing, by itself, does not guarantee a more successful progressive. The free-form lens supplier should be capable of designing progressive lenses in “real time” or “on the fly,” using information specific to the individual wearer.

BENEFITS OF FREE-FORM TECHNOLOGY
As stated, free-form technology can potentially deliver unsurpassed optical performance using a level of customization not possible with semi-finished lenses. Typically, wearers can enjoy enhanced vision and wider fields of view with designs specifically optimized for their individual prescriptions and fitting parameters. Progressive lenses can also be customized for other factors pertinent to the individual wearer, including ergonomic, lifestyle and biometric parameters.

In addition, since the progressive and its optimization can all be cut onto the concave surface of the lens, free-form manufacturing requires a considerably smaller inventory of lens blanks on hand. Often, a dispenser was not able to provide the same progressive design in clear and polarized versions. With free-form, any design can be cut onto a variety of SV semi-finished lens blanks. This allows for a more cost-effective offering of multiple lens materials and even low-demand “orphan” lens designs.

FORMS OF CUSTOMIZATION
With the use of a free-form manufacturing process and a sophisticated optimization program, progressive lenses can be customized based upon a variety of factors specific to the individual wearer. They are:

Semi-Finished Optimization
As stated, a single, semi-finished lens blank can only be optically optimized for an average Rx power (the median sphere). While optical errors in the periphery are well controlled for the optimal power of each base curve, these errors can increase for prescriptions on the same base curve that deviate from this single, optimal power. Powers that deviate from the “Optimal Rx” disadvantage the wearer. Free-form technology allows a lens blank to be optically optimized for every Rx power. The result is the correct amount of asphericity or atoricity to widen clear fields of view.

Customization by Ergonomic Analysis
Conventional progressive lenses are designed to provide correct powers when measured with standard focimeters. These progressive lenses do not take into account factors relevant to the interaction of the wearer with the lens, including frame tilt and vertex distance. As-worn or position of wear optimization maximizes the optics of the lens design as experienced by the wearer with the lens in its intended position of wear using optical ray tracing. This ensures that the design performs consistently, regardless of the ergonomics between the wearer and the lens/frame combination.

Some manufacturers may require you to submit frame-fitting information since the refraction and the as-worn position are different. The visual ergonomics of ocular refraction (lens tilt, vertex distance and near effectivity) differ appreciably from the ergonomics of the as-worn position. Therefore, as-worn optimization refines the optics of the basic lens design by considering the ergonomic interaction between the actual wearer and the position of the fitted lens.

The prescription and ergonomic optimizations discussed so far maintain the basic lens design characteristics or configuration offering wearers enhanced customization of the basic design. Lifestyle, biometric and frame customization may result in significantly modified lens designs for each wearer that may vary considerably from the base design—offering patients extended customization.

Lens design morphing facilitates many forms of extended customization. That means that multiple, discrete designs from a “family” may be employed, or the lens design may be determined “on the fly” from a continuous range of possibilities. For example, a starting base lens design can be manipulated to maximize certain optical attributes for a given wearer, such as the size of viewing zones.

CUSTOMIZATION BY FRAME SHAPE
Roughly half of all frames sold today have a ‘B’ (depth) measurement of 35mm or less as analyzed by the prescriptions received in the Carl Zeiss Vision laboratories. Unfortunately, the near zone of traditional progressive lenses is often edged away in smaller frames styles. “Short-corridor” progressive lenses were eventually introduced to allow more near utility in small frames, however, lenses designed specifically for small frames require unnecessary optical compromises when worn in larger frame styles. Shorter corridor lengths produce either more unwanted cylinder power or smaller viewing zone sizes, as well as reduced mid-range utility.

Free-form manufacturing affords the delivery of multiple corridor-length designs to wearers, or even designs with continuously variable corridor lengths. By providing a wide range of potential corridor length options, progressive lens wearers will always get sufficient near vision utility in small frames, without compromising optical performance any more than necessary.

More advanced applications of free-form customization include progressive lenses customized for facial wrap, frame curvature and other examples of conformal optics. For example, some plano sunglasses use lenses that are cylindrical in surface curvature. Such designs make new frame designs possible. Imagine a lens that conforms to the frame shape requirement yet delivers an optimized progressive prescription correct for fitting requirements.

CUSTOMIZATION BY LIFESTYLE ANALYSIS
Progressive lens designs can be selected or even individually optimized based upon an assessment of the particular visual demands of a given wearer. SOLAOne Ego, currently being tested in Europe, is an example of a free-form progressive lens that varies the balance of near, intermediate and distance-viewing zones of each lens based upon the visual lifestyle demands of the individual wearer. Customization by lifestyle will require another set of tools (questions) that allows the dispenser to assess the patient’s needs with more precision. Here, dispensers use a PDA to assemble all the information and recommend a progressive design.

CUSTOMIZATION BY BIOMETRIC ANALYSIS
Progressive lens designs can also be selected or even individually customized based upon biometric analysis of the wearer using eye/head tracking systems. Here bio-metric data will suggest how wearers use eye-wear based on prescription, posture and learned behaviors. New tracking systems just being introduced in the U.S. market will gain increased awareness and use by practices that offer the best in the category possible and/or differentiate themselves with the newest of lens technologies.

INVENTORY SIMPLIFICATION
A semi-finished progressive lens series typically requires at least 5 base curves with 12 add powers each, resulting in 60 separate lens blanks for each lens material. In its most simplified form, a free-form progressive lens series using back surface technology may require only 4 or 5 semi-finished single-vision base curves, resulting in a considerably smaller inventory for each lens material. The progressive optics, the wearer’s prescription and any optimization are combined and then applied to the concave surface using free-form surfacing.

Moreover, for a fully free-form series that uses spherical blanks, these same blanks may be used for a variety of lens designs, including “one-off” or “orphan” designs that were not possible at all before. Orphan designs could include obsolete progressive lens designs for wearers who want to continue wearing the same product for any variety of reasons.

FREE-FORM MANUFACTURING CHALLENGES
Sophisticated software is required to design and “optimize” a modern progressive lens surface. Progressive lens surfaces must be manufactured or surfaced to a precise shape and to an optical luster to deliver the intended design. Conventional fining and polishing (“cylinder”) machines and tools are not compatible with the complex surfaces used for progressive lenses. Conventional generators are also not capable of producing surfaces to the precision and smoothness required for the use of non-conventional fining and polishing methods. As a result, errors produced by free-form manufacturing are not as readily measurable as errors produced by lens casting. What are the steps?

Free-Form Progressive Lens Optimization
A sophisticated optimization program employs a number of steps to arrive at the final progressive lens design. A sufficiently advanced program begins with a base design. If necessary, the base design is morphed, modified or selected based upon biometric, lifestyle or frame shape parameters to arrive at the actual target design. The prescription and ergonomic parameters are then entered into an analysis program to create an initial progressive lens model. Next, this initial “lens” model is analyzed using optical ray tracing for the as-worn position. The software then minimizes merit functions (such as optical blur, distortion) between the ray-traced lens model and the original target design across the entire surface. A final lens is arrived at, representing the best compromise.

Manufacturing
Conventional generators produce a surface that requires additional fining and polishing using “toric surfacers” or “cylinder machines.” Fining and polishing pads are placed on hard lap tools and the cylinder machine then cycles the lens blank repeatedly over the lap tool and pad combination until smooth and then polished.

Fining and polishing with hard lap tools allows for uniform, yet aggressive material removal. This ensures the shape of even rough surfaces is maintained (or even corrected, such as for elliptical error) during the fining and polishing processes.

In order to understand the principal challenge of free-form manufacturing, it is important to understand the difference between complex and circular lens surfaces. Spherical and toric (sphero-cylinder) surfaces have constant curvatures with circular cross sections that are symmetrical, which allows the use of hard lap tools. Complex surfaces, such as those used for progressive lens designs, have varying curvatures with no symmetry, which precludes the use of hard lap tools. Consequently, free-form progressive lenses require the use of some other form of fining and polishing.

Progressive lenses have changing curves over their entire surface—that is, the surfaces have no symmetry—so hard lap tools are not an option. Once a complex surface is created, it needs to be polished to optical transparency without deforming the shape enough to create power errors or aberrations. Leading free-form manufacturing processes use soft lap polishing to produce the desired surface luster once the lens blank has been generated and finely turned with a single diamond cutter.

Soft lap polishing uses a flexible pad or air bladder to polish the surface. However, material removal isn’t as uniform and excessive soft lap polishing can result in changes to the shape of the surface (or errors in form), creating power errors. To reduce the amount of polishing required, an extremely fine surface is created that requires only buffing to final transparency and luster.

Conventional generators produce rather rough surfaces that must be fined and polished to ensure correct shape and adequate surface luster for optical transparency. Since soft lap polishing is necessary for complex surfaces, free-form generators must produce incredibly smooth surfaces to reduce the likelihood of variations in lens shape during the polishing process. The surface roughness off of conventional generators is quite significant and can be comparable in magnitude to the errors in surface shape necessary to create visible optical effects for the wearer. Errors in surface shape from excessive soft lap polishing of rough surfaces can result in visible power changes. As a result, depending on the prescription and radius of cut required, lenses will be cut multiple times using finer and finer cutting tools. Typically, the surface is milled and then either re-cut by a PCD tool or a single point diamond tool or both. The result is a transparent surface that only requires buffing.

BENEFITS OF MASS PRODUCTION
There are certain benefits to mass production. Once the initial investment in an inventory of molds is made, large-scale casting can be relatively cost effective. Throughput is considerably greater than with individual free-form manufacturing.

Lens designs produced by a well-behaved casting process generally have a high degree of repeatability.

Quality control is simpler, since discrete measurement points for power can be employed to ensure compliance, while these discrete measurements are not always indicative of compliance in free-form manufacturing.

NOT ALL FREE-FORMS ARE CREATED THE SAME
A good base design serves as the critical starting point for free-form optimization. After all, if the base progressive lens design offers only mediocre performance, free-form optimization will simply ensure consistently mediocre performance. It is also important to remember that use of free-form technology provides no assurance that the lens has actually been customized at all for the wearer. While clever application of free-form technology opens up an infinite universe of design possibilities, an innovative design optimization strategy and sophisticated computer software are required to deliver them.

Only with the right tools can free-form technology deliver unsurpassed optical performance with a level of customization and optical optimization not possible in mass-produced, semi-finished lenses.

VERIFYING FREE-FORM LENSES
Conventional progressive lenses are typically designed to read correctly in a standard lensmeter or lensometer. Devices of this type measure the “vertex” power of the lens, not the power of the fitted lens as perceived by the actual wearer. Since lens tilt and vertex distance can produce significant changes to the desired sphere, cylinder and add powers of the prescription, free-form lenses that have been optimized for the position of wear will often necessitate small changes to the original prescription. These power changes are sent to the dispenser as a “compensated Rx” for power verification, which represents the vertex power (for measurement) necessary to produce the desired prescription once the lenses are worn. For many free-form progressive lenses, back vertex power measurements are often used for add power verification as well. Due to the complexities of measuring the add power of progressive lenses, however, use of the semi-visible engravings for add power verification is preferable.

CONCLUSION
While we may live in a world of mass production, consumers are attracted to and prefer more personalized products. Customization has played a vital role in the evolution of progressive addition lens designs. Better and more meaningful forms of customization allow lens manufacturers to differentiate their products and offer the wearer truly unique optical solutions.

Free-form manufacturing and dispensing tools can facilitate and complement new forms of customization. The gap between lens design technology and the individual wearer closes more and more every year.