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Progressive Design in the Digital Age

By Pete Hanlin, ABOM

Is it the design of a building that makes it safe and livable or is it the way it is constructed? There was nothing wrong with the process used to build the Tower of Pisa. However, the design did not take into account the foundation necessary to keep the structure stable. What’s the lesson? Even the best process cannot deliver performance in the absence of a good design.

On the other hand, a good design can benefit from new technologies. For example, in the movie Avatar, part of the film’s magic was created by the extreme high definition provided by audio/visual technology. However, those same technological advances will not “improve” the quality of a lesser-rated film. The same holds true with progressive lenses. A technologically advanced design will fail if not fabricated correctly, and the most advanced lens making processes will not improve a lesser design. The optician must consider both the importance of the design and the process used to create the lens.

Design Foundations

Starting with the design, there are several fundamentals a PAL must provide. The foundation of a great PAL design is built on four things:

• Natural Reading Posture - the wearer must be able to hold his head in a natural posture while reading through the lens.
• Proper Near-Zone Placement - while reading, the near zone is properly located in front of the eyes (and this position is different for each wearer).
• Binocularity - the eyes work together to form one image. This requires binocular peripheral balance. When binocularity is challenged, depth perception is lost.
• Sharp Central Focus- the central zones of the lens must provide sharp vision. This is the number one expectation of progressive lens wearers.

Natural Reading Posture

The “natural reading posture” of a person— who does not wear correction—uses a downward gaze achieved almost solely through head movement. The subject looks down at an angle of about 40 degrees, and uses eye movement primarily for scanning the length of the document. Once the head makes its initial movement, it rarely varies from that position by more than 5-10 degrees.

Most progressives require the wearer to reduce the amount of downward head gaze—relying instead on eye movements. This creates an unnatural posture in which the eyes are looking downward 30 degrees or more. Additionally, the head must be moved when moving from the top to the bottom of a printed page. Although most wearers will eventually “get used to” this position, it will never feel completely natural.

As a result, modern progressives are designed with this in mind. They allow the wearer to move the head downward by 35 degrees or more. The result is to require less than 25 degrees of downward eye gaze for a more natural reading posture. Further, lenses that achieve 85 percent of the add power in about 12mm or less allow wearers to see the top of a normal page. This means that the wearer can read a page without exerting any effort. This is what is meant by “natural reading posture.”

Proper Near-Zone Placement

Even the “widest” progressive designs have a spherical reading zone that is much narrower than a FT28 bifocal. To ensure the wearer receives a suitable width of clear near vision, the reading area must be centered in front of the eye when the wearer is viewing at a near distance. The primary factors used to determine the correct inset are:

1. The focal length of the add (which determines the optimal reading distance and therefore the amount of convergence).
2. The amount of prism generated by the distance Rx (since prism influences convergence as well).

Most modern progressives accomplish this by employing a variable inset. In Varilux progressives, for example, the designs feature an inset that ranges from under 2mm per eye to over 4mm. As a result, the near position varies so only the distance PD has been specified when ordering progressives.

Binocularity

For good binocularity to occur, the optics of a PAL must be similar for both eyes in every viewing position. When the eyes are presented with disparate images, one eye is “suppressed.” The result is the loss of stereopsis, or depth perception.

Even low levels of astigmatism will disrupt vision if it is imbalanced. Premium lens designs support binocular vision by providing “balance.” This is important because when both eyes turn right and left, they are looking through opposite sides of the design. If the optics of each lens is not similar, binocularity is challenged. The visual system can handle peripheral aberration as long as it is the binocularly “equivalent”. This allows the eyes to work together to maintain stereopsis (depth perception). This design feature is available in a number of lenses.

Central Focus

The central zones of the lens must provide sharp focus. While peripheral astigmatism is given a lot of attention, there are also aberrations in the central area of a progressive lens that affect vision. Called higher order aberrations (the most notable being coma), they decrease sharpness (expressed as contrast sensitivity) although they do not typically disturb acuity.

These aberrations are perhaps most noticeable to emmetropic and near emmetropic presbyopes. These patients sometimes refuse to wear progressives for distance vision because the higher order aberrations reduce sharpness. These patients often “see better” at distance without their eyewear due to the blurring effects.

Analyzing the wavefront, for varying pupil sizes for viewing distance, bright to dim lighting, age and the variety of prescriptions (ametropia) presented by the patients that enter your office, the effect of changing pupil size can be included in the calculations completed to design the new Varilux Physio Enhanced lens.

Delivering the Design – Digital Surfacing

All of these foundational design aspects can be delivered using any processing format including traditional surfacing. However, there are three primary limitations associated with traditional surfacing. First, traditional surfacing is limited by the use of physical tools, which limits the number of surfaces that can be created by the designer. Second, these tools also limit the accuracy of the distance portion of the lens due to “rounding”. And finally, traditional surfacing cannot place a design on the rear surface of the lens, so only one surface can be used for the design.

Digital surfacing does not have these limitations. With digital surfacing, labs can produce unlimited shapes and exceptional accuracy on one or two surfaces of the lens to achieve the intended design. It’s still important to remember that the foundation of a great PAL is found in the design, not in the way the lens is processed.

There’s nothing “magical” about digital surfacing. It is just a different way to make a lens. Unlike traditional lens surfacing, digital surfacing is not limited by physical tools. Traditional surfacing uses physical molds and laps/tools. It is impractical to create millions of different tools to create different surfaces, so the lens designer is usually limited to the number of bases and adds (usually 144 molds) and a 1,000 or so laps (sphere and cylinder power).

In digital surfacing, surface designs are stored as computer files and can be modified as needed based on patient data when the actual lens is created. It’s relatively easy to store millions of design files compared to physical tools. Then the computer translates the design exactly to either lens surface and controls the lathe that creates that design. Because of this innovation, lenses can be created in infinite numbers of designs using a digital surfacing process. As a result, the optician can deliver to the patient a better solution for their needs, based on any variety of factors. It also means that specialized lenses can be created for any of the many solutions a patient might want in eyewear.

Don’t be fooled however by the claim that a lens created specifically for an individual, at the time of generating in the lab, is always better. The freedom of digital surfacing is only meaningful if you are using it to customize various qualities of the final lens design being delivered, and that takes technical ability to create customization. For example, the ability to customize the progression length by 0.1mm increments is not helpful if the progression length is over 12mm. Remember, just like the Avatar example, advanced processing technology will not “improve” the quality of a lesser design.

Increased Precision

In traditional surfacing, the molded surface of the lens ensures that design will be almost perfectly accurate every time. However, the physical tools used to create the Rx are inventoried in 0.1D increments, and the resulting “rounding” will result in up to 0.06D deviations in distance power. Since digital surfacing is accurate to within 0.01D, the control of power is tighter. But even with the tightest digital surfacing process, there is slightly greater potential for design variation.

When the surfacing process is not tightly controlled, the variations increase. This is especially problematic for digital surfacing, where even a slight lack of calibration between the surfacing file and the surfacing equipment can result in significant variation in the accuracy of the design.

A digitally surfaced PAL is only as good as its combination of design and process. If the process is bad, the design will not be accurate.

Digital software process control is a lab requirement. It ensures that the surfacing equipment is calibrated every day and the processes used to create the lenses are capable of the highest levels of control and consistency. Without this control, there are no guarantees that you will receive the lens that design calculated because the process changed the final result. Ensure that the lab used has invested significantly in process control and lens design verification after surfacing.

Design Flexibility

Traditionally surfaced PALs are limited to one design (D), placed on the lens’ front surface. Traditional surfacing is not capable of producing a progressive surface on the back of the lens. This limits the designer in the same way having only a single speaker limits how you can listen to music. With one speaker, all you can do is adjust the volume.

A full back surface is also a “single surface” format (called DRx in Varilux lenses). Using digital surfacing, the designer combines the prescription (d) with the design (D) to create a greater number of resulting designs. This method is not limited by an inventory of physical tools. So, the designer can make changes to elements of the design, even though there is still only one speaker. In our music analogy, it would be like adding an equalizer; now in addition to playing the music, we can “tweak” the sound by increasing the base, the treble, or the midrange.

Having the ability to add design on two surfaces increases the flexibility of design (D) and therefore the number of solutions that are possible for wearers. With two surfaces (DualOptix), a designer can create an unlimited number of effects. Beyond just “tweaking” the levels of sound, effects like “stereo” can be included to enhance the experience. What does that mean?

Specifically, the main benefit of allowing the designer to use both surfaces is the ability to simultaneously control focus and distortion. In single surface PALs, the designer must (and does) find a compromise between focus control and the reduction of distortion. When using a single surface progressive, the more distortion is controlled the less focus can be controlled (and vice versa). With two surfaces, it is possible to control both focus and distortion simultaneously.

In a traditional progressive (+2.50 add), the front surface is used to create the add power. Distortion is present because the front curvature cannot be optimized for both the distance and the near power. That is because changing power by 2.5 diopters does not require a 2.5-diopter change in base curve. Moving the add to the back surface does not improve the situation because the same base curve is definitely not optimal for two powers 2.5 diopters apart. However, by disassociating the power change from the curvature change, distortion can be reduced below the levels found in single surface progressives. In addition, the use of asphericity cannot “do away with the base curve effect.” Asphericity can reduce marginal astigmatism and power error; it cannot eliminate distortion.

In review, digital surfacing provides three advantages:

1. It removes tooling limitations but any customization must be made meaningful.
2. There is a potential for greater precision but the process must be controlled.
3. It allows the designer to use both sides of the lens, which can allow simultaneous control of focus and distortion

New Varilux Progressive Lens Formats – Customization, Personalization and Individualization’

Digital surfacing allows for three progressive lens formats. Traditional progressives and full back surface progressives use a single surface to strike a balance between providing good focus, reduction of distortion and increased precision.

DualOptix creates a two design surface progressive. This allows a variety of customization, personalization and individualization segmentation opportunities for ECP and wearer. From the design point of view, it can provide more precise control of both focus and distortion. The results are clearer central vision, exceptionally well-controlled peripheral vision and overall design, based on the preferences the patient chose to include.

Customization of design is achieved using the DualOptix format because the effect of changing pupil size, for a variety of real life conditions, can be compensated for along with the patient’s Rx.

If the patient’s lens positioning information (vertex, tilt and wrap angle) is added, a personalization of the lens’ central power and overall design is created for the wearer.

If one then adds a patient’s biometric data, eye rotation center distance and, reading behavioral data, individualization of the design can be accomplished.

Remember, the DualOptix format by itself does not improve the design and digital surfacing is just the tool to create that design. DualOptix is a format to further improve a fully individualized lens design for the patient. In fact, new digital surfacing technique and design science capabilities can be used to add many features to a product portfolio. Table 1 lists the designing features that deliver vision benefits for ECPs and their patients.

Conclusion

Design is everything. The degree to which one improves the design can add to a patient’s satisfaction. The final progressive delivered to the patient, at this point of digital time, can be improved by adding a variety of new science. Digital surfacing makes it possible.

Recommend the best level of design with customization, personalization or individualization that your patient can afford. They are interested in it and will spend for it. Finally, when they receive their glasses, they’ll thank you for it and come back for more.

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