The Real Details of Vertex, Tilt and Wrap

By Barry Santini, ABOM

For most of the last century, eyeglasses were fabricated using a binocular measurement for pupillary distance, and simply dividing it in half to center the lenses for each eye. This method delivered apparently great patient satisfaction, as visual complaints were not clearly traceable to the lack of using a "proper" monocular PD....even for segmented multifocals. But exactly what does "not hearing complaints" tell us about how well we satisfy our clients, or more importantly, meeting their expectations? Do we really know whether "not hearing complaints," i.e. good enough, is in fact, good enough?

The answer is that we don't, because not hearing complaints is an imprecise metric of customer satisfaction. Ultimately, the best we can say is that an absence of complaints means that our clients are being served "adequately."  But in the online-connected world of today, where consumers are finding information access quick and easy, many are becoming close to experts in their knowledge of the latest in product information about lenses, frames and measuring technologies. For an eye care professional, this means that their customer satisfaction goals have to be ratcheted up to higher levels than ever before.

Offices relying on a performance goal centered on adequate are bound to find it woefully inadequate as an effective, long-term business strategy.

Today, practices must now focus exclusively on achieving absolute excellence, always attempting to exceed client expectations in every pair of glasses sold. It may have previously been sufficient to possess a rudimentary knowledge of the latest lens and fitting technologies, but a contemporary eye care professional needs to go further...much further.

They must readily master and implement the latest skills and technologies into their everyday practice to ensure they are fully delivering on the promise of "wowing" their clients. Optimizing single vision and progressive lenses, using individually tailored measurements for vertex distance, pantoscopic tilt and wrap angle, is therefore absolutely essential in guaranteeing consumers receive all the superlative acuity, comfort and utility possible in today's advanced lenses.

ASSUMING THE POSITION

Although frames and faces may come in a wide variety of styles and sizes, engineers working with traditional single vision and progressive designs had little choice in deciding what distance, tilt and wrap angle values a frame would place the lenses. Therefore they settled on averaged values calculated from vast amounts of real-world fitting data. Examining this data, they found that on average, most lenses would be positioned approximately 13mm in front of the eyes for vertex distance, 10 degrees for Pantoscopic lens tilt and 5 degrees for frame wrap angle, otherwise called position of wear, or POW.

For years, engineers employed these 'default' values in their calculations. But with the arrival of free form processing, manufacturers recognized that significantly higher levels of optimization would be possible if individual position of wear values were included in their calculations. This is not unfamiliar to ECPs who have experience fitting stronger power, post cataract or contact lenses. Having to compensate for the difference in vertex distance between the measured phoropter value and the lens fitting value was a routinely encountered task.

THE EXPANDED OPPORTUNITIES OF FREEFORM PROCESSING

Today, free-form manufacturing enables real-time lens design and optimization of single vision and progressive lenses. Because FF processing enables the use of a variety of molded, semi-finished lens blanks, a tremendous expansion in the number of material, design and treatment options is now available for almost any lens design.

Although FF lenses may be ordered in the traditional way, using PDs, heights and simple frame dimensions, the greatest benefits are only realized when an actual tracing of the frame shape, containing exact dimensions and contour, is uploaded to the lab's lens management system (LMS). You next enter the specific POW values, which were taken during the fitting. With this information, the lens software will automatically optimize the progressive corridor and entire lens surface, according to the design choices made by the lens designer. However, there are more tailored and advanced optimizations possible if ECPs also avoid relying on the default design assumptions created by the original engineers.

For example, even with advanced free-form processed lenses, wearers can often encounter adaptation or utility issues in a single pair, or when switching between multiple pairs of eyewear. This is avoidable if ECPs are more involved in determining the lens's overall design priorities, such as specifying the exact length or slope of the progressive corridor. Eye care professionals are clearly the ones best situated to engage in advanced lens optimization, as they possess a far more intimate and comprehensive understanding of their patient's needs, wants and eyewear style preferences than a lens designer or lab could ever have.

Included amongst the advanced tailoring and personalization options are choosing a distance, intermediate, or near weighted design. By helping patients specify their order of vision priorities, optimization of vertex distance, pantoscopic tilt and wrap angle, individually and together, improve overall lens design and performance.

From "One Size Can't Fit All, (M. Mattison-Shupnick, 2020mag.com/CE), matching the patient to a weighted design lens provides a further tool to get the lenses right. For example, today's mobile lifestyle changes 'normal' reading position and posture. If one asks, "Tell me about your work, where do you do your most reading, show me how you hold the book (tablet and/or smartphone) and the first two reasons that you wear prescription glasses", it's easy to suggest a Near-priority (N), Far-priority (F) or Balanced (B) progressive lens platform. Use Near for indoor/office environments where there's a need for clear, stable, prolonged near vision. Presbyopes who use their eyewear mainly for distance and outdoor work where a clearer and wider periphery enhances the field of view (truck driver, myopes where reading is less in demand) would notice a difference with distance vision optimized. For new presbyopes and people whose demands vary widely, use a balanced design lens.

VERTEX DISTANCE

Ask just about any eye care professional when knowing the fitting vertex distance of a pair of glasses is important, and they'll no doubt respond: "For powers over 6 diopters, you'll have to compensate the power for the difference between the exam vertex distance and the wearing vertex distance."

Although using the power threshold, as the marker for setting the threshold of importance for vertex distance was appropriate in traditional lenses, today's free-form optimized designs use vertex distance measurements as a far more important and essential element of lens optimization than power compensation alone.

In digital lenses, informing the optimization engine of the fitted vertex distance enables the design algorithms to know precisely where the eye's central and peripheral gaze angles intersect the lens surface. This allows the manufacturer's software to calculate optimal corridor length, placement and inset.

Additionally, entering the actual lens tracing complements and leverages entering the fitted vertex distance, which further increases the optimization possible by reducing the lens areas the eye can't see or won't routinely access. In fact, neglecting to enter an exceptional vertex distance, defined as a fitted value less than 12mm or greater than 15mm, will significantly handicap the overall optimization process and reduce the promised performance gains possible in these fully customized lenses. ECPs are additionally advised to consult individual manufacturers to discover whether vertex-based power compensations are executed within the lens design's optimization algorithm. For many designs, traditional vertex distance power compensations are not routinely included in the lens calculations or verification values.

PANTOSCOPIC TILT

Not to be confused with Pantoscopic Angle, which is the angular measure between the frame front and the temple plane, Pantoscopic Tilt describes the vertical angle between a wearer's primary gaze and the intersection of the same with the plane of the lenses.

This is a yoked measure, meaning that both lenses are considered to be in the same plane, I.e. parallel for both eyes. Although the common range of values found is between 0 and 15 degrees, a wearer's natural posture can result in a negative value, defined as retroscopic tilt. A properly positioned, single vision corrected curve lens (compliant with Martin's rule of tilt) requires the lowering of the optical center 0.5mm for each degree of Pantoscopic tilt to ensure the lens surface is properly positioned to correct most off axis aberrations. The addition of the power corridor in progressive lenses compromises off axis clarity, making inclusion of as-worn Pantoscopic Tilt an additionally important priority.

In traditional progressive lenses, when the measured Pantoscopic Tilt value was found not close to the manufacturer's assumed fitting value, no further optimization was possible to compensate for degrading off axis effects. Additionally, both Pantoscopic tilt and vertex distance will often combine in a non-complimentary way, further degrading the utility and comfort of the progressive corridor.

Most eye care professionals under appreciate the importance of Pantoscopic tilt. To illustrate, consider the degree of discomfort a wearer voices about the adjustment of their glasses when the plane of their eyewear is skewed (propellered) - meaning the lenses are no longer parallel to each other in front of the eye. Progressive wearers find even minor amounts of frame-front skew will cause significant visual distress and discomfort.

Should you specify the corridor length? Yes, if it will make a defined difference based on the questions asked of the patient and your observations of them.

For example, SEIKO Superior allows a choice of 11 corridor lengths in increments of 1mm starting with a 8mm length and minimum fitting height of 12mm (8mm, 12mm) all the way to 18mm length and 22mm minimum fitting height. That teaches you that for reading there is a 4mm reading depth. Want more reading? Use a shorter corridor length. Want to mimic the habitual glasses (the patient's current eyewear), consider that the reading is 4mm in depth and subtract that from the habitual glasses fitting height. The result is the targeted corridor length for the new glasses being ordered. The patient didn't have enough corridor width, lengthen the corridor by 2-3mms and corridors widen.

FRAME WRAP ANGLE

Frame wrap angle describes the horizontal angle of the lens plane in front of the eyes. The total frame wrap angle is measured with a common protractor, with one lens placed along the baseline of the protractor, and tracing a line tangent to the nasal and temporal limits of the opposite eyewire. The total wrap angle is then halved to obtain the angle for an individual eye. Unlike Pantoscopic tilt, frame wrap is a non-yoked measurement, meaning the tilt of a lens is opposite for each eye.

Traditional progressive lenses have always benefited from modest wrap angles, in the range of 5-7 degrees. Engineers prefer to use these values over a flatter angle because moderate amounts of wrap decrease the obliquity of the gaze angle in the periphery, which also decreases awareness of image degradation or distortion. Again, it is highly important to include the wrap angle when the measured frame value lies outside this range by two or more units in order to ensure optimal peripheral clarity.

KNOWN UNKNOWNS

Individual POW elements often interact with each other and impact vision in subtle, unpredictable ways. In discussing the interactive effects of vertex, Panto and wrap, it is assumed that the given prescription is accurate, leaving lens size, lens design, pantoscopic tilt, wrap angle and vertex distance as variables to consider when optimizing eyewear designed to deliver the ultimate in vision. For a designer tasked with creating a new lens design, the final lens size, the position of wear, the prescription change and the previous lens design are all considered to be known unknowns. Therefore, the design process begins with what is known: The Rx and how it was determined in the exam room.

AIM FOR THE BULLSEYE

With many variables to sort through in investigating a vision complaint, anything that could help to reduce the pool of possible suspects should be welcomed with open arms.

Luckily, eye care professionals not only recognize the importance of this area, they are highly skilled in it as well. The art of taking measurements, from PDs to Seg heights, ECPs are fiercely proud of the pains they take everyday to ensure the traditional measurements needed for eyewear are as precise and accurate as they can be. That's why it comes as a surprise for lens manufacturers to discover that ECP's often under appreciate all the inherent benefits of taking vertex distance, Pantoscopic tilt and wrap angle measurements in every pair of eyewear sold.

At the front lines of optical — at the dispensing desk — the one given in designing of a pair of eyewear is the Rx. In the recipe of eyewear satisfaction, this is the bedrock upon which all else is built. Given this, it is amazing how many pairs of eyewear, even when their prescriptions have been accurately verified, come back in the hands of the wearer, accompanied by some complaint related to acuity, comfort or utility. The reasons for this should now be clear: Lens designers have had to make many assumptions about how lenses would be positioned and aligned in front of the patient's eyes. And it is this lack of knowledge of the actual position of wear that often moves a pair of eyewear off the bulls eye of perfect to the outer border region between acceptable and unacceptable. By taking and supplying position of wear measurements in the lens ordering process, ECPs obtain the ability to move that pair of eyewear much closer to the bulls eye of perfect, the area the vision trifecta described above calls home.

THE PHOROPTER, WELCOME TO FLATLAND

The main tool today for refining the all-important subjective refraction is the Phoropter. It consists of small diameter lenses that are Flat in curve, positioned perpendicular to the floor at zero pantoscopic tilt, positioned in a flat plane, at zero wrap angle and positioned a fixed distance from the eye, approximately 12mm-14mm.

But outside the exam room, lenses for eyeglasses are larger, requiring a curved profile for better peripheral vision. At the same time, the lens' actual position in front of the eye i.e., the position of wear, differs as well from the frames that are encountered in the exam room. For example, there is Pantoscopic angle; eyewires are tilted inward, toward the cheeks approximately 5-8 degrees. There is also a wrap angle. The frame is wrapped a modest degree, approximately 5 to 7 degrees, around toward the temple. This is done to improve cosmetics and reduce off axis aberrations, including skew or geometric distortion.

These profile and positional differences are the basic elements that Corrected Curve theory and Martin's Rule of Tilt attempt to address to maximize peripheral acuity in single vision lenses. However, in progressives the situation becomes more complicated. Because of the need to blend the progressive corridor into the adjoining lens topography, maintaining good acuity in the peripheral temporal areas becomes far more important. The inferior area also rises in priority as the presbyopic eye relies this region for reading. These two considerations mandate that the precise position of the lenses be seen from a more global perspective in order to deliver fully on the potential of customized free form optimization.

In traditional progressives, entry of individual position of wear parameters into the design process is not possible. Engineers therefore relied on averages gleaned from accumulated fitting data. The advent of using POW values in real-time lens optimization represents an important milestone in the evolution of progressive lenses.

THE MORE THE MERRIER

Faced with the daily task of integrating the above considerations in every pair of progressive glasses can appear daunting to ECPs, and it is easy to see why many continue to rely upon what's worked "good enough" in the past. But continuing to rely on a traditional design, fitted with default position of wear values, a simple PD, height and boxed frame dimensions is quite a bit away from the state of the art.

Further, avoiding the use of customized progressives is not only amounts to leaving "money on the table," it is, more importantly, no longer what today's savvy, online-informed consumer expects when they pay the higher prices asked by their local optical store.

CONCLUSION

The message going forward is clear: Premium prices must be clearly associated with premium products, service, and expertise. And that expertise is only clearly evident when eye care professionals take the time to measure, calculate and explain why all those fancy POW measurements they didn't take before have become important today. When it comes to the real details of vertex distance, Pantoscopic tilt and wrap angle, the more measurements you take, the merrier your clients will be about the terrific acuity, utility and comfort of customized, free-form progressive lenses.