By Barry Santini

For most of its 700-plus year history, eyeglasses have been priced from inexpensive, premade models sold by peddlers to the masses, to custom personalized items that are sold to those with the means and the mind to afford them. Today, despite the appeal of both over-the-counter readers and made-to-order prescription eyeglasses for less than $10, quality conscious consumers are increasingly seeking out those eyecare professionals (ECPs) who are fluent with the technology of custom-fitted, digital free-form lenses. Although they're already familiar with the basics and benefits of monocular PDs and heights, many eyecare professionals have yet to embrace using basic position of wear (POW) measurements—tilt, wrap and vertex distance—and thereby realize the promise of improved acuity and vision utility delivered by customized lenses when using POW values in their optimization.

Rather than just passively being dictated by base curve, ECPs can optimize eyewear in ways rarely considered before. From maintaining a frame's authentic fit and appearance, to reducing anisiekonia and other issues of binocularity originating in even mildly disparate Rxs, using tailored base curves in your lab orders is becoming a valuable new exam and dispensing tool. Further, with the integration of biometric and/or lifestyle data, you can begin to approach the pinnacle of personalization possible in prescription eyewear today.

Then why have so many eyecare professionals shied away from making position of wear values part of their daily routine? One factor is an old friend: risk management. "Why mess around with what's not broken?" and "What difference do these measurements REALLY make?" are comments often heard. To answer these questions, we'll take a new look at how personalized measurements and other lens variables contribute to optimizing the optics in free-form single vision and progressive lenses.

TAKING MEASUREMENTS: WHO'S ‘DEFAULT' IS IT?
Even in the years before ECPs thought about measuring anything other than PD, lens designers were on their side. Since eyeglass optics, fashion and fit make most prescription lenses deviate from the shape, tilt and plane of lenses in the exam room, lens designers had to make certain assumptions about how most eyewear would look and fit. By definition, the values they assumed for base curve and position of wear meant averaging to a mean value.

The common values for position of wear are as follows:

Yet a recent study conducted by Essilor found that five out of six wearers departed from these assumed values. The study concluded that more than 78 percent of eyewear sold will benefit from the use of tailored position of wear measurements. Many eyecare professionals responded by saying, "My patients aren't complaining, so why do all this extra work?" Perhaps they are complaining, but in ways not easily recognized. For example, it is clear that even small adjustments can often illicit patient comments such as "That feels better!" The reason is that eyewear satisfaction is really a complex recipe of many interrelated factors. When we allow that more than three-quarters of all eyewear is not aligned with the lens designer's intentions, we can begin to understand how deviations in just alignment alone—and not including prescription, prescription change, material and lens choice—can bring our patients close to the invisible border that separates "satisfied" from "unsatisfied." Anything we can do to prevent "crossing-over" is paramount to maintaining patient trust and satisfaction.

OPTIMAL LENS MAPPING: USING TRACE DATA AND VERTEX DISTANCE
While lens engineers supported us by optimizing their designs to presumed fitting defaults, they were also faced with the challenge of not knowing just how much the lens' surface may be exposed within the final frame selection, which impacts consideration of eye excursion angles. Therefore allowing the possibility of large eye excursion angles within wide and tall eyewear fashion meant compromising lens optimization. Today, the broad availability of libraries of actual frame shapes together with digitally-enhanced, free-form lenses make this type of compromise completely surmountable. Even without finding a specific frame in a lab's library, a digital frame tracer allows any frame style to enjoy the increased optimization possible from knowing precisely how much lens area is involved, as well as its exact coordinate location. Traditional lens measurements such as A, B, DBL and ED represent five elemental shape defining points. Even just a two-axis frame tracer enables optimization software to use as many as 512 points, an improvement of several magnitudes.

Using a tailored value for vertex distance additionally contributes to optimal lens mapping. Both the actual geometry of the chosen lens shape and its distance from the eye help to define which parts of the visual field should be prioritized in a personalized lens' overall optimization. No longer is taking a vertex distance to be relegated for use only in lens power compensation.

BIOMETRICS AND LIFESTYLE: THE NEXT STEP IN DYNAMIC EYEWEAR PERSONALIZATION
Using position of wear measurements is a great way to start personalizing lenses. But pantoscopic angle, frame wrap angle and vertex distance are, at their most fundamental level, only a set of static values. Knowing the specifics of an individual's proportion of eye-turning versus head turning allows a progressive lens to be properly weighted in design. For example, a person who is primarily an eye turner benefits or prefers a slightly harder progressive design which features larger areas of stable vision characteristics.

Another personal biometric is head cape. Whether due to eye dominance, suppression, postural restrictions or personal habit, a progressive lens engineer knows that some people do not focus their gaze of attention where their nose is pointing. Making assumptions about normal head cape are about as valid as the ones using default position of wear values.

Metrics are but one way to arrive at dynamic lens optimization. Another way is through the use of a weighted lifestyle questionnaire. By asking a prospective wearer to evaluate their various interests, hobbies and visual activities, including cycling, art, reading, computer, sports, etc., and assigning a degree of importance to each one, lens engineers can then use custom software to help calculate an overall utility rating to a lens design. In this way, highly-weighted activities can be optimized while not negatively impacting others receiving ratings of less importance.

DEAR ABBE: HOW POLYCARBONATE GOT A BUM RAP
During the 1980s, as frame fashions evolved to new heights of size and novelty, our trusted non-glass lens offering was centered on standard plastic lenses. But issues of increased thickness, excessive curvature, warping and impact resistance paved the way for polycarbonate to become a dominant player in our lens arsenal. Offering reduced thickness, flatter curves, light weight and resistance to impact and warping, polycarbonate appeared to have it all. After a time however, almost every ECP became acquainted with patients who couldn't or wouldn't tolerate polycarbonate lenses for apparently unknown reasons. These individuals also resisted our efforts to determine in advance whether they might experience problems with polycarbonate lenses.

Today we know why. The optical errors introduced by polycarbonate's chromatic aberration, caused by its low Abbe value, were being layered on top of other unintended optical errors. Most of these originated from one or more of the following:

1. Overly flattened base curves that departed from orthodox best form choices.
2. Inattentive vertical optical center placement violated Martin’s rule for lens tilt, resulting in a compounding of off-axis lens errors.
3. The inherent inability of simple spherical curves to fully correct off-axis aberrations, especially with stronger or more astigmatic prescriptions.

Today, free-form lens technology along with personalized measurements enables optimized digital designs to conquer the compounding effect of these errors. Now, off-axis chroma errors created by using any lower Abbe material including poly, 1.67 and 1.74 are no longer added to errors originating from poor fitting or a lack of global lens optimization. With personalized measurements and good free-form, digitally-optimized design, you may be surprised to find your formerly finicky patients no longer have complaints with polycarbonate lenses.

PERSONALIZING EYEWEAR WITH BASE CURVE
In strict best form/corrected curve fitting, the selection of the proper lens base curve was left up to either the lens manufacturer or the lab fabricating the lenses. One did not normally alter the base curve supplied, except for the following reasons:

1. Eyelash clearance: A steeper base curve would create more vault, but at the expense of departure from ideal off-axis optics.
2. Doctor's instruction to "match base curve": Essentially a catch-all notation, the intent of which was to reduce problems in perceptual adaptation. May have also resulted in poorer, off-axis optics.
3. Overly flat or steep frame curves: The priority to deliver a finished pair of glasses and ensure the lenses tracked the frame and stayed secure often resulted in a departure from best-form optics.
4. Aniseikonia: Some specialists and eyecare professionals would play with base curve to reduce image disparity issues in antimetropic prescriptions. This almost always degraded optics for the sake of optimizing image size.

By using free-form, digitally-enhanced lenses, we no longer need to be a prisoner to base curve matching, or have to accept inferior optics for the sake of the above solutions. In fact, not only can free-form lens technology allow the freedom to choose a base curve easily across a range of 2 diopters on either side of the target best form curve, now we can even address retinal image size disparities and more in moderately anisometriopic prescriptions (see side bar).

THE END OF COMFY, COZY AND COMPROMISE
As we begin to leave behind the introductory "getting to know you" phase of free-form, digitally-optimized lens technology, we're entering a time where consumers have not only heard about these new "HD" lenses from their family, friends or the Internet—they're now coming in and asking for them by name. It behooves every eyecare professional to move out of their traditional comfort zone and become familiar with the tools and techniques of creating truly optimized eyewear through personalized measurements—and sooner than later. The comfy, cozy and simpler times of making eyewear using just PDs and heights are as gone as are the days before Amazon and Google. Sure, there's a lot to learn and master. And it will require investment in the latest tools and training so your office can get started on the right foot. And there will be times that you and your staff will scratch your heads trying to figure out why even this terrific technology doesn't deliver a "wow" factor for every single patient.

But one thing I can promise you: If your practice doesn't ante up soon, your competition certainly will. Personalized lenses are the future of our eyecare industry, and it is wise to get into something new and exciting while it's still on the ground floor. ■

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Barry Santini is a New York State licensed optician based in Seaford, N.Y.