L&T: RxPertise

Jul
2013

Dispensing Trivex, The Decathlete of Lens Materials

 
By Barry Santini

It’s been just over a decade since PPG introduced Trivex, the lightweight, durable mid-index lens material that PPG positioned as an upgrade from its industry-standard CR-39 monomer. Although Trivex has gained many supporters, some eyecare practitioners remain hesitant to use it. Their objections have centered on three concerns: price, thickness and need.

Clearly, controlling one’s cost of goods while delivering fashionable, attractively-finished prescription eyewear are important business concerns. But for those ECPs who have yet to be won over by Trivex, it’s time to reconsider. Can you imagine an optical business trying to survive today by offering only glass or CR-39 lenses, the preferred materials from the 20th century? Trivex’s unique combination of optical and performance characteristics makes a persuasive case that it will become the benchmark against which all other lens materials will be judged in the 21st century.

THE ALL-AROUND OPTICAL ATHLETE

Suppose you asked a customer, “How would you like an extremely lightweight lens, featuring superior optical clarity that won’t crack, fracture, chip, warp or shrink?” I don’t think any eyeglass wearer would answer “no” to this sales opener. But limiting a discussion of Trivex’s attractive qualities to just this short list would completely miss its unsurpassed excellence in the following 10 categories:

  1. Good optics—a catch-all industry term for lens materials known for having good optical characteristics. These include:
    1. Low chromatic dispersion (see “Abbe and Good Optics” sidebar).
    2. Low inherent material stress—Trivex lenses are not injection-molded.
    3. Stable surface curvatures—Trivex has high resistance to circumferential deformation.
  2. Ultra-lightweight—Trivex features the lowest density, 1.11, of any optical-grade material on the market. Trivex lenses are, molecule for molecule:
    1. Eight percent lighter than polycarbonate; 16 percent lighter than CR-39; 19 percent lighter than high-index.
    2. The lightest lens choice for the power range from -3.00D to +3.00D, which encompasses 85 percent of all prescriptions. When dispensed in aspheric or a good free-form design, prescriptions may be made even lighter.
  3. Impact Resistance—no ophthalmic-grade lens material is more impact resistant than Trivex.
    1. Exceeds the FDA minimum impact standard for street/dress eyewear by a factor of 60 times.
    2. With the FDA impact standard now over 40 years old, passing the drop ball test means delivering only the minimum in lens protection.
  4. Dimensional Stability—Because of its unrivaled ability to both deform and return, Trivex lenses also possess high resistance to:
    1. Shrinkage—Unlike CR-39, Trivex lenses won’t shrink and fall out of a frame over time, even in sunglasses. This quality is central to consumer eyewear satisfaction surveys.
    2. Warpage—Even under the uneven pressures that either oddly-shaped frames or non-optimal edging creates, Trivex lenses resist peek-a-boo popping out, making it the best choice for mild prescriptions featuring moderate cylinder powers around axis 90.
    3. Resistant to cracking from long-term compression—In cases where radial fractures are occuring from lenses being mounted too tightly, Trivex has proven to be particularly impervious to this type of consumer satisfaction killer.
  5. Chemical Resistance—Trivex features extreme resistance to common home and office solvents, such as ammonia or nail polish remover, aka acetone. There is far less likelihood that innocent exposure to either of these chemicals will accidentally damage or destroy an otherwise perfectly good lens. Additionally, the more secure and long-lasting thread lock liquids can be used to retain eye wire screws without fear of edge cracking from solvent attack.
  6. Fracture, chip and crack resistance—Particularly when used in frames with exposed lens edges, such as nylon string mounts, Trivex consistently proves to be the most resistant material to fracturing, cracking or chipping in day-to-day handling. Owners of Trivex lenses do not fear the accidental drop onto a bathroom or kitchen’s tile floor.
  7. The best material for making thin lenses—Trivex’s superior impact qualities allow it to be ground to the thinnest edge or center of any lens material.
  8. Heat Resistance—Accidental exposure to high temperatures, such as those found within a closed car during the months of July and August, will not affect the Trivex substrate. Trivex is also very resistant to material deflection brought on by the higher temperatures encountered during hard coat curing processes. These can reach over 266 degrees and can result in optical distortions of the finished lens. Trivex is 90 percent more resistant to this type of induced distortion than polycarbonate.
  9. UV Protection—Trivex offers outstanding UV protection, blocking 100 percent of UV rays all the way to 395 nanometers, well beyond ANSI standards. This is 40 nanometers more than untreated CR-39.
  10. Value—The modest up-charge ECPs see on their wholesale price lists for finished, conventionally-surfaced or free-form manufactured Trivex lenses is easily offset by all its terrific qualities. Reviewing the totality of all of Trivex’s properties, it’s reasonable to say that Trivex lenses actually represent a bargain.

ABBE AND GOOD OPTICS

In the 1970s, when the only lens materials available were glass and plastic, there was no talk of Abbe values or color dispersion. Both glass and CR-39 have similar Abbe values, meaning they are essentially free of the image-degrading effects of transverse chromatic aberration. The Abbe value of a lens value represents an inverse function: The higher the Abbe number, the less the traverse color error.
Below are some Abbe value benchmarks:
CR-39/crown glass
Water/aqueous
Trivex
Polycarbonate
1.60 index
1.67 index
Human eye
59/59
56
45
30
42
32
45


Eyecare professionals have long sought to understand the exact nature of the vision-degrading effects of low Abbe value, and how they can calculate at what point Abbe will have a negative impact on a person’s acuity or contrast sensitivity. In eyeglasses, Abbe’s effect is noticed as the primary gaze moves away from a lens’ optical center (OC). Therefore, Abbe’s impact is related to the total dispersive effect at the off-axis point in question. The lens power, the distance from the OC, and the Abbe value all must be included in our calculations. Let’s take an in-depth look at some Abbe math:

(The following is used with permission from opticampus.com)

The aberrant contribution of Abbe value is referred to as Lateral Chromatic Aberration or LCA. The formula for this is: LCA = Prism ÷ Abbe value For example, consider a +6.00D lens made with polycarbonate, which has an Abbe value of 30. At 15 mm (1.5 cm) from the optical center, the prism is +6.00 × 1.5 = 9.0 prism diopters (∆). The lateral chromatic aberration LCA is equal to 9.0 ÷ 30 = 0.30 ∆.

To help us estimate the actual impact on visual acuity of LCA, vision scientists have given us this table approximating its effect:
Lateral CA
0.05 ∆
0.10 ∆
0.15 ∆
0.20 ∆
0.25 ∆
0.30 ∆
0.35 ∆
0.40 ∆
0.45 ∆
0.50 ∆
0.55 ∆
0.60 ∆
Visual Acuity
20/20
20/22
20/24
20/26
20/28
20/31
20/34
20/39
20/44
20/51
20/60
20/75


Armed with this information, an ECP can finally quantify and appreciate Abbe’s effect on vision. But this is not the whole story.

THE INTERACTING LAYERS OF VISION SATISFACTION
Subjective vision satisfaction is a compendium of all the layered optical effects at work within a single pair of glasses. As the eye is never stationary for long, maintaining peripheral image quality becomes a very important predictor of visual satisfaction. Always keep in mind that any or all of the following “layers” can and will influence peripheral optical quality:

Base curve—Often overlooked, misunderstood or left to others to specify, placing a selected sphere power on the optimal best form base curve is important to getting great vision. Rarely appreciated is the fact that higher-index materials, in spherical form, require steeper base curves, not flatter, to remain compliant with corrective curve design goals.

Astigmatism—Appreciable astigmatism error greater than 0.75D has never been well-optimized off-axis in conventional corrected curve theory. Significant astigmatism, greater than 1.50D, can only be properly optimized peripherally with a well-designed free-form lens.

Fitting—Even though using the appropriate spherical base curve or an aspheric form can help maximize peripheral clarity, the reality is that superior off-axis vision is often compromised by improper fitting. Optimal fitting requires consideration of pantoscopic angle, fitted vertex distance, primary gaze angle and frame wrap angle.

The Rx—Here’s the real unknown: If the prescription supplied is deficient in delivering the best central vision, then good peripheral acuity is bound to suffer.

All of the above factors can sum up to impact peripheral clarity. Some layers, such as modifying the Rx, are often out of the dispenser’s hands. Others, such as base curve or fitting placement, can be compromised by fitting and fashion considerations. In the end, the parts of the acuity success recipe that are under the dispenser’s control are the choice of a lens design and a material with a good Abbe value. Choose wisely.

BUILDING SUCCESS WITH TRIVEX
Today’s savvy ECP is always on the lookout for the next big opportunity to help differentiate and leverage their business. They know that managing the challenges of attracting and retaining clients through a “crisis of the day” approach is no longer a viable business plan. Instead, let’s use the interacting-layer approach from above to help raise the quality of your client’s visual experience through a recipe I call “The Trivex Trifecta.”

Begin by fitting every pair of glasses by measuring pantoscopic angle, vertex distance and frame wrap—whether the lens you ultimately choose actually demands these measurements. Better, buy one of the newer digital centration devices and learn how to use it.

Next, take the time to inform your clients of the benefits of optimized, free-form lenses. And don’t overlook single vision. If the Rx has cylinder power over 1.00D, definitely select a good, free-form single vision lens.

Discuss how Trivex, a new lens material which won’t chip, fracture, crack or break, will combine lightweight and superior optical quality for a tangible visual improvement. When you go to order, consider specifying Trivex in place of other alternatives for prescriptions in the range between -3.00D and +3.00D.

When your client comes in to pick up their new glasses, listen for their “wows.”  Stress how this type of outstanding clarity is a recipe combining digital measurements, optically-enhanced lenses and the good optics of Trivex—all working together to deliver an exceptionally sharp visual experience.

TRIVEX: THE BEST LENS AND THE BEST VALUE
It will soon become clear to your clients that getting the best all-around vision, protection and comfort for their eyes requires a lens that features outstanding performance in more than one category. Trivex’s outstanding balance of qualities makes it the undisputed decathlete of lens materials. And don’t forget: Trivex is great for rimless too.

THE GORILLAS IN THE ROOM: CR-39 AND POLYCARBONATE
A lively discussion topic amongst eyecare professionals, often heard over a few beers at the bar: Which is the best lens material: CR-39, poly or Trivex? Before we try to decide the winner, let’s take a quick look at each material:

CR-39
Developed in the early 1940s during a general investigation and expansion in the creation of plastic material substitutes needed during WWII. One of CR-39’s attractive attributes was that it could be laminated with textiles, thereby reinforcing it while maintaining light weight. These qualities made CR-39 an ideal choice for fuel tanks and especially cockpit fuel lines, which were previously made of glass tubes. Here, CR-39 allowed crew members to note fuel levels and supply while being less apt to shatter in the heat of battle. But just as the developer, PPG, cranked up its manufacture, the war ended, and the company ended up with a tank-car sized load of this expensive monomer, with no immediate customers. The sales force was charged with soliciting the interest of other industries. Soon, established lens optical companies, both domestically and overseas, expressed interest and began learning how to manufacture optical lenses using this unique chemical. The rest, they say, is history. When used for eyeglasses, CR-39 offers lightweight, good scratch resistance, great optics and an easy ability to tint, both for fashion use and sunglasses.

POLYCARBONATE
Also came about from research looking for plastic alternatives to glass, polycarbonate first saw an optical use in the 1950s in the canopies of the then new jet airplanes. Beginning in the 1980s, the development of ophthalmic lenses from polycarbonate progressed, but manufacturers met some roadblocks:
  • Virgin poly was not very scratch resistant, and appropriate coatings had to be developed to ensure optical consumers would be satisfied with their day-to-day handling of poly lenses.
  • Early adopters, such as Gentex Optics, initially found meeting the more stringent quality standards of the ophthalmic community a challenge, especially compared to their experience in the industrial market.
  • In the decade it was launched, tinted lenses were a very popular option for eyeglasses. As the poly substrate itself cannot be tinted, a revised type of tintable scratch-coating required research, development and testing.
Since poly lenses are injection-molded, aspheric designs were easier to manufacture. Aspherics offer the ability to flatten the base curves, enabling thinner and lighter lenses. Polycarbonate looked like a champ amongst lens offerings.

TRIVEX
Trivex, like polycarbonate, has superior impact resistant qualities. But unlike the simpler injection molding used to make polycarbonate, Trivex requires an extensive, ramped-cure manufacturing process, which may preclude smaller companies from entering the market based on price alone.

Coincidentally, the original chemistry that Trivex was developed from was created for military applications: a high-impact, mechanically stable, optically pure, and chemical and heat resistant material for use in helicopter windshields and fighter jet canopies. After tailoring the chemistry specifically for the optical lens market, PPG introduced Trivex in 2002. Adoption was slow at first, compounded by an initially higher price and a lack of education and preparation on how to process Trivex for both surfacing and edging. Optical companies rose to the occasion though, and special surfacing, edging and tinting techniques were created. As far as conventional tinting is concerned, Trivex lags behind CR-39. But companies such as Hoya have developed a full range of fashion and sunglass colors that not only resist fading much better than CR-39, they can be married with a premium AR coating and produce reliable colors and densities. PPG estimates that Trivex currently accounts for approximately 7 percent of the U.S. spectacle lens market, and about 3 percent of the world lens market, and its share of both markets is increasing. Today, Trivex lenses are available from more than 15 global lens manufacturers, and are compatible with the latest free-form and coating technologies and available across the complete range of Transitions photochromic lenses.

THE WINNER?

Well, it’s not that easy. Some 50 years ago, glass was the dominant player in the ophthalmic lens market, but that product is now entering its “end of life” phase, and companies are exiting the glass market because of shrinking demand. In the 1990s, polycarbonate was seen as CR-39’s ultimate successor. But early problems with quality issues, the difficulty of processing polycarbonate with edgers of the day and challenges with tinting pushed many ECPs back to the comfortable familiarity of CR-39. Although today’s polycarbonate lenses are much improved, many ECPs have bypassed it in favor of other higher-index options. But as research continues to yield new lens materials, some ECPs ask: “Do we really need another lens material?” This is not the right question to ask.

With technology rapidly changing our daily lives—from the amount of leisure time we spend on mobile devices, to the transition from media ownership to renting streaming entertainment—it’s literally a new day dawning every day. As technological advancements in both lens design and lens materials impact the world of vision care, it’s time for eyecare professionals to awake from the slumber-inducing lullaby of “everyone will always need glasses.” In today’s dynamically changing marketplace, the wake-up call is that John Q. Public just might not need glasses from you. To compete effectively today, eyecare professionals must start by promoting the latest technology in vision care products to your customers. As the Internet continues to empower information availability, staying ahead of what your clients can easily discover at the click of a mouse—or finger on an iPad—becomes essential in communicating your added value.

It’s time to question whether you’re really buying the best products for your customer—or your own pocketbook.
—BS


COMING SOON: TRIBRID—A NEW AND ADVANCED HIGH-INDEX LENS
From PPG, the same company that brought you Trivex, Tribrid is a new, high-index material combining some of the best attributes of its older brother. Featuring an index of 1.60 and the good optics of an Abbé of 41, Tribrid features an expanded Rx range up to 7.00 diopters. With a specific gravity of 1.21, Tribrid is the lightest of all high-index lenses. Its impact resistance is up to five times greater than other high-index choices. Due to be released this summer in the U.S., look for availability first from X-Cel Optical, who became an early adopter of this exciting new material.
—BS

 

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