The history of lens materials and lens making is an ancient story. Indications of lenses date back to 750 BC where quartz (n = 1.46) and other crystals were polished as lenses. The advent of the printing press around 1450 made reading material much more available to the masses, which increased demand for reading glasses, especially for presbyopic individuals. Essentially, Gutenberg’s printing press made the need for near correcting lenses universal, which placed a higher demand on glass lenses. Ernst Abbe (of Carl Zeiss) and Otto Shott developed top quality optical glass in a joint venture in 1884, mostly higher index materials for precision optics work.
In the first half of the 20th century, eyewear mostly used crown glass (n = 1.53) with some high-index glass. In 1971, the U.S. Food & Drug Administration mandated ophthalmic lens impact testing. The most common (although rarely used) high-index glass materials where dense flint (n = 1.69) and light flint (n = 1.61). Neither flint nor crown glass could pass the impact test without being treated, and many broke during treatment. This ushered in plastic materials, which could pass the impact test without treatment.
In the 1950s, PPG developed a plastic material called CR-39 (n = 1.50), which replaced crown glass as the main lens material in the early 1970s. It took many years before higher index plastic lens materials appeared, but today, there is a good assortment (see Selected Ophthalmic Lens Materials, below).
|SELECTED OPHTHALMIC LENS MATERIALS|
|Refractive Index||Abbe Value||Specific Gravity|
|MR-6 1.6 plastic||1.595||36||1.34|
|MR-8 1.6 plastic||1.592||41||1.3|
|MR-7 1.67 plastic||1.658||32||1.35|
|MR-10 1.67 plastic||1.661||32||1.37|
|MR-174 1.74 plastic||1.732||33||1.47|
|Corning Clear 16 glass||1.601||40||2.63|
Unlike the frame market, where new designs are introduced daily, a new lens material enters the market every few years. For example, MR 1.74 by Mitsui Chemical Co. was introduced about eight years ago and is currently the highest index plastic material available. PPG introduced Trivex in 2001. This mid-index (1.53) material is highly clear (Abbe = 44), highly impact resistant (meets safety and ballistic impact standards if processed correctly) and fabricates well. The fact that it is a mid-index material makes it ideal for 85% of Rxs (from +3.00D to -3.00D), and it’s also a favorite for rimless lenses. Trivex sales represent about 7% of all lenses sold in the U.S. and 3% globally.
Playing off its success with Trivex, PPG introduced Tribrid (n = 1.60) in the U.S. in 2016 and positioned it as having the qualities of Trivex with a higher index. The material boasts a 41 Abbe value, making it about as clear as Trivex, and its impact resistance exceeds the FDA drop ball requirement by 170 times! While it doesn’t meet industrial impact standards, it beats most other lens materials (for example, CR-39 exceeds the FDA drop ball requirement by eight times), making it a far safer option for most Rxs.
Polycarbonate (n = 1.60) continues to be the darling of the optical industry. With a market share of approximately 50% of all lenses sold in the U.S., you’ll find just about any conceivable lens design made with it. It’s impact resistance enables it to meet industrial safety impact standards, and like Trivex, it can meet ballistic (military) impact standards if processed correctly.
Ryan Rogers, North American sales and marketing manager at Mitsui Chemicals America, Inc., sees more functional lenses entering the high-index market. “œLenses should no longer just block 100% at UV 400nm,” Rogers noted. “œWe’re looking at two or three different versions, most notably our UV420 that filters blue light almost 80-90% at 420nm and drops slowly going higher, so it filters a lot of the harmful blue light outdoors, indoors on computer screens, on PDAs and from other sources. The advantage of this product is it’s clear.”
Rogers added, “œOur 1.74 lens material is made with a monomer that uses plant-derived chemicals, which most ECPs don’t realize. This eco-friendly material is USDA certified to be 82% bio-based. Our MR8 (n = 1.60) material is USDA certified at 57%, although no one is casting it at this point.”
Keith Cross, global director of Rx Technologies for PPG Optical Monomers and Coatings, said, “œBig casters advertise higher index because the market associates higher levels of technology with higher index, but that’s not really the case. As you go up in index, you have more tradeoffs such as more weight, lower Abbe value and decreased lens strength. In North America, 85% of prescriptions are in the range of plus/minus 3.00D. Why would you give them decreased optics, heavier, more brittle, more fragile materials? Instead, give the patient a combination of light weight, the best optics (highest Abbe) and the most impact-resistant material. That’s high technology and that’s Trivex.”
Cross added, “œIf you want a higher index material, use Tribrid. It’s clear, thin, lightweight and impact resistant. It drills well and tints great. It’s also very impact resistant. CR-39 exceeds the FDA drop ball test requirements by 18 times, while 1.60 and 1.67 exceed it 30 times. Tribrid exceeds it by 170 times. That’s an exceptional amount of impact resistance. Trivex and polycarbonate exceed the FDA standard by 400 times.”
HIGH INDEX’S FUTURE
Rogers sees opportunities for high-index products in the future. “œMitsui is moving toward materials that improve ocular health and safety,” he explained. “œWe’ve developed MR8 Plus, a high impact-resistant version of our popular MR8, 1.60-index lens material. It passes ANSI Z87.1 (industrial eyewear safety standards) high mass and high impact testing standards, and it’s been certified by COLTS Laboratories for this. It’s very similar to Trivex but not as impact resistant as polycarbonate. While it does meet the Z87.1 industrial safety eyewear standards, we’re not promoting it as a safety lens, but it could be a great option for everyday children’s eyewear. It tints phenomenally, making it a great option for wrap sunglasses. Its high Abbe value (40) facilitates better peripheral vision and better optical clarity in high wrap sunglasses.”
Rogers shared that Mitsui is working on a product called Neo Contrast. “œInstead of filtering out blue light, this product filters out yellow light around 585nm and will be available in two versions: clear and sunglass,” Rogers pointed out. “œWhen you filter yellow light, you increase contrast, and it makes greens and blues pop,” he added.
“œI think lenses will use improved substrate materials with one or more coatings that contribute different functions,” said Cross. “œThese lenses will use a variety of index materials from low to high. I also see wearable electronics as a viable option for lenses, and consumer-based technology will drive the optical industry for the first time in a long time. The consumer electronics industry moves at the speed of light. As these companies get into wearable optical devices, you’re going to see them drive development much more aggressively than we’ve seen in our industry.”
Rob Stevens, chief technology officer of Adlens, sees laminated lenses being part of the lens arsenal in the future. “œIncorporating more technology in a solid piece of plastic is challenging,” he said. “œTo add more features, you’ll have to either add them inside the lens material, coat the lens with the feature, or add multiple layers, creating a composite or laminated lens. For example, you can have a coating to reflect light, you can have LCD displays incorporated within the lens sandwich, and add elements that affect refraction.
“œOn a solid lens, thickness is driven by how rigid the material is and how thick it is to survive the drop ball impact test in the U.S. We have all kinds of things in our laminates. We’ve got silicon oil, a lot of traditional lens materials, and in the future, we’d even consider bringing glass back. It’s very hard, and when it’s thin enough, it’s very flexible. “
Lens materials may be introduced infrequently, but manufacturers are always looking for new ones that fill an unmet need. It’ll be interesting to see how lens materials and lenses evolve as the lifestyles of eyeglass wearers change over time.
Ed De Gennaro, MEd, ABOM, is editor emeritus of First Vision Media Group.
WHERE TO FIND IT: Corning Ophthalmic 800.821.2020 • Corning.com/Ophthalmic // HOYA Vision Care, North America 877.528.1939 • HoyaVision.com • SalesSupport@HoyaVision.com // Mitsui Chemicals America, Inc. 914.253.0777 • MitsuiChemicals.com // PPG Industries, Inc. 800.323.2487 • PPGTrivex.com • PPGTribrid.com • OpticalTech@PPG.com