ALTHOUGH A SMALL PERCENTAGE OF OVERALL LENS SALES, GLASS REMAINS A VIABLE MATERIAL WITH SOME SUPERIOR QUALITIES TO OTHER OPTIONS.
According to the Corning Museum of Glass, glass making dates back millennia to before 2,000 B.C. in Mesopotamia where it was cast in molds or shaped with crude tools. For centuries, glass beads were traded like currency. A hard-to-produce version of hollow glass vessels first emerged in the 16th century B.C. Early glass like this was only for the wealthy.
Glass was the accepted lens material for eyeglasses for centuries and was the only material used in the U.S. until the advent of PPG Industries’ CR-39 ophthalmic plastic lens material in the 1960s. Like most products introduced to the optical industry, the acceptance of plastic lenses was initially very slow. That changed in 1971 when the FDA mandated that all lenses had to be capable of withstand-ing a specified impact. Glass lens-es had to be heat or chemically treated to meet this test (or made extra thick), but plastic lenses were inherently able to pass the test. This catapulted plastic lens usage, and glass usage rapidly declined.
“Glass is the history of the eye-glass industry,” according to Lyle Rubin, sales and marketing manager of specialty glass for Corning. “Of all the commonly utilized oph-thalmic lens materials, crown glass has a high Abbe value, making it remarkably clear and color free. Its exceptionally smooth and hard surface makes it an ideal refracting surface. It also accepts and adheres optical coating better than most plastic material, including some hardcoatings. These are some of the reasons many eyewear compa-nies continue to sell glass lenses.”
Glass is made from three compo-nents: The “former” (usually silicon dioxide, which comes from sand) is the foundational portion of glass; the “flux” helps lower the melting point of glass; and the “stabilizer” (often limestone, which contains calcium oxide), helps the glass avoid unwanted rystals, crumbling or dissolving.
The dry ingredients for glass are heated to about 2,400°F to liquefy them and stirred to make a homogenous melt. Glass was made for millennia in small individual patches. It wasn’t until the indus-trial revolution and the 20th cen-tury that glass was able to be produced in an automated continuous flow process. This enabled glass manufacturers to produce glass in a continuous stream instead of in small batches, which greatly increased production output.
While there are a number of small ophthalmic glass producers in the world, there are two domi-nant and respected ones. Corning, Inc., headquartered in Corning, NY, produces a plethora of glass products such as fiber optic for internet cabling and Gorilla Glass for mobile phone screens. Corning also makes a variety of ophthalmic materials. Like Corning, Barberini, headquartered in Venice, Italy, produces an array of glass materials, including oph-thalmic ones. Founded in the 1950s by the Barberini family, he company specializes in glass sun lenses. There are also some large emerging ophthalmic glass lens producers in India and Asia.
If you work in a retail optical environment, you know that the percentage of Rx glass lens pairs dispensed is very low. Data from The Vision Council indicates that glass Rx lens are less than 0.5% of the lens units used in the U.S. In the U.S., glass lens shipments continue to decline (-2.2% in unit sales during the twelve-month period ending September 2016). In 2015, shipments of all glass lenses reported was 289,458 pairs, down by 40% in unit volume from 2010. A good portion of these glass lens pairs shipped domestically in 2016 were multifocal or PALs without photochromic properties and/or polarization.
On the other hand, glass plano sunglass lenses are doing well, and some professionals in the field indicate that this segment of the market is growing slowly.
Even with low Rx usage, some labs specialize in surfacing glass. For example, Vision Dynamics Laboratory in Louisville, KY, (a division of Vision Dynamics L.L.C.) processes primarily glass lenses, which comprise about 95% of overall production. Michael Yager, VP of sales for Vision Dynamics, explained, “We got into glass lens processing when a large retailer decided to get out of glass processing and asked us if we’d do it for them. I describe us as a lab-to-lab glass lab. Most labs still get a few glass jobs and don’t want to disap-point their customers, so they send the work to us. Their glass business is 1% of their orders; our glass business is 99% of ours. We have a huge glass invento-ry, and we process it expertly because we have so much equip-ment and experience. We do offer free-form glass lenses, but our top sku is flat-top 28 PhotoGray Extra bifocals surfaced the conventional way.” Other labs that process a substantial percentage of glass lenses include Essilor Laboratories of America, Select Optical, FEA Industries, Carl Zeiss Vision, HOYA North America and Precision Optical.
Eyecare professionals know that CR-39 is about 50% lighter than crown glass. One trend eyewear producers use to make their glass lenses lighter is to make them thinner and treat them with an ion exchange process. This makes the lenses substantially light-er and closer in weight to their CR-39 counterparts. The lenses are also stronger because of the ion exchange process, so they pass the FDA-required impact test. Some do this by using Corning’s Clear 15 product (see below), which uses a 1.5mm center thickness instead of the usual 2.2mm +/-0.2mm.
Corning makes a variety of stan-dard and high index ophthalmic glass materials, including clear Unicrown (1.523), 1.7, 1.8 and 1.9. The company also makes standard and high index photochromic glass materials as well as several sun lens colors and specialty lens materials. Introduced last year to the Rx market, Corning Clear 15 is a new high-performance glass that can be produced as thin as a 1.5mm center thickness. “This product is surfaced 25% thinner and therefore lighter than standard glass, which brings plano and lower powered prescriptions much closer to the weight of CR-39 lens-es with a thin profile,” said Lyle Rubin of Corning.
New processing and strength-ening techniques make glass lenses sharper, clearer and more impact resistant, Rubin pointed out, stating, “Glass today is much better than it was years ago. ECPs should also know that there are proper-ty variations among glasses and within the same index of glass, just as there are in the plastic market. Not all glass or plastic lens mate-rials of the same index are made the same.”
Barberini also makes a wide vari-ety of ophthalmic glass materials for the Rx market. While the com-pany specializes in plano glass sun lenses, it obtained the ophthalmic glass business from Schott in 2010 when that company sold off that portion of its business. Now, the company offers an array of ophthal-mic glass comparable to Corning and produced in Germany. Its premium product, Platinum Glass, is manufactured with a highly refined process that uses a column of pure platinum to reduce impurities.
Noted for its sun lenses, Barberini offers them all with UV absorption to 400nm, hydro-phobic, oleophobic and anti-reflective properties. Some reduce infrared light. Offering a range of lens colors as well as mirror treatments, Barberini glass lenses are available in the company’s own sunglasses collection as well as in the lenses the company produces for sunwear sellers around the world such as Tom Ford, Loro Piana and Etnia Barcelona.
While most U.S. lens companies sell very little glass (or none at all), a few sell a great deal. For example, X-Cel Optical has a full line of glass lenses with many design choices, including single vision, several flatop bifocal sizes, two round bifocal sizes, trifocals, quadrafocals, dou-ble segment bifocals and a ribbon segment lens, all offered in differ-ent sun lens and photochromic colors as well as clear.
VISION EASE continues its tradition of offering a wide array of glass lens designs. “We will continue to supply (glass prod-ucts) as long as sufficient cus-tomer demand remains,” said Jay Lusignan, director of marketing communications. The company produces single vision and mul-tifocal lenses in clear and pho-tochromic crown glass, with its top two skus being semi-finished single vision and flat-top 28 bifo-cals. Lusignan theorizes, “There will always be a niche glass market in the U.S. New technologies like tempered glass and electronics eye-wear will perhaps open opportuni-ties, but the glass share of market will likely remain low.”
CORNING’S TAKE ON THE BENEFITS OF GLASS Glass has many appealing qualities, and a number of optical industry professionals mention that some members of the public still see glass lenses as superior to plastic lenses, even after 50 years of plastic lens use, along with improved processes and coatings. Corning offers the following beneﬁcial properties of glass:
STRENGTH: Glass is naturally strong but weakens when surface imperfec-tions are present. Improved processing can mitigate this. Glass can also be heated or chemically treated, which improves its strength and impact resistance.
HARDNESS: Glass is inherently durable and doesn’t require an anti-scratch coating to protect it, so it resists scuff and scrapes better than plastic materials.
ELASTIC: Like most rigid materials, glass ﬂexes under stress, up to its breaking point. When it rebounds, it returns to its original shape without deforming.
CHEMICALLY RESISTANT: Glass is not affected by many household or indus-trial chemicals.
THERMAL SHOCK RESISTANT: Glass tolerates high and low temperatures without deforming as well as sudden changes in temperature such as com-ing into a warm house after being outside in sub-freezing temperatures.
HEAT-ABSORBENT: Glass retains heat instead of conducting it.
ELECTRICAL INSULATING: Glass acts as a good electrical insulator.
OPTICAL PROPERTIES: Optical glass efficiently reﬂects, refracts, transmits and absorbs light with great accuracy.
Lyle Rubin of Corning sees a bright future for glass. While he doesn’t believe it will become the main-stream product it once was, he feels that enhancements to glass from other innovations, such as smart-phone display screens, LCD TVs and spacecraft windows, will find their way into ophthalmic glass. He also feels that there is a seg-ment of the public that will never accept plastic lenses. He also noted that there is still substantial glass usage in foreign countries such as India. That demand will continue to keep the ophthalmic glass mar-ket viable.
Others have indicated that glass has a place in the future of ophthalmic lenses, especially as wearable electronic eyewear emerges. Some of these lenses are already being made with multi-ple layers. Some predict a tough glass will be the outside layer. Some also see composite lenses where each layer of the lens has a specific function. While glass will likely never see the universal usage it once did in eyewear, there is still a viable place for it, and its future looks promising.
Ed De Gennaro, MEd, ABOM, is editor emeritus of First Vision Media Group.
WHERE TO FIND IT: Barberini Eyewear BarberiniEyewear.it • Info@Barberini.it // Carl Zeiss Vision, Inc. 800.358.8258 • Zeiss.com/Lenses • Customer.Service@Zeiss.com // Corning Specialty Glass 813.758.1065 • Corning.com/Ophthalmic • Ophthalmic@Corning.com // Essilor of America, Inc. 800.542.5668 • EssilorUSA.com // FEA Industries, Inc. 800.327.2002 • FEAInd.com • Sales@FEAInd.com // HOYA Vision Care, North America 877.528.1939 • HoyaVision.com • SalesSupport@HoyaVision.com // Luxottica 800.422.2020 • Luxottica.com // Precision Optical Group (P.O.G. Labs) 800.497.9239 • PrecisionOpticalGroup.com // Maui Jim, Inc. 888.666.5905 • MauiJim.com // Select Optical 800.331.1603 • SelectOptical.com // Serengeti 888.838.1449 • Serengeti.Eyewear.com // Vision Dynamics Laboratory 888-900-5503 • VisDynLab.com // VISION EASE 800.328.3449 • VisionEase.com • Info@VisionEase.com // Vuarnet 914.495.3701 • Vuarnet.com // X-Cel Optical Co. 800.747.9235 • X-CelOptical.com
Crown glass has a high Abbe value, making it remarkably clear and color free.