A RANGE OF DIFFERENT LENS MATERIALS MEET THE STANDARD, INDUSTRIAL SAFETY AND BALLISTIC LEVELS OF IMPACT RESISTANCE TO KEEP PATIENTS SAFE UNDER A VARIETY OF CONDITIONS
Requiring lens impact resistance in eyewear is a modern idea. From a durability perspective, eyeglasses have been viewed for the most part as delicate and even sometimes flimsy devices throughout history. Through the centuries, lenses were made of glass, and because of this, people accepted that their eyeglass lenses could scratch, chip and break. Back then, the only way to have made them stronger was to make them thicker, which would make them heavier, uncomfortable and poorly cosmetic.
While the (U.S.) National Bureau of Standards published the Z2 Handbook H24 detailing the eye and face protection of industrial workers beginning in the 1930s, it wasn’t until the 1960s that a dialogue began in America about protecting consumers from eye injuries through federal regulation. Supported by optical industry groups such as The Better Vision Institute and Prevent Blindness, the momentum for such regulation gained strength, and in 1971, the Federal Food and Drug Administration (FDA) established an impact resistance standard for eyeglass lenses (Code of Federal Regulations CFR 801.410). The regulation prescribes a test that uses a stainless steel ball dropped free-fall 50 inches onto the lens’s front surface, which precipitated the rule’s nickname of the “drop ball test.”
Understand that the discovery of glass dates back thousands of years. Contrast that with the implementation of a federal impact resistance regulation to protect consumers for eye injuries and you’ll understand why eyeglass lens impact resistance for consumers is a modern issue. That’s not to say that impact resistance was completely ignored by the optical industry. Several manufacturers produced very capable safety eyewear using glass lenses. Yes, the lenses were thick and heavy, but they did the intended job. It was the advent of more capably impact-resistant plastic lens materials that spurred the industry to consider the impact resistance regulation.
For eyecare professionals, knowing the exact impact resistance of a material is difficult for a few reasons. For starters, lenses break at different stress levels depending on the type of object that strikes them. For example, a lens material may break at a lower impact with a large mass, slow moving projectile then it does with a small mass, high velocity projectile. For another material, it might be exactly the opposite. The shape of the lens material is a factor, as is the material’s thickness. The size, shape and density of the projectile are all factors too. Life doesn’t provide a single impact challenge to eyeglass wearers, which is the problem with having a single impact test for the regulation.
Lens materials manufacturers are more concerned with the overall performance of the materials they produce because they must perform well for all of life’s challenges. That’s why they use additional tests to determine a material’s other physical characteristics in addition to its impact resistance. While their lens materials will pass the FDA regulatory impact test, they are also often capable of much more than that. For example, PPG’s CR-39 exceeds the FDA impact test by 18 times, and its Trivex material exceeds it by 400 times, while 1.60 exceeds it by 30 times.
Instead of detailed specifications on the impact resistance of a lens material, you can place lens materials into three categories: standard impact, industrial safety and ballistic (see chart). Standard impact materials are capable of meeting or exceeding the FDA drop-ball test regulation. Industrial safety lenses meet a higher impact standard defined in the ANSI Z87.1 – 2010 standard. Ballistic level lenses meet a U.S. military standard (MIL-PRF-31013) that requires a ¼-inch odd-shaped projectile to be fired at the lens at 640 to 660 feet per second (about 400 mph) from different angles. Placing lens materials into categories such as these makes it much easier to decide what to provide, depending on the needs of the wearer.
CHOOSING A MATERIAL
The first decision eyecare professionals (ECPs) must make about any lens material they wish to recommend to a patient is the degree of impact resistance the patient requires. Ophthalmic lens materials fall into three fundamental impact categories: standard impact, safety (industrial) impact and ballistic. Once this is decided, other factors such as weight, thinness and scratch resistance should be considered. Understand that if an ECP chooses a safety or ballistic impact material, the number of material choices becomes very limited. Even so, it’s the ECP’s responsibility to recommend what’s best for the wearer.
Despite manufacturer efforts encouraging ECPs to use higher index materials regularly, even for lower powered Rxs, ECPs continue to use material indices suited for an Rx’s needs. Another trend worth mentioning is the growth of ballistic level lens materials. Their initial use by police, fire and military personnel has widened to the sports market for anyone who wants that level of protection. For example, that level of protection would be comforting to a parent of a young child.
It’s important to know that coating a lens weakens it. This includes coatings such as anti-scratch, mirror and anti-reflective treatments. Manufacturers take this into account when creating a new lens material because the finished lens (the coated, cut and edged lens) is what must be capable of passing the impact testing.
The future of impact-resistant lenses is a bit unclear. There is no dialogue in the U.S. about the need for increased impact resistance or a cry from ECPs or industry representatives to ease the FDA regulation. On the other hand, consumers are increasingly interested in safety. For those who want it, ballistic level lens materials are available for everyday Rxs.
Some lens materials companies are working on composite lens materials, which are essentially different materials and coatings laminated/bonded together. This lens “sandwich” can make for an exceptionally impact-resistant lens that has multiple features because of all the layers.
Ed De Gennaro, MEd, ABOM, is editor emeritus of First Vision Media Group.
WHERE TO FIND IT: COLTS Laboratories 727.725.2323 • ColtsLaboratories.com // 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 // Wiley X, Inc. 800.776.7842 • WileyX.com • Info@WileyX.com