|VSP’s Reveal Free-Form progressive lens comes in polycarbonate which has superior impact strength compared to CR-39.|
|HELPFUL HINTS ECPs should be aware of the FDA’s regulations regarding lens impact resistance. Here’s a link to the regulation: accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=801.410. The federal government posts questions and answers to help ECPs understand the rule: fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm070579.htm.|
|Vision-Ease Lens’ LifeRx is made of polycarbonate, which is up to 50% thinner and 50% lighter than standard lenses.|
|Trivex material can be used to make lenses with less than a 2.0mm center and still meet the impact regulation.|
|HOYA’s iD MyStyle comes in a high-index plastic lens material which has become increasingly popular.|
There are standards in place to ensure that thin lenses are safe.
How thin is thin when it comes to lenses? That may sound like rhetoric but in the world of eyewear it’s a seriously important question. It also begs the question: how much thickness reduction is enough between two lens materials? No matter how you slice it, when it comes to the optics of eyeglass lenses, thin is not as simple as it appears.
How thin is too thin? This question was initially answered in 1987 when the Food and Drug Administration (FDA) looked into the safety of eyeglass lenses. After years of investigations and testimony from countless stakeholders and the public, the FDA promulgated rules that detailed what needed to be done to make eyeglass lenses safe to a defined standard. The standard is commonly known as the “drop ball” test. This test consists of dropping a 5/8-in. stainless steel ball free fall for 50 in. onto the lens surface (the lens rests on a support with a neoprene gasket). If the lens doesn’t break, it passes. Glass lenses must be individually tested (there are some exceptions) while plastic lenses must be batch tested.
It’s interesting to note that this federal regulation was designed to protect the public from the potential hazard of shattering glass lenses. Even though the vast majority of lenses sold in the U. S. today are made from plastic materials, this consumer protection regulation survives.
Lenses that meet the drop ball test are considered impact resistant. When the regulation was first implemented, the way to achieve that impact-resistance level with glass lenses was to make the lenses no less than 2.0mm thick (2.2mm +/- 0.2mm) and either heat or chemically treat them. This thickness guideline became an industry standard for many years. In the ’70s anything less than 2mm became the common thinking about thinness. Many lenses still use this minimum thickness level because from a practical standpoint, it works.
As you read the FDA regulation, you’ll discover that the organization never specified a minimum lens thickness, instead, it defined the drop ball test. The 2.0mm concept came from industry groups attempting to meet the standard. This distinction is important. It was never the FDA’s intention to tell the industry how thin lenses could be, instead, it gave the industry a level of impact to meet; the industry imposed the thickness idea on itself. This means that according to federal regulations there is no minimum thickness standard.
TOO THIN TODAY
New lens materials ushered in new possibilities for lens thickness. The first to do so was polycarbonate. While it was very popular for use in safety eyewear, its superior impact strength in comparison to CR-39‘ (and similar ADC materials) meant that it could meet the drop ball test at less than 2.0mm. You soon found polycarbonate lenses being made in the 1.3mm and 1.2mm thickness range. Today a number of lens materials are capable of meeting the drop ball impact test at less than 2.0mm. For example, Trivex‘ material can be used to make lenses with less than a 2.0mm center and still handily meet the impact regulation.
WHAT’S TOO THIN?
Lens manufacturers and suppliers must test lenses and keep records of that testing. If you’re buying lenses from a nationally known company and you’re concerned that the lenses may not meet the impact regulation, ask for documentation. A simple note from the company saying they comply is fine. If you’re getting them from a lab, you can do the same thing. When in doubt, ask for documentation. That’s the only way you’ll know.
High-index plastic lens materials have become popular all around the world. In places like Japan they are all the rage, due in part to the high-minus power distribution in the population. U.S. eyeglass wearers love thin lenses and ECPs and their labs work hard to make eyeglass lenses appear as thin as possible. One way to do this is to use a high-index lens material.
How much thinner will a lens be if you use a high-index lens material? Table 1 provides some fundamental indications. Note that the table is simply a guide and does not represent any individual lenses. To answer this question specifically for two lenses, the design of those lenses must be take into consideration with factors such as base curve, type of lens, the degree of asphericity used, the impact capabilities of the lens (so it can be made thinner), etc. Even so, the table will be helpful when trying to assess which lens material might meet your patient’s needs.
There are two ways to use this table. As you recommend a lens material to a patient, mention that the material you’re recommending is 12%, 20%, 34%, etc., thinner than “conventional” plastic (CR-39 and similar ADC materials). By itself, that is pretty helpful, and having percentages like this will help the patient envision the difference. I’d suggest you also use a graphic that has thickness comparison lens profiles to reinforce this information.
Another way to use this table is to have a lens thickness for CR-39 and use the percentage in the table to determine the thickness reduction. For example, you have Mr. Ricks’ old lenses made of conventional plastic that has a +3.00D lens in it that has a 4.2mm center thickness. If you use 1.60 material for his new sun lenses of the same Rx going into this frame, you’ll reduce that to 3.4mm.
Let’s assume you’re sitting at the dispensing table and you’re thinking of recommending a higher-index lens product to help your patient achieve thinner lenses. Which one will you recommend? Table 1 might be handy as you explore this option. Polycarbonate (1.59 on the chart) will provide lenses that are 18% thinner. Is that enough thickness reduction? Going with 1.67 will almost double that amount of thickness reduction so perhaps that’s the way to go. It depends on how high the Rx is and whether your patient is willing to pay for the thickness reduction.
Some lens materials have a terrific advantage at reducing thickness because their impact strength enables them to meet the FDA requirement at less than 2.0mm. This means that higher-powered lenses can be made thinner than the table indicates because the lenses will meet the FDA impact requirement with a thinner center. This can be a real advantage for Trivex material and polycarbonate.
How thin is thin? Now you know.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.
WHERE TO FIND IT:
HOYA Vision Care, North America
877-528-1939 • hoyavision.com
PPG Industries, Inc.
800-323-2487 • ppgtrivex.com
800-328-3449 • vision-ease.com
VSP Optics Group
800-852-7600 • vspopticsgroup.com