|According to Optima, its 1.74 lenses are 45% thinner and 35% lighter than CR-39 material.|
Super high-index lens materials can have an important place in your practice.
With all the index of refraction choices on the lens material market, eyecare professionals (ECPs) are confronted with a wide array of possibilities. One is super high-index materials. Lenses made from these materials have the highest index of refraction available for ophthalmic purposes. This gives them the potential for being the thinnest choice for a given Rx, but this sometimes comes at the expense of other material characteristics that are not quite as appealing. The following helps clear the air about this lens material category.
WHAT’S SUPER HIGH?
While there is no official subdivision of the index of refraction of lens materials in the U.S., the Index of Refraction Categories chart (at bottom right) may be useful.
When choosing a lens material, the first decision an ECP must make is whether the material has to be highly impact resistant. If it does, as it might in the case of children’s eyeglasses or providing eyewear for a patient with useful vision in only one eye, then the ECP is limited to those materials that meet ANSI Z87.1 standards such as polycarbonate and Trivex® lens material. This means these patients will only have the 1.60 index of polycarbonate as their highest option. Rx’s not requiring the highest level of impact resistance can be used with all materials.
WHEN TO GO SUPER HIGH
While there are a number of factors that enter into the decision of going super high with the index of a lens material, most ECPs will start by looking at the power of the Rx. If it is high, they will consider higher index materials.
The categories in the Rx Categories chart (at right) are pretty conservative and many ECPs will probably extend the powers in each category higher. But with these categories in mind, how often would you need to utilize a super high-index lens material, those that serve powers of +/-5.25D or higher?
A recent compilation of dispensing data from Younger Optics reported that the vast majority (85%) of Rx’s you use everyday fall between +/-3.00D. Between +/-4.00D, 92% of Rx’s are found. And between +/-5.00D, 95% will be utilized.
SUPER HIGH IN PERSPECTIVE
Based on the data of the Younger study, the vast majority of prescriptions do not require high- or super high-index materials. In fact, the data indicates that in the average office, not more than five out of 100 patients will require super high-index lenses. That’s not very often, but when they’re needed, they provide some pretty terrific features.
At the top of the features list for super high-index lens materials is their ability to slenderize the highest of Rx’s. This is important because most patients prefer their eyewear to have a “normal” appearance. In other words, the closer Rx eyewear resembles plano eyewear, the more patients seem to like them. Thick lenses hint to observers that the wearer has “weak eyes,” something most people don’t wish to convey.
CR-39® material is usually what other materials are compared to. If you compare 1.74 to CR-39, 1.74 lens material makes lenses about 30% thinner. You can improve this greatly by making the lens flatter and using aspheric curvatures. For example, Optima, Inc. claims its 1.74 lenses are 45% thinner and 35% lighter than CR-39 material. Putting this into perspective, if your patient had a 7mm edge thickness utilizing a CR-39 lens material, replacing it with a 1.74 option would lower that thickness to less than 3.8mm.
|UVCO’s Chemilens accommodates a high Rx range.|
Super high-index lens materials tend to be heavier than most other materials. Even so, because you’ll use less of the super high-index material for a particular Rx, the super high-index lenses usually are lighter than other lens options once they are surfaced—so don’t be alarmed at the higher specific gravity numbers for super high-index materials.
One recurring comment from ECPs about super high-index materials is the lower Abbe values, meaning patients may detect colored ghost images, especially in bright light. Dispersion is inherent in lens materials and cannot be eliminated. Higher index materials usually have more of this property, which is indicated by its Abbe value. There is no way to avoid it or reduce it other than by using a lens material that has a higher Abbe value.
There are a number of suppliers of 1.67 and higher materials as seen in the Super High-Index Materials table (at bottom). Since these lens materials are designed for very high prescriptions, they utilize aspheric frontside surfaces in order to reduce optical aberrations. The Rx range of these products usually goes pretty high too.
For example, UVCO’s Chemilens has a finished power range up to -15.00 SPH to -4.00D cylinder and a semi-finished range available in more than 10 base curves from 0.50D base to 12.00D base with both spheric and aspheric design. Optima’s 1.74 Rx range reaches up to -15.00D. Some lenses like Essilor of America, Inc.’s Thin&Lite® 1.74 can be produced down to a 1.0mm minimum center thickness and still pass the impact requirement of the FDA regulation.
Look for both stock and semi-finished options in this category, and many of the stock lenses come with an anti-reflective treatment.
While you might not use them very often, it’s good to know that there are so many options available in the super high-index lens category when you do.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.
WHERE TO FIND IT
Carl Zeiss Vision Inc.
800-358-8258 • zeiss.com/lenses
Essilor of America, Inc.
800-542-5668 • essilorusa.com
HOYA VISION CARE, North America
877-528-1939 • hoyavision.com
Nassau Vision Group
800-526-0313 • nassauvisiongroup.com
800-621-1216 • optima-inc.us
Seiko Optical Products of America, Inc.
800-235-5367 • seikoeyewear.com
Shamir Insight, Inc.
877-514-8330 • shamirlens.com
Signet Armorlite, Inc
800-950-5367 • signetarmorlite.com
212-356-0010 • inovixlens.com
800-328-3449 • vision-ease.com
800-366-5367 • youngeroptics.com