What is the Highest Index Lens Available in the U.S.?
If you’re reading this you’re probably familiar with the characteristics of high index lenses.
Highest Index Lens Available in the U.S.
You know that vision-correction prescriptions that fall within the stronger prescription range can benefit from the reduction in thickness and possible reduction in weight that high index lenses allow. You also know the trade-off is that high index lenses are more expensive, more fragile than most standard-material lenses, and feature more chromic aberration (designated by the Abbe value scale), which can be distracting to wearers sensitive to such anomalies.
You may also know that as lens index increases, both weight and reflected light values increase. For instance, a 1.74 high index plastic lens has a specific gravity of 1.47 and a reflected light rating of 14.36, while a 1.80 high index glass lens bumps those numbers up to a specific gravity of 4.02 and a reflected light rating of 18.85. Still, for those who require extremely strong prescriptions, high index lenses at the upper end of the spectrum can certainly be worthwhile alternatives to the thick, “Coke Bottle” lenses that lower-index materials produce.
So how high does the high index scale go? 1.9 is currently the highest available material worldwide, but it is not approved for sale in the United States by the FDA (Food and Drug Administration). 1.8 high index lens material is the highest level that is FDA-approved for U.S. sale and distribution.
The reason for this is the more stringent impact-resistant standards set by the FDA. In accordance with FDA regulation 21 CFR 801.410(d)(1), a “Referee” or “Drop Ball” test – interchangeable terms for the approved impact test – must be performed on all lens material types. The regulation states, “In the impact test, a 5/8-inch steel ball weighing approximately 0.56 ounce is dropped from a height of 50 inches upon the horizontal upper surface of the lens. The ball shall strike within a 5/8 inch diameter circle located at the geometric center of the lens. The ball may be guided but not restricted in its fall by being dropped through a tube extending to within approximately 4 inches of the lens. To pass the test, the lens must not fracture….”