The Facts about High Index Glass Lenses
Glass is regarded by the public as fragile. It conjures up images of baseballs flying through glass windows or falling glassware shattering on the kitchen floor. These images are hard to dispel when some consumers consider glass lenses for their eye-wear. On the other hand, few think about the tough windshield glass of their cars that readily deflect small rock impacts at highway speeds.
The technology of glass manufacture has come a long way in the last few decades. However, it seems the technology of plastics gets all the attention. This isn’t surprising given the array of amazing plastics available today.
Unfortunately, glass has an image problem. It’s underestimated even as a choice of lens material for eye-wear, an application it once dominated before plastic lenses became popular. However, glass lenses have improved in important ways while also retaining the properties for which they’ve always been valued.
Glass Lenses Are Available with Very High Indexes
A classic disparagement of glass as a lens material is the description: “coke bottle glasses.” This refers to bulky and heavy lenses that make the wearer look bug-eyed or beady-eyed. This happens when low index glass such as crown glass is used for high prescription lenses. However, glass with very high indexes of refraction is available for making thin lenses for high prescription eye-wear. In fact, glass is the only lens material commercially used for 1.8 and 1.9 high index lenses.
Glass Lenses Are Tough and Impact Resistant
Although glass lenses don’t have the same impact resistance as those made from polycarbonate or Trivex, they must have sufficient impact strength to pass the drop-ball test. This means they must withstand the impact of a 5/8 stainless steel ball (0.56 ounce) dropped from a height of 50 inches onto the center of their upper lens surface. Every lens used by the customer is tested this way.
Glass Lenses Are Highly Abrasion Resistant
No other lens material can match the scratch resistance of glass. Unlike plastic lenses, glass lenses require no anti-scratch coating. This property makes them more durable than plastic, even those with protective anti-scratch coatings. Glass is harder than many substances and won’t scratch as badly as plastic when it comes into contact with substances harder than itself. In short, it takes a lot more to scratch glass than plastic.
Glass Has High Optical Quality
This means glass lenses give you greater clarity than plastic lenses. More precisely, glass is a more transparent material than any of the plastics used for lenses. This is why precision measurement instruments use glass lenses.
The Drawbacks of High Index Glass
All lens materials have their drawbacks and glass is no exception. One of these is weight. Glass is a heavier material than plastic lens materials across all indexes of refraction. While lenses made from high index glass are thin, they have a heftier feel than high index plastic. However, this is a vast improvement over coke bottle glass lenses of the same prescription level, which are both thick and heavy. In addition, high index glass eye-wear doesn’t distort the appearance of the wearer’s eyes.
Other drawbacks of high index glass, are its limited index, color, and frame selections. High index glass lenses are available at 1.80 and 1.90. Because holes cannot be drilled in glass lenses, rimless frames are not available for them. Glass lenses are used only in full frame glasses.