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Why Glass? Why Not?

I’m fairly sure that most people will agree that glass has the best optical properties out of any material currently used to make optical lenses. There is a reason most equipment, such as a phoropter, tends to have glass lenses instead of using a material like polycarbonate. Having a higher optical quality is important when you’re measuring, and it should be important to the care of the patient as well. Some patients may not notice any difference, and some patients can tell right away. For the more discerning patients, the superior quality of glass is something that they may not want to do without once they try it.

Scratch resistance is one of the most sought-after qualities in a lens, which is why so much effort has gone in to developing all kinds of scratch coatings for plastic lenses. All of this effort in coating development to simply try and get back to the natural scratch resistance of glass. There is still no other optical material that comes close to the punishment that you can inflict on a glass lens. There’s a reason that cell phones have glass screens and not plastic screens. I’m fairly certain that I want my eye wear to be at least as durable as my phone.
This is not just in terms of scratching, but chemical resistance as well. This makes glass perfect for harsh conditions where other materials wouldn’t last. Take polycarbonate, for example. It performs very well on impact testing. When you look at scratch resistance, however, it often comes up lacking. It can also do especially poor in places with a lot of chemicals, paints, and solvents – especially things like acetone. While we like it when a patient buys a second pair, we usually don’t want it to be because the lenses had a problem.

Glass has a certain feel that other materials just can’t come close to. It’s not just their weight, but there’s a mystique around glass lenses that just make it seem like it’s worth more than plastic. This is a great thing for the Eyecare Professional, as it makes it easier to use glass to fill a role in high-end lenses. Having this as a high-end product for something like sunwear, for example, helps to give added value to the final product. This is an important part in selling glass – the idea that it’s a super-premium product. Since usage of glass isn’t as widespread as plastic, glass has the ability to command a higher price – as it should, given the benefits that it has over many other materials.

The Bad Side
There are, of course, some negative things that have become associated with glass. One of these things is that glass has a very limited availability. This is in terms of both material/color selection, as well as prescription range and lens size. The second concern over glass tends to be about its weight – that it’s too heavy for people to want to use.
Thankfully, full-backside freeform has the same benefits in both plastic and glass, and greatly helps our availability. Technological advances in glass also make it possible to have thinner, lighter glass than ever before.

Glass progressives have traditionally had a fairly restrictive range in terms of their color and size availability. Free-form helps us to eliminate that problem, since it’s now possible to use single vision lenses to make a progressive. Just like with plastic free-form, this greatly opens up the color and lens size options that can be used, as well as a much quicker turn-around. This means that a full range of fixed-tint lenses – including gray, green, brown, rose, and yellow – are now available as progressives. Full back-side progressives in glass also help to overcome the issue of cut-out. Since the progressive design can be decentered on the back of the lens, it makes it possible to put a progressive in a much larger frame than was previously possible. Free-form also helps with the prescription range, as it’s possible to get a glass free-form lens up to a +4.50 add power.
Beyond general availability, it’s important to note that there are also several major free-form designers that are currently available in glass. Apart from simply offering a ‘house brand’ lens, its important to be able to use brand-name designs as well. So far, lens designs from the following companies can be found in glass: IOTKODAK, and Shamir.

One of the major drawbacks to glass, compared to plastic, is weight. Glass can be heavy, especially with a high prescription. For low or plano prescriptions, however, there’s not much of a noticeable difference between the weight plastic and glass. For higher prescriptions, there are some other options. High index glass is one option, though this can sometimes be a problem as well. As you use higher index glass, lenses do become thinner, but they can also become more dense. A better option is by using a brand new product called Thin & Clear. This material has all of the same properties as regular glass, but Thin & Clear can be made 25% thinner while still passing drop-ball. This means Thin & Clear can have a 1.5mm thickness, while regular glass usually has a 2.0-2.2mm minimum thickness.

I sometimes hear that people don’t like glass because they think that it’s easy to break it, or that it’s going to shatter while someone is wearing it. Most glass (unless it’s polarized) is either air- or chemically-tempered to strengthen it. This ensures the glass is extremely tough and impact-resistant and is able to pass the drop-ball test. To this point, it’s important to note that there’s no requirement for every individual plastic lens to undergo drop-ball testing. The FDA only requires that a ‘statistically significant’ portion of them are tested. Glass lenses, on the other hand, must undergo testing with each and every lens. This means you can be sure that every glass lens – safety or dress wear – has been tested and won’t shatter from an impact. Some people think having to drop-ball every lens means glass isn’t safe. I prefer to think that it ensures that you know that when a lens is sent out, that it’s already been tested and there’s nothing to worry about. When it comes to plastic lenses, however, we just assume that if it’s at a certain minimum thickness that it’s going to be okay. It’s also important to note that should a lens shatter, that tempered glass will crumble into dull pieces, while plastic tends to splinter in shards that can often be sharp.

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