When you’re in the market for new glasses, the million dollar choice usually comes down to the frames. I mean, what better way to showcase your personal style? Bold, metallic, bright, clear, flashy, blingy… the choices never end! But for most, little thought goes into the actual lenses. If they function the way they’re supposed to, what else is there to consider?
As it turns out: a lot!
A crash-course on eyewear history
Just like frames, there’s a lot of different types of lens that are used in eyewear. Since the emergence of eyewear in the 13th Century, glass would be blown into shape to magnify objects and would be used in early forms of glasses. Early glasses were hard to come by and usually only afforded by the absolute wealthiest people, not to mention the refractive power of these early lenses were pretty limited. Over the centuries, glass became more affordable and accessible, thanks to huge improvements in understanding optics, refractive measurements, and glass manufacturing processes.
But in the 20th Century, eyewear would see it’s most innovative jumps in technology. Where glass was once the only option for eyewear, today there are several lenses to choose from; all of which have made improvements where glass fails. Today, glass is still a choice for lenses, but is rarely used due to its drawbacks.
The story of alternate lenses starts in the 1940s. Introduced during and after WWII, acrylic lenses were cheaper to produce, more lightweight, and could be manufactured at a mass scale – a critical necessity during wartimes. However, early acrylics were brittle; they would scratch and crack easily, not to mention how they would often discolor and become cloudy over time. Acrylic (as a material) is still used in frames and other things, but aren’t really used as eyewear lenses anymore, thanks to improvements in plastics like CR-39.
CR-39, originally created in 1945, is a lightweight plastic that promised improvements in every way over acrylics while still maintaining the attractive affordability and scalability that acrylic offered. CR-39 lenses wouldn’t become discolored or cloudy over time and were more lightweight than their predecessor, but are still scratch-prone unless a scratch-resistant coating is applied to the lens. Today, they’re the basic plastic lens type that most eyewear and optometry offices offer, most used for lighter prescriptions.
In 1983, the first polycarbonate lens was introduced! In their debut, they were the lightest and thinnest lenses available, with a high, 1.59 refractive index (more on that later). They were also incredibly durable and didn’t require the same coatings and upkeep that CR-39 had. But with all these pros, there was one major drawback during their introduction: they had terrible optics, which meant they wouldn’t transfer light as efficiently as other types of lenses. Despite this pitfall, polycarbonate’s high resistance to damage has made them ideal for children and people with active lifestyles.
After polycarbonate lenses were introduced, the conversation about eyeglass lenses changed. Previously, the race was on for what material could replace glass – something cheaper, more durable, and lighter. But after polycarbonates came onto the scene, it was all about who could achieve the higher refractive index!
Understanding the refractive index
The refractive index is a measurement of how light bends through a medium. It’s a measurement of how efficient a lens can be (I like to compare it to the MPG of a car). Everyone wants a thinner, lighter lens. The higher a lens can score on the refractive index, the thinner a lens can be while still maintaining an accurate prescription for a patient. The refractive index can tell us how accurate and powerful a lens can be. The second benefit of having a higher refractive index is being able to handle stronger prescriptions.
For example: the human eye, without the necessity of eyewear, has a natural refractive index measurement of about 1.37. That number acts as a baseline for how efficient 20⁄20 vision is. The goal of any pair of glasses is to make sure your vision reaches 20⁄20, so lenses with stronger refractive indexes can be used when your eyes cannot. Even though they had poor optics at their introduction, polycarbonate lenses could achieve a high, 1.59 refractive index – making them more capable than glass and early plastics.
Shortly after their introduction, companies like Optima Inc. started to produce more high index lenses, like their Hyperindex 1.60 lenses in 1987. As the name would suggest, Hyperindex lenses had a refractive index of 1.60, ushering a new era of “high index lenses”. They refined what was already done by polycarbonates, making them stronger, lighter, and more efficient. Since then, various iterations of these high index lenses have come out. They constantly improve to be thinner and more powerful, perfect for patients who need stronger prescription lenses!
Choosing the right type
Today, there’s many different types of lens to choose from. Each has their pros and cons and can be recommended depending on variables like your prescription, your activity level and lifestyle, or just personal preference.
Believe it or not, we actually don’t recommend glass! Sure, they have made a lot of improvements over the years and they have better optics than their plastic cousins, but despite these advantages, they’re outweighed by their disadvantages. Glass lenses are heavier, can still break on direct impact, and can cause major damage to the eyes if they crack and shatter.
In most cases, CR-39 lenses are widely acceptable as the standard. They’re very affordable and durable, but they need to have special coatings applied in order to be scratch-resistant, blue-light reflecting, and UV reflecting. Nevertheless, they’re still the norm and offer a lighter, more durable eyewear experience than glass.
Polycarbonates are lighter than both glass and CR-39, offer superior durability and refractive index, all the while being cheaper than glass lenses. These make plastic lenses perfect for active lifestyles and children. The down side to plastics is how easily they can scratch. Even with anti-scratch coatings, plastics can pick up nicks and scratches easier than glass – even if they don’t shatter or break like glass does. High index lenses are generally only used for stronger prescriptions that would otherwise require thick glass or plastic in order to achieve the right level of refraction. With high index lenses, stronger prescriptions can be applied to thinner materials.
Lighter than polycarbonates, equipped with 100% UV protection without needing a special coating, and with incredible durability, Trivex is what we really enjoy recommending to patients. They have great optics, are heat resistant, and maintain their dimensional stability (meaning they won’t become cloudy or warp over time). They have a lower refractive index than polycarbonates (1.53 vs 1.59), which makes them perfect for most prescriptions, safe for higher prescriptions that really benefit from high index lenses. Trivex is the preferred material for most eye doctors in their own lenses. Lighter, clearer and stronger.
When our talented team of opticians find the perfect frames for you, they’re also working to find the perfect lenses for you, too. Special coatings, tints, transitions, and materials all come into play when you look at lenses choices. Next time you come in, ask us about the different types of lens materials we offer and we’ll be happy to help you find what perfectly fits your wants and needs!