Whether you’re buying glasses for the kids that will need to withstand some knocks on the playground, glasses for playing sports where they might take a bump, or glasses that just need to stand up to tougher than normal treatment, high impact lenses might be a good idea.
This article will explain the benefits and drawbacks of different lens types to bear in mind when making your next purchase.
CR39 Plastic Lenses
Currently CR39 is the plastic most commonly used in lenses, its name is an abbreviation of Columbia Resin 39. This particular plastic was the 39th formulation of a thermal-cured material developed by PPG industries in the 1940s.
This plastic is very lightweight, just half the weight of glass, it’s also cheaper, making it an ideal lens material for mass produced eyewear. Despite this, perhaps the most important attribute of CR39 is its remarkable optical qualities.
Next to glass, CR39 has the highest Abbe value of any lens. An Abbe value defines the chromatic aberration of a material, which is an optical error that causes different wavelengths of light to appear as ‘halos’ around objects. The effect of chromatic aberration is generally more noticeable in peripheral vision than when looking through the centre of a lens.
All of these qualities make CR39 a cost effective option that has a higher impact resistance than glass, without compromising on quality of vision. However, CR39 is not without its drawbacks. This type of lens requires an additional UV coating to provide the same protection that other high impact lenses do from UV rays. As well as this, of the plastic lenses CR39 is by far the thickest, which can distort the wearer’s eyes, an effect that some find cosmetically unappealing.
High Index Plastic Lenses
An option for those looking to curb some of the drawbacks of CR39 lenses could be found in high index plastic lenses. These lenses are much thinner than CR39 lenses, with a refractive value of between 1.60 and 1.74 (the thinnest lens currently available), compared to CR39’s value of 1.498.
A refractive value describes how efficiently a lens bends light. The higher the refractive value of a material, the thinner that material is able to be whilst still allowing the wearer to see clearly. For those who are mathematically inclined, this can be described as the ratio of the speed of light in a vacuum, divided by the speed of light in the lens material.
As well as being much thinner than CR39, high index lenses are often made with an aspheric design, meaning that lens manufacturers can use flatter curves to achieve a less magnified image of the wearer’s eye. Many wearer’s find this more visually appealing, but as well as this aspheric designs can also improve peripheral vision.
Whilst aspheric designs are available in other lens types, they tend to be more expensive (particularly in CR39 and polycarbonate), so if an aspheric lens appeals to you, then a high index plastic lens would likely be the most stylish and cost effective route to take.
Polycarbonate is a very impact resistant material. It was developed in the 1970s for aerospace applications including astronaut helmet visors and spacecraft windshields, so if nothing else, that’s pretty cool…
By the 1980s polycarbonate was being used for lenses as it was thinner, lighter and more impact resistant than glass. Nowadays it is the standard for safety goggles, children’s glasses and sports goggles, owing to it’s excellent impact resistance.
Polycarbonate is a thermoplastic that starts the lens making process as pellets which are shaped in a process called injection molding. During this process the pellets are compressed under very high pressure into lens molds, then cooled to form a hard plastic lens.
It’s resilience as a material means that polycarbonate lends itself to being used for rimless glasses like these above from Mont Blanc, where drill mountings are the only method of attaching the lens to the frame, as it is much less likely to crack than other materials.
As well as its toughness, polycarbonate lenses naturally block 100% of the sun’s UV rays without the need for a coating, meaning your eyes are properly protected. These lenses are also offered in a wider range of options (such as progressive lenses) than the other high impact lens materials.
Whilst polycarbonate undoubtedly makes a truly impact resistant lens, it’s durability comes at a price. Polycarbonate has significantly more lens reflection than plastic or glass, which means that an anti-reflective coating could be necessary. Further to this polycarbonate has an Abbe value of just 30, meaning it offers comparably poor optical quality to the previously discussed options.
Trivex or NXT
A similarly tough but optically superior material is Trivex. This material was invented in 2001 by Pittsburgh Pennsylvania (the same people who invented CR39). Trivex was initially developed for military applications as it is bulletproof, transparent and lightweight.
Trivex is a urethane based monomer and undergoes a similar cast molding process to polycarbonate, again making it suitable for rimless styles. Where it differs from polycarbonate lenses is it’s crisper optics, Trivex has an Abbe value of 45. This makes these lenses suitable for those with anything from -4 to +5 prescriptions as it offers significantly better central and peripheral clarity. This superior clarity comes at a price, as Trivex tends to be more expensive than polycarbonate.
Both Trivex and polycarbonate lenses are very thin and light compared to glass and CR39. Next to each other the lenses are extremely close in both attributes, however polycarbonate can boast being about ten percent thinner than Trivex, whilst Trivex is about ten percent more lightweight than polycarbonate. As well as their similar weight and thickness, both lenses provide comparable impact resistance.
The impact resistance of both materials can be attributed to their ‘softness’ compared to other high impact lenses. This softness means both Trivex and polycarbonate can absorb impact rather than fracturing. Of course, this means that both require a scratch resistant coating, but with this coating, they are rendered as scratch resistant as glass.
Finding the right high impact lens for you, will largely depend on the role the glasses need to fulfil. Whilst CR39 plastic lenses are not bulletproof, for everyday use they prove a cost effective choice with an excellent Abbe value. Do not forget to check for a UV protective coating as these are the only lenses that do not provide any UV protection on their own.
If true impact resistance is what you’re aiming for then Trivex or Polycarbonate lenses are the best option, with Trivex providing better optical quality for a slightly higher price. It’s important to make sure to get these lenses treated with an anti-scratch coating.
High-index plastic lenses are an expensive but fashionable choice, allowing ultra thin shapes to be achieved. These lenses provide crisp optics, especially if shaped aspherically, but don’t have the impact resistance of trivex or polycarbonate.
Featured Image: MattysFlicks