Discovering the perfect pair of glasses isn’t just about finding great looking frames that you’ll love to wear.
The difference between loving your glasses and hating them is also due to the type of lens you wear and the material that the lenses are made of.
Now, this topic is huge and quite technical, so in this short article, I’m going to try and keep things super-simple.
A commonly-held belief is that spectacle lenses are made of glass, and that all lenses are pretty much the same.
In fact, the majority of spectacles lenses sold these days are made of plastic, and all spectacles lenses are definitely not the same. The optical quality of spectacle lenses varies significantly, just like the quality of any product you might use every day varies significantly.
Because glass lenses are dispensed so rarely, we’re not going to discuss them in this article, other than to say that the main advantage of glasses lenses over plastic lenses is their superior optical quality and better scratch resistance. The main disadvantage of glass lenses, though, is that they are very heavy, they are brittle and can break/shatter easily, and they are not compatible with modern frames, modern lens technologies, and glazing techniques.
For the remainder of this article, we’ll be talking about plastic lenses.
Plastic lenses originated in the 1940s after the Second World War. The material is called CR39 (or Columbia Resin Number 39), which was used to make plastic-reinforced fuel tanks for US bombers during WW2. At the end of the war, there was a vast surplus of CR39, which had no apparent use. The Armorlite Company in California is credited with being the first company to manufacture spectacle lenses using CR39 due to its optical clarity (being almost the same as optical glass), and because it’s lightweight (roughly half the weight of glass lenses).
Despite being an old technology (almost 80-years since its creation) CR39 is still the most commonly used material to manufacture spectacle lenses. Although it does have some good properties, it also has a number of downsides, including being brittle (making it unsuitable for rimless/semi-rimless glasses and other glazing techniques which are needed for a lot of modern frames) and it’s a soft material which is easy to scratch. All plastic lenses are easier to scratch than glasses lenses, which requires all plastic lenses to have a scratch-resistant coating applied to them. (Note: scratch-resistant coating, not scratch-proof, no lens material is truly scratch-proof.)
One of the things our eyewear stylists will chat with you about when choosing spectacle lenses is the lens index. The lens index describes how efficiently a spectacle lens bends the light.
The lowest, or most basic, lens index for a plastic lens is 1.5, which is CR39.
The highest lens index available for a plastic lens is 1.76, although this is only produced by one manufacturer in the world, Tokai. For all other lens manufacturers, the highest index material is 1.74.
Between these two, there are other lens materials including Trivex (1.53), polycarbonate (1.59), 1.6 and 1.67 index lenses.
But what do these numbers mean? The higher the number, the more efficient the lens is at bending the light, which means that for any given prescription, the lens can be made thinner and lighter, which will provide a more pleasing cosmetic result. This is especially helpful if you have a high or more complicated prescription, where the cosmetic look of your lenses, and the weight of them, is even more important.
And, as you might imagine, there are pros and cons to all of these different lens materials. So you can see that it’s not a simple case of “a lens is a lens”, and not every lens type is going to be right for you.
Polycarbonate - 1.59 Index
Polycarbonate lenses have their origins in the 1990s, which saw a trend towards rimless glasses.
Because CR39 is inherently brittle, lens manufacturers had to find a new lens material to allow for this growing trend. Enter polycarbonate into the optical lens market.
Polycarbonate is extremely strong compared to CR39. If it’s thick enough it can stop a bullet in its tracks, and is actually used to make bullet-proof glass. This property makes it perfect for drilling holes into lenses for rimless glasses, as well as other modern lens glazing techniques. Polycarbonate is also very lightweight, being roughly half the weight of CR39.
Polycarbonate does have its drawbacks, though. Optically, it’s not that great to look through, because it has a low Abbe value. This Abbe value describes how evenly the different wavelengths (or colours) of light will pass through a lens. Natural daylight is made-up of different colours of light, with blue light at one end of the spectrum and red light at the other end of the spectrum. Lenses with a low Abbe value will slow down some colours passing through the lens compared to other colours. What that means in reality is that you get separation of the colours, so that if you are looking at something, instead of seeing a clear, sharp and distinct focus you may see different colours around that object. This is called “colour fringing” or chromatic aberration.
Polycarbonate has the lowest Abbe value of any spectacle lens material, and therefore also has the highest chromatic aberration or colour fringing. In other words, it’s not a great lens material if you’re wanting the clearest and most precision vision possible.
Consequently, it’s not a material that we use or recommend at Clarke & Roskrow.
Trivex - 1.53 Index
Trivex is another by-product of the military, being used to manufacture windscreens for military helicopters in the 2000s. Trivex is extremely strong (stronger than polycarbonate) with high-impact resistance. Again, bullet-proof glass can be made from Trivex. It’s also extremely lightweight and very optically clear, being almost the same as CR39 and optical glass, which makes it a perfect spectacle lens material. It also has good compatibility with modern lens technologies and glazing techniques.
A downside of Trivex is that it’s only available through certain lens manufacturers, including Carl Zeiss Vision, one of our preferred lens partners.
1.6 & 1.67 Index
Lenses made from 1.6 index plastic are considered a mid-index lens. They have good optical clarity, similar to CR39 and Trivex, as well as having good impact resistance, which makes them suitable for rimless/semi-rimless glazing as well as other modern glazing techniques and lens technologies. They can be up to 20% thinner than standard CR39 lenses, for the same prescription, so the cosmetic outcome can be superior, as well as the lenses being more lightweight.
1.67 lenses are considered to be a high-index lens. These lenses can be up to 30% thinner than standard CR39 lenses, so again the overall cosmetic appearance can be much improved, as well as the lenses being more lightweight.
The downside of 1.67 index lenses is the optical clarity is not as good because of its lower Abbe number, so there is some sacrifice in optical quality for the benefit of having thinner, more lightweight lenses. The optical clarity is not as poor as polycarbonate, but it’s not as good as 1.5, 1.53 or 1.6. Some of this downside can be mitigated by choosing the right frame shape and size, as well as taking other physical fitting characteristics into account. Our team of expert eyewear stylists will give you more advice on this.
1.74 Index
1.74 index is classed as a very high index lens, and is the highest lens index available for a plastic lens (with the exception of 1.76 from Tokai, Japan). For any given prescription, 1.74 index lenses will provide the thinnest, flattest and most lightweight option, offering as much as a 50% reduction in the thickness of the lenses.
Unfortunately, though, 1.74 plastic lenses are more brittle than their high and mid-index counterparts, which makes them unsuitable for rimless/semi-rimless glazing and some other modern glazing techniques and frames. They are not fully compatible with some lens technologies such as polarised sunglasses. 1.74 index lenses also have a low Abbe value, which means the optical quality of the lens is not as good as the basic and mid-index options. People with more complicated prescriptions will often happily accept a reduction in optical clarity for the benefits of having lenses that are as thin, flat, and lightweight as possible so that the lenses look great when worn. Again, some of this reduction in optical clarity can be mitigated by choosing the right frame shape, size and other fitting characteristics.
Another downside to all mid, high and very high index lenses is that they are highly reflective compared to CR39. All spectacle lens materials will reflect a proportion of light from the front and back surface of the lens. The higher the lens index, the higher the reflectance. These reflections can be annoying for the wearer, as well as being visible to other people. High reflectance also means that less light passes through the lens and into the eye, so the clarity of the image we see through the lens is degraded (which is why camera lenses, binoculars and telescopes all have anti-reflective coatings applied to them.)
All of our lenses come with premium coatings as standard, which include durable scratch-resistant, anti-reflection, smudge, oil, and water repellent coatings, to ensure the best possible optical clarity and performance. And all of our lenses come with a 2-years manufacturer’s warranty against faulty manufacture.
Conclusion
So, that’s a very brief overview of lens materials and refractive index.
Our expert team of dispensing opticians will be able to give you lots more information and advice when they meet you in person.
And just in case you’re wondering, we don’t automatically recommend the highest lens index just because it’s the highest-priced option. You’ll get honest and transparent advice on the best lens index for your needs. Oftentimes the higher index material isn’t actually the best option, which is why you need expert advice and guidance.
As a general rule, we don’t recommend glass lenses due to its weight, it’s incompatibility with modern glazing techniques and lens technologies, as well as its poor impact resistance. We don’t usually recommend a 1.5 index lens; this is now very old technology and is an unsuitable material to use for most of the frames that we work with. And we never use 1.59 polycarbonate – despite its useful properties it’s just horrible to look through.
If you would like to book a no-obligation Eyewear Styling Consultation to discover your perfect pair of glasses, call the team on 01858462996 or email hello@clarkeandroskrow.co.uk
