Should You Buy CR-39 Lenses or Polycarbonate?
The Polycarbonate Sales Pitch
Thinner, lighter, 10x more shatterproof.
These and other virtues of polycarbonate lenses are extolled by your optician, when you are shopping for new glasses.
Odds are that you have no idea about lens materials. And why should you? Unfortunately, there are more than a few opticians who don't actually know a whole lot about lens materials. Others may, but still want the larger profit margin that comes from selling a poly lens. All this leaves you with less than perfect advice on what lenses to buy.
Here Is The Odd Fact: In The Case Of Lenses, Cheaper Might Actually Be Better.
Glasses started out with glass lenses. And glass is actually by far the best lens material, in terms of optical quality. You can buy a thousand dollar pair of poly lenses, and they won't have anywhere near the optical quality of a $15 glass lens.
Think I might be joking? Consider the lenses used in expensive cameras and in telescopes. Are those ever any type of polycarbonate or plastic? Never! The highest quality lens is always made of glass.
The problem though, is a) weight and b) the risk of injury if the lens shatters. This is how the road down alternative materials got its start.
CR-39 is the name of the monomer base that makes up the lens material in inexpensive plastic lenses.
Owned by PPG, this material revolutionized lens making. Half as heavy as glass, far less likely to shatter, and optical quality nearly as good as glass. CR-39 is heated and poured into optical quality glass moulds - adapting the qualities of glass very closely.
Most lenses in glasses today are made of CR-39 plastic, or very close copies (since the material is owned by PPG).
And Then Came Poly
Polycarbonate is still lighter, and can be made in higher index - meaning that a strong prescription lens can still be quite thin.
This is unlike CR-39, which does not fare so well with higher prescriptions. Since the refractive index is only 1.49, these types of lenses get very thick, if you need a strong correction. That's where polycarbonates begin to make sense.
Anything Below -2.00 or -.175 Diopters, Doesn't Require High Index Lenses.
Polycarbonate is also even more shatter resistant, making it ideal for activities that have a strong risk of high impacts to your lenses. Though in reality this is not much of an issue, since CR-39 itself is quite impact resistant. The sales pitch of "shatterproof" is just that - a pitch.
There are reasons, in addition to high prescriptions, that warrant poly lenses:
CR-39 Cracks Easily, When Drilled.
This only matters if you choose rimless glasses, where the mounting point of the lenses has to be drilled into the side of the lens. Polycarbonate takes well to this treatment, where CR-39 does not. So if you want a rimless design, you need to go with polycarbonate.
Optics As Priority
- Your eyesight tends to get worse, with high prescriptions.
- Your perception of your environment is affected by optical quality.
- Light is handled poorly by polycarbonate materials.
- Poly lenses are a significant optical quality compromise.
What Matters: Optics
Polycarbonate seems to be an easy sale, and optic shops will always steer your to poly, if you appear to have the extra cash to blow. And as we already discovered, if you have high myopia or want rimless glasses, this may indeed be the way to go. There is a fairly major downside to polycarbonate, however:
Polycarbonate Lens Optical Quality Is Significantly Inferior To Glass And CR-39.
And the difference is significant. What you gain in impact resistance and higher index, you pay for in how the lens transfers light into your eye. And much like a poor quality camera lens translates into a poor quality photograph, that poly lens transfers an image through your eye, into your brain, that is far, far less than your visual cortex expects.
Lens Material Affects Light Transmission
Glass vs. CR-39 vs. Poly
highest optical quality
good optical quality
poor optical quality
1/2 weight of glass
thin & light
high shatter risk
low shatter risk
very low shatter risk
Breaking a Cr-39 Lens
The Shatter Risk Myth
If you are low myopia and don't use rimless glasses, you might still be sold on the "upgrade" to a lower optical quality and more expensive lens, with the shatter risk sales pitch.
Is it really worth the higher price and much lower optical quality, though?
CR-39 has particular issues with cracking, if you drill them. Beyond that though, breaking this type of material requires an impact far higher than you are likely to ever experience. Even then, it does not shatter the way glass does. In my 40 years in the optometry field, I have never seen or heard of an eye injury from a shattered CR-39 lens.
Take a look at the YouTube video on the right. Look at just how much force he has to exert, on the whole of the lens, before it breaks (clean break also, no shatter). What would cause that much force? Nothing, most likely. Shatter risk is insignificant, by all practical accounts.
Aside from lenses themselves, you also want to educate yourself about myopia causes, and ways to prevent your eyesight from further deteriorating:
Trivex Lens, And Alternatives
There is also Trivex now, quickly becoming an industry darling.
It combines the positives of polycarbonate with higher optical quality, which makes it a step in the right direction for lens prescriptions.
The main question should always be, what the biggest priority of your lenses is.
#1 Priority Should Be Optical Quality.
You can buy a great poly lens, but it will cost 10x to 40x as much as the equivalent in an inexpensive CR-39. If you are highly myopic, looking at the various high index options makes sense, to give you a thin and light lens, while reducing the optical quality compromise.