Sunglasses lens materials: glass, polycarbonate, TAC and CR-39, compared honestly

Almost every sunglasses lens is one of four materials. Glass gives the clearest view and resists scratches best, but it is heavy and can shatter. Polycarbonate is the toughest, light, and blocks UV by its chemistry, at the cost of a soft surface that needs a hard coat. Tri acetate cellulose, TAC, is very light and inexpensive, and because it is built in laminated layers it is where a polarizing film gets sealed inside a lens rather than coated on. CR-39 is a plastic that sits between them. None is best. Each is a trade, and the trade that suits a mountaineer is wrong for a runner.

Glass

Mineral glass was the original lens material and it is still the optical benchmark. It has the highest clarity and the least distortion, and its surface is hard enough that it resists scratches better than any plastic without a coating.

The costs are weight and fragility. A glass lens weighs roughly twice what a plastic one does, which you feel on the nose by afternoon. And glass can shatter on impact, which rules it out for any sport or for children; our impact ratings post covers what a tested lens has to survive. Glass does not block UV by itself; it needs a coating or a UV-absorbing additive.

Choose it for: long hours of still viewing where clarity matters most, and you are not going to drop them. Avoid for: sport, driving where impact is a risk, anyone who wants light.

Polycarbonate

Polycarbonate is the plastic in safety glasses and sports eyewear. It is extremely impact-resistant, roughly ten times tougher than glass or CR-39, and it is light. It also absorbs UV by its chemistry, so the UV protection cannot wear off. Our lifespan post explains why that matters.

The trade is a soft surface, which is why every polycarbonate lens carries a hard coat, and slightly lower optical clarity than glass or CR-39: a little more chromatic fringing at the edges, noticeable in thick prescription lenses and rarely in a plano sunglass.

Choose it for: sport, cycling, children, anything where the lens might take a hit. Avoid for: nothing in particular; it is the safe default.

TAC: tri acetate cellulose

TAC is a cellulose-based plastic that is very light and inexpensive to produce. What makes it distinctive is that a TAC lens is made as a laminate: several thin layers bonded together. That construction is why TAC is the usual home of a laminated polarizing film. The film is sealed between two layers of the lens, so it cannot be scratched off or worn through; only scratching through the outer layer reaches it.

The trade is surface hardness, which is lower than glass and lower than coated polycarbonate, so a TAC lens depends on its hard coat and on how you clean it. It is also less impact-resistant than polycarbonate, though it flexes rather than shatters. Optical clarity is good in a plano lens.

Choose it for: light, affordable polarized sunglasses where the polarization must not wear off. Avoid for: heavy impact use.

Our lenses are TAC. That is the frame maker's stated material, it is why our polarizing film is laminated inside, and it is why our cleaning post says rinse before every wipe.

CR-39

CR-39 is the plastic most prescription lenses were made from before polycarbonate. It is lighter than glass, clearer than polycarbonate, and more scratch-resistant than polycarbonate before coating. It is not as tough as polycarbonate and does not block UV without an additive or coating.

Choose it for: prescription sunglasses where clarity matters and impact does not. Avoid for: sport.

Side by side

How to tell which you have

• Weight. Glass is unmistakably heavy.
• Tap it. Glass rings; plastic thuds.
• Flex it gently at the edge. TAC flexes visibly; polycarbonate flexes slightly; glass does not.
• The label. The honest answer. If the seller does not state the material, ask.

Frequently asked questions