After years of fitting glasses and handling different lens materials, I’ve learned that the choice between CR-39, polycarbonate, and high-index plastics isn’t really about which one is “best.” It’s about understanding what each material actually does under the conditions where you’ll wear it. Most people don’t think much about what their lenses are made from until something goes wrong – a scratch, a crack, or the realization that their prescription is thicker than it needs to be.
The optical industry settled on these three materials because they each solve a different problem. None of them is perfect. All of them have trade-offs that become obvious only after you’ve worn them for a while or seen what happens when they fail.
CR-39: The Workhorse
CR-39 is a thermoset plastic that’s been around since the 1940s. It’s still the most common lens material globally, and there’s a practical reason for that. It’s cheap to produce, easy to cut and shape, and it works reliably for most prescriptions and lifestyles. If you’ve worn glasses at any point without thinking about the material, you were probably wearing CR-39.
The optical properties are excellent. CR-39 has a refractive index of 1.498, which means it bends light reasonably well without needing to be extremely thick. For mild to moderate prescriptions – say, up to ±4.00 diopters – the thickness difference between CR-39 and fancier materials isn’t noticeable to most people. The material also has good impact resistance for everyday wear, which is why it’s still used in safety glasses and industrial eyewear.
Where CR-39 shows its age is in scratch resistance. The surface scratches easily, and once it does, there’s no real fix. You can apply hard coatings to help, but the material itself is softer than polycarbonate or high-index plastics. I’ve seen people with CR-39 lenses develop a fine web of scratches after two or three years of normal use, especially if they don’t clean them carefully or keep them in a case. The material also absorbs water and oils more readily than alternatives, which means fingerprints and smudges stick around longer.
For someone with a stable, moderate prescription who doesn’t mind replacing glasses every few years, CR-39 remains practical. The cost is low, the optical clarity is genuine, and the material doesn’t require special handling. It’s also the easiest material to tint if you want colored lenses, and the tinting tends to be more even and vibrant than with other plastics.
Polycarbonate: The Impact-Resistant Choice
Polycarbonate emerged in the 1970s and was marketed as shatterproof. This is accurate. The material is significantly more impact-resistant than CR-39, which is why it became standard for safety glasses, sports eyewear, and children’s frames. It can take a real hit without cracking or shattering.
The refractive index of polycarbonate is 1.586, slightly higher than CR-39. This means lenses can be somewhat thinner for the same prescription, though the difference is modest – maybe 10 to 15 percent thinner than CR-39 for moderate prescriptions. For most people, this isn’t a game-changer in terms of appearance or weight.
What I’ve noticed over time is that polycarbonate has its own set of quirks. It’s more prone to internal stress and can develop stress marks or internal cloudiness if it’s not manufactured carefully or if the frame puts pressure on the lens edges. Some people also report that polycarbonate lenses can feel slightly softer or more flexible than CR-39, which can be unsettling if you’re used to rigid lenses. The material also yellows more easily with UV exposure and age, particularly if it’s not protected with a good UV coating.
Scratch resistance is better than CR-39 but still not exceptional. Polycarbonate benefits from hard coatings, but the material itself is more resistant to surface damage. For active people, parents of young children, or anyone who spends time in environments where glasses might get knocked around, polycarbonate makes practical sense. The durability advantage is real, even if it’s not dramatic.
High-Index Plastics: Thinness at a Cost
High-index materials – typically with refractive indices ranging from 1.60 to 1.74 – were developed to address one specific problem: thick lenses for strong prescriptions. Someone with a prescription of ±8.00 diopters or higher will notice a substantial difference in lens thickness and weight when moving from CR-39 to high-index plastic. The lenses can be 30 to 50 percent thinner, which affects both the appearance of the glasses and how they sit on the face.
The optical quality is good, though not flawless. Higher-index materials tend to have slightly higher chromatic aberration – a subtle color fringing effect at the edges of the lens – but for most people, this is imperceptible in daily wear. The real trade-off is cost. High-index lenses are significantly more expensive than CR-39 or polycarbonate, sometimes two to three times the price.
Durability is where high-index materials show variation. Some high-index plastics are quite durable; others are softer and scratch more easily than polycarbonate. This depends on the specific material and the coatings applied. I’ve seen high-index lenses that held up beautifully after years of wear, and I’ve seen others that looked beaten up after a year. Much of this comes down to the quality of the hard coating and how carefully the wearer treats them.
Reflectivity is another consideration. High-index materials tend to reflect more light at the lens surface, which is why anti-reflective coatings are almost essential with these lenses. Without them, the lenses can look noticeably reflective, which affects both appearance and the amount of light that actually reaches your eyes. With a good anti-reflective coating, the lenses perform well optically.
What Actually Matters in Practice
The choice between these materials comes down to a few practical factors. If your prescription is mild to moderate and cost is a concern, CR-39 is still a sensible choice. You’ll get good optical quality and low cost, with the understanding that you’ll probably need to replace the lenses or glasses within a few years due to scratching.
Polycarbonate makes sense if durability and impact resistance are priorities. Athletes, people who work in rough environments, and parents buying glasses for children benefit from the extra toughness. The optical performance is good, and the material is less expensive than high-index options.
High-index plastics are worth considering if you have a strong prescription and want noticeably thinner, lighter lenses. The cost is higher, and you’ll need to be more careful about scratches, but the aesthetic and comfort benefits can be significant for people with prescriptions above ±6.00 diopters.
One thing I’ve learned is that the coating matters as much as the base material. A well-coated CR-39 lens can outperform a poorly coated polycarbonate lens. Hard coatings, anti-reflective treatments, and UV protection are often more important than the underlying plastic. A lens that scratches easily but has excellent optical properties might be a better choice for someone than a more durable lens with mediocre clarity.
The material you choose is less about finding the “right” answer and more about understanding what trade-offs you’re willing to accept. Thinner lenses cost more. Softer materials are cheaper but scratch. Impact resistance adds weight and cost. There’s no material that excels at everything, and recognizing that helps you make a choice that actually fits your life rather than chasing a theoretical ideal.
