The simplest way to tell standard plastic lenses (CR-39) from polycarbonate is to gently flex them: polycarbonate bends noticeably, while CR-39 feels rigid and resists bending. Several other hands-on tests can confirm what you’re dealing with, from comparing weight and thickness to listening to the sound the lens makes when tapped. The distinction matters more than you might expect, because the two materials differ sharply in impact resistance, scratch behavior, and optical clarity.
Check Your Paperwork First
Before you start bending or tapping anything, the fastest route is to look at the receipt from your optician or the order confirmation if you bought online. Most optical shops list the lens material on the invoice, sometimes abbreviated as “CR-39,” “plastic,” “poly,” or “PC.” If you have a current prescription, the optician’s office can usually pull up your lens specifications in their system with a quick phone call. This is especially helpful if the lenses have anti-reflective or scratch-resistant coatings, because coatings can mask some of the physical differences you’d otherwise feel by hand.
If you’re dealing with old glasses, safety eyewear, or a pair with no documentation, you’ll need to rely on physical tests. The good news is that CR-39 and polycarbonate differ enough in basic physical properties that you can often distinguish them without any specialized equipment.
The Flex Test
This is the go-to method used informally in optical shops. Remove the lens from the frame if you can, or if it’s still mounted, hold the frame steady and press gently on the center of the lens with your thumb. Polycarbonate is a thermoplastic, meaning it softens and flexes under moderate pressure. You’ll feel it give and spring back. CR-39, by contrast, is a thermoset resin. Once it’s cured during manufacturing, it becomes rigid and brittle. Pressing on a CR-39 lens feels like pressing on a piece of hard candy: it doesn’t flex, and if you pushed hard enough (don’t), it would crack rather than bend.
A word of caution: don’t overdo the pressure. The goal is to feel a subtle difference in flexibility, not to stress-test the lens to failure. A light press in the center is enough. If the lens moves under gentle thumb pressure and bounces back, you’re almost certainly holding polycarbonate. If it feels unyielding, it’s likely CR-39 or another thermoset plastic.
Weight and Thickness
Polycarbonate has a higher refractive index than CR-39, which means it bends light more per millimeter of material. The practical result is that for the same prescription power, a polycarbonate lens can be made thinner. CR-39 has a refractive index of about 1.498, while polycarbonate sits around 1.586.1Aston Publications Explorer. High Refractive Index Plastic Optical Materials That difference doesn’t sound dramatic, but in a moderate-to-strong prescription it translates to visibly thinner edges on the polycarbonate lens.
Polycarbonate is also lighter. Its specific gravity is around 1.20, compared to roughly 1.32 for CR-39. If you hold two lenses of the same prescription and diameter side by side, the polycarbonate lens feels noticeably lighter. The combination of thinner edges and lower weight is actually one of the reasons polycarbonate became popular for children’s glasses and sports eyewear in the first place. If your lenses feel surprisingly light for their prescription strength, that’s a soft clue pointing toward polycarbonate.
The Tap Test
Hold the lens near your ear and tap it lightly with a fingernail or the end of a pen. CR-39 produces a higher-pitched, sharper “click,” closer to the sound of tapping on a ceramic plate. Polycarbonate gives a duller, slightly lower-pitched “thud,” more like tapping on a thick piece of flexible plastic, which is exactly what it is. The difference comes down to the materials’ stiffness: CR-39 is harder and more rigid, so it vibrates at a higher frequency when struck. Polycarbonate absorbs more of the impact energy, damping the vibration.
This test works best in a quiet room and takes a little practice to calibrate your ear. If you have a known pair of CR-39 lenses and a known pair of polycarbonate lenses to compare, the difference becomes obvious quickly. Without that reference point, the tap test is best used as a confirming check alongside the flex test rather than a standalone method.
Surface Scratches as a Clue
If you’ve worn the glasses for a while without a scratch-resistant coating, the condition of the lens surface tells you something. Polycarbonate is inherently softer than CR-39 and scratches much more easily. Research on polycarbonate’s surface properties has consistently noted that its low hardness is a significant limitation, and that hard anti-scratch coatings are essentially required to make polycarbonate practical for everyday use.2PubMed Central. Preparation of transparent hard GPTMS-Al(2)O(3) thin films on a polycarbonate substrate Early work on plastic lens materials made the same observation, noting that polycarbonate in particular needed surface coatings to achieve acceptable wear resistance.1Aston Publications Explorer. High Refractive Index Plastic Optical Materials
So if your uncoated lenses are covered in fine hairline scratches after a few months of use, that’s consistent with polycarbonate. If they’ve held up reasonably well without coatings, CR-39 is more likely. Of course, most modern lenses come with at least a basic scratch-resistant coating, which muddies this test. A well-coated polycarbonate lens can look pristine for years. But if you’re examining a pair of older or budget glasses where the coating has worn through in spots, the scratch pattern underneath can be revealing.
Why You Should Not Use Acetone
You’ll sometimes see advice online suggesting you can identify polycarbonate by dabbing a small amount of acetone (nail polish remover) on the lens surface. The logic is real: acetone attacks polycarbonate aggressively but leaves CR-39 unharmed. A documented case of polycarbonate ophthalmic lenses shattering after accidental acetone exposure demonstrated just how destructive the interaction is. The acetone caused solvent-induced cracking, swelling, and crazing that progressed to complete lens failure.3ASM International. Solvent-Induced Cracking Failure of Polycarbonate Ophthalmic Lenses
That same case report recommended protecting polycarbonate lenses from any contact with solvents, including those found in marking pens, adhesives, and some soaps.3ASM International. Solvent-Induced Cracking Failure of Polycarbonate Ophthalmic Lenses So while acetone sensitivity is a genuine chemical difference between the two materials, using it as a test means risking the destruction of a perfectly good lens. The flex test and tap test give you the same information without the collateral damage. If you’re cleaning your glasses, this is also worth keeping in mind: never use acetone-based cleaners on any pair where you’re not sure of the material.
Impact Resistance and Why It Matters
The biggest functional difference between CR-39 and polycarbonate isn’t something you can see or feel on a casual inspection, but it’s the reason many people end up in polycarbonate lenses in the first place. In impact testing using a range of projectiles, CR-39 (called “allyl resin plastic” in technical literature) shattered at impact energies below what those projectiles would deliver during normal use. Polycarbonate, by contrast, resisted impacts from every tested projectile at energy levels exceeding what would be expected in routine conditions.4PubMed. Shatter resistance of spectacle lenses
This is why polycarbonate is the default material for children’s eyeglasses, sports goggles, and safety glasses in many countries. The U.S. FDA has required impact resistance testing for eyeglass lenses since 1972, and that regulation helped drive the shift away from glass and toward impact-resistant plastics.5PubMed Central. Historical Development, Applications and Advances in Materials Used in Spectacle Lenses and Contact Lenses If your glasses were specifically sold as safety-rated, sports-rated, or children’s eyewear, they’re very likely polycarbonate. That context alone can settle the question without any physical testing.
For everyday adult dress eyewear, the distinction is less critical from a safety standpoint. Most adults aren’t taking high-velocity impacts to the face on a regular basis. But if you’re someone who works around machinery, plays racquet sports, or just tends to be rough on your glasses, knowing you have polycarbonate gives real peace of mind.
Optical Quality Differences
Here’s where CR-39 has a genuine advantage. Optical clarity isn’t just about refractive index; it also depends on a property called the Abbe number, which measures how much a lens material disperses white light into its component colors. A higher Abbe number means less color fringing at the edges of your vision. CR-39 has an Abbe number around 58, which is quite good. Polycarbonate sits around 30, which is among the lowest of any common lens material.
In practice, this means polycarbonate lenses produce more chromatic aberration, visible as slight color fringes or rainbow-like halos around high-contrast edges, especially in your peripheral vision. Most people wearing a mild prescription barely notice it. But if you have a strong prescription, work in visually demanding conditions, or are just sensitive to optical quality, the difference between polycarbonate and CR-39 can be noticeable and annoying. Some people who’ve switched from CR-39 to polycarbonate report a subtle degradation in visual “crispness” that they struggle to pinpoint until they learn about the Abbe number difference.
If you’re trying to identify your lens material and you notice unusually prominent color fringing when you look through the edges of the lens, that’s a soft indicator of polycarbonate. It’s not definitive on its own, since lens design and coatings also play a role, but combined with other tests it adds to the picture.
What About Trivex and High-Index Lenses
The question “plastic or polycarbonate” assumes those are the only two options, but a few other materials have carved out meaningful market share. Trivex, introduced in the early 2000s, offers impact resistance comparable to polycarbonate but with a higher Abbe number (around 43-45), meaning better optical clarity. It’s slightly lighter than polycarbonate and has become popular for drill-mounted rimless frames because it resists cracking around drill holes better than either CR-39 or polycarbonate. If your lenses flex like polycarbonate but seem unusually clear, Trivex is a possibility.
High-index plastics are another category. These are designed for strong prescriptions, with refractive indices ranging from about 1.60 to 1.74, which makes for very thin lenses. They don’t flex the way polycarbonate does but are thinner than you’d expect from CR-39 at the same prescription. High-index lenses tend to have low Abbe numbers (similar to or worse than polycarbonate), so the same color-fringing issue applies.
If none of the tests above give you a clear answer, or if the lens seems to share characteristics of multiple materials, there’s a decent chance you have Trivex or a high-index plastic rather than the two most common options. An optician with a lensometer can measure the refractive index directly and identify the material within seconds, which is the definitive answer when DIY tests leave you uncertain.
Edge Characteristics and How Opticians Tell
When an optician cuts and edges a lens to fit a frame, the material reveals itself immediately. CR-39 produces a fine, powdery dust when cut, and the edges are slightly rough, almost chalky. Polycarbonate, because of its flexibility and toughness, tends to produce long stringy shavings rather than dust, and the cut edges come out smoother and slightly glossy. If you pop a lens out of its frame and examine the edge closely with a magnifying glass, you may be able to see this difference. A cleanly cut, smooth, almost waxy-looking edge suggests polycarbonate. A matte, slightly textured edge is more consistent with CR-39.
Opticians also sometimes use a polariscope, a device that shines polarized light through the lens. Polycarbonate often shows internal stress patterns visible under polarized light, appearing as colored bands or swirls. This is a byproduct of the injection-molding process used to manufacture polycarbonate lenses. CR-39 lenses, which are cast rather than injection-molded, typically show far fewer or no stress patterns. You can approximate this at home with two polarizing filters (or two pairs of polarized sunglasses): hold one in front of and one behind the lens, rotate them, and look for colored stress patterns. If you see vivid bands of color, you’re almost certainly looking at polycarbonate.
Solvent Sensitivity Beyond Acetone
The acetone vulnerability of polycarbonate isn’t a quirk limited to nail polish remover. The failure analysis that documented acetone-induced shattering also noted that marking pens, certain adhesives, and some soaps contain solvents capable of attacking polycarbonate.3ASM International. Solvent-Induced Cracking Failure of Polycarbonate Ophthalmic Lenses The mechanism is called environmental stress cracking: the solvent weakens the polymer chains at the lens surface, and any existing stress in the lens (from the frame gripping it, from manufacturing, or from being tightened too aggressively) provides the force that turns those weakened spots into cracks.
CR-39, being a cross-linked thermoset, is far more chemically resistant. Most common household solvents won’t damage it. This means that if you’ve ever accidentally gotten hand sanitizer, perfume, or household cleaner on your lenses and noticed hazing or tiny cracks developing afterward, the lenses are very likely polycarbonate. It’s a diagnostic clue you probably wish you didn’t have, but it’s a reliable one. For ongoing care, the safest cleaning approach for any lens material is warm water with a drop of dish soap, rinsed and dried with a microfiber cloth. Skip anything alcohol-based or solvent-based unless you’re certain the lenses are CR-39.
When Children’s and Safety Glasses Complicate the Question
If the glasses in question belong to a child, the answer is almost always polycarbonate (or Trivex). Pediatric eyecare guidelines in many countries strongly recommend impact-resistant materials, and most optical labs default to polycarbonate for children’s prescriptions unless specifically instructed otherwise. The impact resistance data makes this an easy call: polycarbonate withstood projectile impacts that shattered every other tested lens material at energies encountered in everyday scenarios.4PubMed. Shatter resistance of spectacle lenses
Occupational safety glasses follow the same logic. Lenses marked with “Z87” or “Z87+” on the temple or lens indicate compliance with ANSI impact standards, and virtually all lenses meeting those standards are polycarbonate or Trivex. If you see that marking, you don’t need any of the physical tests described above.
The one category where CR-39 still dominates is standard adult dress eyewear in mild-to-moderate prescriptions, especially when the wearer prioritizes optical clarity and isn’t concerned about impact protection. Budget glasses from online retailers may also default to CR-39 because it’s less expensive to manufacture. If you ordered the cheapest lens option from an online eyewear shop, CR-39 is the most likely material unless polycarbonate was specifically listed as an upgrade.