CR-39 is a lightweight, thermosetting plastic that has served as the default material for prescription eyeglass lenses for decades. Its name, short for Columbia Resin #39, reflects its origins as an industrial polymer developed in the mid-twentieth century, and its optical properties still make it a go-to choice for standard-prescription eyewear. What keeps CR-39 relevant in a market flooded with higher-index plastics and polycarbonate alternatives comes down to a specific combination of visual clarity, weight savings over glass, and adaptability to coatings and tints.
Where the Name Comes From
CR-39 was the thirty-ninth formula in a series of resins developed by the Pittsburgh Plate Glass Company (now PPG Industries) during the early 1940s. The material was initially explored for military and industrial applications, including use as an optical cement for bonding glass elements in gunsights and other instruments. A 1944 paper in the Journal of the Optical Society of America described its properties in that role, establishing the resin’s optical potential early on.1Journal of the Optical Society of America. The Use of Columbia Resin No. 39 as an Optical Cement Over the following decades, lens manufacturers recognized that CR-39 could be cast directly into ophthalmic lenses, and by the 1970s and 1980s it had largely displaced glass as the standard lens material for everyday eyeglasses.
How CR-39 Lenses Are Made
CR-39 is chemically known as allyl diglycol carbonate, a thermoset polymer with the formula C₁₂H₁₈O₇ and a density of about 1.31 grams per cubic centimeter.2Nuclear Instruments and Methods in Physics Research Section B. Measurement of thermal and optical properties of CR-39 solid-state nuclear detector by photothermal deflection Unlike thermoplastics such as polycarbonate, which can be melted and injection-molded, CR-39 is produced through a casting process sometimes called bulk polymerization. A small percentage of a catalyst is mixed into the liquid monomer, and the mixture is poured into a mold assembly shaped to the desired lens curvature. The mold then goes into a forced-convection oven, where controlled heating triggers the catalyst to initiate polymerization. The monomer gradually converts into a rigid, highly cross-linked glassy polymer.3International Journal of Machine Tools and Manufacture. Modelling the casting process of plastic ophthalmic lenses
The casting cycle is slower than injection molding, often taking many hours to complete, because the polymerization needs to proceed gradually. Rushing the cure can introduce internal stresses that distort the lens or cause cracks. This slower production method is part of why CR-39 lenses tend to be less expensive in standard prescriptions but are not always the cheapest option for mass-produced safety eyewear, where injection-molded polycarbonate dominates.
Optical Clarity and the Abbe Number Advantage
The single biggest selling point of CR-39 in optical terms is its Abbe number of 58.2Nuclear Instruments and Methods in Physics Research Section B. Measurement of thermal and optical properties of CR-39 solid-state nuclear detector by photothermal deflection The Abbe number measures how much a material spreads white light into its component colors as light passes through it. A higher Abbe number means less color spreading, which translates to sharper, more color-neutral vision. CR-39’s Abbe value of 58 is considerably better than polycarbonate (around 30) and most higher-index plastics (typically in the low-to-mid 30s or 40s). Crown glass, the traditional standard, sits in a similar range to CR-39, which is one reason the switch from glass to CR-39 felt optically seamless for wearers.
This difference is not just a number on a spec sheet. A photographic simulation study compared off-axis image quality through CR-39 (refractive index 1.50), polyurethane (1.60), and polycarbonate (1.59) lenses. The researchers found that chromatic aberration was more prominent in the higher-index materials, especially polycarbonate, while the lower-index CR-39 produced less color fringing when looking away from the lens center.4PubMed. Photographic simulation of off-axis blurring due to chromatic aberration in spectacle lenses In practical terms, people wearing CR-39 lenses are less likely to notice colored halos around bright lights or blurry color fringes at the edges of their field of view. For anyone who has switched to polycarbonate or a high-index lens and felt that something was slightly “off” about their vision, chromatic aberration is often the culprit.
CR-39’s refractive index of about 1.50 is, however, a trade-off.5Aston University. High Refractive Index Plastic Optical Materials A lower refractive index means the lens needs more curvature to bend light the same amount, so CR-39 lenses are thicker than higher-index alternatives at the same prescription. For people with mild to moderate prescriptions, the difference is small and the lenses look fine in most frames. For stronger prescriptions, the extra thickness becomes noticeable, and this is where higher-index materials earn their place despite their optical compromises.
When CR-39 Works Best and When It Does Not
CR-39 is an excellent match for prescriptions in roughly the ±4 diopter range. In that zone, the lenses stay thin enough to look good in standard frames, and you get the full benefit of the material’s optical clarity. Once prescriptions climb higher, the physics of a 1.50-index material work against you. Lens edges for nearsighted prescriptions get noticeably thick, and lens centers for farsighted prescriptions bulge outward. This is purely a cosmetic and weight issue; the optics are still fine, but most people prefer a thinner profile.
CR-39 is also not the best choice for rimless or semi-rimless frames, where the lens itself needs to bear structural loads from drill mounts or nylon cords. CR-39 is rigid and can crack or chip under point stress more easily than polycarbonate, which is much more impact-resistant. For children’s eyewear and safety glasses, polycarbonate or Trivex are usually recommended because they can absorb impacts without shattering. CR-39 does not meet the same impact-resistance standards without extra thickness.
Weight Compared to Glass
One of the original appeals of CR-39 was that it cut lens weight roughly in half compared to crown glass. Glass has a density of about 2.5 grams per cubic centimeter, while CR-39 sits at 1.31. For a full day of wearing glasses, that difference adds up in comfort, especially for people who wear larger frames or have stronger prescriptions that require more material. The lighter weight also means less pressure on the nose bridge and behind the ears, which reduces the headaches and red marks that heavier glass lenses can cause.
Polycarbonate is lighter still (about 1.20 g/cm³), and Trivex is in a similar range. So CR-39 is not the lightest option available. But the weight difference between CR-39 and polycarbonate is modest enough that most people do not notice it, whereas the optical clarity gap between the two materials is something sensitive eyes can feel immediately.
UV Protection
Untinted CR-39 blocks some ultraviolet light on its own, but it does not provide complete UV protection without help. The good news is that CR-39 takes UV-absorbing tints extremely well. A study testing UV-tinted CR-39 lenses found that all tested lenses absorbed all UV-B radiation and at least 99% of UV-A. The tints were also remarkably durable: after a full year of daily washing and drying, the lenses continued to block all UV-B and at least 99% of UV-A.6PubMed. Efficacy and durability of ultraviolet tints in CR-39 ophthalmic lenses So while you should confirm that your CR-39 lenses include a UV coating or tint, once they do, the protection is thorough and long-lasting.
Polycarbonate, by contrast, inherently blocks UV without any added coating, which is one of its genuine advantages. If UV protection is your primary concern and you want it built into the lens material rather than added as a treatment, polycarbonate or Trivex has the edge. But in practice, virtually all CR-39 lenses sold today come with UV treatment included, so the functional difference at the point of purchase is usually zero.
Scratch Resistance and Hard Coatings
The one clear weakness of bare CR-39 is that it scratches more easily than glass. Glass is a hard, brittle material that resists surface abrasion well; CR-39 is softer and picks up fine scratches from everyday handling. This is why virtually every pair of CR-39 lenses sold in the last several decades comes with a hard coating applied to both surfaces.
Modern hard coats use sol-gel chemistry to deposit a thin, durable protective layer on the lens surface. One well-known industrial hard coat designed for CR-39 substrates achieved a Bayer abrasion ratio of 4.8, meaning it resisted scratching nearly five times better than an uncoated reference surface.7Journal of Sol-Gel Science and Technology. Industrial applications of sol-gel derived coatings Anti-reflective coatings, hydrophobic treatments, and oleophobic (anti-smudge) layers can all be stacked on top of the hard coat. CR-39’s surface chemistry bonds well with these coatings, which is part of why it remains a preferred substrate for multi-layer lens treatments.
If you have CR-39 lenses and find they scratch quickly, the coating may have worn through or may have been a lower-quality product to begin with. Replacing or upgrading the hard coat when ordering lenses is inexpensive and makes a significant difference in longevity. Wiping lenses with a dry paper towel or shirt fabric is the fastest way to grind through any hard coat; a quick rinse with water before wiping removes the grit particles that cause most everyday scratches.
Chemical and Environmental Stability
CR-39’s thermoset structure gives it notable stability. Unlike thermoplastics, which can soften or deform with heat, CR-39 holds its shape well at the temperatures glasses typically encounter, including being left in a hot car. Its cross-linked molecular network also resists most common solvents, so splashes of household cleaners, perfumes, or hair products are unlikely to damage the lens material itself (though they can degrade coatings). Researchers have noted CR-39’s stability against various environmental factors and its high degree of optical clarity as reasons it continues to be used not just in eyewear but in scientific instruments like nuclear particle detectors.2Nuclear Instruments and Methods in Physics Research Section B. Measurement of thermal and optical properties of CR-39 solid-state nuclear detector by photothermal deflection
That said, CR-39 can yellow very slightly over many years of UV exposure, especially if the UV-absorbing tint degrades. This is a slow process and unlikely to become noticeable within a normal lens replacement cycle of two to three years. If you hold onto the same pair for a very long time, you might notice a faint warm tint developing, which is a sign it is time for new lenses.
Tinting and Specialty Applications
CR-39 is one of the easiest lens materials to tint to a custom color and density. The casting process produces a slightly porous surface structure that absorbs dye evenly, allowing opticians to create gradient tints, solid fashion tints, or functional tints for specific activities. Polycarbonate, by comparison, resists dye penetration and typically needs to be tinted during manufacturing or through a surface coating rather than a soak.
This tinting versatility makes CR-39 popular for prescription sunglasses, shooting glasses, and specialty sport eyewear where a specific tint color or density is part of the functional design. Yellow or amber tints for low-light shooting, grey or green tints for general sun protection, and rose tints for certain visual conditions can all be applied after the lens is ground to prescription.
The Environmental Cost of Lens Grinding
One aspect of CR-39 lenses that rarely comes up in optical discussions is the environmental footprint of manufacturing. When a cast CR-39 blank is ground to match a prescription and fit a specific frame, a substantial amount of material is removed. Research into lens-grinding wastewater found that roughly half of each initial lens blank’s mass is lost during the cutting and grinding process, and the process consumes about 20 liters of water per pair of lenses. That wastewater, carrying fine plastic particles and coating residues, is typically discharged into wastewater systems. Conservative estimates put the global figure at around 5,770 tonnes of plastic waste released into the environment each year from lens grinding alone.8PubMed Central. Physical and chemical characterisation of ophthalmic lens-grinding wastewater: uncovering environmental implications
The waste stream is not just inert plastic dust. Analysis has detected titanium dioxide particles from lens coatings, heavy metals, bisphenol A, phthalates, and fluorine-containing compounds in the grinding wastewater.8PubMed Central. Physical and chemical characterisation of ophthalmic lens-grinding wastewater: uncovering environmental implications These are recognized environmental contaminants with endocrine-disrupting properties. The issue is not unique to CR-39; polycarbonate and higher-index lenses generate similar waste streams. But given how many billions of lenses are ground worldwide each year, it is a pollution source that the optical industry has only recently started to examine. Filtration systems for grinding wastewater exist but are not yet standard in most retail optical labs. If sustainability matters to you, asking your optician whether they filter their grinding waste is a reasonable question, though you may get a blank stare.
CR-39 Beyond Eyeglasses
CR-39’s combination of optical clarity, chemical stability, and radiation sensitivity has given it a second life far from the optician’s office. In nuclear and particle physics, thin sheets of CR-39 are used as solid-state nuclear track detectors. When a charged particle passes through the material, it leaves a trail of molecular damage. Chemical etching widens those trails into visible tracks under a microscope, letting researchers count and characterize particles. This application exploits the very properties that make CR-39 a good lens material: its uniform molecular structure, transparency, and environmental stability ensure that the tracks left by particles are clean and reproducible.
CR-39 track detectors have been used to measure radon levels in homes, monitor radiation exposure for workers in nuclear facilities, and even detect cosmic rays on high-altitude balloon flights. The material’s sensitivity to alpha particles is especially valued in radon testing, where a small piece of CR-39 is placed in a room for a set period and then chemically etched to reveal the density of alpha tracks. The same optical clarity that lets you read a book through CR-39 lenses lets a technician count individual particle tracks under magnification. It is one of those nice cases where a material’s consumer and scientific applications reinforce each other, with quality improvements driven by one market benefiting the other.