Chloe Zhang
Plastic lenses are generally made from a strong material called CR-39, but there are also a range of hi-index lenses available, which are thinner and lighter. The most common plastic lens materials are PMMA (acrylic resin), PC (polycarbonate), COC (cyclo-olefin copolymer), and COP (cyclo-olefin polymer). Each material has its own advantages and disadvantages, and the choice of lens material depends on the wearer's specific needs. For example, polycarbonate lenses are strong, durable, and impact-resistant, while CR-39 lenses are more impact-resistant than glass lenses but lack shatter resistance. Plastic lenses are compatible with a wider variety of frames and can be tinted more easily than glass lenses. However, glass lenses offer superior clarity, scratch resistance, and a thinner, more attractive appearance.
| Characteristics | Values |
|---|---|
| Plastic lens materials | CR-39, Polycarbonate, Trivex, Styrene, SAN, ABS, High-Index |
| CR-39 characteristics | Tough, scratch-resistant, heat-resistant, chemical-resistant, highly tintable, lightweight, high optical quality, blocks 80% of UV light |
| Polycarbonate characteristics | High impact resistance, lightweight, thinner, stronger, lower optical quality, scratch-prone, total UV protection, more expensive |
| Trivex characteristics | High impact resistance, lightweight, thin, strong optical quality, minimal chromatic aberration, blocks nearly all UV light, thicker for higher powers, scratch-prone |
| Styrene characteristics | Lower UV transmission, lower UV resistance, easily scratched, low tendency to stress cracks, low cost, excellent molding properties |
| SAN characteristics | Glass-like polymer, low tendency to stress cracks, slightly more chemical durability than PS |
| ABS characteristics | Developed from polymerization of styrene and acrylnitrile |
| High-Index characteristics | Can be plastic or glass, thinner, lighter, higher refractive index, lower Abbe value, higher chromatic aberration, cosmetically attractive, lower impact resistance |
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What You'll Learn
CR-39 plastic lenses
While CR-39 lenses have many advantages, there are some drawbacks to consider. For prescriptions higher than +/-2.00, CR-39 lenses will become thicker and heavier, and a polycarbonate or high-index lens may be more suitable. Similarly, for three-piece rimless eyeglass frames, a regular plastic lens is not recommended as it may crack under strain. For children or adults engaged in sporting activities, polycarbonate lenses are recommended for their impact resistance and better eye protection.
CR-39 lenses are also used in trifocal lenses, which have three viewing areas for different distances. The standard CR39 flat-top 28 trifocal has a 28mm wide reading area at the top. These lenses are suitable for corrections up to +/-3.00 and offer various options such as UV protection, sunglasses tint, and anti-reflective coating.
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Polycarbonate lenses
Polycarbonate was initially developed for the aerospace industry and is used for astronaut helmet visors, among other applications. It has a high transparency, second only to acrylic resin, and offers excellent durability and protection against impacts. Polycarbonate lenses are a popular choice for eyeglasses due to their combination of strength, lightweight feel, and impact resistance. They have become one of the most popular lens choices since their introduction in the 1980s, offering a sleek and modern alternative to traditional glass lenses.
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Trivex lenses
In terms of impact resistance, both Trivex and polycarbonate lenses are impact-resistant materials. However, polycarbonate is generally more impact-resistant, which is why it is often used in safety glasses and sports goggles. Trivex lenses, on the other hand, offer better scratch resistance than polycarbonate, making them a good choice for individuals who are rough on their glasses. They are also easier for manufacturers to shape while maintaining sturdiness.
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High-index lenses
The degree to which high-index lenses are thinner than regular lenses varies based on their material, the given prescription, and the frame they are used in. For example, a standard plastic lens with a refractive index of 1.50 and a 1.67 high-index lens might have the same prescription, but the high-index lens could be up to 30% thinner. High-index lenses are generally scratch-resistant and can fit into almost any type of frame.
Polycarbonate lenses are a type of high-index lens made from impact-resistant plastic with a refractive index of 1.59. They are thinner than standard plastic lenses but not as thin as high-index lenses with a higher refractive index. For instance, 1.67 high-index lenses are about 20% thinner than polycarbonate lenses, while 1.74 high-index lenses are up to 10% thinner than 1.67 lenses. 1.74 high-index lenses are recommended for especially strong prescriptions of +/-8.00 or higher.
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Acrylic resin lenses
Plastic lenses are generally made from a strong material called CR-39, a type of acrylic resin. Acrylic resin lenses, also known as PMMA, are characterised by their excellent transparency and weather resistance, good mechanical strength, and high surface hardness. They are widely used in optical lenses because they transmit more light than glass and have a low refractive index.
Acrylic resin is sometimes called "organic glass" due to its excellent surface lustre and ability to be freely coloured. It has excellent strength and transparency, and does not deteriorate outdoors due to its weather resistance. However, it has some disadvantages, such as high water absorption, a change in the refractive index with water absorption, and low heat resistance.
Despite these drawbacks, acrylic resin lenses offer some unique advantages over glass lenses. They are much lighter and more comfortable to wear, especially if you need to wear your glasses all day. Their lightweight nature also helps them stay in place longer, reducing the chance of slipping down your nose. Acrylic resin lenses are also very durable and difficult to crack or break, making them a safer option for both children and adults.
In addition, acrylic resin lenses are less likely to cause glare, as they are naturally less reflective than glass. They are compatible with a wide variety of frames in different shapes, sizes, thicknesses, and colours, providing a broader selection for consumers.
Overall, while acrylic resin lenses may have some minor drawbacks, they offer significant benefits in terms of comfort, durability, glare reduction, and design flexibility, making them a popular choice for optical lenses.
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Frequently asked questions
The most common plastic lens materials are CR-39, polycarbonate, Trivex, and high-index. CR-39 stands for Columbia Resin #39, a plastic polymer developed in the 1940s for military planes in World War II. Other plastic lens materials include PMMA (acrylic resin), COC (cyclo-olefin copolymer), and COP (cyclo-olefin polymer).
Plastic lenses are half the weight of glass lenses, offer high optical quality, and are compatible with a large variety of frames. They are also easy to tint and provide a wide range of transition lens options. Plastic lenses gained popularity in the 1970s and replaced glass lenses in the 1980s due to their durability.
Plastic lenses are more prone to scratches compared to glass lenses. They have a lower refractive index, requiring thicker lenses. Some plastic lenses, like polycarbonate, are the most easily scratched plastic and require scratch-resistant coatings.
The best lenses for glasses depend on your prescription, visual needs, and desired benefits. Consider factors such as optical clarity, eye safety, frame selection, prescription type (single-vision or multifocal), comfort, and durability. Discuss your specific needs with an optometrist to make an informed decision.
