Polycarbonate lenses are a specific type of plastic material used in eyeglasses, originally developed for aerospace applications like helmet visors and space shuttle windshields. This thermoplastic polymer is characterized by its exceptional toughness and resilience. The primary reason for its widespread popularity in eyewear is its unmatched resistance to impact, offering a significant safety advantage over traditional glass or standard plastic (CR-39) lenses. Understanding the properties and trade-offs helps determine if it is the right choice for your vision needs.
Defining the Safety and Durability of Polycarbonate
The key benefit of polycarbonate is its high impact resistance, making it virtually shatterproof. Polycarbonate lenses are up to ten times more resistant to impact than standard plastic lenses, stemming from the material’s inherent flexibility. When struck, the material absorbs the energy by flexing rather than fracturing. This makes it the default choice for safety eyewear.
Polycarbonate also offers practical benefits for everyday wear, as it is a high-index material with a refractive index of approximately 1.59. This higher index allows the lenses to be made significantly thinner and lighter than CR-39 plastic for the same prescription strength. The lighter weight and thinner profile contribute to greater comfort, especially for individuals with higher prescription powers. Polycarbonate naturally blocks 100% of the sun’s harmful ultraviolet (UV) radiation without requiring an additional coating.
Scenarios Where Polycarbonate Lenses Are Essential
Polycarbonate lenses are widely considered the standard of care where impact risk is a constant factor. Children and adolescents are one primary demographic where this material is strongly recommended. Due to their high activity levels and increased risk of accidental impacts during play, the fracture resistance of polycarbonate provides necessary protection against potential eye injuries.
Athletes and individuals who participate in active sports (such as basketball, soccer, or racquet sports) also benefit from the material’s durability, as the ability to withstand high-velocity impact is paramount. Additionally, those who work in occupational or hazardous environments must often use polycarbonate lenses, as they meet strict safety criteria, such as the American National Standards Institute (ANSI) Z87.1 standard for impact resistance.
Beyond safety, the material is also a practical choice for people with strong prescriptions, particularly those with a power of plus or minus four diopters or more. The higher index allows these lenses to be made thinner and flatter, reducing the noticeable thickness that can affect the appearance of the eyewear. This ensures the final product is both more comfortable and aesthetically pleasing.
Optical Trade-Offs and Alternatives
While polycarbonate excels in safety and thinness, there is a recognized trade-off concerning its optical clarity. The material has a lower Abbe value, typically around 30, which measures how much a lens material disperses light. A lower Abbe value means light separates more as it passes through the lens, potentially causing chromatic aberration, which manifests as color fringes around objects. This effect is usually more noticeable in stronger prescriptions.
For users sensitive to this optical phenomenon, alternatives offer similar safety with improved clarity. Trivex is a urethane-based lens material that provides comparable impact resistance to polycarbonate and is similarly lightweight. Trivex boasts a significantly higher Abbe value (typically around 43 to 45), resulting in less chromatic aberration and sharper peripheral vision. Although Trivex may be slightly thicker than polycarbonate, its superior optical performance often makes it the preferred choice for those prioritizing visual quality.
Another alternative is high-index plastic, which can be made even thinner than polycarbonate due to a higher refractive index, such as 1.67 or 1.74. While these materials minimize lens thickness for very strong prescriptions, they generally do not match the same level of impact resistance as polycarbonate or Trivex. Consequently, the decision comes down to balancing safety and thinness against the highest possible optical clarity.