High index lenses are a popular solution for individuals with strong vision correction needs. They are engineered to be significantly thinner and lighter than conventional plastic, providing a more aesthetic appearance and greater comfort for high prescriptions. However, the advanced material science that makes these lenses effective also raises questions about their durability. This article examines the composition of high index lenses and clarifies their impact resistance relative to regulatory standards and other specialized lens materials.
The Composition of High Index Lenses
High index lenses derive their name from a higher refractive index, which is a measure of how efficiently a material bends light. Standard plastic lenses typically have an index of 1.50, but high index materials begin at 1.60 and can go up to 1.74. These advanced materials are specialized plastic polymers, often containing elements like sulfur, which increase the material’s density. This higher density allows the lens to be ground much flatter to achieve the same corrective power, resulting in the desired thinness and lighter weight.
The internal structure of these polymers introduces a material trade-off. While the increased density provides superior light-bending ability, it can also inherently lead to a material that is more brittle and less flexible than lower-index plastics. For instance, the thinnest 1.74 index lenses are more brittle than 1.67 index lenses, and both are generally less flexible than standard plastic resins. This reduced flexibility is a direct result of prioritizing thinness and lightness for the strongest prescriptions.
Mandatory Safety Standards and Impact Testing
In the United States, the Food and Drug Administration (FDA) regulates all spectacle and sunglass lenses as medical devices, mandating a minimum standard for impact resistance. This regulatory requirement is verified using the “Drop Ball Test,” which all lenses sold must pass before reaching the consumer. The test procedure involves dropping a 5/8-inch steel ball from a height of 50 inches onto the surface of the mounted lens.
To pass this test, the lens must not chip, crack, or break upon impact, confirming it meets the basic threshold for safety. This standard aims to ensure that lenses can withstand minor, low-energy impacts encountered during normal, daily use. Passing the Drop Ball Test confirms a lens is impact-resistant, which is not the same as being shatter-proof. The certification indicates a minimum level of protection and does not guarantee the lens will survive a high-energy impact event.
The FDA requires that manufacturers test a statistically significant sample of lenses from each production batch, including those finished to minimal thickness. Therefore, a high index lens, such as a 1.74 resin, must pass this identical test requirement. The method of achieving that compliance may differ from a more inherently durable material. Less impact-resistant materials often rely on additional coatings or a slightly thicker profile to meet this mandated performance level.
Comparing High Index Resins to Polycarbonate and Trivex
Comparing standard high index resins directly to materials specifically engineered for superior durability, namely Polycarbonate and Trivex, clarifies the question of impact resistance. Polycarbonate is a thermoplastic polymer often cited as being up to ten times more impact resistant than standard plastic lenses. Trivex, a urethane-based material, offers comparable impact protection and is often chosen for its slightly better optical clarity.
These two materials are the industry standards for safety eyewear, children’s glasses, and active sports lenses because of their superior tensile strength and ability to absorb energy without fracturing. In contrast, standard high index resins, particularly the thinnest 1.74 index, sacrifice some inherent material strength to achieve their thinness and lightness. The tensile strength of a 1.74 resin is significantly lower than that of Polycarbonate or Trivex.
It is possible to obtain high index lenses in Polycarbonate, typically up to a 1.60 index, offering a balance of thinness and superior durability. However, the very highest index options, such as 1.74, are not typically available in Polycarbonate because the material composition does not permit the same degree of light-bending efficiency. For users whose primary concern is maximum eye protection against significant force, prioritizing Polycarbonate or Trivex is generally recommended, even if it means accepting a slightly thicker lens profile compared to a 1.74 resin.