Protective eyewear is a fundamental safety measure in industrial, construction, and laboratory settings where eye hazards are present. Protecting vision from flying debris, chemicals, or blunt force requires lenses that are structurally sound enough to withstand impact. In the United States, the performance of safety eyewear is governed by strict, standardized regulations established by the American National Standards Institute (ANSI). These standards dictate the minimum performance requirements, including lens thickness, which is directly tied to impact resistance. The standard requires that a lens must not fracture, chip, or dislodge from the frame upon impact.
Minimum Thickness for Basic Impact Protection
The most fundamental level of defense, known as basic impact protection, requires a specific minimum thickness for the lens. For prescription safety lenses, the standard dictates a minimum thickness of 3.0 millimeters (mm) at the thinnest point. This requirement applies to lenses made from traditional materials like glass or standard plastic, such as CR-39, which are not inherently as impact-resistant as newer materials.
The 3.0 mm thickness ensures these materials possess the necessary mass and structural integrity. An exception exists for certain prescription powers: a lens with a plus power of 3.00 diopters or greater may have a minimum thickness of 2.5 mm. Eyewear meeting only this basic impact rating must have a warning label affixed, informing the wearer of the protective gear’s limitations.
High-Impact Certification and Material Thickness Variations
Not all safety lenses must adhere to the 3.0 mm minimum thickness if they achieve a higher level of certification. Eyewear rated for high-impact protection is marked with a “Z87+” designation on both the lens and the frame. This certification signifies that the lens has passed a more rigorous set of impact tests, allowing for a reduction in the required material thickness.
High-impact prescription lenses made from materials like polycarbonate or Trivex can be manufactured with a minimum thickness of 2.0 mm at their thinnest point. These advanced resins perform better under stress. Polycarbonate is a thermoplastic polymer known for its toughness and ability to withstand high-speed impacts without shattering.
Trivex is another advanced lens material that offers high impact resistance and superior optical clarity, also meeting the high-impact standard at the 2.0 mm minimum. The ability of these materials to pass high-impact testing at a thinner profile offers wearers lighter and more comfortable eyewear while maintaining a higher level of protection.
Validating Safety: The Role of Impact Testing
The thickness requirements established by the ANSI Z87.1 standard are validated through a series of physical tests designed to simulate real-world hazards. These tests confirm that the lens, regardless of its material or thickness, will not fail or be dislodged from the frame. The standard includes two main impact resistance evaluations that lenses must pass to earn certification.
The first is the High-Mass Impact Test, which simulates the impact of a heavy, slower-moving object, such as a falling tool. For this test, a pointed, 500-gram (1.1 lb) projectile is dropped from a height of 50 inches (127 cm) onto the complete protective device. To pass, the lens must remain intact, and the frame must retain the lens without fracture or contact with the eye of the headform.
The second evaluation is the High-Velocity Impact Test, which mimics fast-moving debris like metal shavings or grinding particles. This test involves shooting a 1/4-inch (6.35 mm) steel ball at the lens at a speed of 150 feet per second (approximately 102 mph). Passing this test requires that the lens does not fracture, detach any pieces from its inner surface, or make contact with the eye of the headform upon impact. Successful completion of both the High-Mass and High-Velocity tests earns the lens the Z87+ high-impact rating.