Polycarbonate is the material to recommend. It is the material of choice for safety lenses because of its impact resistance, which means it resists breaking when something strikes it. Polycarbonate was used in face shields for astronauts, motorcyclists, and firefighters long before it was made into eyeglass lenses. For a child playing ball, that strength is the whole point.
Strength lets the lens be thin. The usual thinnest center is 1.0 mm for polycarbonate and 2.0 mm for glass. Polycarbonate also has a high index of refraction, 1.59, so it bends light strongly and keeps strong prescriptions from getting bulky. Trivex is a second impact-resistant choice. Glass is very scratch resistant but is heavier, and it now accounts for only about 2% of lenses sold. The textbook says to insist on polycarbonate or Trivex for children and one-eyed patients, because a person with one good eye cannot afford to lose it. More than 40% of disabling eye injuries happen at home or during sports. Many people never think about the risk until an injury happens. A child with two good eyes today still deserves the stronger lens.
For any patient who could take a hit, reach for polycarbonate or Trivex first. Ask what the patient does for work and for fun, then match the material to that answer. Matching lens material to a patient's risk is a core part of the COA optics and spectacles content area.
Incorrect Answers
- A. Glass is scratch resistant but breaks on impact, which is why every glass lens must pass a drop ball test before dispensing.
- B. Plexiglass is a rigid acrylic plastic. It is not one of the impact-resistant lens materials the textbook recommends for eyewear.
- D. Crown glass is still a glass lens material. It carries the same weight and breakage problem as any other glass.
Testing Pearls
- Polycarbonate and Trivex are the impact-resistant materials of choice for safety lenses.
- Insist on them for children and monocular patients, even with no refractive error.
- Polycarbonate index is 1.59; its thinnest center is about 1.0 mm.
- Glass is heavy, scratch resistant, and now only about 2% of lenses sold.
- Over 40% of disabling eye injuries occur at home or during sports and outdoor activities. Modern scratch-resistant and antireflective coatings bring plastic and polycarbonate close to glass for abrasion resistance.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 12, Fundamentals of Practical Opticianry.
