Lightweight, sturdy plastics such as polycarbonate (PC) and hard resin (CR-39) have replaced glass as the preferred materials for eyeglass lenses. These materials are less scratch-resistant than glass, so anti-scratch coatings have been developed to counteract this problem. This type of coating is applied in a thin film to both sides of an eyeglass lens to increase its resistance to abrasion.
Facts
•Of the two basic types of plastic lenses, the CR-39 is the more scratch-resistant option. Polycarbonate is designed to be softer so it will absorb an impact rather than shattering. This softness makes it very prone to being scratched. Because of this, anti-scratch coatings come standard on all PC eyeglass lenses. For CR-39 lenses, it is an option that can be selected at the time the lenses are ordered. It may also be added later at an optometrist’s office. However the quality of the coating is generally higher when applied by the manufacturer.
Benefits
•Anti-scratch coatings prevent eyeglass lenses from acquiring minor scratches during normal use. Wearers who find imperfections in their visual field annoying can upgrade their CR-39 lenses at minimal cost. Scratch-resistant lenses are a popular option for children’ glasses since they can withstand a certain amount of rough handling. People who enjoy outdoor activities and sports often choose coated PC lenses to prevent incidental damage to their eyewear.
Application Process
•The protective coating consists of a thin layer of silicon dioxide and titanium dioxide. These substances are applied as a transparent film over the surface of the lens. Methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and plasma CVD are used to apply anti-scratch coatings during lens production. These are nanotechnology-based processes that vaporize the molecules and bond them to the surface of the polycarbonate or resin lens. When lenses are coated at retail locations, they are simply dipped in a liquid silicon mixture.
Considerations
•Polycarbonate plastic offers automatic UV protection because of the internal molecular structure of the material. These lenses are highly resistant to shattering and are ideal for environments where there is a risk of impact against the glasses. CR-39 resin lenses have a lower level of distortion than PC lenses. Users who want eyeglasses that offer the best visual clarity prefer the CR-39 material for this reason. Both anti-scratch and UV-protective coatings can be added to these resin lenses upon request.
Warning
•Scratch-resistant coatings are not scratch proof. They also wear off over time. The useful lifespan of the protective film is about two years. Recoating the lenses generally doesn’t work well, and over-the-counter scratch fillers have limited effectiveness. Scratches cannot be polished out since even minor polishing can alter the prescription. Using a soft cotton cloth or lens cleaning kit to clean eyeglasses reduces the risk of scratching the coating. Any material used to wipe off smudges should be free of grit or other particles that might damage the anti-scratch film.