PLASTIC LENSES SINGLE-VISION (SCRATCH-RESISTANT COATING LENSES)

This higher density also generally makes high-index lenses harder than standard lenses, making them more scratch resistant. This does not mean that they’re more impact-resistant; actually, the opposite is true. This hardness is often accompanied by brittleness and the possibility of chipping. Here are some useful things to know about lens materials and scratch resistance:

• High-index plastic is more scratch-resistant than standard plastic or polycarbonate. Generally, the higher the index of the plastic, the more scratch-resistant it is.
• High-index glass is more scratch-resistant than crown glass.
• Even the highest-index plastic is still less scratch resistant than any glass material. You trade glass’s unmatched scratch resistance for lightness and impact resistance when you choose plastic.
• Of all the plastics, polycarbonate is the least scratch-resistant but the most impact resistant.
• In general, as the index increases, hardness increases, and lenses become more brittle. So, the highest index glass lenses are extremely brittle but thin and scratch-resistant. Conversely, polycarbonate, one of the lowest-index lenses, is extremely impact resistant and flexible but very soft and easy to scratch.
• Most high-index plastics rate relatively the same for scratch resistance, and any high-index plastic is going to be more scratch resistant than standard plastic or polycarbonate.

High-index plastic is sometimes chosen not for its thinness, but specifically for its scratch resistance and durability. It is a good choice for a pair of glasses that you want to wear for a long time.