What hard resin is commonly used in plastic lenses?

B. CR39 is a type of hard resin that is commonly used in the manufacturing of plastic lenses due to its excellent optical clarity, lightweight nature, and good impact resistance. It is a thermosetting plastic, specifically a type of allyl diglycol carbonate, which offers several benefits for everyday eyewear. This material can easily be molded into different shapes and thicknesses, making it versatile for various lens prescriptions.

CR39 lenses are favored for their affordability and comfort compared to other types of lenses, such as glass or polycarbonate. They also provide UV protection and are less prone to shattering, which enhances safety for wearers.

While polycarbonate and Trivex are also materials used for lenses, they are not classified as common hard resins like CR39. Polycarbonate is known for its high impact resistance and thinner profile but is not typically referred to as a hard resin. Similarly, Trivex offers various benefits similar to polycarbonate but is less widely recognized than CR39 in standard optical practices. Polyurethane, on the other hand, is more commonly associated with specific applications in coatings rather than as a primary material for eyewear lenses.