Differences Between Plastic and Polycarbonate Lenses

The first known use of a device to increase magnification dates to 65 A.D. Eyeglasses are first specifically mentioned in Italy in 1289. The lenses of these early eyeglasses were, quite naturally, made of glass. However, as time progressed, different materials were developed that offered numerous advantages over glass lenses. Two such materials in common use are plastic and polycarbonate, which account for 50 percent and 20 percent of all lenses sold respectively. There are several significant differences between the two.

Composition

• Plastic lenses are cast from a resin into the correct shape and baked over a period of time to set into shape as a lens. Plastic resin for lenses was first introduced in 1945. It has remained one of the most popular lens materials since that time. Polycarbonate was developed in the 70’s for the aerospace industry. Small pellets of the material are heated and injection molded under high pressure to quickly produce a lens.

Safety

• All prescription lenses sold in the U.S. must meet certain safety specifications set by the FDA. A metal ball is dropped on the lens from a set height to measure its impact resistance. Polycarbonate lenses are softer than plastic lenses, and flex more on impact. This absorbs shock better, making them less likely to shatter. On average, polycarbonate lenses are about 10 times more impact resistant than plastic lenses. Polycarbonate lenses are popular for safety and sports glasses due to this characteristic.

Characteristics

• There are important considerations about both lens materials concerning comfort and look. Plastic lenses are generally as thick as glass lenses of the same correction, although they are much lighter. Polycarbonate lenses can be as much as 40 percent thinner and 30 percent lighter than plastic lenses with the same correction. Due to its ability to flex more than plastic, polycarbonate can be used in rimless frames that use drilled screws to secure the lenses.

Optical Clarity

• The characteristics that make polycarbonate lenses safer and more flexible also make them less clear optically. While polycarbonate lenses can be manufactured with precision, they tend to flex in the frame, which affects optical clarity. Plastic lenses deliver greater optical clarity, comparable to those of glass lenses. Additionally, the relative softness of polycarbonate causes them to scratch more easily than plastic lenses.