Choosing Eyeglass Lenses: By Material
For complete information on eyeglass lens materials, visit the Consumer Guide to Eyeglasses.
Today, you have more choices in lens materials than just glass or plastic:
Plastic versus glass lenses
Glass is the hardest lens material and offers the best protection against scratches. But glass lenses can shatter more easily than plastic lenses, are twice as heavy as plastic lenses, and don't provide as much protection from the sun's harmful ultraviolet (UV) rays.
Unless your main concern is scratch protection (and you don't mind wearing heavy glasses that might slide down your nose frequently) heavy, your better choice in a lens material for your eyeglasses is one of the options available in plastic.
For many years, CR-39 plastic has been the standard lens material for eyeglasses. (The "CR-39" is the name given to the resin formulation by the original manufacturer.) CR-39 plastic lenses are half the weight of glass lenses, have better ultraviolet (UV) light protection, are less prone to shattering, and can be tinted more easily. And with an added scratch-resistant coating, they are nearly as resistant to scratches as glass lenses. CR-39 plastic lenses are about the same thickness as glass lenses.
Polycarbonate was one of the first "thinner, lighter" lens materials. Polycarbonate lenses are both thinner and lighter than CR-39 plastic lenses. But the most important feature of polycarbonate lenses is their impact resistance. Polycarbonate lenses are 10 times more impact-resistant than glass or regular plastic lenses, making them the preferred choice for safety glasses and eyewear for children and adults who desire lightweight, protective lenses.
High index plastic lenses
High index plastic is a term used to describe a variety of plastic lens materials that are thinner and lighter than regular CR-39 plastic. For a given prescription power and lens design, the factor that determines lens thickness is the index of refraction (also called refractive index) of the lens material. Materials with a higher refractive index bend light more efficiently than materials with a lower index of refraction. CR-39 plastic lenses have a refractive index of 1.498. Any lens material that has a refractive index higher than glass (1.523) is considered a high index material.
Today's high index plastic lenses sold in the United States range in refractive index from 1.54 to 1.74. One of the most popular high index lens materials has a refractive index of 1.67. Lenses of this material are about 50% thinner than lenses made of standard CR-39 plastic.
Your professional optician will be able to help you determine which high index lens material is best for your prescription and budget.