Modern eyeglasses are truly a marvel of technology and engineering. The lenses we take for granted today have evolved over centuries, and are a far cry from the crude glass lenses of old. Eyeglass lens technology has been moving away from glass due to the weight, thickness, and the dangers associated with the lenses shattering. Eyeglass lenses are now almost exclusively made out of different types of plastic, each with their own specific optical and physical characteristics. The four main types of lenses utilized are CR39 plastic, polycarbonate, Trivex, and high index plastic.
Before discussing different lens materials, it is important to understand two properties that dictate how they look and behave; index of refraction and abbe value. The index of refraction of a material is a measure of its ability to bend light. The higher the index of refraction, the more the light will bend when it passes through the material. This is important when talking about lens thickness, as higher index lenses can achieve more with less material, allowing lenses to appear thinner. Abbe value relates to the optical quality of the lens material. As light passes through a lens, each wavelength of light reacts uniquely, which can cause the wearer to experience blurring or color abnormalities.
CR39 was the first application of a plastic eyeglass lens. Its use in the eyeglass industry was a revolutionary step that freed eyeglass wearers from their heavy glass lenses. Both the index of refraction and abbe value of CR39 are very near to glass, allowing excellent optical quality at a similar thickness. As CR39 is a plastic, it is significantly lighter than glass, however, what you gain in weight reduction, you lose in scratch resistance. Wearers of glass lenses were historically very happy with the durability of glass. Modern plastic lenses will inevitably scratch, usually leaving the wearer seeking replacement after two to three years.
Polycarbonate and Trivex are plastics with a higher index of refraction than CR39, offering the benefits of plastic, in a thinner and lighter package. These materials also address the fragility of CR39, offering improved scratch and impact resistance. These harder plastics are the standard for safety eyewear and are recommended for patients participating in activities where eye injury is possible. Polycarbonate, although durable, suffers in its optical quality. Due to a lower abbe value, polycarbonate can cause visual disturbance and distortion in some patients. Trivex addresses that issue with an abbe value similar to CR39 and glass. The only downside to Trivex is its cost and availability. Of the two, polycarbonate will be less expensive and more widely available.
High index plastic is just as it sounds. A plastic lens material focused on maximizing the index of refraction. These lenses are recommended for patients with higher eyeglass prescriptions. High index plastic is not impact resistant and is the most expensive of the lens options available. High index lenses are often marketed as an optical retailer’s “premium” lenses and thus will come standard with things like anti-reflective coating, pushing the price up even further.
I recommend Trivex to most patients. Trivex has a good cost-to-benefit ratio, offering great optics and durability in a reasonably priced package. That being said, the other lens materials certainly have their place, and I suggest discussing your particular needs with your doctor and/or sales associate. For more information on lens materials, see the links below.