An Introduction to Lens Options and Definitions
If you’re getting ready to purchase new glasses, you’ve probably thought a lot about what kind of frames you want. You will probably try on dozens of frames to make sure you find one you love. But when it comes to eyeglasses, frames really are just that — they merely support the important part, the lenses! Lenses disappear before your eyes, but they are the most important part of your eyeglass purchase. To make sure that your glasses don’t just look good but also help you see well and feel comfortable, keep reading.
To select the right lenses, you should understand the difference between all the choices.
- Standard Plastic/CR-39: When they were introduced in 1947, plastic lenses (made from a polymer called Columbia Resin 39) offered significant improvements over the older glass options because they were safer and more affordable. These newer lenses also provided better UV protection than standard glass lenses. CR-39 lenses remain popular today for those same reasons, but they also have a relatively low refractive index (1.498) — it’s okay if you don’t know what that number means, we’ll get to it.
- Polycarbonate: The next generation of plastic lenses were introduced in the early 1980s. The material had been designed about a decade before that for use in the aerospace industry. In fact, this material is still used for the “helmet visors of astronauts and for space shuttle windshields.” These lenses are even more impact resistant than CR-39 lenses as well as thinner and lighter, with a refractive index of 1.586. They also block 100% of UV light. All of these benefits make polycarbonate lenses an excellent choice for children’s and athletes’ glasses.
- High Index: Consumer desire for even thinner and lighter lenses—to avoid the “bug eye” look and to prevent heavy glasses from sliding down the wearer’s nose — lead to the development and production of high index lenses. These lenses have a refractive index of 1.67 or 1.74. For a little perspective, high index lenses with a refractive index of 1.74 are about 50% thinner than CR-39 lenses. Like polycarbonate lenses, high index lenses block 100% of UV rays.
So now let’s talk more about the refractive index, which is what defines the “high index” of high index lenses. Often, our vision problems are caused by refractive errors, meaning the light does not travel through our eyes the way that it is supposed to. Corrective lenses do some of the work of bending light for our eyes. The refractive index defines how light moves through a material, and lenses made from material with a high index refract light more efficiently, meaning “less material can be used in high-index lenses to correct the same amount of refractive error, which makes high-index plastic lenses both thinner and lighter than conventional glass or plastic lenses.”
There are a few drawbacks to high index lenses — they are more expensive and have a lower Abbe value, meaning there might be a very slight decrease in clarity of objects seen at particular angles. However, for many people, high index lenses offer far more benefits than drawbacks.
According to AllAboutVision.Com, an independent informational website devoted to providing consumers with unbiased information, high index lenses are especially beneficial for people who suffer from hyperopia (farsightedness). If your prescription is above a +1.00, you may be a good candidate for high index lenses, and whether you should choose 1.67 or 1.74 high index lenses will depend on the strength of your prescription. Certain types of frames may also be more suited to thinner lenses.
If you have questions about what type of lenses are right for you, make an appointment to talk to your eye care professional. In the meantime, you can visit highindexlenses.com, where you can read a variety of informational articles or live chat with an expert.