We are not big promoters of polycarbonate lenses at Portland Eye Care. Though it does have a lower density than standard ophthalmic plastic, meaning your glasses will be lighter when made with polycarbonate lenses. And, it does have a higher refractive index than standard ophthalmic plastic, meaning it bends light more so we can use less of it to make your glasses, thus making them thinner. So why, don’t we usually recommend polycarbonate lenses?
The answer is that polycarbonate has a much lower Abbe number than either standard ophthalmic plastic or Trivex. The Abbe number of a material is a measure of how coherent light rays stay as they pass through the material. The fact that polycarbonate lenses have a low Abbe number means that light passing through the lenses will tend to spread out and not stay focused. This means you may notice little fringes or even tiny “rainbows” around images when you look through them. Usually, the effect is not strong enough for patients to actually notice these fringes, but every once in a while someone in polycarbonate lenses, especially at higher prescriptions, will tell us that “something doesn’t feel quite right.” And, when we leave everything else the same, but switch materials, the problem goes away.
While it isn’t as good in terms of optical quality, polycarbonate is impact resistant and it is less expensive than other thin light materials. That’s why it is often used, especially in children’s glasses. However, a good alternative to polycarbonate lenses for children (and adults) is a material called Trivex. Trivex is also thin, light, and impact resistant. And, Trivex has much better optical quality (the Abbe value for trivex is 46 whereas it is only 30 in the case of polycarbonate).
Trivex lenses also have a lower reflectance than polycarbonate lenses, which is good, but nowadays most patients opt for an antireflective coating on their lenses that eliminates virtually all reflections anyway, so this probably won’t impact your decision. However, if you aren’t getting an antireflective coating on your lenses, then this is another aspect to consider.