Welcome to question of the day #195

Welcome to question of the day #195

Many of my patients work in the construction industry. I’m thinking of offering polycarbonate lenses instead of CR39 to those at risk of ocular trauma. Is this a good idea?

I think that is a very good idea. Polycarbonate lens along with another material called Trivex have many advantages over CR39 and other types of plastic lens materials. Here are the major ones:

More resistant to impact than plastic so they don’t shatter or break as easily

Refractive index of polycarbonate lenses at 1.59 is higher than CR39 at 1.49; polycarbonate lenses can be 20 to 25% thinner than the equivalent powered CR39 lens.

Weigh around 30 % less than the equivalent powered CR39 lenses.

Polycarbonate material naturally absorbs UV, while CR39 needs to have a UV filter added.

Polycarbonate lenses come in a wide range of refractive corrections and lens forms.

Polycarbonate also has some disadvantages:

Polycarbonate lens material is more expensive than CR39 and this makes prescription lenses cost more than the equivalent power in CR39.

Another material which is impact resistant and that is Trivex. This is also available as a lens material. Polycarbonate lenses may be thinner and lighter than CR39 but some experts claim that Trivex is even better than in these attributes and provides increased clarity.

Polycarbonate has a lower abbe value than CR39 and high-index materials. Abbe value indicates the dispersion of light for different lens materials. The lower the abbe value the higher the chance of chromatic aberrations, that is coloured fringes around the objects viewed. Many people wearing polycarbonate lenses don’t notice this and when they do only when they looking through the periphery of their lenses. I cannot recall a single patient in my clinical experience that rejected polycarbonate lenses because of coloured fringes.

Polycarbonate is softer than CR39, which makes it more prone to scratches and therefore less durable. A simple way to counter this problem is to have the lenses hard coated. However, this adds further to the cost.

Overall, taking the advantages and disadvantages into account I think that patients at risk of ocular trauma deserve the opportunity to purchase polycarbonate lenses.