The most common lens material used in eyeglasses today are any one of several plastic formulations. Of the plastics used in eyeglasses, Columbia Resin #39, or CR39, is the most common and least expensive plastic available for eyeglasses in common use today. Columbia Resin no. 39, or “standard plastic” as it is most commonly known as, is the 39th plastic polymer formulation to come out of the Columbia Resins project in 1940. The plastic developed from the Columbia resins project, like most other plastic materials formulations, were not originally destined to be eyeglass lens material.
CR39 was developed for military use as a way to reinforce fuel tanks and lower the weight of bomber aircraft being manufactured for the WWII war effort. After the war it was realized that the CR39 plastic resin would be ideal for eyeglass lenses because it was crystal clear, transparent to visible light and because it was half of the weight of the more common glass lens material. Since then CR39 plastic lens material has almost single handedly brought about the near extinction of glass as a lens material. For over six decades CR39 standard plastic lenses have become the most prevalent lens in ophthalmic eyewear.
Like most plastics, CR39 is tough and resists scratching, heat and most chemicals. It is the clearest lens material in common use on the Abbe scale at an average value of 58. Comparatively, Trivex is the next clearest to CR39 at 45, and polycarbonate is 32 (higher numbers are better). Most high index plastic blends fall below an Abbe value of 40.
CR39 is a perfectly suitable material for appearance and aesthetics in many low to medium power prescriptions (Rx). For higher power prescriptive corrections CR39 will yield the thickest edge or center profile of any commonly used lens material because of its low refractive index value of 1.50. However, for the right Rx, and carefully considered frames, CR39 can be used with few compromises because of its low cost and other noteworthy qualities including the fact it can easily be tinted to accomplish many fashion colors. These and its low cost make it an ideal choice for eyeglasses today.
CR39 is not usually the best choice for rimless drill-mount eyeglasses because it tends to chip when drilled. And, it is not as impact resistant as other materials like polycarbonate and Trivex lens plastics. As a result, in eyewear made for children it is usually not the material of choice, nor does it protect eyes well enough for industrial, safety, sport or avocation uses. In those instances polycarbonate or Trivex lens materials are chosen for their superior impact resistance but, not because they have superior optical qualities over CR39. Standard plastic lenses should also be treated to reject UV radiation (sunlight) as it does not have this capability naturally. Adding a UV coating is available at a modest additional cost. Plastic lenses also are not as scratch resistant as glass but anti-scratch coatings and proper cleaning can make CR39 lenses nearly as durable and a great budget choice for many prescriptions.
For shear bang for the buck, CR39 lens material is unbeatable as a lens material. CR39 can be made into many prescriptions but does not have the wider range of corrective capabilities inherent in higher index plastic lenses. When everything is considered, including the superior optical qualities of CR39, it would be difficult to find a better overall material for the majority of wearers than CR39. In high power lens prescriptions, CR39 is usually not the best choice to avoid unappealing edge profiles or thick lens centers of farsighted prescriptions. In those cases higher index plastic materials are better choices. For general every day eyewear needs in your eyeglasses you’ll find standard plastic CR39 lens materials is the most affordable option for almost any budget, and a very good choice for every day eyewear.