Plastic CR-39
CR-39 is a plastic polymer commonly used in eyeglass lenses. It was developed by Columbia Resins in 1940. The Armorlite Lens Company started manufacturing eyeglass lenses with CR-39 in 1947. Today CR-39 is made by PPG industries.
CR-39 has an index of refraction of 1.498 and an Abbe number of 58. Today there are many materials that are thinner, lighter and more impact resistant that CR-39. One advantage of CR-39 is that the surface is very tough. It is resistant to scratches, most solvents, and material fatigue.
For safety glasses, CR-39 is rated only for low-impact applications and must be ground to a 3 mm center thickness. Lenses for nearsightedness are thicker on the edges, so using CR-39 produces a lens with a thick edge.
