What is the difference between CR-39, polycarbonate, and glass lenses?
Various materials have been used over the year to make lenses. Glass was a common material for many years but has fallen out of favor as new lightweight and high index material have become available. CR-39 or Columbia Resin #39 is a registered trademark of Columbia Laboratories and commonly used as the material in “plastic” lenses. It is a much lighter weight material with a refractive index of 1.498 making it almost equivalent to glass’ 1.52. The basic difference in material weight allows for a much lighter weight lenses than an equivalent lens manufactured in glass. CR-39 is a thermoset plastic meaning it can not be bent by heat so lenses are made from blanks and ground to match the frames and prescription. The other advantage of plastic lenses is they are less breakable than glass but less scratch resistant if not coated. With coatings they can be made very scratch resistant. Polycarbonate is a higher index material (1.52) which is lighter and 10 times more impact resistant than pla
