CR-39, or allyl diglycol carbonate (ADC), is a kind of plastic that was developed in 1940 and first used to help create a new type of fuel tank for B-17 bombers during WWII. Since then, its dominant use has become much more civilian – in making lenses for eyeglasses.
Apparently, eyeglass lenses haven’t commonly been made of glass for decades, because of the high weight. CR-39 has half the weight of glass, good resistance to ultraviolet light (which causes cataracts), and a refractive index nearly as high as that of crown glass (meaning lenses can be fairly thin). Unlike polycarbonate lenses (which offer more safety), CR-39 doesn’t scratch too easily. It does, however, produce more chromatic aberration than crown glass. CR-39 is pretty good when it comes to how much light reflects off rather than passing through; normally, lenses made of CR-39 involve a 7.97% loss of light, compared with 8.59% for crown glass.
I am quite happy with CR-39 plastic lenses myself. My only wish is that they could be made more resistant to dust, rain, and fingerprints. In particular, it would be nice if water would bead and roll off of them, rather than sticking in droplets that become smudges.
