 CR-39 is transparent in the visible spectrum and is almost completely opaque in the ultraviolet range.It has high abrasion resistance, in fact the highest abrasion/scratch resistance of any uncoated optical plastic.CR-39 is about half the weight of glass with an index of refraction only slightly lower than that of crown glass, and its high Abbe number yields low chromatic aberration, altogether making it an advantageous material for eyeglasses and sunglasses.