CR-39 material

CR-39 material 

Allyl diglycol carbonate or Columbia Resin. CR-39 is a light transparent plastic material (refractive index 1.498, V = 59) used in the manufacture of spectacle lenses and much harder than polymethyl methacrylate. It is not quite as hard as glass. Syn. hard resin. See constringence; plastic lens.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann