CR-39 (allyl diglycol carbonate, index 1.50) is the original plastic lens, optically excellent and cheap; it is the base of stock single vision lenses worldwide. 1.56 is a mid-index resin popular in Asia. Polycarbonate (1.59) is injection moulded, impact resistant and the standard for children's, safety and rimless work but has lower optical quality. Trivex (1.53) is impact resistant with better optics.
High-index plastics 1.60, 1.67 and 1.74 come mostly from Mitsui Chemicals' MR series of monomers (MR-8, MR-7, MR-174). They allow thinner, lighter lenses for strong prescriptions and command higher prices. Mineral (glass) lenses, once universal, survive in sunglasses and in 1.90 index for extreme prescriptions.
Every lens starts as a blank. Finished stock lenses are moulded to power for common prescriptions; semi-finished blanks have a moulded front and are surfaced to the prescription on the back in a laboratory. Blank casting is concentrated in China (Danyang), Thailand, Korea, Japan and Mexico.