Lens Materials, Designs, and Treatments
Materials and trade-offs
Every material choice balances refractive index (thickness), Abbe value (chromatic aberration), specific gravity (weight), and impact resistance. CR-39 has an index near 1.498 and an Abbe value near 58 with excellent optics but poor impact resistance. Polycarbonate (1.586, Abbe near 30) is extremely impact resistant but optically softer and prone to stress cracking at drill holes and rimless mounts. Trivex (1.53, Abbe 43 to 45) offers impact resistance with better optics and is a favorite for drill mounts. High-index 1.60, 1.67, and 1.74 give progressively thinner lenses with lower Abbe values and higher cost. Glass gives the best optics but is heavy and shatters.
Designs
Single vision, bifocals (flat-top 28, round segment, executive), trifocals, and progressive addition lenses, which use a corridor with soft or hard designs, prism thinning, and unavoidable peripheral unwanted cylinder. Also occupational and computer designs, aspheric and atoric curves, and free-form digital surfacing that compensates for wrap and tilt.
Treatments
Hard coat for scratch resistance, anti-reflective multilayer coating (must be applied to the back surface as well as the front), hydrophobic and oleophobic layers, UV blocking, photochromic (activated by UV, so it performs poorly behind a windshield), polarized filters that remove horizontally oriented reflected glare, fixed tints, gradients, and mirrors.
Traps and rule of thumb
Questions imply that the highest index is always best; in fact lower Abbe and higher cost are real penalties, and polycarbonate is thicker than 1.67 at the same power. Safety and children's eyewear should be polycarbonate or Trivex to meet impact standards. Rule of thumb: high plus prescriptions benefit most from high index, while high minus prescriptions benefit from a small eyesize first.