Lens Materials, Designs, and Treatments
Materials
Key numbers: CR-39 is n = 1.498, Abbe ≈ 58, specific gravity ≈ 1.32. Crown glass is 1.523 with Abbe ≈ 59, but heavier and less impact resistant. Polycarbonate is 1.586 with Abbe ≈ 30 and is the impact benchmark. Trivex is 1.53 with Abbe ≈ 44–45 and good impact strength. High-index 1.60 gives Abbe ≈ 41–42; 1.67 and 1.74 are thinner and lighter but lower in Abbe (roughly 32 and 33). Higher index means thinner and flatter lenses, but also more dispersion and more surface reflection.
Designs
Single vision, bifocal (flat-top 28/35, round seg, executive), trifocal, and progressive. Progressives require a corridor length choice, a fitting cross height, and patient counseling about peripheral distortion. Aspheric and free-form designs flatten the front curve and reduce aberration and edge thickness.
Treatments
Hard coat for scratch resistance, anti-reflective stacks for transmission, hydrophobic and oleophobic top coats, photochromic, polarized, tint, UV, and mirror coatings. Safety standards require impact-resistant material or a documented treatment.
Traps
- Assuming the highest index is always best; Abbe, impact, and cost matter just as much.
- Using polycarbonate for a tight rimless drill mount where stress cracking is a risk.
- Confusing photochromic (darkens with UV) with polarized (blocks reflected glare).
Worked example
A −6.00 D job in 1.74 will be markedly thinner than the same job in CR-39, but the patient may notice off-axis color fringing and stronger surface reflections unless AR coating is specified.