Spectacle Lens Properties
Modern spectacle lenses come in a wide variety of materials, designs, and coatings. The CPOA helps patients understand their lens options and assists in verifying and dispensing spectacles. Understanding the optical and physical properties of lenses helps you explain trade-offs and make appropriate recommendations based on a patient's prescription, lifestyle, and visual needs.
Lens Materials
| Material | Index | Abbe # | Key Properties |
|---|---|---|---|
| CR-39 (plastic) | 1.50 | 58 | Most common; excellent optics; lightweight; affordable; not impact-resistant |
| Polycarbonate | 1.586 | 30 | Impact-resistant (ANSI Z87.1); required for children & safety; high chromatic aberration |
| Trivex | 1.53 | 45 | Better optics than polycarbonate; also impact-resistant; lighter |
| High-index (1.60) | 1.60 | 40 | Thinner, lighter for moderate prescriptions |
| High-index (1.67) | 1.67 | 32 | Very thin for higher prescriptions |
| Ultra high-index (1.74) | 1.74 | 33 | Thinnest available; moderate chromatic aberration |
| Glass | 1.52 | 59 | Excellent optics; heavy; shatters easily; rarely used today |
Index of Refraction
The index of refraction (n) describes how much a material bends light compared to air. Higher index = greater bending per unit thickness = thinner lens for the same prescription. This matters most for patients with high prescriptions (+/- 4.00 D or more), where lens thickness becomes cosmetically and physically significant.
💡 Clinical Tip: Higher-index lenses are thinner and lighter, but they also have more internal reflections due to their higher reflectivity. This makes anti-reflective (AR) coating even more important on high-index lenses. Without AR coating, high-index lenses can produce annoying internal reflections.
Abbe Number (Chromatic Aberration)
The Abbe number (also called V-number) measures a lens material's chromatic aberration -- how much it splits white light into different colors at the periphery. A higher Abbe number means less chromatic aberration and better peripheral optical quality.
- CR-39 has a high Abbe number (58) -- excellent peripheral vision
- Polycarbonate has a low Abbe number (30) -- noticeable chromatic fringes for some patients, especially with high prescriptions and large frames
For patients who notice colored fringes or halos at the edges of their polycarbonate lenses, Trivex or CR-39 may be preferable if safety requirements allow.
Lens Coatings
Anti-Reflective (AR) Coating
AR coating reduces reflections from the lens surface, improving visual clarity and reducing glare, especially for nighttime driving and computer use. It also improves the cosmetic appearance of lenses (people can see the wearer's eyes rather than reflections). AR coating is highly recommended for high-index lenses and for anyone who uses screens or drives at night.
Scratch-Resistant Coating
Applied to the front and back surfaces, scratch-resistant coating increases lens durability. Most modern plastic lens materials come with this coating standard. It does not make the lens scratch-proof -- only more resistant.
UV Protection
UV-blocking is built into polycarbonate and Trivex materials and can be added to other plastics. Glass does not inherently block UV. UV protection is important for long-term ocular health (cataract and macular degeneration risk reduction). Clear lenses with UV protection do not appear different from those without.
Photochromic Lenses
Photochromic lenses (e.g., Transitions) darken in UV light (outdoors) and return to clear indoors. They provide convenience by eliminating the need for separate prescription sunglasses. Limitations:
- Do not darken well inside a car (windshields block UV that activates the photochromic reaction)
- Darkening is temperature-dependent (darker in cold, less dark in heat)
- Take a few minutes to fully transition
Mirror and Tinted Coatings
Sun lenses are tinted to specific visible light transmission levels (VLT%). Polarized lenses contain a filter that blocks horizontally polarized light (reflections from flat surfaces like water and roads), reducing glare significantly for driving, fishing, and outdoor activities.
🔑 Key Point: Polarized lenses may make certain LCD screens (GPS units, ATMs, some car dashboards) appear blank or distorted because these screens emit polarized light. Warn patients before recommending polarized lenses if they use such devices.
Lens Designs
- Single vision (SV): one prescription power; for distance, near, or intermediate
- Bifocal: two zones (distance and near); visible line; flat-top 28 (FT-28) most common
- Trifocal: three zones (distance, intermediate, near)
- Progressive addition lens (PAL): no visible line; gradual power change from distance to near through a corridor; requires fitting height measurement; narrower reading zone than bifocals but cosmetically preferred
- Occupational lenses: customized for specific tasks (e.g., office bifocal with near at top for upward gaze, as when working at an overhead monitor)
Key Takeaways
- Higher index = thinner lens; lower Abbe number = more chromatic aberration
- Polycarbonate is impact-resistant (required for children/safety) but has high chromatic aberration (Abbe 30)
- CR-39 has the best optics for a plastic lens (Abbe 58) but is not impact-resistant
- AR coating reduces reflections; critical on high-index lenses; improves night driving
- Photochromic lenses don't darken well inside cars; polarized lenses interfere with some LCD displays
- Progressive lenses require precise fitting height measurements for proper function