Overview of Spectacle Lens Types
Spectacle lenses come in a range of designs and materials, each suited to different prescriptions, visual needs, and patient preferences. As a CPO, understanding lens types helps you communicate options clearly to patients and assist in appropriate selection during the dispensing process.
Lens Materials
| Material | Index | Impact Resistance | Best For |
|---|---|---|---|
| CR-39 plastic | 1.50 | Moderate | Low-power prescriptions, cost-effective |
| Polycarbonate | 1.586 | Highest | Safety eyewear, children, sports, rimless frames |
| Trivex | 1.53 | High | Similar to polycarbonate with better optics |
| High-index 1.67 | 1.67 | Lower | Moderate-to-high prescriptions |
| High-index 1.74 | 1.74 | Lowest | Very high prescriptions, maximum thinness |
Single Vision Lenses
Single vision (SV) lenses have one focal power throughout the entire lens. They are used for:
- Distance correction only
- Near correction only (reading glasses)
- Full-time wear for patients without presbyopia
Multifocal Lenses
Bifocals
Bifocal lenses have two distinct zones separated by a visible segment line. The upper zone contains the distance prescription; the lower segment contains the near addition. The most common design is the flat-top (D-segment) bifocal, with a D-shaped near segment.
- Executive bifocal: full-width segment across the entire lens
- Round segment bifocal: older design, circular segment
Trifocals
Trifocal lenses add a third, intermediate zone between the distance and near segments. Two visible lines separate the three zones. Useful for patients who need clear arm's-length vision (computer, dashboard).
Progressive Addition Lenses (PALs)
Progressive lenses provide a smooth, continuous power gradient from distance at the top through intermediate in the middle to near at the bottom, with no visible lines. They require:
- Accurate monocular PD and fitting height measurements
- Minimum B measurement (frame height) of 28-30 mm
- Patient adaptation time, especially for peripheral distortion in the lateral zones
Lens Coatings
- Anti-reflective (AR) coating: reduces surface reflections, improves clarity and cosmetics, essential for high-index lenses
- Scratch-resistant coating: hardens plastic lens surfaces; prolongs life
- UV protection: blocks UV-A and UV-B; essential for long-term ocular health
- Photochromic (Transitions): darkens outdoors in UV light, clears indoors
- Blue light filter: reduces high-energy visible light; relevant for digital screen users
- Hydrophobic coating: repels water and smudging for easier cleaning
Key Takeaways
- Polycarbonate: highest impact resistance; required for safety, children, and sports
- High-index materials: thinner for strong prescriptions but require AR coating
- Single vision: one power; bifocal: two zones with visible line; progressive: seamless gradient
- Progressive lenses require minimum 28-30 mm frame B measurement and precise measurements
- Photochromic lenses do not darken behind automotive glass