What Are High-Index Materials?
High-index lens materials have a refractive index higher than the standard CR-39 (1.498). By bending light more efficiently per unit of surface curvature, they achieve the same optical power with flatter curves, resulting in thinner, lighter lenses. This is especially beneficial for strong prescriptions where standard materials produce thick, heavy lenses.
Available High-Index Options
| Index | Abbe Value | Thickness Reduction vs. CR-39 | Material Type |
|---|---|---|---|
| 1.56 | 37-39 | ~15% | Plastic |
| 1.59 (polycarbonate) | 30 | ~20% | Thermoplastic |
| 1.60 | 36-42 | ~25% | Plastic |
| 1.67 | 32 | ~35% | Plastic |
| 1.70 | 36 | ~40% | Glass |
| 1.74 | 33 | ~45% | Plastic |
| 1.80 | 25 | ~50% | Glass |
| 1.90 | 30 | ~55% | Glass |
The Index-Abbe Trade-Off
As the refractive index increases, the Abbe value generally decreases. This means thinner lenses come at the cost of more chromatic aberration (color fringing), particularly noticeable in the lens periphery and in higher prescriptions.
Chromatic aberration appears as colored fringes around high-contrast edges. It is most bothersome for:
- High-prescription patients looking through peripheral lens areas
- Patients who are optically sensitive or detail-oriented
- Situations with high-contrast lighting (night driving, looking at screens against dark backgrounds)
Higher index = thinner lens but more chromatic aberration. The sweet spot for most patients is 1.60 or 1.67, which provides significant thickness reduction while maintaining reasonable optical clarity. Reserve 1.74 for the strongest prescriptions where thickness is the primary concern.
When to Recommend High-Index
- Prescriptions above ±4.00 D: Thickness becomes cosmetically and functionally noticeable
- Minus prescriptions above -6.00 D: Edge thickness in CR-39 becomes very thick and heavy
- Plus prescriptions above +4.00 D: Center thickness becomes bulky
- Patient concerned about cosmetics: Thinner lenses look better in the frame
- Large frame selections: Bigger frames expose more edge thickness in minus lenses
Dispensing Considerations
- Anti-reflective coating is essential: High-index materials reflect more light than lower-index materials (due to the higher index). Without AR coating, the internal reflections can be distracting and reduce contrast.
- Smaller frames reduce thickness further: Frame size has a dramatic effect on edge thickness for minus lenses. Combining high-index with a smaller frame maximizes the cosmetic benefit.
- Aspheric designs: High-index materials paired with aspheric front surfaces produce the thinnest possible lens profile.
- Weight may increase: Some high-index plastics have higher specific gravity than CR-39, so the lens may be thinner but not necessarily lighter. The overall weight depends on both density and volume.
When recommending high-index materials, always include AR coating in the recommendation. The higher surface reflectance of high-index lenses (up to 50% more than CR-39) makes AR coating even more beneficial than with standard materials. Many labs offer high-index and AR as a package for this reason.
Recommending 1.74 index for a -3.00 D prescription. The patient pays a premium price but sees minimal thickness improvement over 1.60 and may notice more chromatic aberration. Match the index to the prescription: 1.60 for ±4 to ±6, 1.67 for ±6 to ±8, and 1.74 for ±8 and above.
Key Takeaways
- Higher refractive index means thinner lenses with flatter curves
- The trade-off: higher index = lower Abbe value = more chromatic aberration
- AR coating is essential for high-index materials due to increased surface reflectance
- Match the index to the prescription strength (1.60 for moderate, 1.67-1.74 for high)
- Combining high-index with small frames and aspheric designs maximizes thinness