High Refractive Index (HRI) Lenses
High Refractive Index (HRI) lenses are made from denser materials that bend light more efficiently than regular lenses. This allows the lens to achieve the same prescription with significantly less thickness—often up to 50% thinner than standard plastic lenses.
Because less curvature is required for the lens power, HRI lenses provide improved aesthetics (no "coke-bottle" look) and increased comfort for the wearer. The higher your prescription, the more noticeable these benefits become.
Understanding Refractive Index
The refractive index describes how fast light travels through a material. The higher the number, the slower light moves and the more it bends:
- - Air: 1.0 (baseline)
- - Water: 1.33
- - CR-39 Standard Plastic: 1.498 (thickest lens)
- - Trivex / Mid-Index: 1.53 – 1.56
- - Polycarbonate: 1.61
- - High-Index Plastic: Up to 1.74 (thinnest lens)
- - Diamond: 2.4 (the densest clear material)
Lens Material Options
High-Index Plastics (1.60 – 1.74)
The modern standard for strong prescriptions. They are thinner, lighter, and generally less brittle, making them ideal for rimless and semi-rimless frames.
Polycarbonate & Trivex
While slightly thicker than high-index plastic, these are highly impact-resistant. They are the preferred choice for children, athletes, and safety eyewear.
Glass (Crown & Flint)
Glass offers exceptional optics but is significantly heavier and more brittle. It is rarely used today compared to modern lightweight polymers.
Tips for achieving the thinnest profile
Lens material is only one part of the equation. To get the best cosmetic result, consider:
- - Frame Shape: Smaller, round, or oval frames result in thinner edges.
- - Aspheric Design: These lenses have flatter curves, reducing magnification and thickness.
- - Frame Material: Plastic (acetate) frames hide edge thickness better than thin metal frames.
Note: Because HRI lenses reflect more light due to their density, an Anti-Reflective (AR) coating is essential for visual clarity and comfort.