High index lenses are made from specialized plastic resins engineered with sulfur, nitrogen, and sometimes halogen atoms woven into their molecular structure. These elements raise the material’s refractive index, which is the measure of how efficiently it bends light. The higher that number, the less material you need to correct your vision, and the thinner and lighter the finished lens becomes.
The Chemistry Behind High Index Plastics
Standard plastic lenses use a resin called CR-39, which has a refractive index of 1.50. It’s been the workhorse of the eyeglass industry for decades. To push beyond that index, lens manufacturers modify the polymer chemistry by introducing specific atoms that bend light more aggressively.
Sulfur is the most important ingredient. Incorporating sulfur into a polymer chain raises the refractive index while keeping the material lightweight, optically clear, and heat-stable. The most common high index plastic lenses, particularly those at the 1.67 index, are made from polythiourethane. This material combines sulfur-containing compounds (called polythiols) with isocyanates, creating a dense but light resin that bends light far more efficiently than CR-39.
Beyond sulfur, manufacturers also use halogen atoms like chlorine or bromine, nitrogen-containing compounds like urethane, and aromatic ring structures to boost refractive index. The lenses are produced through cast polymerization: liquid monomers are poured into a mold shaped to the prescription, then heated until they harden into a solid lens. This process allows precise control over the final shape and optical quality.
How Index Levels Differ by Material
High index lenses come in several tiers, and each step up uses a slightly different formulation to achieve a higher refractive index:
- 1.60 index: The entry point for high index. These lenses are roughly 30% thinner than standard CR-39 and use sulfur- or halogen-modified resins. They work well for moderate prescriptions.
- 1.67 index: The most popular high index option. Typically polythiourethane, these lenses are about 45% thinner than CR-39. They strike a good balance between thinness and cost for stronger prescriptions.
- 1.74 index: The thinnest plastic lens commercially available. These use the most heavily modified sulfur-based polymers and are reserved for the strongest prescriptions where minimizing thickness matters most. The specific formulation is sold under trade names like MR-174.
For context, polycarbonate (index 1.59) and a material called Trivex (index 1.53) also exist in the mid-range. These are valued more for impact resistance than thinness, which is why they’re popular for children’s glasses and safety eyewear rather than for reducing lens thickness.
What About High Index Glass?
Glass lenses can actually reach higher refractive indices than plastic, going up to 1.8 or even 1.9. At those levels, glass can produce extremely thin lenses. The tradeoff is weight. Glass is significantly heavier than plastic at every index level, and wearing high index glass lenses all day can become uncomfortable, especially in stronger prescriptions. Glass is also more fragile and shatters more easily on impact. For these reasons, high index plastic has almost entirely replaced glass in modern eyewear.
The Optical Tradeoff: Chromatic Aberration
Every lens material has a property called the Abbe value, which measures how cleanly it transmits light across different wavelengths. A lower Abbe value means more chromatic aberration: slight color fringing around high-contrast edges, especially in your peripheral vision. Standard CR-39 has an Abbe value of 58, which is excellent. High index materials score considerably lower. A 1.60 index lens has an Abbe value of 36, while 1.67 and 1.74 lenses both sit around 32.
In practice, most people wearing high index lenses don’t notice this color fringing during normal activities. It’s most apparent when looking through the outer edges of the lens at sharp contrasts, like white text on a dark background. If you’re sensitive to this effect, a 1.60 index lens offers a better balance between thinness and optical clarity than the higher tiers.
Why High Index Lenses Need Coatings
The same dense molecular structure that makes high index materials bend light efficiently also makes their surfaces more reflective. A standard CR-39 lens reflects a small amount of light from its surface, but high index lenses reflect noticeably more. This creates visible glare on the lens surface and reduces the amount of light reaching your eyes.
Anti-reflective coating is strongly recommended for any high index lens. It reduces surface reflections, lets more light pass through to your eyes, and makes the lenses look nearly invisible on your face instead of showing distracting reflections. The coating also reduces halos around lights at night and improves clarity for driving after dark. Most optical shops include anti-reflective coating as a standard add-on when you order high index lenses, and skipping it undermines much of what you’re paying extra for.
Choosing the Right Index for Your Prescription
The material you need depends largely on your prescription strength. For mild prescriptions (roughly up to plus or minus 2.00 diopters), standard CR-39 or polycarbonate lenses won’t be noticeably thick, and upgrading to high index adds cost without much visible benefit. Between plus or minus 2.00 and 4.00, a 1.60 index lens offers a meaningful reduction in thickness and weight. For prescriptions stronger than plus or minus 4.00, 1.67 index makes a real difference in comfort and appearance. At plus or minus 6.00 and beyond, 1.74 index lenses keep the edges from becoming conspicuously thick, particularly in larger frame styles.
Frame choice also matters. Smaller, rounder frames naturally hide lens thickness better, so you may not need the highest index if you pick a compact frame. Larger or rimless frames expose more of the lens edge, making high index materials more worthwhile for cosmetic reasons alone.