Refractive index
A higher index bends light more, so the same prescription can be ground thinner. Gains are most visible in stronger minus powers.
Eyeglasses and Optical · Lens Materials
Spectacle Lens Materials Polycarbonate, Trivex, high-index and glass.
The lens material sets how thin, light, sharp and impact-resistant a pair of glasses can be. Knowing the trade-offs between index, Abbe value and weight lets your opticians recommend with confidence and explain the price difference in one sentence.
1.50Index of standard CR-39 plastic
1.74Highest common plastic index (MR-174)
1.11Trivex specific gravity, the lightest
50 inFDA drop-ball height for impact testing
How materials differ
Four numbers decide the recommendation
Every lens material can be described by a few properties. Index drives thickness, Abbe value drives off-axis clarity, specific gravity drives weight, and impact behavior drives where the lens can safely be used.
Abbe value
Higher Abbe means less chromatic aberration. CR-39 sits near 58, while polycarbonate sits near 29 to 30, which some wearers notice as color fringing toward the edge.
Specific gravity
Density in grams per cubic centimeter. Trivex is about 1.11 and polycarbonate about 1.21, compared with roughly 2.5 or more for glass.
Impact behavior
Polycarbonate and Trivex are the go-to materials when impact protection matters. Trivex also holds up well in drilled rimless mounts.
Matching material to the patient
A short decision path keeps the conversation simple at the dispensing table.
- Children and active wearers: polycarbonate or Trivex
- Drilled rimless: Trivex or a 1.60 material
- High minus: 1.67 or 1.74 to manage edge thickness
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Polycarbonate
Index around 1.58 to 1.59 with Abbe near 29 to 30. Strong impact resistance; it can stress-crack in drill mounts, so check the lab and frame guidance.
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Trivex
Index 1.53, Abbe about 45 to 46, specific gravity 1.11. Lightweight and tough, with high tensile strength that suits drilled rimless frames.
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Mid and high index
Mitsui Chemicals lists MR-8 at 1.60 and Abbe 41, MR-7 and MR-10 at 1.67 and Abbe 31, and MR-174 at 1.74 and Abbe 32.
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Glass
Crown glass offers about 1.52 index and Abbe near 59 but is heavy, and in the US it must be treated and tested to meet the federal impact rule.
At the dispensing table
How opticians present materials
The goal is to recommend one material clearly, explain why, and show the visible benefit rather than reading out a spec sheet.
Start from the Rx and frame
Strong powers, large eyesizes and rimless mounts narrow the field before lifestyle is even discussed.
Name the trade-off
Thinner lenses can mean a lower Abbe value. For a patient sensitive to peripheral blur, a mid-index option may serve better than the thinnest one.
Pair with the right coating
Higher-index lenses reflect more light at each surface, so anti-reflective treatment becomes part of the material recommendation.
Material reference table
Index, Abbe and density by material
| CR-39 (standard plastic) | Index 1.50 · Abbe 58 · SG 1.32 |
|---|---|
| Trivex | Index 1.53 · Abbe 45–46 · SG 1.11 |
| Polycarbonate | Index 1.58–1.59 · Abbe 29–30 · SG 1.21 |
| 1.60 (MR-8) | Index 1.60 · Abbe 41 (Mitsui) |
| 1.67 (MR-7 / MR-10) | Index 1.67 · Abbe 31 (Mitsui) |
| 1.74 (MR-174) | Index 1.74 · Abbe 32 (Mitsui) |
| Crown glass | Index 1.52 · Abbe 59 · SG 2.54 |
| 1.70 glass | Index about 1.71 · Abbe 35 · SG 3.20 |
US impact-resistance rule (21 CFR 801.410)
- Eyeglass and sunglass lenses sold in the US must be impact resistant unless the prescriber documents a reason otherwise and notifies the patient in writing.
- The referee test drops a 5/8-inch steel ball, about 0.56 oz, from 50 inches onto the lens; the lens must not fracture.
- Finished prescription glass lenses are tested individually; plastic prescription lenses may be tested on a statistical basis by the manufacturer.
- Manufacturers keep test records for three years.
Notes on the numbers
Values are representative figures published by the material maker or an independent optical laboratory and vary slightly by supplier and measurement method. Confirm the exact material and its properties with your lab when ordering.
Common questions about lens materials
Is a higher index always better?
Not always. Higher index makes a thinner lens but usually lowers the Abbe value and raises reflections, so the best fit depends on the prescription, frame and patient.
Polycarbonate or Trivex for kids?
Both are the impact-resistant choices. Trivex is lighter and has a higher Abbe value; polycarbonate is widely available and slightly thinner at the same power.
Why do some labs avoid polycarbonate in drill mounts?
Polycarbonate is susceptible to stress cracks around drill holes. Trivex has high tensile strength, which is why many labs prefer it for rimless work.
Do all plastic lenses block UV?
In published laboratory data, polycarbonate, Trivex and the MR high-index materials transmit essentially no UV, while untreated CR-39 lets a small share through.
Is glass still dispensed?
Yes, mainly for patients who want its scratch resistance and optics, but it is heavier and must meet the federal impact rule after treatment.
References
- Laramy-K Optical. Lens materials: index, Abbe, specific gravity and UV data. Laramy-K
- Mitsui Chemicals. MR series lineup: refractive index, Abbe number, heat distortion temperature. Mitsui Chemicals
- Younger Optics. Trivex-based Trilogy lens material properties. PDF
- US Food and Drug Administration. 21 CFR 801.410, impact-resistant lenses in eyeglasses and sunglasses. eCFR
Recommend one material, and say why
A clear material recommendation makes every other lens decision easier to explain.
Brand and material names are trademarks of their respective owners. Property values are representative published figures; confirm specifications with the lens manufacturer or laboratory. Clinical and dispensing decisions rest with the eye care professional.