Lens Materials and Index: CR-39, Polycarbonate, Trivex, High-Index and Glass

Two pairs of glasses with exactly the same prescription can look and feel completely different. One may have thick, heavy edges that make your eyes look small; the other may be thin, light and barely noticeable. The difference usually comes down to the lens material, and in particular a number called the refractive index. Opticians often offer a menu of options, from standard plastic to 1.74 high-index, with prices rising at each step, and it is not always clear what you are paying for.

This guide explains the main spectacle lens materials in plain language: what refractive index and Abbe value mean, how much thinner high-index lenses really are, which materials are safest for children and sport, and when paying more makes sense. It is meant to help you have an informed conversation with your optician, not to replace their advice for your particular prescription and frame.

The basic language: index, Abbe value and density

Refractive index

Light slows down when it passes from air into a lens. The refractive index (n) is a measure of how much it slows and therefore how strongly the material bends light. Water has an index of about 1.33, standard plastic lenses about 1.50 and the thinnest plastic lenses 1.74. A material with a higher index can achieve the same focusing power with flatter curves, so the lens becomes thinner. For a myopic (minus) lens, this means thinner edges; for a hyperopic (plus) lens, a thinner centre.

Abbe value

Every lens material splits white light into its colours a little, just like a prism. This is called chromatic aberration, and it shows up as faint coloured fringes around high-contrast objects when you look through the outer parts of a lens. The Abbe value (also called the V-number) measures this: the higher the number, the less colour fringing. Most people do not notice fringing with Abbe values above about 40, especially with weak prescriptions, but some sensitive wearers with strong prescriptions do notice it with low-Abbe materials when looking off-centre.

Density and weight

Density, often quoted as specific gravity, tells you how heavy the material is per unit of volume. A thinner lens is not automatically lighter: a dense material may be thinner but weigh almost as much. In practice the weight you feel depends on lens size, prescription and material together, which is why small frames help as much as an expensive material.

Impact resistance

All spectacle lenses sold in many countries must pass a basic impact test. In the United States, for example, the FDA requires prescription lenses to withstand the impact of a steel ball dropped onto them from a set height. Safety eyewear standards, such as ANSI Z87.1 in the US or EN 166 in Europe, use much tougher high-velocity tests that only certain materials, mainly polycarbonate and Trivex, reliably pass. For more on protective eyewear, see our page on safety and sports eyewear.

Comparison of common lens materials

The figures below are typical approximate values; exact numbers vary between manufacturers and product lines.

CR-39: the standard plastic lens

CR-39 is a hard resin plastic that replaced glass as the everyday lens material from the 1970s onwards. With an index of about 1.50 and an Abbe value close to that of glass, it gives very clear, distortion-free optics at a modest price. It also tints evenly, which makes it popular for sunglasses.

Its weaknesses are thickness and toughness. With prescriptions beyond roughly minus 3 or plus 3 dioptres, CR-39 lenses start to look noticeably thick, particularly in larger frames. CR-39 is also more brittle than polycarbonate or Trivex and can crack if drilled, which makes it a poor choice for rimless frames. It needs a scratch-resistant coating to stay clear, and is usually a sensible, economical choice for adults with weak prescriptions in full-rim frames.

Polycarbonate: tough and light

Polycarbonate was originally developed for aerospace and is the material used in many safety visors and riot shields. As a lens material it is roughly ten times more impact resistant than standard plastic, lighter than CR-39, and thinner thanks to its index of about 1.59. It naturally blocks ultraviolet radiation, so lenses provide UV protection even without an additional treatment.

The trade-off is optical: polycarbonate has the lowest Abbe value of common materials, around 30. Most people with weak to moderate prescriptions never notice, but some with strong prescriptions see colour fringes or slightly softer vision when looking through the edges of the lens. Polycarbonate is also soft and scratches easily, so it always needs a hard coating. It is the standard recommendation for children's glasses, people who are monocular (have useful vision in only one eye), and sports and safety eyewear.

Trivex: the balanced choice

Trivex is a urethane-based material introduced in the early 2000s. It offers impact resistance comparable to polycarbonate, but with better optics: its Abbe value of around 43 to 45 is noticeably higher. It is also the lightest common lens material. Its index of about 1.53 means it is a little thicker than polycarbonate for the same prescription, so it is best for weak to moderate prescriptions. Trivex is strong enough to be drilled for rimless frames without cracking, blocks UV and is often chosen for children, active adults and anyone who wants safety without compromising clarity. It is usually somewhat more expensive than polycarbonate and not every laboratory stocks it in every design.

High-index plastics: 1.60, 1.67 and 1.74

High-index lenses use special resins, often thiourethane-based, to reach indices above 1.60. They are the main option when you want thin, attractive lenses for a strong prescription.

1.60

The 1.60 index is a very good all-rounder. It is roughly 20 to 25 percent thinner than standard plastic, has a reasonably good Abbe value, and is tough enough to drill for rimless and semi-rimless frames. It is often the sweet spot for prescriptions of roughly plus or minus 2 to 4 dioptres, or for people with moderate prescriptions who want rimless glasses.

1.67

The 1.67 index makes lenses about 30 to 35 percent thinner than CR-39. It is popular for prescriptions in the range of roughly minus 4 to minus 8 dioptres and for moderate plus prescriptions. The Abbe value is lower, but most wearers adapt without problems.

1.74

The 1.74 index is the thinnest widely available plastic, around 40 to 45 percent thinner than CR-39. It is mainly worth paying for with very strong prescriptions, usually beyond minus 8 dioptres or with high astigmatism. For weaker prescriptions, the extra thinning compared with 1.67 is often less than a millimetre and may not justify the considerable extra cost.

What high-index lenses cannot do

The thinning effect is not linear: going from 1.50 to 1.60 makes a large visible difference, while going from 1.67 to 1.74 makes a smaller one. High-index lenses also reflect more light from their surfaces than low-index materials, so an anti-reflective coating is strongly recommended; without it, lenses look shiny and reflections can bother you. Our guide to lens coatings and treatments explains why. Finally, no material can make a strong lens in a large frame look truly thin. Frame choice matters at least as much.

Glass lenses

Glass was the standard lens material for centuries. Crown glass has outstanding optical clarity, with an Abbe value close to 59, and its hard surface resists scratching better than any plastic. However, glass lenses are roughly twice as heavy as standard plastic, so they can leave marks on the nose and slide down the face. Most importantly, glass can shatter on impact, sending sharp fragments towards the eye. Even toughened glass is far less impact resistant than polycarbonate or Trivex.

For these reasons glass is now rarely used for everyday spectacles and should not be used for children or sports. It still has niche uses, for example in some very high prescriptions using special high-index glass, in certain laboratory settings where chemical resistance matters, and for wearers who strongly prefer its scratch resistance and accept the safety trade-off after discussing it with their optician.

How to choose the right material for you

Start with your prescription

As a general rule of thumb, which your optician may adjust:

• Up to about plus or minus 2.00 D: CR-39 or Trivex is usually sufficient. Higher index adds little visible benefit.
• About plus or minus 2.00 to 4.00 D: 1.60 offers a good balance of thinness and optics; Trivex or polycarbonate if safety is a priority.
• About plus or minus 4.00 to 8.00 D: 1.67 is usually the best value.
• Beyond about minus 8.00 D or strong plus: consider 1.74, plus a carefully chosen small frame.

Astigmatism counts too: a prescription of minus 3.00 with minus 2.00 cylinder has a strongest meridian of minus 5.00, so it behaves like a stronger prescription in terms of edge thickness. If you are not sure how to read these numbers, see how to read your glasses prescription.

Then consider who will wear them and how

• Children: polycarbonate or Trivex, always.
• Sport, workshops, outdoor work: polycarbonate or Trivex, ideally in certified safety or sports frames.
• Rimless or semi-rimless frames: Trivex, 1.60 or polycarbonate, because they tolerate drilling and stress well.
• Sensitive to optical quality, strong prescription: consider 1.60 or Trivex rather than polycarbonate, and ask about aspheric designs.
• Tight budget, weak prescription: CR-39 with a good hard coating is perfectly adequate.

Aspheric designs

Traditional lenses have spherical front curves. Aspheric and atoric lenses use flatter, gradually changing curves that reduce thickness and bulging further and give a more natural appearance, especially for plus lenses, where eyes can otherwise look magnified. Many high-index lenses are aspheric by default. The design and the material together determine how a lens looks.

Common myths about lens materials

• Myth: higher index is always better. For weak prescriptions the difference is negligible, while optics (Abbe value) can actually be slightly worse.
• Myth: thinner means lighter. Not necessarily; density and lens size matter just as much.
• Myth: plastic lenses do not block UV. Polycarbonate, Trivex and most high-index materials block UV inherently, and CR-39 is usually treated. Check that lenses are labelled as blocking UV up to 400 nm.
• Myth: glass is the premium choice. Glass has excellent clarity but is heavy and less safe; modern plastics are the standard for good reason.

When to see an eye care professional

Lens materials are a comfort and appearance choice, but a sudden change in how well you see through your glasses is a medical question. If your prescription is increasing quickly, or you have very high myopia, regular eye examinations are important, because strong myopia is linked to a higher risk of retinal detachment, glaucoma and myopic macular degeneration.