Lens Materials: Index, Abbe Value and Impact Rules

A higher refractive index makes a lens thinner for the same power but usually lowers the Abbe value, which means more color fringing away from the optical center. Specific gravity drives weight. Separately, FDA rules in 21 CFR 801.410 require impact-resistant eyeglass and sunglass lenses, checked by a drop-ball test or an approved equivalent.

Last checked Nov 2, 2026. Confirm fees and dates with the certifying body. optical.education is independent and is not affiliated with or endorsed by the American Board of Opticianry (ABO-NCLE) or the FDA. Disclaimer.

Lens materials fall under Ophthalmic Products (20%) and impact rules under “Laws, Regulations, and Standards” (10%) in the ABO test specifications (checked Nov 2, 2026).

Index and thickness

Refractive index (n) is how much a material bends light. A higher-index material reaches the same power with flatter curves, so the lens is thinner. Most of the benefit shows up on stronger prescriptions.

Abbe value and chromatic aberration

Abbe value (v) measures dispersion. A low value means colors separate more, seen as color fringes and blur when looking through the side of the lens. Wearers notice it more with higher powers, and good centration and a correct PD help.

Comparison table

Typical values from our lens materials comparison, which lists its published sources. Individual products vary.

Material	Index	Abbe value	Specific gravity
CR-39	1.498	58	1.32
Trivex	1.530	45	1.11
Polycarbonate	1.586	30	1.20
1.60 high index	1.600	41	1.30
1.67 high index	1.670	32	1.36
1.74 high index	1.740	33	1.47

Specific gravity and weight

Specific gravity is density compared with water. A lower number means a lighter lens for the same volume. Finished weight also depends on volume, so a thinner high-index lens can weigh about the same as a thicker low-index one.

FDA impact resistance: 21 CFR 801.410

Quoted from eCFR (opened Nov 2, 2026):

“…eyeglasses and sunglasses must be fitted with impact-resistant lenses, except in those cases where the physician or optometrist finds that such lenses will not fulfill the visual requirements of the particular patient, directs in writing the use of other lenses, and gives written notification thereof to the patient.” (§ 801.410(c)(1))

“…a 5⁄8-inch steel ball weighing approximately 0.56 ounce is dropped from a height of 50 inches upon the horizontal upper surface of the lens.” (§ 801.410(d)(2))

The lens must not fracture. Under paragraph (c)(3), glass prescription lenses are tested one by one, while plastic lenses can be tested by statistical sampling of batches. Paragraph (i) says the rule does not apply to contact lenses.

Dispensing scenarios

Child’s first glasses, active sports: start with impact-resistant materials such as polycarbonate or Trivex, then weigh Abbe value for the prescription.
High minus, wants thin edges: a higher index reduces thickness; talk through Abbe value, coatings and a smaller frame.
Complaints of color fringes: check centration and PD first, then consider a higher-Abbe material.

Practice questions

Which material in the table has the lowest Abbe value?
Show answer
Polycarbonate, about 30.
A patient with a strong prescription complains of color fringes when looking through the side of new polycarbonate lenses. Which property explains it?
Show answer
Low Abbe value (more chromatic aberration away from the optical center).
Rank CR-39, Trivex and polycarbonate from lightest to heaviest by specific gravity.
Show answer
Trivex (1.11), polycarbonate (1.20), CR-39 (1.32)
From what height is the steel ball dropped in the FDA test?
Show answer
50 inches (21 CFR 801.410(d)(2)).
Does 21 CFR 801.410 cover contact lenses?
Show answer
No. Paragraph (i) says it does not apply to contact lenses.

Frequently asked questions

What is Abbe value in eyeglass lenses?

A number that describes how much a material spreads white light into colors. The higher the Abbe value, the less chromatic aberration. CR-39 is about 58 and polycarbonate about 30 in our comparison table.

Does a higher index lens have a lower Abbe value?

Usually, but not always. Polycarbonate (1.586) has a lower Abbe value than 1.60 high index, and 1.74 is listed slightly higher than 1.67. Compare the actual numbers rather than assuming.

Which lens material is lightest?

By specific gravity, Trivex is the lowest of the common materials in our table at about 1.11, then polycarbonate at about 1.20. Finished weight also depends on thickness, so a thin high-index lens can end up close.

What does the FDA drop ball test require?

Under 21 CFR 801.410(d)(2), a 5/8-inch steel ball weighing about 0.56 ounce is dropped from 50 inches onto the horizontal upper surface of the lens, and the lens must not fracture.

Source: eCFR — 21 CFR 801.410 (impact-resistant lenses in eyeglasses and sunglasses) (opens in a new tab)

Are all prescription lenses required to be impact resistant?

Eyeglass and sunglass lenses must be, with one exception: a physician or optometrist can find that such lenses will not meet the patient’s visual needs, direct other lenses in writing, and notify the patient in writing. The rule does not apply to contact lenses.