Written and maintained by CASRAI Editorial Board
Last updated
General product and regulatory information only. It is not medical advice and does not replace an eye care professional’s guidance.
Short answer
CR-39 is PPG’s trademarked name for an allyl diglycol carbonate (ADC) monomer that is cast into clear plastic eyeglass lenses. PPG describes it as a 1.5 index lens material. In the United States, FDA’s impact-resistance rule, 21 CFR 801.410, applies to plastic lenses whatever the material, so it never singles CR-39 out.
Definition and scope
“CR-39” is used in two ways. Strictly, it is the name of a monomer, the liquid starting material. PPG states that “there is only one CR-39 monomer, and it only comes from PPG.” In everyday optical talk, “CR-39 lenses” means finished plastic lenses made from that monomer, which PPG calls a thermoset material (one that is cured into a fixed shape instead of being re-melted).
This page covers spectacle lenses and the regulation that applies to them. Contact lenses are outside it: 21 CFR 801.410(i) says the impact-resistance policy does not apply to contact lenses (see contact lens materials).
How CR-39 lenses are used in practice
PPG says lenses made from its CR-39 monomer have been relied on by wearers “for more than seven decades” and that they are scratch-resistant, lightweight, and able to stand up to heat and household chemicals and cleaning products. These are the manufacturer’s own statements, not independent test results. The same page says the material “combines the optics of glass with the durability of a thermoset material.”
The lens itself is only one part of a finished pair. For where a lens sits in the frame and what the other components are, see parts of eyeglasses; for how lens material relates to vision type and coating, see types of eyeglass lenses. A cast plastic lens can be made as single vision, covered in single vision lenses.
Related material: polycarbonate
The only direct material comparison on PPG’s CR-39 page is optical: it states that CR-39 lenses “have less chromatic aberration than polycarbonate lenses.” Chromatic aberration is the colour fringing a lens can produce. PPG gives no figure for it on that page, so this page does not either.
Where it matters for labs, clinics and supply decisions
The regulation is the useful anchor for anyone documenting or stocking eyewear. Under 21 CFR 801.410(c)(2), a physician or optometrist may order “glass lenses, plastic lenses, or laminated glass lenses made impact resistant by any method”, and all of them must withstand the impact test in paragraph (d)(2). The rule is material-neutral, so neither “CR-39” nor any other brand name is a substitute for the test requirement.
What the regulation says, and what it does not
| Question | What 21 CFR 801.410 says (checked 2026-10-31) |
|---|---|
| Which lenses must be impact resistant? | Eyeglasses and sunglasses must be fitted with impact-resistant lenses, unless the practitioner finds such lenses will not fulfil the patient’s visual requirements, directs other lenses in writing and gives the patient written notice (c)(1). |
| Which materials are allowed? | Glass, plastic or laminated glass lenses, made impact resistant by any method (c)(2). |
| What is the test? | A 5/8-inch steel ball weighing approximately 0.56 ounce is dropped from 50 inches onto the upper surface of the lens (d)(2). |
| What counts as a pass? | The lens must not fracture. It fractures if it cracks through its entire thickness across a complete diameter into two or more separate pieces, or if lens material visible to the naked eye becomes detached from the ocular surface (d)(2). |
| Is there a minimum thickness? | None is set in the text. Manufacturer batch testing must include finished forms of minimal lens thickness (c)(3). |
| Do contact lenses fall under it? | No (i). |
Checklist: reading a claim about a CR-39 lens
- Is the statement about the monomer or about a finished lens? PPG’s page describes the monomer and lenses made from it.
- Is it a manufacturer statement (scratch-resistant, lightweight, 1.5 index) or a regulatory requirement (impact test)? Only the second is set out in 21 CFR 801.410.
- Does it name a comparison? PPG’s only stated comparison is less chromatic aberration than polycarbonate.
- Does it imply an exemption? The text of the rule contains none for any plastic material.
For safety-rated eyewear, which is a different standard, see prescription safety glasses and ANSI.
Common misunderstandings
“CR-39” is a generic name for any plastic lens
It is not. PPG presents CR-39 as its own proprietary monomer. Other plastic lens materials exist, and a lens in one of them is not a CR-39 lens.
“Impact resistant” is a property of the material
In the regulation it is a tested outcome. Section 801.410 defines it by the drop test above and applies the same test to glass, plastic and laminated lenses.
The rule sets a thickness for plastic lenses
It does not. It requires that testing cover the thinnest finished forms.
Frequently asked questions
What does CR-39 mean on a lens?
It refers to a lens made from PPG’s CR-39 allyl diglycol carbonate monomer, a plastic that PPG describes as a 1.5 index lens material.
Is CR-39 plastic?
Yes. PPG describes lenses made from it as plastic lenses, made from a thermoset material.
Does FDA’s impact-resistance rule apply to CR-39 lenses?
Yes, as to any plastic prescription or nonprescription eyeglass or sunglass lens: 21 CFR 801.410 requires impact-resistant lenses and sets the drop-ball test, without naming a material.
How does CR-39 compare with polycarbonate?
PPG states only that its CR-39 lenses have less chromatic aberration than polycarbonate lenses.
Sources
Checked 2026-10-31.
- U.S. Food and Drug Administration, 21 CFR 801.410, Use of impact-resistant lenses in eyeglasses and sunglasses, as reproduced by Cornell Legal Information Institute from the Electronic Code of Federal Regulations: https://www.law.cornell.edu/cfr/text/21/801.410
- PPG Industries, PPG CR-39 Monomer for High-Quality Plastic Lenses: https://www.ppg.com/en-US/optical/cr-39-monomer. Manufacturer statements are attributed to PPG.
