What Is CR-39?
CR-39 (Columbia Resin #39) is the standard plastic ophthalmic lens material, chemically known as allyl diglycol carbonate (ADC). Introduced in the 1940s as a lightweight alternative to glass, it remains one of the most widely used lens materials worldwide due to its excellent optical quality and versatility.
Key Properties
| Property | Value | Significance |
|---|---|---|
| Index of refraction | 1.498 | Standard reference; moderate thickness |
| Abbe value | 58 | Excellent clarity; minimal chromatic aberration |
| Specific gravity | 1.32 | About half the weight of glass |
| Impact resistance | Moderate (passes FDA drop ball test when >2mm center thickness) | Less impact-resistant than poly or Trivex |
| UV protection | Blocks UV-B, transmits some UV-A | UV coating recommended |
Advantages of CR-39
- Best optical clarity among common plastic materials (Abbe 58)
- Lightweight: About half the weight of crown glass
- Tintable: Accepts dye evenly and in a wide range of colors and densities
- Cost-effective: One of the least expensive lens materials
- Good scratch resistance: Harder surface than polycarbonate (though softer than glass)
- Chemical resistance: Resists most household chemicals and solvents
- Easy to fabricate: Edges and drills well
CR-39 has the HIGHEST Abbe value (58) of any commonly used plastic lens material. This means it produces the LEAST chromatic aberration (color fringing), making it the optically clearest option. For patients who notice color fringes in their current high-index or polycarbonate lenses, CR-39 is the best optical upgrade.
Limitations of CR-39
- Thickness: At n=1.498, lenses are thicker than high-index alternatives for strong prescriptions
- Impact resistance: Does not meet the impact requirements for safety eyewear or children's glasses (unless special tempered or thickened)
- UV protection: Does not fully block UV-A without an additional UV coating
- Not suitable for: Drill-mount (rimless) frames without reinforced drill holes (can crack at drill points)
When to Recommend CR-39
- Low to moderate prescriptions (up to about ±4.00 D) where thickness is not a concern
- Patients who prioritize optical clarity and are sensitive to chromatic aberration
- Tinted lenses: CR-39 tints more evenly and in more color options than other materials
- Budget-conscious patients who want a good-quality lens without premium pricing
- Adult patients in non-hazardous environments (not recommended for children or safety applications)
CR-39 is an excellent choice for prescription sunglasses because it tints beautifully and evenly. Polycarbonate and Trivex require special tinting processes or coatings and do not achieve the same gradient or fashion tint results. When a patient wants custom-tinted lenses, CR-39 is usually the best substrate.
Assuming CR-39 is outdated or inferior because it has a lower index than newer materials. CR-39 actually has BETTER optical quality (higher Abbe value) than polycarbonate, Trivex, or most high-index materials. For low prescriptions, it provides the clearest, most distortion-free vision of any plastic lens.
Key Takeaways
- CR-39 (n=1.498, Abbe 58) is the standard plastic lens material
- Highest Abbe value among common plastics means best optical clarity
- Lightweight, tintable, cost-effective, and easy to fabricate
- Not impact-resistant enough for children or safety applications
- Best for low-to-moderate prescriptions and custom tinted lenses