Understanding Ultraviolet Radiation
Ultraviolet (UV) radiation is invisible electromagnetic energy with wavelengths shorter than visible light. It is divided into three bands:
| Type | Wavelength | Reaches Earth | Ocular Damage |
|---|---|---|---|
| UV-C | 100-280 nm | Blocked by ozone layer | Not a natural concern |
| UV-B | 280-315 nm | Partially | Corneal damage (photokeratitis), contributes to cataracts |
| UV-A | 315-380 nm | Yes (most reaches surface) | Penetrates to lens and retina; cataract, macular degeneration risk |
Why UV Protection Matters for Eyes
Cumulative UV exposure contributes to several ocular conditions:
- Cataracts: UV accelerates crystalline lens protein changes
- Macular degeneration: UV and short-wavelength visible light may damage the RPE
- Photokeratitis: Acute UV-B overexposure causes painful corneal inflammation (snow blindness, welder's flash)
- Pterygium and pinguecula: UV-related growths on the conjunctiva
- Eyelid skin cancer: UV damage to the thin eyelid skin
UV Protection by Lens Material
| Material | Inherent UV Protection | Additional Coating Needed? |
|---|---|---|
| Polycarbonate | 100% UV-A and UV-B | No |
| Trivex | 100% UV-A and UV-B | No |
| CR-39 | Most UV-B, partial UV-A | Yes (UV coating recommended) |
| High-index plastics | Varies by formulation | Some need UV coating, some inherently block UV |
| Crown glass | Most UV-B, transmits UV-A | Yes (UV coating needed) |
Polycarbonate and Trivex inherently block 100% of UV radiation without any additional coating. CR-39 and glass require a UV-absorbing coating or additive to achieve full UV-A protection. This is an important dispensing consideration when a patient's UV protection needs are a priority.
UV Coatings and Additives
For materials that do not inherently block UV, protection is added through:
- UV-absorbing coating: Applied to the lens surface (most common method)
- UV-absorbing monomer: Mixed into the lens material during manufacturing
- Photochromic dyes: All photochromic lenses provide UV protection (darkening is triggered by UV)
A lens providing full UV protection blocks radiation up to 380 nm (sometimes labeled as "UV400," meaning it blocks wavelengths up to 400 nm, including some short blue-violet light).
UV Protection and Lens Tint
An important distinction:
- Tint/darkness does NOT equal UV protection. A dark lens without UV coating can actually be WORSE than no lens at all, because the dark tint dilates the pupils, allowing more UV to enter the eye.
- Clear lenses CAN provide full UV protection with proper coating or inherent material properties.
- All sunglass lenses should block 99-100% of UV regardless of tint darkness.
Educate patients that UV protection is important year-round, not just in summer. UV exposure is significant on cloudy days (up to 80% penetration), at high altitudes, near water and snow (reflection doubles exposure), and during winter when the sun is lower. Clear prescription lenses with UV coating provide protection during everyday wear.
Assuming dark sunglasses automatically provide UV protection. Lens darkness (visible light absorption) is independent of UV blocking ability. A dark lens without UV protection dilates the pupil and actually increases UV exposure to the internal structures of the eye. Always verify UV protection in sunglass prescriptions.
Key Takeaways
- UV radiation contributes to cataracts, macular degeneration, photokeratitis, and pterygium
- Polycarbonate and Trivex inherently block 100% of UV; CR-39 and glass need UV coating
- Lens darkness does NOT indicate UV protection level
- Full UV protection means blocking up to 380-400 nm
- UV protection is important year-round, not just in summer