What Is a UV Meter?
A UV meter (also called a UV spectrophotometer or UV tester) is an instrument that measures how much ultraviolet radiation a lens blocks. It works by shining UV light through the lens and measuring how much passes through. The result is expressed as a percentage of UV blocked or transmitted.
UV protection is a critical quality of spectacle lenses because UV radiation contributes to cataracts, macular degeneration, pterygium, and other eye conditions. Verifying UV protection ensures patients receive the protection promised.
Types of UV Radiation
| Type | Wavelength | Eye Risk |
|---|---|---|
| UVC | 100-280 nm | Absorbed by atmosphere; rarely reaches the eye |
| UVB | 280-315 nm | Corneal and lens damage; most energetic reaching the eye |
| UVA | 315-380 nm | Penetrates deeper; contributes to cataract formation |
Quality ophthalmic lenses should block 99-100% of both UVA and UVB radiation.
Inherent UV Protection by Material
Some lens materials provide UV protection inherently, without additional coatings:
- Polycarbonate: Blocks virtually 100% of UV up to 380 nm. No UV coating needed.
- Trivex: Blocks virtually 100% of UV up to 380 nm. No UV coating needed.
- CR-39: Blocks most UVB but only about 60% of UVA. Needs a UV coating for full protection.
- Crown glass: Provides minimal UV protection. Requires UV coating.
- High-index plastics: Most high-index materials provide good UV protection, but coverage varies by specific material.
Visible Light Transmittance
UV meters often also measure visible light transmittance (VLT), expressed as a percentage of visible light that passes through the lens. VLT determines how dark a tinted lens appears:
| VLT Range | Appearance | Typical Use |
|---|---|---|
| 80-100% | Clear or very light tint | Indoor, cosmetic tint |
| 43-80% | Light to medium tint | Fashion tint, variable conditions |
| 18-43% | Medium to dark tint | General-purpose sunglass |
| 8-18% | Dark tint | Bright sunlight, beach, snow |
| Below 8% | Very dark | Specialty use only; not for driving |
Using the UV Meter
To test a lens:
- Turn on the instrument and verify calibration (zero reading with no lens)
- Place the lens in the measurement area
- The instrument displays UV blocking percentage and/or VLT percentage
- Record the values and compare to the expected specifications
Some instruments provide separate readings for UVA and UVB blocking, while simpler models give a combined UV blocking percentage.
Tint Verification
The UV meter's VLT measurement is useful for verifying that tinted lenses meet specifications:
- Sunglass lenses should typically transmit 15-25% of visible light for general outdoor use
- Driving lenses should transmit at least 8% (very dark lenses below this threshold are unsafe for driving)
- Fashion tints may transmit 70-90% of visible light
- Gradient tints will give different readings at different points on the lens
Clinical Relevance
UV meters provide objective verification of lens UV protection, supporting patient education and quality assurance. They also verify tint specifications, ensuring that sunglass lenses and specialty tints meet the ordered density. Having a UV meter in the dispensary adds credibility and demonstrates a commitment to eye health.
Key Takeaways
- UV meters measure how much ultraviolet radiation a lens blocks
- Quality lenses should block 99-100% of UVA and UVB
- Polycarbonate and Trivex inherently block 100% of UV; CR-39 and glass need UV coatings
- Visible light transmittance (VLT) measures tint darkness, independent of UV protection
- Dark tint without UV protection is more dangerous than no lens
- UV meters are valuable patient education tools in the dispensary