Do clear lenses block UV light?

It depends on the material. Clear polycarbonate and Trivex lenses block virtually 100% of UV radiation without any tint or special coating. Clear CR-39 lenses block most UVB but only about 60% of UVA unless a UV coating is applied. Clear crown glass provides minimal UV protection without a coating. Lens color and UV protection are independent properties.

What is visible light transmittance?

Visible light transmittance (VLT) is the percentage of visible light that passes through a lens. It determines how dark or light a tinted lens appears. A VLT of 100% means all visible light passes through (clear lens). A VLT of 15% means only 15% of visible light passes through (dark sunglass lens). VLT is separate from UV protection.

Can dark sunglasses be bad for your eyes?

Yes, if they do not block UV radiation. Dark lenses cause the pupil to dilate (open wider), allowing more light to enter the eye. If the lens does not block UV, the dilated pupil actually increases UV exposure compared to wearing no sunglasses at all. This is why UV protection must always be verified independently from tint darkness.

How do you test UV protection on glasses?

Use a UV meter (spectrophotometer) to measure how much UV radiation passes through the lens. Place the lens in the instrument's measurement area, and it displays the percentage of UV blocked. Quality ophthalmic lenses should block 99-100% of both UVA and UVB radiation. The test takes only seconds per lens.

UV Meter & Light Transmission: Verifying Lens UV Protection and Tint Levels

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.

Polycarbonate and Trivex provide complete UV protection without any additional coating. CR-39 and glass require a UV coating to provide adequate protection. This is an important selling point for polycarbonate and Trivex and a common ABO exam topic.

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.

Use the UV meter as a patient education tool. Demonstrating that a clear polycarbonate lens blocks 100% of UV while their old uncoated CR-39 lens blocks only 60% is a powerful way to show the value of lens material upgrades. Many patients assume clear lenses cannot protect against UV, so showing them the measurement changes their perception.

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

Assuming that a dark tint means good UV protection. Tint darkness (VLT) and UV protection are independent properties. A very dark lens without UV protection is actually more dangerous than no lens at all, because the dark tint causes the pupil to dilate, allowing more UV radiation to enter the eye. Always verify UV blocking independently from tint darkness.

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