Browsing sunglasses online, you’ve probably noticed terms like VLT% or light transmission factor and wondered what they actually mean - and more importantly, how they affect what you’ll see through your lenses.
Light transmission factor (LTF), is the percentage of visible light that passes through a lens. It’s what determines how light or dark a lens appears when you’re wearing it. The lower the percentage, the darker the lens. The higher the percentage, the lighter it will be.
It’s a useful number to understand when buying glasses online. Photos can show you the colour of a lens, but they don’t convey how dark it will feel in real life. LTF gives you a reliable way to judge that before you buy.
Does LTF affect UV protection?
Visible light transmission has nothing to do with how much UV protection your sunglasses provide. Visible light is what we see, while ultraviolet (UV) light is invisible and responsible for long-term damage to your eyes and skin. A darker lens might feel more comfortable in bright conditions because it reduces glare, but it doesn’t automatically make it safer.
In fact, a dark lens without proper UV protection can be worse, as your pupils dilate behind the tint, potentially allowing more harmful UV rays in. That’s why all quality sunglasses should be rated UV400, meaning they block 99–100% of ultraviolet light between 100 and 400 nanometres.
Whether your lenses are light or dark, proper UV protection should always be a given.
Why does visible light transmission matter?
While LTF doesn’t affect safety, it still plays a huge role in how comfortable your vision feels. Get it right, and your lenses will feel effortless in bright conditions. Get it wrong, and you’ll either be squinting in the sun or struggling to see clearly.
Low LTF lenses (around 80%) allow less light through, making them darker and better suited to bright sunlight.
High LTF lenses (above 40%) allow more light through, making them lighter and better suited to indoor or low-light environments.
Most sunglasses sit somewhere in the middle of this range, depending on how and where they’re intended to be worn.
The 5 sunglasses lens categories
To simplify things, sunglasses are grouped into five categories based on their LTF percentage. Each category is designed for a specific level of brightness and use.
See the table below.
|
Category |
0 |
1 |
2 |
3 |
4 |
|
LTF |
80-100% |
43-80% |
18-43% |
8-18% |
3-8% |
|
Colour tone |
Very light |
Light |
Medium |
Dark |
Very dark |
|
Sun use |
Overcast |
Low |
Moderate |
Strong |
Very strong |
|
Driving suitability |
Day & night |
Day only |
Day only |
Day only |
Never |
Category 0 lenses (80–100% LTF)
Very light or clear lenses with minimal tint. These offer little to no reduction in sun glare and are typically used for everyday prescription glasses rather than sunglasses.
Category 1 lenses (43–80% LTF)
Lightly tinted lenses that provide limited glare reduction. More aesthetic than functional, but can offer mild comfort in bright conditions.
Category 2 lenses (18–43% LTF)
A moderate tint that sits between spectacles and sunglasses. Suitable for mild sunlight, overcast days, or general outdoor use where full sun protection isn’t needed.
Category 3 lenses (8–18% LTF)
The most common category for sunglasses. These lenses absorb a significant amount of visible light, making them ideal for everyday use such as driving, walking, and bright outdoor conditions.
Category 4 lenses (3–8% LTF)
Very dark lenses designed for extreme brightness, such as mountains or glaciers. These are not suitable for driving due to reduced visibility.
What LTF is best for me?
Category 3 lenses, with a LTF of around 20%, are the best all-round choice. They provide enough darkness to stay comfortable in bright sunlight while still offering clear, usable vision for everyday activities like driving.
If you prefer a lighter look or spend more time indoors, category 0 or 1 lenses will feel more natural. For mixed weather or softer light, category 2 lenses can be a good middle ground. Category 4 lenses are designed for extreme environments and aren’t practical for everyday use.
When buying online, this is where LTF becomes genuinely useful. It removes the guesswork. Instead of relying on how a lens looks in a photo, you can understand how it will actually perform in real life.
How is visible light transmission measured?
Visible light transmission is measured using a device called a photometer, which compares the intensity of light before and after it passes through a lens.
The result is expressed as a percentage. For example, a lens with a LTF of 15% allows 15% of visible light through and absorbs the remaining 85%. Certain lens manufacturers or eyewear brands may label this as 85% ABS (absorption).
It’s a simple measurement, but it gives you a clear and accurate understanding of how dark a lens will feel to wear.
How is visible light transmission controlled?
Lenses typically start out completely clear and are then darkened through a tinting process.
They’re dipped into a heated dye bath, where the colour gradually absorbs into the lens material. The longer the lens is left in the dye, the darker it becomes. This is how different levels of LTF are achieved.
By controlling the timing and dye process, technicians can also create gradient lenses (darker at the top, lighter at the bottom) or more complex two-tone finishes.
Once tinted, the lenses are measured again to ensure they meet the intended LTF. This consistency is especially important when ordering lenses online, where accuracy matters.
FAQs
Which LTF is best for driving?
Category 3 lenses (around 20% LTF) are considered the sweet spot for driving. They reduce a significant amount of brightness and glare from the road, sky, and surrounding surfaces without making your vision feel too dark or restricted.
This balance is important, as you still need to clearly see dashboard displays, shaded areas, and changing light conditions. Anything lighter may not cut glare effectively, while anything darker (like category 4) can reduce visibility too much, especially when moving between bright and shaded environments.
Category 4 sun lenses are illegal for driving your car in the UK.
Are darker sunglasses always better?
Not necessarily. While darker lenses can feel more comfortable in intense sunlight, they’re not always the best choice for every situation. If a lens is too dark for the conditions you’re in, it can actually make it harder to see detail, especially in shaded areas or indoors. The key is choosing a lens that matches your environment. For most day-to-day use, a mid-range LTF (category 3) offers the best balance.
Can I wear category 4 lenses while driving?
No, category 4 lenses are not suitable for driving and are typically restricted for road use. They’re designed for extreme brightness, such as high-altitude environments or snow-covered landscapes where light intensity is significantly higher. Because they block so much visible light, they can reduce your ability to react quickly to changes in road conditions, shadows, or hazards. For safety and legal reasons, it’s always best to stick with category 2 or 3 lenses when driving.
Which LTF is best for sensitive eyes?
If you have sensitive eyes, lower LTF lenses (typically category 3) will generally feel more comfortable, as they reduce the overall brightness reaching your eyes. However, it’s not just about darkness. Polarised lenses can make a noticeable difference by reducing glare from reflective surfaces like water, roads, and glass, which is often what causes discomfort in the first place. A combination of category 3 tint and polarisation is usually the most comfortable setup for bright conditions.
Do polarised lenses affect LTF?
Polarised lenses are tinted just like regular, non-polarised lenses. However, their key difference is their ability to filter out horizontal glare caused by reflected light bouncing off surfaces like water, roads, or snow. Over long durations of sun exposure, polarised lenses reduce eye strain while improving clarity, contrast, and overall visual comfort without necessarily making the lens feel dramatically darker.
Is LTF the same as lens colour?
No, LTF and lens colour are two separate things. LTF tells you how much light passes through the lens, while colour affects how you perceive contrast and tones. For example, a grey lens and a brown lens can have the exact same LTF but feel completely different to look through. Grey tends to offer more neutral colour perception, while brown can enhance contrast and depth. This is why choosing a lens is about more than just how dark it looks—it’s also about how you want the world to appear through it.
For a guide to the different sun lens colours available, check out this blog post.
Hopefully you found this article helpful. Please check out our other sunglasses blogs for more useful advice. Thanks for stopping by.