A sunglass lens's category indicates how much light it lets through, on a scale from 0 to 4. This figure, often overlooked when buying a pair, is actually the most important factor in choosing sport sunglasses that suit you: it determines which light conditions and which activity a lens is built for. Here's what each category means and how to match it to your needs.
What does a lens category actually mean?
The filter category measures the percentage of visible light the lens transmits. The higher the number, the less light gets through, and the stronger the protection against glare. This classification is set by the CE standard and applies to all sunglasses.
Don't confuse it with UV protection: category relates to visible light, while the UV400 standard guarantees that ultraviolet rays (UVA and UVB) are blocked. A clear, category 1 lens can perfectly well be UV400 rated. The two are entirely independent.
The 5 lens categories in detail
- Category 0 (80 to 100% transmission): clear or very lightly tinted lens, for low light or indoor use.
- Category 1 (43 to 80%): light tint, for cloudy days, woodland trails and early morning outings.
- Category 2 (18 to 43%): moderate tint, for medium brightness.
- Category 3 (8 to 18%): high protection, recommended for outdoor sport: cycling, running, trail running, hiking. This is the most versatile category.
- Category 4 (3 to 8%): maximum protection, reserved for environments with very high glare (glaciers, high mountains). Not recommended for driving.
Which category for which activity?
Category 3 covers the vast majority of outdoor outings in normal to intense conditions. It's the standard Cairn builds into most of its sport models. The other categories address more specific situations:
- Category 1: forest trail running, dense woodland mountain biking, dawn outings
- Category 2: overcast days, mid season conditions, partly shaded environments
- Category 3: road cycling, running, hiking in full sun, skiing
- Category 4: glacier trekking, high mountains above 2,000m
For activities where light changes constantly, EVOLIGHT photochromic lenses automatically cover categories 0 to 3 depending on ambient UV: a single pair for the whole outing.
Why category 4 isn't recommended behind the wheel
Category 4 lets through too little light for safe driving. With just 3 to 8% transmission, it significantly reduces your ability to perceive contrast and light signals: a real risk in a car or even cycling on an open road. It's designed exclusively for environments with extreme glare, where the light intensity justifies that level of filtering.
Choosing the right lens category: the essentials
- Category 3: the default choice for outdoor sport in full sun
- Category 1 to 2: low light, woodland, mid season
- Category 4: high mountains and glaciers only, never behind the wheel
- UV400: worth checking regardless of category
- EVOLIGHT photochromic: covers categories 0 to 3 in a single pair
At Cairn, category 3 UV400 lenses come as standard, with high performing technology at a fair price. To choose based on your activity, explore the sport sunglasses collection or the cycling sunglasses models.
