Lens darkness is measured two ways: VLT, the percentage of visible light that passes through, and a category number from 0 to 4 that bands those percentages. Both describe darkness only — neither tells you anything about UV protection, which is a separate specification entirely.
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What VLT actually measures
Visible light transmission is the share of visible light reaching your eye. A 15% VLT lens passes 15% and blocks 85%. Lower numbers mean darker lenses. This is a comfort and glare specification and it is unrelated to UV blocking, which good lenses provide at every tint level.
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The five categories
Category 0 (80-100% VLT) is essentially clear. Category 1 (43-80%) suits overcast and low light. Category 2 (18-43%) covers average conditions. Category 3 (8-18%) is the standard bright-sun range that most sunglasses fall into. Category 4 (3-8%) is for extreme glare — glaciers, high altitude snow, and open water.
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Why category 4 is restricted for driving
Category 4 lenses are dark enough that traffic signal colours and shadowed hazards become hard to distinguish, and they are widely marked as unsuitable for road use. If a lens carries that warning, take it seriously — it is not a liability formality.
Glacier glasses explainedWhere category 4 lenses genuinely belongRead Guide → - 4
Matching category to your hiking
Forest trails and overcast days suit category 2. General bright-sun hiking suits category 3, which is why nearly every sport sunglass sits there. Snow, glacier, and sustained high-altitude travel are where category 4 earns its place — and it is genuinely too dark for anything else.
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Where photochromic lenses fit
Photochromic lenses shift across categories as light changes, typically spanning category 1 to 3. That covers a forest-to-ridge transition in one lens. They generally do not reach category 4, and they respond slowly in cold and poorly behind car windscreens.
Photochromic vs fixed tintWhether an adaptive lens suits your hikingRead Guide →
Categories at a Glance
| Category | VLT | Conditions | Example use |
|---|---|---|---|
| 0 | 80-100% | Indoor, night | Clear safety lenses |
| 1 | 43-80% | Overcast, dusk | Low-light riding lenses |
| 2 | 18-43% | Average daylight | Forest trails, variable light |
| 3 | 8-18% | Bright sun | Most hiking sunglasses |
| 4 | 3-8% | Extreme glare | Glacier, high altitude snow |