The short answer: every sunglasses lens carries a filter category from 0 to 4, defined by how much visible light it lets through (VLT — visible light transmission). Category 2–3 covers almost all real life. Category 4 is for glaciers and open water — and it is not legal for driving. The category says nothing about UV: that is the separate UV400 filter.
The Cat 0–4 reference chart
| Category | Light transmitted (VLT) | Tint | Made for | Driving? |
|---|---|---|---|---|
| Cat 0 | 80–100% | Clear to barely tinted | Fashion frames, wind protection, night sports | Yes (day and night) |
| Cat 1 | 43–80% | Light | Overcast days, dusk, fog — the yellow-lens zone | Yes (day); not at night |
| Cat 2 | 18–43% | Medium | Variable weather, city sun, most gradient lenses | Yes (day) |
| Cat 3 | 8–18% | Dark | Bright sun, beach, driving south — the default sunglasses category | Yes (day); never at night |
| Cat 4 | 3–8% | Very dark | Glaciers, high altitude, open ocean | No — restricted for road use |
How to read this when buying
- One everyday pair: Cat 3. It handles US summer sun and stays comfortable.
- Commuting in mixed weather: Cat 2, or a Cat 2–3 gradient.
- Mountains and water: Cat 3 polarized for activity; Cat 4 only for serious glacier or offshore exposure — and take them off before the drive home.
- The category is usually printed inside the temple arm or on the lens sticker as “Cat. 3” or “Filter category 3”.
From our test bench. We spot-check VLT with a light meter because mislabeled categories are a real supplier problem — a “Cat 3” that meters at 25% transmission is a Cat 2 sold as darker than it is. Batches that miss their label by more than a step are returned.
Part of The Complete Guide to Sunglasses Lenses · See also: the lens color guide · Last updated: July 19, 2026
