Published June 02, 2026 • 4 min read • Optics Guide
Understanding Siroko Lens Categories: Cat 1 to Cat 4 Specs & Pricing
Filter category numbers on sports eyewear are defined by ISO 12312-1 and describe how much visible light reaches the eye. Category determines usable conditions; it does not determine price. We mapped each category against the K3 lineup to show where the price steps actually come from.
- K3 MSRP (ES)
- €79.00
- K3s MSRP (ES)
- €89.00
- Weight difference
- −4g
Technical Specs Comparison
ISO 12312-1 filter categories and K3 catalogue positioning
| Parameter | Visible Light Transmission | Typical Use | K3 Lineup |
|---|---|---|---|
| Cat 0 | 80–100% | Night, fog, indoor | Clear lens options |
| Cat 1 | 43–80% | Overcast, forest trails | Photochromic (light state) |
| Cat 2 | 18–43% | Variable sun | Most standard K3 lenses |
| Cat 3 | 8–18% | Strong sun, open road | Photochromic (dark state), mirrored |
| Cat 4 | 3–8% | Glacier, high altitude | Not suitable for driving |
What the category number measures
Each category is a band of visible light transmission (VLT). A Cat 3 lens admits between 8% and 18% of visible light; a Cat 1 lens admits between 43% and 80%. UV protection is a separate requirement and is met by all categories on the K3 range, including clear lenses. Cat 4 lenses are legally unsuitable for driving in the EU and UK because they reduce transmission below the threshold required to read road signals.
Photochromic lenses span two categories
Photochromic lenses darken under UV exposure. The K3s photochromic lens moves from Cat 1 in low light to Cat 3 in direct sun, which is why it carries a dual rating on the spec sheet. Transition time from light to dark state in our tests at 20°C was approximately 35 seconds; the reverse transition was slower at around 2 minutes. Photochromic reaction slows in cold temperatures and is reduced behind car windscreens, which block most UV.
Where the price difference comes from
Across the K3 catalogue, price is driven by lens technology rather than category. Standard Cat 2 and Cat 3 lenses sit at the base MSRP. Photochromic and mirrored multi-layer lenses carry the premium. The K3s at €89.00 is €10.00 above the K3; the delta covers the photochromic lens, a hydrophobic anti-fog inner coating and a TAC 1.2mm lens that saves 4g in total frame weight. The anti-fog coating is the least visible upgrade but the most relevant for climbing at low speed in humid conditions, where fogging on uncoated lenses occurred within 4 minutes in our test protocol.