Alongside the ultraviolet specification, most sunglasses carry a filter category from 0 to 4. It is a different measurement answering a different question, and mixing the two up is easy.
What it describes
The category describes how much visible light the lens transmits. It says nothing about ultraviolet, which is invisible and covered by the separate UV400 or UVA/UVB specification. A lens can be a dark category 3 and still filter ultraviolet poorly, and a very light category 1 lens can filter it completely.
The five categories
- Category 0 — transmits roughly 80 to 100% of light. Clear or very lightly tinted. Cosmetic or for indoor use; useful as a wind and debris shield for cycling at dawn.
- Category 1 — roughly 43 to 80%. A light tint for overcast conditions and low sun.
- Category 2 — roughly 18 to 43%. Medium tint, suited to average daylight.
- Category 3 — roughly 8 to 18%. The standard for bright sunshine, and where the large majority of sunglasses sit. For most South African daytime conditions this is the sensible default.
- Category 4 — roughly 3 to 8%. Very dark. Intended for extreme glare: high mountains, glaciers, open ocean.
The one that matters legally
Category 4 lenses are not suitable for driving, and this is not a matter of preference. They transmit so little light that they compromise your ability to see into shadow, and passing under a bridge or into an underpass produces a genuinely dangerous drop in vision. Manufacturers mark them accordingly.
Category 0 is also a poor driving choice in bright sun, for the opposite reason. Categories 1 to 3 are all fine behind the wheel.
Darker is not better
It is worth repeating, because the category number is the one figure that genuinely does describe darkness, and it is easy to read it as a quality scale. It is not. A category 4 lens is not a superior lens; it is a specialist one, appropriate on a glacier and inappropriate on the N1.
Choosing a category is about matching the lens to the light you are actually in. For everyday South African sun, category 3 is the answer. If you spend a lot of time in changeable or overcast light, a category 2 will be more comfortable more of the time.
Where to find it
The category is usually printed on the tag at the point of sale, and often etched on the inside of a temple arm alongside the model code. If a pair carries no category marking and no ultraviolet specification, you are being asked to take both on trust.
