What UV400 Actually Means — And Why Dark Lenses Aren't Protection
Here's the part that catches people out: a very dark pair of sunglasses with no ultraviolet filtering is worse for your eyes than wearing nothing at all.
It sounds wrong, and the reason it's true is simple. Your pupils respond to visible light. In bright sun without sunglasses they contract, which limits what gets in. Put a dark tint in front of them and they open up, because as far as your eye is concerned the sun just went behind a cloud. If that dark lens isn't also filtering ultraviolet, you've widened the door and let more of the harmful part through.
Tint and protection are two separate things. One you can see. The other you can't.
What the number means
Ultraviolet light is measured in nanometres. UVB sits roughly between 280 and 315nm, UVA between 315 and 400nm. Both reach the ground, and both reach your eyes.
UV400 means the lens blocks wavelengths up to 400 nanometres — which covers the whole of that range. It's not a brand, a grade, or a marketing tier. It's a threshold, and it's the one worth looking for. Every pair we sell carries UV400 protection against harmful UVA & UVB rays.
What it doesn't tell you
UV400 says nothing about how dark a lens is, or how it handles glare. Three things get confused constantly:
Tint is how much visible light the lens cuts. It affects comfort and how much you squint. It has no bearing on ultraviolet filtering — a very pale lens can block all UV, and a very dark one can block none.
Polarisation cuts glare bouncing off flat surfaces: water, wet roads, car bonnets, snow. It's excellent for driving and being near water, and it's also unrelated to UV. Polarised lenses usually carry UV protection too, but one doesn't imply the other.
Lens category runs from 0 to 4 and describes how much light the lens transmits. Category 2 and 3 cover most everyday wear. Category 4 is very dark, intended for high altitude and open water, and isn't legal for driving in many countries.
Why any of this matters
Ultraviolet exposure to the eye accumulates over a lifetime, and it doesn't announce itself. The effects that build up quietly — clouding of the lens, growths on the surface of the eye — are the ones worth avoiding early rather than treating late. A short, intense dose has a more obvious version too: sunburn of the cornea, which is what people get from a day on snow or water without protection and which is as unpleasant as it sounds.
None of this is a reason for alarm. It's a reason to make sure the pair you already reach for is doing the job you assume it's doing.
Checking a pair you own
Look on the inside of the temple. Most frames carry their protection marking printed there — UV400, or a CE mark with a category number, or wording naming the standard. If you find it, you're fine.
If there's nothing printed anywhere, that isn't proof the lens fails, but it does mean nobody has claimed otherwise. On an inexpensive pair with a very dark tint and no marking at all, the safe assumption is the unhelpful one.
An optician can test a lens properly in about a minute, usually for nothing, if you want to know rather than guess.
The short version
Dark is comfort. UV400 is protection. Polarised is glare. They're three separate features and a lens can have any combination of them — so check for the one that matters rather than assuming the tint has it covered.
Any questions about a specific pair, ours or anyone else's, email sales@callenford.com and we'll tell you what we know.