Sunglasses and UV protection

The category measures light. UV400 measures something else

A pair of sunglasses carries two numbers that never speak to each other. The category, 0 to 4, states how much visible light the lens lets through: that is what you perceive as lighter or darker. The UV400 mark states how far up the spectrum it filters ultraviolet, which is invisible and is the part that does harm.

The two come apart. There are near-colourless lenses that block ultraviolet entirely, because the filter sits inside the material and tints nothing. And there is the very dark lens with no certified filter, which dims the view and lets the ultraviolet straight through.

That second combination is worse than no sunglasses at all. Behind a dark lens the pupil opens to compensate for the gloom, and through that open pupil comes more ultraviolet than would arrive in full sun with your eyes screwed up.

The five categories, and which one you need here

The categories come from the European standard EN ISO 12312-1 and are printed on the temple or the label. On this coast, with glare off the sea and off white walls, category 3 is the usual answer.

• Category 0: lets through 80 % to 100 % of the light. A fashion or indoor lens
• Category 1: 43 % to 80 %. Overcast days and winter light
• Category 2: 18 % to 43 %. Moderate sun, town, spring
• Category 3: 8 % to 18 %. Beach, boat, most of the year here
• Category 4: 3 % to 8 %. High mountain and snow. Not permitted for driving

There is no miracle lens for driving at night

No sunglasses work after dark, and neither does the yellow lens sold for exactly that. Every one of them removes light, and at night the problem is that there is too little of it already. Taking more away only worsens what you can make out down the road.

Polarised lenses have their place and this is not it. They cut the glare bouncing off a lit horizontal surface: the sea, wet tarmac in sunshine, the bonnet of the car ahead. After dark there is none of that to cut.

What genuinely improves night driving is duller: an anti-reflection coating, a clean unscratched lens, and a prescription that is up to date. If you do a lot of night driving, say so at the examination and it will shape which lens we suggest.

Prescription sun, without giving up on seeing

If you wear spectacles all day, the comfortable answer is a pair of sunglasses made to your prescription, varifocal included. The sensible alternative is a sun clip over your everyday frame: light, and it saves carrying two pairs.

One technical limit is worth knowing before you fall for a frame: the more wrapped and curved it is, the less prescription it will take without distorting towards the edges. We will tell you whether that particular frame will carry yours before anything is ordered.