Sunglasses: what the label really means

There is a belief repeated on every beach on the Costa Cálida: the darker the lens, the more it protects. It is false, and it is the reason a pair of market-stall sunglasses can leave the eye worse off than if it were bare.

The confusion comes from the label. On the same arm there live two numbers that measure different things, and that nobody explains at the point of sale.

Two labels, two different things

The filter category (0 to 4) measures the visible light the lens lets through. It is the darkness. It is defined in the international standard ISO 12312-1, which governs general-use sunglasses throughout Europe.

UV400, or “100% UV”, measures the ultraviolet the lens blocks. Ultraviolet is invisible: you do not see it, you do not feel it and it does not bother you.

The Swiss Federal Office of Public Health puts it without circumlocution in its factsheet on sunglasses: the degree of ultraviolet protection depends on the properties of the material the lens is made of, and not on the colour or the tint.

In other words: an almost clear lens can block 100% of ultraviolet, and a jet-black lens from a street stall may block none at all.

Why a dark lens without a UV filter is worse than nothing

Your pupil reacts to visible light. With a dark lens in front of it, the eye thinks there is little light and opens the pupil. If that lens does not filter ultraviolet, you have just opened the door and let in more radiation than would get in under full sun with your eyes screwed up.

That is the mechanism. There is no pain, no warning and no symptoms that day. Ultraviolet accumulates over decades and is associated with cataract, pterygium, solar keratitis and macular degeneration, as the American Optometric Association sets out.

The table of categories

The values are those of ISO 12312-1:2022, as reproduced in the Swiss factsheet.

That last line matters more than it seems. Category 4 lenses are so dark that they do not allow the colours of a traffic light to be distinguished, and that is why they are prohibited at the wheel. They must carry a printed symbol of a crossed-out car. If you bought some very dark glasses for driving along the coast at midday, take a look at them.

For normal use in Puerto de Mazarrón, category 3 is the one that applies. Category 2 falls short on an August day on Bahía beach. Category 4 is more than you need unless you are going out sailing.

What to look at before paying

• The CE mark. Without it, there is no declaration of conformity with any standard. In the testing commissioned by the Swiss health authority, the 57 CE-marked pairs analysed all met 100% ultraviolet protection, and no differences in optical quality were found between the expensive ones and the cheap ones. The problem is not the price. It is the absence of a mark.
• The category printed on the arm or on the label.
• A reference to the standard ISO 12312-1:2022 in the labelling.
• That they cover. The same factsheet recommends that the lens reach up to the eyebrows and cover the width of the face. Ultraviolet coming in from the side, bounced off sand or water, is stopped by no lens.
• Brown or grey. These are the tints that maintain correct colour perception. Blues and reds worsen contrast sensitivity.

The figure that explains the problem

A pilot test carried out on beaches by the Visión y Vida association together with the Spanish glasses, lenses and contact lenses forum concluded that 40% of people cannot tell a quality pair of sunglasses from a street-stall imitation. The same release from the Spanish national college of opticians-optometrists notes that only 27% of the population protects their eyes adequately, because most put design before protection.

It is not ignorance. It is that the difference, quite simply, cannot be seen. A good pair and a bad pair look exactly alike in the hand and in the mirror.

Things the label is not going to tell you

Polarised is not ultraviolet protection

A polarised lens has a filter that absorbs light reflected off horizontal surfaces: the sea, wet tarmac, the car bonnet. It removes the glare and rests the eyes when driving. But polarising and filtering ultraviolet are two independent functions. A lens can be polarised and not give full ultraviolet protection. Check the two things separately.

Sunglasses expire

A lens’s resistance to ultraviolet degrades over time. The Swiss factsheet indicates that protection is guaranteed for two years, according to an ageing test that assumes a maximum of two hours of use a day. On the Murcian coast, two hours a day is a conservative estimate.

And scratches reduce that protection. A scratched pair that has spent five summers in the car glovebox at forty degrees is no longer the pair you bought.

Light eyes need more

An iris with less pigment lets more radiation through. Many of our British and German patients arrive with blue or green eyes from a country with half the hours of sunshine a year, and here they spend their afternoons on the terrace. It is a combination that deserves a better lens, not just a darker one.

Children too

Children’s eyes have less pigment and a more transparent crystalline lens, so radiation penetrates further. Up to the age of thirty there is also an additional transmittance window around 320 nanometres that lets some radiation reach the retina. A cap is not enough.

If you wear prescription glasses

It is the question we are asked most and the one that is answered worst. Yes, you can have your prescription in a sun lens, including a progressive one. And no, contact lenses with an ultraviolet filter do not replace sunglasses: they protect the cornea they cover, not the rest of the eye nor the eyelids.

If you are going to order prescription sunglasses, ask explicitly whether 100% ultraviolet protection is guaranteed in the lens they are going to fit. It is literally the recommendation the Swiss health authority gives to wearers of corrective lenses.

How to settle it with the ones you already have

The first part you do at home, this afternoon. Look for the CE mark, the category and the reference to the standard. Hold the lens up to the light against a white wall, looking for fine scratches. And check the approximate date of purchase: if they are more than two summers old and have lived in the car, they are no longer the pair you bought.

The second part we do here. If you have several pairs of sunglasses and do not know which to keep, bring them to your appointment and we will go through them with you one by one, along with your prescription. In many cases the conclusion is that the ones you had were fine and the one that was surplus was your favourite.