Nearly every pair of sunglasses leaves the shop with a small sticker on one lens carrying a single digit between 0 and 4. Most people peel it off in the car park without reading it. That digit is the one claim on the packaging tied to a written laboratory test, and at the dark end it marks the difference between a pair you can wear anywhere and a pair you are not permitted to drive in.
Five bands cut by how much light gets through
The digit comes from ISO 12312-1, the international standard for sunglasses for general use, now in its 2022 edition. It sorts lenses by luminous transmittance, meaning the share of visible light that reaches the eye behind them. Colour, coating, price, and brand play no part in the sorting.
Category 0 passes more than 80 percent of visible light. Category 1 covers 43 to 80 percent. Category 2 covers 18 to 43 percent, category 3 covers 8 to 18 percent, and category 4 sits between 3 and 8 percent. The Swiss Federal Office of Public Health, which commissioned laboratory testing of shop-bought pairs against the standard, calls categories 0 and 1 unsuitable as sunglasses and puts everyday wear in categories 2 and 3. Category 4 it reserves for sea, desert, high mountain, and snowfields.
So a pale grey lens that flatters a face in a mirror may be a category 1 filter doing almost no glare work. The tint you like and the protection you assume are two separate purchases that happen to arrive in the same frame.
Tint depth and ultraviolet blocking are separate specifications
The most common assumption about sunglasses is the one the standard contradicts fastest. Darkness has nothing to do with ultraviolet protection. That property depends on what the lens material absorbs, not on how much visible light it holds back, which is why a nearly clear lens can block ultraviolet completely and a very dark one can fail.
The figure behind the phrases printed on hangtags is specific. A lens counts as offering full ultraviolet protection when transmittance at wavelengths up to 380 nanometres stays below 0.05 percent. That is the number underneath a claim like 100 percent UV or UV 400, and it is unrelated to the category digit sitting next to it.
Protection also has a shelf life that nobody mentions at the till. The ageing test built into the standard is designed around two years of use at a maximum of two hours a day, and scratches cut into the result. A pair worn every day through three summers has outlived the conditions it was certified under.
The darkest band carries a crossed-out car
Labelling requirements sit in the same standard, and they are more prescriptive than the average product page suggests. A pair has to carry the manufacturer or supplier identity, the filter category, a description of that category, and a statement about suitability for road use, along with a reference to the standard itself.
Pairs unsuitable for driving must be marked with a symbol of a crossed-out car. Category 4 lenses are the ones that earn it, because at 3 to 8 percent transmittance they are too dark to allow reliable recognition of traffic signals. That mark travels on the label, which is exactly the part of the purchase most likely to be binned before the first wear.
| Category | Visible light through | What the standard puts it to | Road use |
|---|---|---|---|
| 0 | Above 80 percent | Fashion tint, not a sunglass | Permitted |
| 1 | 43 to 80 percent | Light tint, limited glare work | Permitted |
| 2 | 18 to 43 percent | Everyday wear | Permitted |
| 3 | 8 to 18 percent | Bright sun, beach, mountains | Permitted |
| 4 | 3 to 8 percent | Sea, desert, snowfields | Prohibited |
There is a second, quieter limit in the same territory. Any lens passing less than 75 percent of visible light is not meant for driving in twilight or at night, which takes in categories 2 and 3 and therefore most of the sunglasses people actually own. The prohibition on category 4 is absolute; everything below the top band comes with a time of day attached.
A red light has to survive the lens
Traffic signal recognition gets its own requirement, and it is the part of the standard that explains why some tints are quietly bad ideas. Across the 475 to 650 nanometre range, spectral transmittance must not fall below 0.2 times the lens luminous transmittance. That band holds the colours a driver has to read at speed.
A strongly coloured lens can pass on overall darkness and fail here, because it flattens one part of the spectrum while leaving the rest alone. The Swiss factsheet reaches the same conclusion from the other direction, noting that brown and grey tints give the cleanest visual clarity while blue and red tints reduce contrast sensitivity and visual acuity. The fashionable rose lens is the one to check hardest.
Fifty-seven pairs and one useful null result
The Swiss testing itself is worth knowing about, because of what it failed to find. Ten pairs of baby sunglasses, seventeen children's pairs, and thirty adult pairs went to an accredited laboratory for optical testing against the European version of the standard. All fifty-seven carried the CE mark, and all fifty-seven met the requirement for full ultraviolet protection.
More useful for shopping: no quality difference emerged between expensive and low-cost pairs. Money buys frames, fit, lens clarity at the edges, and how long a hinge lasts. On the two properties the standard measures, a cheap compliant pair and a costly compliant pair are the same purchase.
Two hundred and thirty-two pairs, fifty-seven wrong for the road
Compliance is not universal, though, and one study puts a figure on the gap. Artur Loureiro and Liliane Ventura measured 232 sunglasses from the Brazilian market and published the results in BioMedical Engineering OnLine in April 2021. Fifty-seven of them, or 24.6 percent, were unsuitable for driving.
The breakdown matters more than the headline. Forty-four were category 4 filters, which is a labelling and buying problem rather than a manufacturing fault. Six failed the red signal requirement. Seven passed so little light that they fell below category 4 entirely and could not be classified. One pair in four on those shelves was wrong for the road, and nothing about the way they looked said so.
Reading the sticker before it comes off
Handle the label as part of the product rather than as packaging, and the whole question takes about a minute in the shop.
- Find the category digit and check it against what you will use the pair for, then photograph the label before you remove it so the road use marking survives the purchase.
- Treat the crossed-out car symbol as a hard stop for anything that lives in a glovebox, and keep category 4 pairs for skiing, sailing, and altitude.
- Read the ultraviolet claim separately from the tint, and take a missing statement as a missing feature rather than an oversight.
- Hold a candidate pair up and look at a red brake light or a red sign through it, since a tint that dulls red is one you will resent at a junction.
Then decide how many pairs the year actually needs. One category 2 or 3 pair in a neutral tint covers commuting, walking, and most holidays. A category 4 pair earns its place only if you spend real time on snow or water, and it should live somewhere other than the car. Buying that way costs less than buying one dark pair for everything and finding out at dusk which band it belongs to.