Almost every pair of sunglasses sold today carries a UV claim somewhere on the tag, and almost nobody is told what the number refers to. It is worth two minutes, because it is the single specification on a pair of sunglasses that affects your eyes rather than your appearance.
It is a wavelength, not a score
Light is measured in nanometres. Visible light, the part you can see, runs from roughly 400nm at the violet end to about 700nm at the red end. Ultraviolet sits just below that visible range, and it is divided into UVA, which runs from about 315nm to 400nm, and UVB, from about 280nm to 315nm.
UV400 means the lens blocks wavelengths up to 400nm. Because 400nm is the top of the ultraviolet band, a lens that filters everything below it is filtering the whole of UVA and UVB. That is why the figure is 400 and not something more impressive-sounding: there is nothing above it left to block.
Tint has nothing to do with it
This is the part that catches people out. How dark a lens looks and how much ultraviolet it filters are two entirely separate properties. A lens is tinted by adding colour, and it is given ultraviolet protection by a treatment or by the material itself. One does not imply the other.
The consequence is genuinely counterintuitive. A very dark lens with no ultraviolet filter is worse for your eyes than wearing nothing at all. In bright light your pupil contracts to limit what gets in. Put a dark lens in front of it and the pupil opens up again, because as far as your eye is concerned it has just walked into the shade. If that lens is not filtering ultraviolet, you have simply opened the aperture and let more of it through.
That is the reason a cheap dark pair from a street stall is a bad trade rather than a neutral one.
Why South Africa is a stronger case than most
Ultraviolet exposure rises with altitude and with proximity to the equator, and much of South Africa scores on both counts. Highveld cities sit well above sea level, and the country gets a great deal of clear-sky sunshine. Reflection adds to it: water, pale sand and light-coloured concrete all bounce ultraviolet back upwards, which is why glare off a road or a dam feels harsher than open sky.
What to actually check
- Look for the wavelength figure. UV400, or a statement that the lens blocks 100% of UVA and UVB, are saying the same thing. A vague claim of “UV protection” with no number is telling you less than it appears to.
- Buy where the claim is accountable. A brand with a distribution chain behind it has something to lose from a false specification. An unbranded pair does not.
- Do not judge by darkness. A light brown lens from a reputable manufacturer will usually be doing more for your eyes than an opaque black one of unknown origin.
- Do not assume it wears off. Ultraviolet filtering in a modern lens is generally built into the material rather than sprayed on the surface, so it does not scrub away with cleaning. Deep scratches are a reason to replace a lens; ordinary use is not.
The short version
UV400 is the number worth caring about, tint is a comfort and styling decision, and the two should not be confused. If you take one habit from this, make it checking the wavelength figure rather than holding the lens up to the light.
