There is a persistent idea that sunglasses should be replaced every couple of years because the ultraviolet protection wears out. For a decent modern lens that is not really how it works, and it is worth knowing what actually matters instead.
Does ultraviolet protection wear off?
In most current lenses the ultraviolet filtering is built into the lens material itself, or bonded into it, rather than sprayed onto the surface as a separate film. That means ordinary wear and ordinary cleaning do not remove it, and a well-kept pair does not quietly stop protecting you on a schedule.
The caveat is that a cheap lens with a surface-applied coating is a different proposition, which is one more argument for buying from a manufacturer with something to lose.
The four signals that do matter
1. Scratches in your line of sight
This is the main one. A scratch in the centre of the lens scatters light and forces your eye to work around it, which is genuinely tiring on a long drive. Scratches cannot be polished out of a coated lens, because polishing takes the coating with them. Once they are in your central field of view, the lens is done.
2. A frame that will not hold its fit
If they slide down your nose no matter how often they are adjusted, the frame has been stretched or the hinges have gone slack. An optometrist can often re-tension a frame, and that is worth trying before replacing anything. If it will not hold, sunglasses that sit halfway down your face are not protecting your eyes.
3. Heat damage
A pair left in a parked car through a South African summer takes real punishment. Look for a faint rainbow sheen, cloudiness, or fine crazing across the lens: these suggest the bond between the lens and its coatings has been stressed. A frame that has visibly warped will never fit properly again.
4. Delamination
On polarised lenses the polarising layer is bonded within the lens. If it starts to separate you will see patches, bubbling or a blotchy look, particularly at the edges. At that point the polarisation is failing and the lens is compromised.
What is not a reason
- Age alone. A ten-year-old pair kept in its case can be in better condition than a one-year-old pair kept loose in a bag.
- Fine surface haze. Cosmetic, and usually the accumulated result of dry-wiping. Annoying, not dangerous.
- Fashion. A perfectly good reason to buy another pair, but not a reason to throw the first one away.
The quick check
Hold them at arm’s length and look through each lens at a straight line, such as a door frame, moving your head slowly. Distortion, a wobble, cloudiness or scratches crossing the centre are your answer.
