Polarised Glasses

Glare removed rather than the whole scene dimmed

Polarised glasses work on a different principle from a plain tinted lens. Instead of cutting every wavelength evenly, the filter blocks the horizontally aligned light bouncing off water, wet tarmac, snow and car bonnets. The selection covers wraparound frames, driving lenses, photochromic models that darken in sunlight, yellow low light lenses and UV400 sunglasses, which makes the conditions, not the frame shape, the sensible place to start.

What does polarisation actually do?

Light reflected off a flat surface leaves it mostly aligned in one plane. A polarising filter is a grid that lets one orientation through and stops the other, so that reflected glare disappears while the rest of the scene is left alone. The effect is obvious on water, where the surface reflection drops away and the eye suddenly sees through it, and on a wet road, where the sheet of white ahead thins out and the tarmac reappears.

When is a polarised lens the wrong choice?

Whenever a screen matters. Phone, dashboard and instrument displays are polarised themselves, so they can dim or black out entirely at certain angles, and tilting the head brings them back. Anyone reading ice on a road or judging the state of a hard surface may also want that reflection left in place, since glare is information. In those cases a plain tinted lens is easier to live with.

Which tint suits which conditions?

• Grey: the most neutral option, dimming everything evenly and keeping colours honest under bright sun.
• Brown and amber: lifts contrast and depth perception, which helps under patchy cloud and on broken ground.
• Yellow: brightens a dull or misty scene by raising contrast. It does not amplify light, so it adds nothing in genuine darkness.
• Photochromic: darkens under ultraviolet and clears again indoors. Windscreen glass filters much of that ultraviolet, so the reaction is weaker inside a car.
• Mirrored: a reflective coating that cuts how much light reaches the lens at all, which suits snow and open water.

What does UV400 mean?

It describes where the filter stops, blocking ultraviolet wavelengths up to four hundred nanometres, which covers the UVA and UVB range reaching the eye. It is a separate property from both tint and polarisation, and that distinction explains why a dark lens without ultraviolet protection is worse than wearing nothing: the pupil opens behind the tint and admits more of the harmful wavelengths, not fewer.

How is the coating kept intact?

Rinse grit off under a tap before wiping, because dragging a dry cloth across sand is what puts the fine scratch marks across the centre of a lens. Dry them with a microfibre cloth rather than a shirt tail, store them in a case instead of face down on a table, and keep them out of a car parked in the sun, since sustained heat lifts coatings and warps a frame.