How to check if sunglasses are polarised (the phone or laptop screen test)

Polarised lenses cut glare from roads, water and car bonnets, and sellers often claim them. You can test the claim in under a minute with any LCD screen.

Look at a white LCD screen, like a laptop or ATM display, through one lens and slowly turn the sunglasses about 90 degrees. If the screen goes much darker or black, the lens is polarised; if nothing changes, it is not. On Ray-Ban, polarised lenses also carry a small P next to the logo on the right lens.

• 1. Open a white screen: Bring up a blank white page on a laptop or monitor. LCD screens give off polarised light, which is what makes this test work.
• 2. Look through one lens: Hold the sunglasses about 30 cm from the screen and look through one lens, as if you were wearing them.
• 3. Rotate slowly: Turn the sunglasses gradually up to about 90 degrees while watching the screen. A polarised lens makes the screen turn much darker, often almost black, at some angle. A plain tinted lens stays the same throughout.
• 4. Try the two-pair test: If you have a pair you know is polarised, hold one lens in front of the other and rotate one pair. Two polarised lenses block nearly all light when they are at right angles.
• 5. Check for glare outdoors: Look at a shiny car bonnet or a puddle in sunlight and tilt your head. With polarised lenses, the bright glare fades noticeably as you tilt.
• 6. Look for the markings: On genuine polarised Ray-Bans, a small P appears beside the logo on the right lens. Treat it as a helpful sign rather than proof, since copies can print a P too.