Lenses
How to tell if sunglasses are polarized: three tests that take a minute
To tell if sunglasses are polarized, hold one lens in front of a phone or laptop screen and slowly rotate the glasses. If the screen darkens or goes black at some angle, the lens is polarized. If it stays the same brightness all the way round, it is not. The test works because most LCD screens emit polarized light, and a polarizing lens blocks it once the two are crossed. Two other tests, one using a second pair of sunglasses and one using nothing but daylight, are below.
Test 1: the screen test
- Turn on a phone, tablet or laptop and show a bright, mostly white screen.
- Hold the sunglasses about a hand's width in front of the screen, looking through one lens.
- Rotate the glasses slowly through 90 degrees, as if turning a dial.
Polarized: at some angle the screen dims sharply or goes black, then brightens again as you keep turning. Not polarized: the screen stays the same at every angle.
Why it works: an LCD makes its picture by polarizing light, so the light leaving the screen is already vibrating in one plane. Turn a polarizing lens so its axis is at right angles to that plane and almost nothing gets through.
One caution: some newer OLED phones and screens with anti-reflective films give a weaker effect, and a few will not darken at all. If the screen test is inconclusive, use test 2 or 3.
Test 2: the two-pair test
If you have a pair you know is polarized, hold the two pairs lens to lens, one rotated 90 degrees to the other, and look through both. If the overlap goes dark, both are polarized. If light passes freely through the overlap at every angle, at least one is not.
This is the crossed-polarizer effect from any optics textbook. The first lens lets through only light vibrating in one plane. The second, turned 90 degrees, blocks exactly that plane. Almost nothing survives both.
Test 3: the daylight test
No screen, no second pair. Go outside on a bright day and find glare: sun on a car bonnet, a puddle, a window, a lake.
- Put the sunglasses on and look at the glare.
- Tilt your head slowly to one side, about 45 degrees.
Polarized: the glare visibly fades as you tilt, then returns as you straighten up. Not polarized: it stays the same.
Reflected glare is horizontally polarized, which is why polarized sunglasses are built to block the horizontal plane. Tilt your head and the lens's axis no longer lines up with the glare, so it lets more through. This is also the most useful test, because it shows you the exact thing polarization is for.
What none of these tests tell you
UV protection. Polarization and UV protection are separate properties. A lens can pass all three tests above and block no UV at all, and a clear lens can block UV completely. There is no home test for UV; you need the manufacturer's label, or an optician with a UV meter. Our UV400 vs polarized post explains what the labels mean.
Quality. These tests tell you the film is present. They do not tell you how well the lens is made, whether the film is laminated inside the lens or coated on the surface, or how dark the tint is.
Why some polarized sunglasses fail the screen test
Occasionally a genuinely polarized pair shows only a faint change against a screen. Two reasons:
- The screen is not strongly polarized. Some OLED and some anti-glare-coated screens emit light with less of a single orientation.
- The film is oriented at an unexpected angle. Almost all sunglasses block the horizontal plane, but a badly assembled pair can have the film rotated. Test 3 will still show the effect at some tilt.
If a pair fails all three tests, it is not polarized, whatever the sticker said.
What we sell
All five pairs on this site are polarized, and you can run any of these tests on them the day they arrive. If you want the plainest one to start with, that is the T01 Grey. The polarized vs non-polarized post covers when the filter helps and when it does not.
Frequently asked questions
How do I check if my sunglasses are polarized and UV protected?
Check polarization with the screen test above. UV protection cannot be tested at home; look for a label stating UV protection or UV400, or ask an optician to check with a UV meter.
Is there a polarized sunglasses test picture I can use online?
Some sites show an image designed to appear only through a polarized lens. They rely on the same principle as the screen test, so a plain white screen works just as well and does not depend on the image being made correctly.
Does it matter if sunglasses are polarized?
For driving, water, snow and long days outside, yes: the glare removal is the point. For reading screens, flying, or skiing in flat light, non-polarized can be the better choice. It matters; it is just not always better.
Can polarized lenses wear off?
A film laminated inside the lens, as in tri acetate cellulose lenses, cannot wear off; it is sealed between two layers. A polarizing coating applied to the surface can scratch through over time. The lens page for each of our pairs describes the construction.
Sources
- American Academy of Ophthalmology. Sunglasses: Polarized lenses. www.aao.org
- Hecht E. Optics, 5th ed. Pearson, 2017. Chapter 8, Polarization: Malus's law and crossed polarizers. www.pearson.com
Measurements on this site are the frame maker's published figures, attributed on every page that prints one. This is general information about sunglasses, not medical advice.
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