Polarised Lenses and Screens: Why a Filter Can Turn a Display Black
A polarising filter passes one direction and blocks the rest
Light is a transverse wave, which means its oscillation is perpendicular to the direction it travels. That leaves an entire plane of possible directions for the wave to vibrate in, and for light arriving from most sources those directions are all present at once and constantly changing.
Polarisation is what you get when that jumble is reduced to a single vibration direction. A polarising filter is a material whose molecular structure transmits waves oscillating along one axis of the filter and absorbs the rest, so light leaving a polarising filter is polarised. Sunglasses use that behaviour on purpose.
Their transmission axis is normally oriented vertically. The reason is that glare reflected from a horizontal surface arrives with a strong horizontal component, so a filter that passes only vertical vibration rejects most of it. This is why polarising lenses cut the flat glare off water, wet roads and car bonnets so effectively while leaving the rest of the scene alone.
What a display has to do with it
Liquid crystal displays are built on the same physics. An LCD works by placing a liquid crystal layer between two polarising filters and using voltage to twist the crystals, which controls how much of the backlight passes the second filter. Which means the light that leaves the screen is not a jumble of directions at all. It is polarised.
Malus's law explains the black screen
What happens when polarised light meets a second polarising filter is described by a relationship called Malus's law: the fraction of light transmitted equals the square of the cosine of the angle between the two polarisation axes. Written as a formula, transmission = cos²θ.
- At θ = 0 degrees the axes are aligned, cos²0 = 1, and everything passes.
- At θ = 45 degrees, cos²45 = 0.5, so half the light passes.
- At θ = 90 degrees the axes are crossed, cos²90 = 0, and in principle nothing passes at all.
That is the whole mystery of the phone that goes black in one orientation and not the other. Your sunglasses are a polariser with a vertical axis. If the phone's screen happens to emit light polarised along the same vertical direction, the screen looks normal. Turn the phone ninety degrees, or tilt your head ninety degrees, and the axes cross. The display does not stop working; you have simply inserted two crossed polarisers in the same light path.
Why some screens dim instead of vanishing
Not every display behaves the same way, because not every display leaves its light linearly polarised in the same manner.
Classic liquid crystal panels, including most computer monitors, televisions and dashboard displays, are the ones that go dramatically dark. Screens built on a different emissive technology often carry a circular polariser on the front surface instead. That layer is there to control reflections, and it changes the polarisation state of the light leaving the screen from linear to circular. A linear filter in front of circularly polarised light dims it whatever the angle, so those screens look slightly grey through polarising lenses but do not disappear, and rotating the device does not bring them back.
It also depends on the device's own orientation. Many phones are held in portrait and polarise vertically in that position, so the effect only shows up when the phone is turned sideways. Others are built the other way round. That inconsistency is why two people can own the same sunglasses and disagree about whether their phones are affected.
What the standard says about the axis
Because the orientation of a polarising filter determines what it does, the axis is not left to chance. ISO 8980-1, the standard covering uncut finished single-vision and multifocal spectacle lenses, includes requirements for the orientation of polarising lenses and for how they are marked. In other words, a polarised sunglass lens has a defined axis, and a lens supplied with that axis in the wrong orientation will not reject horizontal glare properly, even if its tint and its UV performance are fine.
Common misconceptions
- "Polarised means UV protection." They are separate properties. Polarisation is about the direction of vibration; UV attenuation depends on the lens material and its UV absorber. A polarised lens without adequate UV absorption is still a poor sun lens.
- "Polarised lenses are the darkest." Darkness is the tint level, which is a different specification entirely. You can have a lightly tinted polarised lens and a very dark non-polarised one.
- "A polarised lens that blacks out my screen is faulty." It is working exactly as designed. The display is the second polariser in the optical path.
- "If one phone is fine, my new phone will be too." The behaviour depends on the screen technology and the orientation of the device, so it is worth checking before you buy rather than after.
- "Polarised lenses are always the better choice for driving." They remove reflected glare beautifully, but removing glare also removes some of the cue that a road surface is wet, and they complicate reading digital instrument panels and heads-up displays. For many drivers the trade is worth it; it is still a trade.
How to tell whether it will bother you
The test takes a minute and needs no equipment beyond the glasses and the screen. Put on the pair, look at a screen you use regularly, then rotate the device slowly through a quarter turn. If it dims and recovers as you rotate it, you have a linear-polarised display and you will notice the effect whenever you hold it at the wrong angle. If it stays the same brightness whatever you do, that screen is not going to be a problem.
It is worth doing the same check on the displays you look at while driving, since a dash panel that fades behind polarising lenses is a bigger annoyance than a phone that does.
What to ask for
If you want the glare reduction but not the screen problem, it is fair to ask about the options rather than accepting whichever lens is on the shelf. Tint level and polarisation are separate choices, and polarised sunglasses are available in lightly tinted versions and in non-polarised versions with excellent UV performance. If you use a heads-up display or read a digital dashboard regularly, say so when you are choosing: it is a concrete requirement, and it is much easier to satisfy before the lenses are made than after.
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Luenx Aviator Polarized Sunglasses for Men: Square UV400 Driving
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FEIDUSUN Sunglasses Men Polarized Sunglasses for Mens and Womens,Black Retro Sun Glasses Driving Fishing UV Protection
A $9.99 deal pick (list $15.99) holding the #1 Best Seller slot in Amazon's Men's Sunglasses chart - 4.4 stars from 5,024 ratings with 10K-plus bought in the past month; the listing credits TAC lenses with UVA/UVB protection plus an anti-glare HD polarized coating it says cuts glare off roads, water and snow, in a retro square matte-black frame it describes as lightweight, flexible and durable.
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