Are privacy screen protectors any good?

Privacy protectors do exactly what they claim, and the trade-off is larger than most buyers expect. Whether that trade is worth making depends almost entirely on where you use the phone.

How they work

A privacy protector contains a layer of microscopic vertical louvres, in effect a very fine venetian blind laminated into the glass. Light leaving the display straight ahead passes between the louvres and reaches your eyes. Light leaving at an angle strikes the side of a louvre and is absorbed.

The result is a viewing cone of roughly 60 degrees total, so about 30 degrees either side of straight-on. Beyond that the screen goes dark rather than merely dim. On a train or in an open-plan office it works as advertised, and the effect is more complete than people expect from the photographs.

The cost, which follows from the same mechanism

Those louvres can't distinguish between a stranger's eyes and yours. A substantial proportion of the light heading towards you is absorbed too, and the practical consequence is that you'll run the display brighter, which costs battery, and which has a ceiling.

Outdoors in bright sun, where you were already at maximum brightness, there's no headroom left. This is the situation where privacy protectors are most disappointing: the phone becomes genuinely hard to read, and no setting fixes it.

There's also a slight loss of sharpness, because the louvre layer sits between the pixels and your eye, and on some films a faint vertical texture is visible against a plain white background.

Two-way and four-way

Most privacy protectors are two-way: they block horizontal off-axis viewing but not vertical, so someone standing over you can still see the screen. Four-way films block both axes.

Four-way sounds better and is often worse in practice, because it dims the display noticeably more and it interferes with landscape use — turn the phone sideways to watch something and the previously-horizontal louvres are now blocking your view from above and below.

If you mainly want to stop the person in the next seat reading your email, two-way is the right choice.

The social friction

This is the objection people don't anticipate. Showing someone a photograph becomes a small negotiation about angles. Watching something with another person on a train is awkward. Handing the phone to a colleague to look at a document means they hold it directly in front of them or see nothing.

On a personal phone used at home, this friction shows up daily and the privacy benefit shows up rarely, which is why a lot of privacy protectors end up removed.

Who should buy one

Buy it if you regularly read work email, client data or anything confidential on public transport, in a coffee shop or in a shared office. Buy it if your employer requires it, many do for handset access to corporate mail, and this is a legitimate compliance purchase.

Don't buy it if your actual complaint is glare. A matte anti-glare protector is cheaper, doesn't dim the display and solves reflections properly. A meaningful number of people who buy privacy filters wanted anti-glare and didn't realise the two were different products.

And think twice if your phone has an always-on display, because the combination makes the standby screen hard to read at a glance, which removes the point of having it.