Short answer: on a crowded train, privacy glass stops the person beside you and beside them. It does not stop the person directly behind you reading over your shoulder, and it never will, because they are looking at the screen from the same angle you are.
How the louvres work
Under the etched face there is a layer of microscopic vertical blinds — the same idea as the slats on a window, several thousand times smaller. Look at the screen straight on and you are looking down the gaps, so you see the panel. Move off-axis and the slats close the view, and by roughly 30 degrees off-centre the screen reads as a dark rectangle.
That angle is the whole specification, and it is worth being precise about what it covers. Thirty degrees is about the difference between the seat you are in and the seat next to it. It is not the difference between you and someone standing behind your seat.
What it actually stops
The passenger in the next seat. Somebody standing to one side in a queue. The person across the aisle. Anyone glancing sideways at a desk. In practice that is most casual over-reading, because most casual over-reading happens from the side.
What it does not stop
Anyone behind you looking at the same angle you are — on a standing train, that is the realistic threat and privacy glass does nothing about it. It also does nothing about a camera pointed at your screen from behind, and nothing at all if you tilt the phone to show someone something and forget to tilt it back.
It is a filter for angles, not a curtain. Treat it as something that removes the accidental audience, not something that makes the screen secret.
What it costs you head-on
Light that the louvres block is light that does not reach your eye either. At the same brightness setting the screen is dimmer than it would be under a regular sheet. Indoors this is barely worth mentioning; in direct sun you will notice, and you will run the brightness higher, which costs battery.
There is no version of this technology that narrows the viewing angle without taking some brightness with it. Anything sold as privacy glass with no brightness cost is either not a louvre filter or is not doing much.
Who it is genuinely for
People who read messages on public transport. People who work on a laptop-sized amount of information on a phone in an office where others walk past. Anyone handling other people's data on the move — field staff, health workers, anyone with a customer list on the screen.
If you mostly use the phone at home and in a car, you are paying for an angle you never need. Regular will look better and cost less.
Questions people ask
Does privacy glass stop people behind me seeing my screen?
No. Someone directly behind you looks at the screen from almost the same angle you do, and privacy glass only blocks side angles. It stops the person beside you, not the one over your shoulder.
At what angle does a privacy screen protector go dark?
On a filter cut at about ±30°, the screen reads as a dark rectangle once you are roughly 30 degrees off-centre — about the difference between your seat and the one next to it. Head-on, you see the screen normally.
Does privacy glass make the screen dimmer?
Yes, a little. The louvres that block light going sideways also block some of the light coming to you, so at the same brightness setting the screen looks dimmer than under clear glass. Indoors it is barely noticeable; in direct sun you will turn the brightness up.
Is privacy glass worth it for a daily commute?
If you read messages, banking apps or work chats on a train or bus, yes — most casual over-reading happens from the side, and that is what it stops. If your phone mostly stays at home or in the car, clear glass looks better and costs less.
Does privacy glass stop someone photographing my screen?
Not from behind you. A camera pointed over your shoulder sees the screen at the same angle you do. Privacy glass removes the casual audience to the side; it does not make the screen secret.