There are many different manufacturers of these filters although the best known (and possible inventor?) of them is 3M. Basically they use a “micro louvre” system to ensure that when placed onto the screen the image can only be viewed from directly in front. Someone sat next to you can not see the screen at all, just a black image. The louvres work in a similar way to venetian blinds but in a vertical arrangement; when they are open you can see through them but the moment you move to one side the blind slat itself blocks the way. The principle is the same in the filter – vertical slats that allow enough light out to see the image but block the view fro the side.
As a technology they are very simple, albeit expensive – you can expect to pay upwards of £50/$70USD for a 3M one. That seems rather expensive, so what are the real world benefits?
Most people nowadays will travel for over an hour to their place of work, and with the increasing number of people using a laptop as their primary computer, that travel time can be more effectively utilised by working. Being able to do so without fear of someone viewing the strategy or bid document you are working on gives great peace of mind. Without wishing to countenance the transport of sensitive/confidential documents in open, it does provide an extra level of protection in addition to encryption etc..
Social engineering is also significantly reduced. Someone wishing to engage in a conversation with you to get hold of information has ready access to your screen for topics, interests, even personal details (from your wallpaper?) and has a “hook” to start that conversation. By blocking that view, they have to work much harder for those personal details.
There are downsides to using a screen filter though;
Risk homeostasis, i.e. you begin to think nobody can see your screen, and so let your guard down elsewhere. Bearing in mind that you can only view the screen from directly in front of you, that means that the person peering from between the seats directly behind you can also see the screen.
You are also highlighting the fact that you have something worth looking at! I have experienced interested stares from people in a restaurant in Washington D.C., (where I thought security techniques such as a screen filter would be de rigour) as they saw the lovely golden sheen on my new 3M filter; it was gold as it allegedly helped increase the clarity and privacy at the same time. i certainly drew attention to myself!
Of course the Pros far outweigh the Cons, and so for me the inclusion of a screen filter into my BYOS arsenal is certainly one of the most important pieces of kit to have.
As an aside, filters are also available for phones and tablets. I have one on my iPhone and it is very effective when holding the phone in portrait. If I need to show someone something on my phone i simply rotate it to landscape, and people either side of me can see the screen fully.