15" to 16"

A privacy filter darkens the screen to anyone viewing from either side, which is what a laptop needs on a train, in a café, in an airport lounge or across an open-plan office. For anyone working with personal data, client records or financial information away from a desk, it is a basic precaution.

Measuring is where mistakes happen. The stated size is the diagonal of the visible screen, and it has to be paired with the right aspect ratio. A 15.6in 16:9 filter and a 16in 16:10 filter are different shapes, and modern business laptops increasingly use 16:10. Check your machine’s specification rather than assuming from the nominal size.

Attachment. Frameless filters use adhesive tabs or slot into side clips. Magnetic versions attach and detach in seconds, which is worth the premium if you only need privacy some of the time or share the screen in meetings.

Touchscreens need a filter rated as touch-enabled; a standard one interferes with touch response.

Most filters also reduce glare and cut blue light.

The range covers Fellowes, Kensington, StarTech, Targus, Lenovo, HYPER and PREVO.