How they work

LCDs are made up of lots of tiny cells (subpixels) each filled with a liquid that can block light or let it through depending on whether or not an electric charge is applied. The charge determines how much light can get through.

Each subpixel can produce 256 levels of brightness and when grouped together in threes - with a red, green or blue filter - make an LCD screen capable of producing 16.8 million colours (256 shades of red, green and blue, respectively).

If a screen has a faulty transistor you get a pixel that may appear permanently white, black or any variation of the two primary colours that are still working. This is known as a bad or dead pixel.

Normally one bad pixel is unlikely to be a problem, unless it's in the central part of the screen. However, if bad pixels are obvious and grouped together you should ask for a replacement.