Dead pixels are either whole pixels or subpixels that do not turn on. These pixels always look black. When LCD panels first started to become popular, stuck and dead pixels were quite common and would often appear in clusters.  Too many faulty pixels can render a monitor worthless, so the ISO 13406-2 standard was created to define the maximum number of faulty pixels that are permitted for any given class of display. A Class One panel can't have any faulty pixels, but a Class Four panel can have up to 50 permanently white pixels. “In order to generate all the colours of the spectrum, each pixel is made up of three subpixels – one each for red, green and blue light. When all three subpixels are on, the pixel is white, and when all three are off , the pixel is black.”

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