A dead pixel is a defective pixel that remains
unlit on an LCD screen, monitor, Camera Charge-coupled device or
CMOS sensor.
The term "dead pixel" is often erroneously applied to other
defective pixels, but they have separate terms. A permanently lit
(white) pixel is called a hot pixel, and a pixel that stays on a
solid color (red, green, or blue) is known as a stuck pixel.
In LCD manufacture, it is common for a display to be manufactured
that has a number of sub-pixel defects (each pixel is composed of
three primary-colored sub-pixels). The number of faulty pixels
tolerated before a screen is rejected is dependent on the "Class"
which the manufacturer has given the display (although officially
described by the ISO 13406-2 standard, not all manufacturers
interpret this the same way, or follow it at all). Some
manufacturers have a zero-tolerance policy with regard to LCD
screens, rejecting all units found to have any number of sub-pixel
or pixel defects, meaning the display is a "Class I" display.
Others reject them according to the number of total defects, or the
number of defects in a given group, or other definitions. Some
screens come with a leaflet stating how many dead pixels they are
allowed to have before you can send them back to the manufacturer.
Dead pixels can also occur in clusters and these are particularly
annoying and in most cases these can be sent back the
manufacturer.
In some cases, the manufacturer sends all screens to sale, and then
replaces the screen if the customer reports the unit as faulty and
the dead pixels meet their minimum requirements for return.
Recently, this has become a hot button topic with the Sony
Playstation Portable system as many units have shipped to sale with
dead pixels to varying degrees, and there has not been a clear
policy on return requirements (see that article's Criticisms
section).
The majority of dead pixels are only noticeable on a solid color
background, for example through the use of the "Dead Pixel
Checkers" which consist of 4-5 solid-color images.