A "Dead Pixel" is defined as a pixel that is improperly lit when the screen has an all black background.
Other "problem" pixels are referred to as "Missing Pixel" (i.e. improperly lit pixels against other background colors are also defined as missing pixel). ISO 13406-2 distinguishes between three
different types of defective pixels:
Hot pixels (always on)
Dead pixels (always off)
Stuck pixels (one or two sub-pixels are always on or always off)
Similar defects can also occur on CCD or
CMOS image sensors in digital cameras. In these devices, defective pixels fail to sense light levels correctly, whereas defective pixels in LCDs fail to reproduce light levels correctly.
A permanently lit white pixel is called a glowing pixel. Hot pixels are usually best seen against a dark background. In
digital photography a
dark frame is sometimes used to allow correction for hot pixels.
A dead pixel is a defective pixel that remains unlit. Dead pixels are usually best seen against a white background.
A stuck pixel will usually be most visible against a black background, where it will appear red, green, blue, cyan, magenta, or yellow, although stuck red, green, or blue pixels are most common. Each pixel on an LCD monitor is composed of three
subpixels (one red, one green, and one blue) which produce the visible color of the pixel by their relative brightness. A stuck pixel results from a manufacturing defect, which leaves one or more of these sub-pixels permanently turned on or off.
Stuck pixels are not guaranteed to be correctable, and can remain faulty for the life of the monitor. Anecdotal evidence suggests that such pixels can sometimes be fixed by flashing numerous colors with a very rapid intensity.
Defective pixels in LCD screens can sometimes be improved by mechanically manipulating the area around a defective pixel by pressing or tapping. This can help to evenly distribute the oil inside the screen, but it can also damage it.
Stuck versus dead pixels
Stuck pixels are often incorrectly referred to as dead pixels, which have a similar appearance. In a dead pixel, all three sub-pixels are permanently off, producing a permanently black pixel. Dead pixels can result from similar manufacturing anomalies as stuck pixels, but may also occur from a non-functioning transistor resulting in complete lack of power to the pixel. Dead pixels are much less likely to correct themselves over time or be repaired through any of several popular methods.
Stuck pixels, unlike dead pixels, have been reported by LCD screen owners to disappear, and there are several popular methods purported to fix them, such as gently rubbing the screen (in an attempt to reset the pixel), cycling the color value of the stuck pixel rapidly (in other words, flashing bright colors on the screen,) or simply tolerating the stuck pixel until it disappears (which can take anywhere from a day to years). While these methods can work on some stuck pixels others cannot be fixed by the above methods. Also some stuck pixels will reappear after being fixed if the screen is left off for several hours.
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 that 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 defects. Displays meeting this standard are deemed Class I. Other manufacturers reject displays according to the number of total defects, the number of defects in a given group (e.g. 1 dead pixel or 3 stuck sub-pixels in a 5x5 pixel area), or other criteria.
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 defective pixels meet their minimum requirements for return. 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 may tend to occur in clusters; in most cases displays with such a problem can be sent back to the manufacturer.