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A defective pixel or a dead pixel is a pixel on a liquid crystal display (LCD) that is not functioning properly. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have further distinguishing types.Apple Technician Training
Similar defects can also occur in charge-coupled device (CCD) and 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.
Another cause of bright dot may be the presence of impurities in the liquid crystal. On the one hand, impurities will affect the alignment of liquid crystal molecules, and on the other hand, they can reflect light to form bright spots.
TAB is one of several methods employed in the LCD-manufacturing process to electrically connect hundreds of signal paths going to the rows and columns of in layer 6 (the transparent electrode layer) in the LCD to the video integrated circuits (ICs) on the driver board that drives these electrodes.
If an LCD is subjected to physical shock, this could cause one or more TAB connections to fail inside the display. This failure is often caused by horizontally flexing the chassis (e.g., while wall-mounting or transporting a display face up/down) or simple failure of the adhesive holding the TAB against the glass. TAB faults require the replacement of the LCD module itself. If these connections were to fail, the effect would be that an entire row or column of pixels would fail to activate. This causes a horizontal or vertical black line to appear on the display while the rest of the display would appear normal. The horizontal failure runs from edge to edge; the vertical failure runs from top-to-bottom.
Hardware manufacturers and distributors claim that TAB faults, as opposed to other physical defects found in an LCD, do not allow for repair.
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., one dead pixel or three stuck sub-pixels in a five-by-five pixel area), or other criteria.
In some cases, a manufacturer sends all screens to sale 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 the owner can send them back to the manufacturer. Dead pixels may tend to occur in clusters; in most cases, displays with such a cluster problem can be sent back to the manufacturer.
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