Dead Pixel Test

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Defective pixels are pixels on a liquid crystal display (LCD) that are not performing as expected. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have further distinguishing types.

A photograph taken with a damaged image sensor

Similar defects can also occur in a charge-coupled device (CCD) or CMOS image sensor 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.

Variations[edit]

Dark dot defects[edit]

A dark dot defect is usually caused by a transistor in the transparent electrode layer that is stuck "on" for TN panels or "off" for MVA/PVA and IPS panels. In that state, the transistor places the liquid crystal material in such a way that no light ever passes through to the RGB layer of the display.

Bright dot defects[edit]

Hundreds of hot pixels appear in a 32 minute long exposure photograph taken in virtual darkness. Note: Must be viewed at full size.

A bright dot defect[3] is a group of three sub-pixels (one pixel) all of whose transistors are "off" for TN panels or stuck "on" for MVA/PVA panels. This allows all light to pass through to the RGB layer, creating a bright white pixel that is always on. This is commonly known as a "hot pixel".

Partial sub-pixel defects[edit]

A partial sub-pixel defect[3] is a manufacturing defect in which the RGB film layer was not cut properly.

Tape automated bonding (TAB) faults[edit]

A TAB fault[3] is caused by a connection failure from the TAB that connects the transparent electrode layers to the video driver board of an LCD.

TAB is one of several methods employed in the LCD display-manufacturing process to electrically connect hundreds of signal paths going to the rows and columns of electrodes in layer 6 (the transparent electrode layer) in the LCD display to the video ICs on the driver board that drives these electrodes.

If an LCD display 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 replacement of the LCD display 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 tend to claim that TAB faults, as opposed to other physical defects that may be found in an LCD, do not allow for repair.[3]

Stuck sub-pixels[edit]

A stuck sub-pixel[3] is a pixel that is always "on". This is usually caused by a transistor that is not getting any power, and is therefore continuously allowing light at that point to pass through to the RGB layer. This means that any given pixel will stay red, blue, or green, and will not change when attempting to display an image. These pixels may only show up using certain applications, or they may be on all of the time.

Stuck versus dead pixels[edit]

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 pixel which is permanently black. 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.[citation needed] Dead pixels are much less likely to correct themselves over time or be repaired through any of several popular methods.[citation needed]