Defective pixel
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Defective pixels are pixels on a liquid crystal display (LCD) not performing as expected. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels,[1][2] while hardware companies tend to have further distinguishing types. [3]
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.
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[edit] Variations
[edit] Dark Dot Defects
A dark dot defect [4] is usually caused by a transistor in the transparent electrode layer that is stuck "on." It continuously runs a charge across the liquid crystal material, so no light ever passes through to the RGB layer.
[edit] Bright Dot Defects
A bright dot defect [5] is a group of three sub-pixels (one pixel) all of whose transistors are not working. This allows all light to pass through to the RGB layer, creating a bright white pixel that is always on.
[edit] Partial Sub-Pixel Defects
A partial sub-pixel defect [6] is a manufacturing defect in which the RGB film layer was not cut properly.
[edit] TAB Faults
A TAB Fault [7] is caused by a connection failure from the TAB that connects the transparent electrode layers to the video driver board of an LCD.
TAB stands for Tape Automated Bonding, which 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 drive these electrodes.
If an LCD display is subjected to extreme heat or cold or physical shock, this could cause one or more TAB connections to fail inside the display. This failure requires 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. In the examples below, a horizontal or vertical black line would 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.
[edit] Stuck Sub-Pixels
A stuck sub-pixel [8] 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.
[edit] 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,[9] 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.
Hardware manufacturers and distributors tend to claim [10] that TAB faults, in opposition to other forms of defective pixels, will not allow for repair or reliving of the issue, in opposition to other physical defects that may be found in an LCD.
[edit] Manufacturer policy
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.[11] 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.
[edit] References
- ^ BeHardware article explaining ISO 13406-2
- ^ About article discussing pixel defects
- ^ Apple Technician Training
- ^ Apple Technician Training
- ^ Apple Technician Training
- ^ Apple Technician Training
- ^ Apple Technician Training
- ^ Apple Technician Training
- ^ WikiHow article: how to fix stuck pixels
- ^ Apple Technician Training
- ^ BeHardware article discussing defects on LCD displays and manufacturer's policies.
