Defective pixels are pixel

Pixel

In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

s on a liquid crystal display

Liquid crystal display

A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

 (LCD) that are not performing as expected. The ISO standard ISO 13406-2

ISO 13406-2

ISO 13406-2 is an ISO standard, with the full title "Ergonomic requirements for work with visual displays based on flat panels -- Part 2: Ergonomic requirements for flat panel displays". It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers...

 distinguishes between three different types of defective pixels, while hardware companies tend to have further distinguishing types.

Similar defects can also occur on CCD

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 or CMOS

CMOS

Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 image sensor

Image sensor

An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

s 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.

Dark dot defects

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.

Bright dot defects

A bright dot defect 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 "Hot Pixel".

Partial sub-pixel defects

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

Tape automated bonding (TAB) faults

A TAB fault 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 electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s in layer 6 (the transparent electrode layer) in the LCD display to the video IC

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s on the driver board that drive 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.

Cold start

Commonly called cold start fault, another type of TAB failure shows up when the display has been turned off long enough for the LCD to cool down/shrink enough so the bond is in open circuit

Open circuit

The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...

. This fault is often seen as ghosting

Ghosting (television)

In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...

 double, or triple, image on one side of the display with shadowing or dulling of the other part of the screen. A common test for this problem is to use a 50% and a 75% grey test signal. The fault often disappears as the LCD sheet heats up and expands into the frame; flexing the TV when this is happening will immediately tell you if this is a TAB fault. Cold start TAB faults tend to always get worse as the regular heating and cooling of the bond from powering the display on and off causes further cracking.

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.

Stuck sub-pixels

A stuck sub-pixel 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

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

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 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.

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

ISO 13406-2

ISO 13406-2 is an ISO standard, with the full title "Ergonomic requirements for work with visual displays based on flat panels -- Part 2: Ergonomic requirements for flat panel displays". It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers...

 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 5×5 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.

See also

• ISO 13406-2
  ISO 13406-2
  ISO 13406-2 is an ISO standard, with the full title "Ergonomic requirements for work with visual displays based on flat panels -- Part 2: Ergonomic requirements for flat panel displays". It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers...
  
  , the ISO standard that defines classes of devices, based on a certain maximum number of defective pixels

External links

• Repair stuck pixel, screen burn, dead pixel (Warning: Will cause rapid flashing of monitor)
• Web-based tests for dead pixels
• Common pixel defect deplantation methods