![Lcd display dead pixel](https://upload.wikimedia.org/wikipedia/commons/2/20/Lcd_display_dead_pixel.jpg)
A defective pixel is a
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the ...
on a
liquid crystal display (LCD) that is not functioning properly. The
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
standard
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 ...
distinguishes between three different types of defective pixels,
while hardware companies tend to have further distinguishing types.
[Apple Technician Training]
![Damaged photo sensitive chip](https://upload.wikimedia.org/wikipedia/commons/b/b4/Damaged_photo_sensitive_chip.jpg)
Similar defects can also occur in
charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
(CCD) and
CMOS image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...
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.
Types
Dark dot defect
A dark dot defect is usually caused by a
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
in the transparent
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
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 so that no light ever passes through to the
RGB
The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...
layer of the display.
Bright dot defect
![Hot pixels at 32 minutes ISO 400 Nikon D5100](https://upload.wikimedia.org/wikipedia/commons/6/6f/Hot_pixels_at_32_minutes_ISO_400_Nikon_D5100.JPG)
A bright dot defect or hot pixel is a group of three sub-pixels (one pixel) all of whose transistors are "off" for TN panels or stuck "on" for MVA and PVA panels.
This allows all light to pass through to the RGB layer, creating a bright white pixel that is always on.
Partial sub-pixel defect
A partial sub-pixel defect is a manufacturing defect in which the RGB film layer was not cut properly.
Tape automated bonding fault
A tape automated bonding fault (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-manufacturing process to electrically connect hundreds of signal paths going to the rows and columns of
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s 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.
Stuck sub-pixel
A stuck sub-pixel is a pixel that is always "on".
This is usually caused by a transistor that is getting power all the time (VA/IPS) or not getting any power (TN) and is therefore continuously allowing light at that point to pass through to the
RGB
The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...
layer. 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 the time.
Manufacturer policy
In LCD manufacture, it is common for a display to be manufactured with several 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 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 ...
standard, not all manufacturers interpret this standard 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., 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.
See also
*
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 ...
, the ISO standard that defines classes of devices based on a certain maximum number of defective pixels
References
{{Reflist
External links
Web-based tests for dead pixels 1Common pixel defect deplantation methodsIsMyLcdOK - check for dead, paralyzed, stuck pixels on LCD monitors for Windows.
Digital imaging