Problem with Pixel Error

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Revision as of 09:21, 21 October 2005 by 144.136.91.219 (Talk) (Don't panic!)

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This page provides information about pixel and subpixel errors.

What are dead pixels or dead subpixels?

Dead Pixels are a phenomenon caused by the technology of LCD displays. TFT displays consist of a huge number of individual display cells, each controlled by its own transistor. I.E. a display with a resolution of 1024x768 pixels consists of 1024x768x3 = 2,36 million cells. A such enormous number makes the production of a display without any faulty cell difficult. The more strict the quality restrictions are, the more expensive the panels get, because this simply means raising the number of garbage panels.

Types of pixel errors

Here is a listing of possible types of dead pixels / subpixels:

pixel type
state type
pixel red subpixel green subpixel blue subpixel
always lit   ·     ·     ·     ·  
always unlit   ·     ·     ·     ·     ·     ·     ·  

You can notice here, that always lit complete pixels, red subpixels and green subpixels are the most annoying ones. Also, dead pixels located closer to the center of the screen are more annoying than such located closer to the edge of the screen.

Dead pixels do not always have to be a constant phenomena. You might in fact experience your pixel in question be faulty at times and working perfectly at other times. Or you might experience it to be a little flickering. This is only logical since a faulty transistor might sometimes work and sometimes not, depending on environmental conditions or even just randomly.

Hint:
The table above also shows you a way of detecting dead pixels and subpixels. Just use plain colored screens.


The ISO 13406-2 norm

However, to give the concerned customer more transparency when buying LCD displays, the ISO norm 13406-2 was introduced, providing a basis for warranty claims. By assuring one of the classes of this specification, the producer is bound to keep that quality standard throughout the whole production series. However, Manufacturers are not necessarily going with this standard. For values for IBM displays look at this page.

The values in the following table state the acceptable errors per 1 million pixels. Dead pixels form a cluster when they are within a square of 5x5 pixels.

Error class Type 1
(always lit pixel)
Type 2
(always unlit pixel)
Type 3
(always lit or always unlit subpixel)
Type 4
(clusters with more than one Type 1 or Type 3 faults)
Type 4
(clusters of Type 3 faults)
I 0 0 0 0 0
II 2 2 5 0 2
III 5 15 50 0 5
IV 50 150 500 5 50

Causes for dead pixels/subpixels

  • As stated above, dead pixels are an inherent feature of the technology/production process of TFT displays. How many of them are in the final displays sold is just a mater of quality control standards.
  • Furthermore, they can also accur at random time after production. Transistors can have faults that are not recognizable at production time. These faults may make the transistor stop working after it has been in use for a certain amount of time.
  • Mechanical or environmental stress (i.e. heat) can possibly damage a transistor, leading to pixel errors.


Don't panic!

Dead pixels are a really common phenomena on TFT displays. In fact there are other things about TFT displays that matter a lot more, like brightness and contrast, picture stability, color reproduction and response time. People just tend to focus on those things that are the most obvious, that's why everyone cries for dead pixels and response time when it comes to TFT displays.

However, as stated above there can be truely annoying types of dead pixels. If their count is higher than the displays quality class permits you might get the device replaced under warranty. The best thing, however, is to look for dead pixels right after purchase (you might have a chance to return it without special reason then):

  • Do it three times over two days - different moods and different environemental light have an influence on your perception.
  • lcdtest utility can generate predefined patterns, and may help you to detect damaged pixels.
  • Be concious about how hard it was to find the dead pixel - if it was really hard it can't be too bothering.
  • Also make sure that what you consider a dead pixel isn't just a piece of dirt or dust.
  • Try pressing the (blue) scroll button on the UltraNav. There is a bug in the UltraNav driver which causes fake Bright pixels to appear on the screen.

Solutions

Besides some april jokes there isn't really any solution except looking for a replacement.

However, for Windows an interesting program exists that can help you making annoying subpixels and pixels more bearable. It works by trying to compensate a dead subpixel by adjusting the values for the remaining working subpixels. This way you can i.e. get rid of a most annoying always lit red subpixel. The program is german only and a download link can be found in its german PixChanger discussion.

External sources