LCD Panel Defects : Dead Pixel and Stuck Pixel

Presented by HDTV Shop

 

No product is completely free of defects, but LCD panel monitors have brought the definition of defects to the public light. Imagine how frustrating it can be to bring home a new monitor, turn it on and see a black or white dot in the middle of this new display. Worse yet, there could be three of four of these dots scattered around the display.

As a consumer looking to purchase an LCD panel monitor for a computer or even to be used as a TV, it is very important to understand your rights with regards to the quality of the item you are purchasing. This article looks to help inform consumers what a pixel defect is, why they are allowed and how to look at warranties.

In semiconductor manufacturing process, it is impossible to reach zero-defect production. In normal semiconductors, this is not a problem, because the defective devices are rejected and don’t reach the market. 

But in LCD panels, all transistors are joined in the same panel. Unfortunately, a defective transistor cannot be repaired nor changed. Rejecting every screen with one or more damaged sub-pixels would make a too expensive product. The LCD panel makers has it’s own criteria to reject or accept the panel. That’s why “defective” screens reach the market.

 To understand how pixel problems develop in an LCD panel, it is important to have a basic understanding of the underlying technology of the LCD panel monitor. Unlike CRTs that are actively generating the light through phosphors, an LCD actually takes white light and filters it to get the desired colors. To do this for each given pixel, three sub pixels are defined for red, green and blue light. When the sub-pixel is off, the filter will block the specified color of light. When the sub-pixel is on, it will open the filter to let a desired amount of light through.

Pixel defects for an LCD panel monitor are defined as one of two types: dead or stuck. Technically, both types of pixel defects are stuck pixels but it really deals with the electrical currents and the state of each pixel or sub-pixel.

A dead pixel is defined as a pixel or set of sub-pixels that has failed and is permanently in the off position. This condition means that the pixel will not let any light through. This can be observed as a dark or black spot on a brightly colored or white background.

A stuck pixel is defined as a pixel or sub-pixel that has failed and is permanently in the on position. This can be either with a single or multiple sub-pixels for a given pixel and is best observed on a dark or black background. A white pixel means all three sub-pixels have failed while a green, red or blue pixel means one of the sub-pixels has failed.

When the TV set is turned Off, you cannot notice the defect because the backlight is turned Off.

LCD panels are very complicated devices to put together. Every sub-pixel has associated a transistor. This means that in a WXGA panel (1280 pixels x 768 pixels) there are 1280 x 768 x 3 = 2949120 transistors. Monitors with higher resolutions will have an even greater number of pixels. Even a small impurity in the layers that make up these pixels can result in a pixel defect. Semiconductors have child death effect. This means that during the beginning of the screen life, some transistors (sub-pixels) can die. Once the initial period passes, it is more difficult to get more damaged transistors. This means that your new TV set can show new defective sub-pixels during the initial period.

It is very important to be a well informed consumer when purchasing an LCD panel monitor. Research the warranty and replacement policies from the manufacturer and retailer before making a purchase. Be sure to understand what the terms of the different types and locations of pixel defects mean. All of this can help alleviate frustration if the monitor that you purchase does not meet your standards of quality.

My advice is to return the TV set to the shop if you have several lit dots, and get another one which may have less defects (or zero-defects if you are very lucky). But consider that the seller can argue that the shop cannot  replace the TV set for this reason.

It is important to find out what the replacement policy is on any LCD panel monitor prior to purchase. In general, the manufacturer will have a policy with regards to the number of stuck or dead pixels allowed before it can be returned. To make things even more confusing, the location of the pixels can also play an important role in whether the manufacturer will accept it as defective. The number of defective pixels that define a panel as meeting the replacement criteria will also vary with the size and even model of the monitor.

In addition to the manufacturer warranty, look at the replacement policy of the retailer that you are purchasing the monitor from. It is very likely that the policy of the retailer may be better than that of the manufacturer allowing another avenue if the newly purchased monitor has a number of defective pixels.

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This entry was posted on Monday, March 22nd, 2010 at 12:05 am and is filed under Technical Information. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.