LCD display quality has greatly improved since they were first introduced to the market. Back then, it wasn't uncommon for displays to have several flaws in at the time of shipping or show up shortly afterwards and still be deemed acceptable and thus not valid for warranty repairs or replacements. Today those problems are mostly cleared such that most LCds have no defects and manufacturer's offer zero-pixel warranties. Still, it is important to know the types of defects that can show up in an LCD display so you can properly handle any warranty or returns of a computer display.
To understand how pixel problems develop in an LCD monitor, it is important to have a basic understanding of the underlying technology of the LCD 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.
Dead and Stuck Pixels
Pixel defects for an LCD 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. Another term for this is a hot pixel.
Why Were Defects Allowed?
LCD panels are very complicated devices to put together. Each pixel in an LCD monitor is controlled by a number of transistors embedded into sheets of material. For example, a 24" LCD with a resolution of 1920x1080 has 2,073,600 pixels comprised of 6,220,800 sub-pixels. 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.
When the production costs for these panels was extremely high, manufacturers would not be able to produce enough perfect panels to offset the costs of productions. As the production costs got smaller and quality control improved, more and more companies have started to take the zero-pixel defect policy which means that the defects are less likely to make it to market. This is still the major reason why the larger display panels or 30-inches and larger are much more expensive because the increased size means that there is a higher risk of defects and thus more panels have to be rejected which in turn increases the costs.
It is important to find out what the replacement policy is on any LCD 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.
It is very important to be a well informed consumer when purchasing an LCD 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.