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LCD screens dead pixels
by Vincent Alzieu
Published on September 30, 2004

Introduction
Some manufacturer specifications such as response time, contrast ratio, viewing angles, and refreshing rate are easily found. Others, however, such as dead pixels are more difficult. In fact, this is a very sensitive subject for flat screen manufacturers as warranty policies vary considerably!

If you plan to purchase a screen in the near future, consider first taking a look at the exchange policies followed by each manufacturer. If you have already bought a LCD screen and are satisfied, or you need to use your warranty, here are the rules followed by manufacturers.

The future of all LCD screens


The evolution from CRT (tube screen) to LCD technology has at least one major problem; dead pixels. Like all industrial products, LCD screens follow several norms. The main one comes from the international organization ISO, who made such norms as the ISO 9002. For LCD screens there is the 13406-2 norm and from it comes several screen characteristics; response time (see 19” comparative test + response time article), viewing angles, contrast ratio, brightness (see the focus on LCD article), etc.

Based on the fact that a liquid crystal panel couldn’t be perfect, the ISO 13406-2 also introduced rules regarding defective pixels in relation to screen size.

What is a dead pixel?
The norm defines three types of defects. Type 1 is for pixels stuck in “up position” while type 2 is for those stuck in “low position”. In other words, this corresponds to pixels that remain in white or black regardless of the picture displayed. It may sound disturbing, but this is compensated by the brightness of surrounding pixels which reduces this defect to some effect. There is one type of dead pixel left…


Type 3 concerns sub-pixels. If you take a closer look at your screen you will see that each pixel is actually made of three sub-pixels: one red, green and blue. A type three error designates the malfunctioning of one of these red, green, or blue components. This time the result is a very bothersome red, green or blue dot on your screen!

ISO answered our question regarding the defect rate for LCD screens. The norm was developed for small sizes screens inferior to 15” designed for professionals. But since then, LCDs are omnipresent, sizes have increased and the resolution has changed from 1024 x 768 for the 15" LCD, to 1280 x 1024 for the 17 and 19", to 1600 x 1200 for the 20 and 21", and finally up to 3840x2400 pixels for the top of the line 22".


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