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A look into dead pixels- 2007
by Vincent Alzieu
Published on May 22, 2007

A standard norm : The ISO 13406-2
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 standards. The main ones come from the international organization, ISO, who made such norms as the ISO 9002. For LCD screens there is the 13406-2 and from it comes several screen characteristics; response time (see this article on this subject), viewing angles, contrast ratio, brightness (more on this here ), etc. Based on the fact that a liquid crystal panel can’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 (“hot pixels”) in the “up position” while type 2 is for those stuck in a “low position”. In other words, this corresponds to pixels that remain in white or black regardless of the picture displayed. This may sound alarming, 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 sitting side by side: 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, called a “stuck pixel”.

ISO answered our questions regarding the defect rate for LCD screens. The norm was developed for small screen sizes inferior to 15” intended for use by manufacturers. Since then, however, LCDs are omnipresent, sizes have increased and 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 finally up to 2560x1600 pixels on 30”s.
What the norm says
The ISO 13406-2 norm defines 4 classes of screens. The strictest and best one, “class 1”, allows no defects. The worst, “class 4” authorizes up to 1344 pixels and faulty sub-pixels on a 20” screen! Fortunately, no manufacturers use this class and almost all use class 2 to establish their warranties.

A case in point : BenQ and its FP series (not the T).

On the FP series, BenQ applies the Class II of the ISO norm. Their 19” monitors are equipped with 1280 x 1024 panels, and thus have 1280 x 1024 = 1,310,720 pixels.

Type 1 : 2 x 1,310,720 / 1,000,000 = 2.62144 dead pixels tolerated
Type 2 : 2 x 1,310,720 / 1,000,000 = 2.62144 dead pixels tolerated
Type 3 : 5 x 1,310,720 / 1,000,000 = 6.5536 dead pixels tolerated

If we first take type I errors into account this means 2.62144 white pixels are acceptable.
2 white ones and BenQ doesn’t exchange the screen
3 whites and we are above the tolerance level and the guarantee comes into play.

If we continue with this principle, BenQ will make an exchange if you have:
  • 3 white pixels or
  • 3 black pixels, or
  • 7 defective sub-pixels.
    On the other hand, if your screen has two white pixels, two black and six colored, BenQ could reply that they are still within the norm and refuse a return.

    The norm applied to 17, 19, 20, 22, 23, 24, 26, 27, and 30 inches

    Applied to current diagonals, Class II tolerance allows the following errors :

    By default some screens are Class I

    If a screen is ISO 13406 certified, but the manufacturer doesn’t indicate the class, it’s automatically a Class I, or in other words, is guaranteed for zero dead pixels

    Excerpt of the norm :

    ISO 13406-2:2001(F), p39, chapter 7.20, Pixel defects
    It’s agreed that flat panel-type screens are in the Pixel class I concerning defects (refer to 3.4.13 Pixel defects). In the contrary case, the manufacturer must specify the defect Pixel class of the screen.

    Now, let’s move on to what each manufacturer says...

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