LCD screens dead pixels - BeHardware
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Written by Vincent Alzieu

Published on March 23, 2005

URL: http://www.behardware.com/art/lire/519/


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Dead pixels

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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What the norm says

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 549 malfunctions for a 15” screen, and 1344 pixels and faulty sub-pixels on a 20” screen. Fortunately no manufacturers use this class. Almost all manufacturers use class 2 to establish their warranties:


Practical case : Sony

Sony strictly applies the ISO Class II norm. Their 17” monitors are equipped with 1280 x 1024 panels and have: 1280 x 1024 = 1 310 720 pixels.

Type 1 : 2 x 1 310 720 / 1 000 000 = 2,62144 dead pixels allowed
Type 2 : 2 x 1 310 720 / 1 000 000 = 2,62144 dead pixels allowed
Type 3 : 5 x 1 310 720 / 1 000 000 = 6,5536 dead pixels allowed

So for type 1 errors, 2,62144 white pixels would be tolerated.
2 white pixels, Sony will not exchange the screen.
3 white pixels, the maximum number of faulty pixels is exceeded, Sony exchanges your screen.

So according to this rule Sony would exchange your screen if you have:

  • 3 white pixels, or
  • 3 black pixels, or
  • 7 faulty sub pixels

    However, if your screen has two white pixels, two black pixels and six faulty sub pixels, Sony could say that they are still within the norm and refuse to exchange your screen.

    The norm applied to 15, 17, 19, 20 and 21” screens

    Applied to different screen sizes, Class II tolerance authorizes the following number of faulty pixels:


    By default LCD screens are Class I

    If a screen has the ISO 13406-2 certification, but the manufacturer doesn’t indicate the screen class, it’s possible to consider it a Class I screen. Therefore your screen shouldn’t have any dead pixels.

    The ISO 13406-2:2001 says that a flat screen should be considered a Class I screen unless another is specified by the manufacturer.

    Now let’s see what the manufacturers say:


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    A, B, C : Acer, Apple, BenQ...

    A, B, C : Acer, Apple, BenQ...


    Apple : So far Apple only dealt individually with dead pixel issues. Even if reader’s feedback weren’t that bad, we found quite strange that Apple’s client didn’t beneficiate from a minimum policy.

    That just changed today. The new official statement is: Apple displays meet the ISO 13406-2 specification. Any customer who is concerned about pixel anomalies with an Apple display should contact an Apple Authorised Service Provider (it also include people who doesn’t meet the ISO 13406-2 requirements).




    BenQ was one of the only few manufacturers not to answer our questions for the first article release on dead pixels policies. Today, by looking at their answer we understand why. They are the only one not to use the class 2 of the ISO norm, but the class 3 for entry level products: the T series. With this series the number of dead pixels tolerated explodes!

    We find the norm 2 already inappropriate, but the use of the class 3 is really shocking. This choice is in complete contradiction with the policy of a manufacturer who is trying to push their products’ quality image. This attitude is really a shame for a LCD manufacturer especially if he sales products to the general public. We hope that BenQ will change their dead pixel policy soon!





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    D, E, F : Dell, Eizo, Formac...

    D, E, F : Dell, Eizo, Formac...
    Daewoo Yes and then No for the answer to the question of will you exchange the monitor if the pixel is in the middle of the screen, 2/2/4 then 3/3/5 for 15” monitors, 3/3/7 then 5/5/8 for 19" monitors, Daewoo policy has changed but not in a good way. Some reader reported problems with Daewoo for monitor exchanges and we better understand them today.





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    H, I : HP, Hyundaï, Iiyama...

    H, I : HP, Hyundaï, Iiyama...

    GNR Two years ago the French representative told us on the phone to have a zero dead pixel policy. Today the policy has changed and came back to more reasonable terms.

    Hercules : this French manufacturer ceased screen production this year. It’s unfortunate because we liked their ProphetView… Although they don’t sell screens anymore recent purchases still have a warranty. This is why Hercules is present in this article.




    Hyundaï :This manufacturer admits they are more flexible than their policy for dead pixels. But the main problem is that Hyundai is one of the worst for after sales service according to users. Faulty screens are replaced but with considerable delays. One of our readers had to wait 6 months to receive a new screen! We hope this situation won’t last and they will improve their warranty’s efficiency…



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    J, K, L : LaCie, LG, LiteOn...

    J, K, L : LaCie, LiteOn, LG...





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    M, N, O : Medion, Nec, Neovo...

    M, N, O : Medion, Nec, Neovo...





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    P, Q, R : Packard Bell, Philips, Proview...

    P, Q, R : Packard Bell, Philips, Proview...


    Philips: Philips said that from April some of the new LCD range will no longer have a zero dead bright pixel warranty, but a dead pixel warranty! Whether if they are black, white, colored… all pixels will be covered by this warranty. This new warranty covers the B2B, B2C, 170B6, 170P6, 190B6, 190P6, 170X5 and 190X5 monitors.

    For other products it is a little bit trickier. Here is Philip’s policy:

    15”, 17” & 20” LCD:
    The acceptable level is inferior to:
    - 4 bright sub pixels
    - 2 bright contiguous sub pixel
    - 0 white pixel (which corresponds to 3 bright sub pixels)
    - Distance between 2 faulty bright dots superior to 15mm
    - Number of faulty dead bright pixels within 20 mm of the center inferior to 3
    Total of any type of bright dead pixels inferior to 4.

    19” & 23” LCD:
    The acceptable level is inferior to:
    - 3 bright sub pixels
    - 2 bright contiguous sub pixel
    - 0 white pixel (which corresponds to 3 bright sub pixels)
    - Distance between 2 faulty bright dots superior to 25mm
    Total of any type of bright dead pixels inferior to 3.

    It would be a good thing if Philips simplified this policy. It must be tricky for example for after sales services to explain that they can’t proceed to the exchange because the distance between the two faulty pixels is superior to 15 mm…






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    S à Z : Samsung, Sony, ViewSonic...

    S à Z : Samsung, Sony, ViewSonic...

    Samsung has launched a new policy last January. Samsung will provide 14 days exchange service from the actual date of purchase. After this delay the warranty is back to normal conditions.

    This Zero Bright Dot warranty will be realized in several steps. The first one took place in January: all X and P series are now covered by the ZBD warranty. In February, the ZBD warranty was supposed to be extended to TV and DVI monitors equipped with a numerical interface (DVI), but Samsung has put this new program in stand by. Out of the four steps announced last January, only the first one has been applied.

    Sharp offer two 20" LCD screens. On the top of the line product their policy on white pixel is 0. This is the reason why two responses are included in the table.




    ViewSonic recently changed their dead pixel policy. Since last spring, their screens have a “no fully malfunctioning dead pixel” warranty, meaning permanent white or black pixels. For sub-pixels, ViewSonic sticks to the ISO 13406-2 norm.





    Page 10
    Conclusion

    Conclusion
    The main question: Why not a zero dead pixel warranty? All manufactures answered that monitor prices would steeply increase. This answer brings us to another question. How many screens are affected by this problem? Is it really one out of two screens? Even if it is very difficult to obtain official figures, one manufacture told us that 3% of screens had problems with dead pixels. This is a small number and if it is that amount, the manufacturers’ answer regarding the potential rise in prices really isn’t consistent…

    Stores that offer money back guarantees

    One possibility is to buy your flat screen from a shop that offers a money back guarantee. If a dead pixel is found, you will be able to take it back without even calling the manufacturer’s after sales service.

    The last option: after sales services

    It’s possible that dead pixels will appear after a warranty expires, and only one dead pixel located in the middle may be enough to ruin your flat screen. Your last possibility is to call the manufacturer’s after sales service. We advise you to remain calm and polite. Explain that it’s very unpleasant use your computer with such a bright pixel on the screen. Some manufacturers will respond that according to the ISO norm they don’t have to make an exchange but they will make concessions if the dead pixel is located in the middle of the screen. If the dead pixel is located near the edge this may be more difficult.

    The future?

    ISO said last year that they were working a new version of their norm. Of course we think that flats screens should have a zero dead pixel warranty, or at least this should be applied for bright dead pixels. Permanent black pixels are less disturbing and sometimes even invisible.
    We also hope that this new version will bring new rules regarding response time and contrast ratio measurements…


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