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Pixel defects explained

When people call us for advice or with questions on buying a new TFT monitor, one of the ‘fears’ that is common is that they’ll end up with a monitor with some kind of pixel defect, so I thought it would be useful to go into things in a bit more detail.

Firstly, the incidence of pixel defects on Eizo or NEC screens is very, very small. I don’t have any official figures to show you, but from our own experience, it’s way under 0.1%. So it’s important to put the possibility of a defect into perspective – people get very hung up worrying about it.

No monitor manufacturer guarantees that their display will have absolutely no pixel defects*. The reason is simply that with the way the panels are made, there is always a slight chance of a speck of dust getting between the various layers that make up the panel, which in turn can cause a display issue. Factories are meticulously clean to counter these problems, but the possibility remains.

If the manufacturer agreed to replace a panel that had a pixel defect, this would add significantly to their costs, which in turn would make the product more expensive, and in all probability make you less inclined to buy it due to the higher price.

* When NEC launched the latest SpectraView Reference 241, 271 and 301, they introduced a 6 months pixel defect warranty. Although we’ve not seen the small print to see how ‘watertight’ their claims are, it is a step in the right direction for consumers. The NEC warranty does not cover partial sub-pixel defects, also known as ‘cut’ pixels, however, we are told that this defect is incredibly rare.

What’s a Pixel?

This is an over simplified explanation for the sake of brevity and clarity, so please excuse me! A TFT (Thin Film Transistor) display works by shining light (the backlight can be either CCFL or LED lamps) through a panel made up of ‘pixels’. Each pixel or dot (actually it’s a square) is really made up of 3 sub-pixels – red, green and blue which in effect are tiny switches or transistors, that can alter the amount of light that passes from the backlight, through the coloured sub-pixel filters and therefore to your eye. By allowing maximum light transmission through the red, green and blue sub-pixels, the effect to the observer (ie you) is that the pixel is white. It follows that black is achieved by stopping light coming through. Every other colour and shade can be achieved by altering the amount of light let through the red, green and blue sub-pixels individually.

Now if this is all sounded complicated, try this – a typical 24″ display has 1920 pixels across by 1200 down, which is 2.3 million pixels to control! Now multiply this by 3 (remember each pixel is made up 3 sub-pixels) and you realise that the electronics in the monitor has to control nearly 7 million sub-pixels on a 24″ display. Perhaps now you can see why manufacturers are reluctant to make guarantees against pixel defects!

Now we know what a pixel and a sub-pixel is, let’s look at what can go wrong…

Pixel Problems

Dead Pixels

People generally worry about a dead pixel, but in reality, truly dead pixels are very uncommon. A dead pixel is usually the result of a failure in the electronics that drive the screen or of the individual transistor. It’s when a pixel (ie all 3 sub-pixels) is permanently off which results in a black dot.

Stuck Pixels

A stuck pixel can be stuck on or off. If it’s stuck ‘off’. it can look like a dead pixel. If it’s stuck ‘on’, the pixel appears constantly white. There are reports that stuck pixels are not necessarily permanent and can be ‘massaged’ back to life, however we cannot recommend people try this as a remedy as you could invalidate your warranty and cause more problems that you had to start with!

Sub-Pixel Failure

A sub-pixel failure is where one of the sub-pixels is stuck on or off, resulting in a tiny red, green or blue dot on the screen. This is the most common type of failure that people experience.

Partial Sub-Pixel Defect

Finally, just when you thought you’d heard it all, there is also a partial sub-pixel defect, but we’re getting into the extremely rare end of the defect spectrum.

The ISO Standard

As of July 2011, both NEC and Eizo adhere to the ISO 9241-307 specification (revised 2008) as class 1 products.

Class 1 panels permit any or all of the following:

  • 1 full bright (“stuck on white”) pixel
  • 1 full dark (“stuck off”) pixel
  • 2 single or double bright or dark sub-pixels
  • 3 to 5 “stuck on” or “stuck off” sub-pixels (depending on the number of each)

As mentioned above, NEC also offer a 6 months pixel defect warranty on some of the SpectraView models (currently the SpectraView 241, 271 and 301) which guarantees replacement/repair for any pixel defects (except for sub-pixel defects aka ‘cut’ pixels).

So if you have a monitor and it’s class 1 ISO 9241-307, then it can have any or all of the problems mentioned and still be within specification ie the manufacturer may not consider it to be faulty. But, as I mentioned at the start, all monitor manufacturers have the same policies (broadly speaking) and the incidence of defects is incredibly small. I’d also say that you do actually get used to a defect – we use an old monitor in our dispatch area that has a permanently red pixel on the screen – when we first used it, it stood out like a sore thumb, but now, I don’t notice it and have to hunt around the screen to see it. Pixel defects are annoying, but they’re not the end of the world!

 

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