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Dead Pixels: black, white or colorful?

When talking about Dead Pixels, we do not mean the mildly popular British sitcom about three friends fixated on a fictitious online role-playing game, but pixels on an LCD that aren’t working properly: failing to reproduce the correct light levels.

Starting with the basics, on a standard TFT display, a pixel consists of three sub-pixels: red, green and blue. When for some reason one of those sub-pixels is stuck in the wrong position blocking the light from passing through that layer, a small dark dot appears on the screen – that’s the bad dead pixel. When, however, the entire pixel is stuck in the wrong position opposing to the one above, this allows all light to pass through that layer and creates a white pixel which we can call a bright or lit bad pixel. There is another version of that lit bad pixel, and that is a little dot that is either red, green or blue depending on which sub-pixel happens to be stuck permanently letting the light through. While there are more scenarios where a pixel can be faulty, these three summarise the most common situations pretty well.

dead pixel
bad sub-pixels

We have created a simple test that lets you check if your screen has any bad pixels. If you’d like to check it out click the button below. It basically lights up the entire screen in either one of the five colors concerned so that a tiny dot that is different becomes apparent. This is also how technicians sometimes fix the problem; by switching all those colors on your screen you are resetting the stuck crystals. Another method would be applying a slight pressure to where the bad pixel is located on the turned off screen, turning it back on and then releasing the pressure. If they are not completely dead, hopefully, one of those methods can put them back to the right position.

Mother glass has class

Displays consist of various layers, one of them being glass. Glass comes from TFT factories in large sheets called mother glass and is sorted by grades or classes. These classes are governed by the ISO-9241-3xx series of standards which identify three levels of allowable number of defects.

The table below shows the maximum number of bad pixels permitted in each class:

Class 3up to 5 full bright pixels, 15 full dark pixels and 50 single or double sub-pixels stuck on or off
Class 2up to 2 full bright pixels, 2 full dark pixels and 5-10 single or double bright or dark sub-pixels (depending on the number of each; no more
than 5 bright subpixels are permitted)
Class 1up to 1 full bright pixel, 1 full dark pixel, 2 single or double bright or dark sub-pixels and 3 to 5 “stuck on” or “stuck off” sub-pixels
(depending on the number of each)

Class zero allows no defects whatsoever.

The number of bright pixels that are accepted is lower because they shine and so are more visible. Dark pixels, on the other hand, are more acceptable because they are simply black and as such, less visible.

What all that means in practice is that when ordering a certain amount of displays, depending on the class agreed upon, a certain percentage of them will have those defects while the majority will be defect-free. That percentage depends on every manufacturer but is usually really small which is why we do not come across it very often. Riverdi’s displays have practically no bad pixels as we test them all; we haven’t had any such cases even though they are allowed. However, in our 10.1” displays we add an additional layer of tests as standard so we do not miss a single item with a bad pixel. There is a 100% ZERO bad pixels guarantee.