# Question on testing LCD display

## satimis

Hi folks,

Just purchased a new Samsung 24" display, model 2494HM.  I need to test it for dead pixels.  Because the shop didn't test it, saying that if there are more than 3 dead pixels, Samsung will replace a new LCD display within 7 days.

Google "how to test LCD display" found following link;

LCD dead pixel test - Dead Pixels Test

http://www.gdargaud.net/Hack/DeadPixels.html#LCD

But I have no idea how to proceed.  Could you please help.  TIA

B.R.

satimis

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## NeddySeagoon

satimis,

First of all - don't test it for dead pixels.  Use the display for a week or two and see if you notice any in normal use. If not its fine.

The thing about dead pixels is that once you know where they are, you will see them and they will annoy you. Even if its just one or two, which is inside the tolerance for dead pixels.

Its rare that a whole pixel is dead, normally its single subpixels. The may be dead in two ways, hot, always on and cold always off. 

To spot hot pixels, look at the srceen very carefully while its displaying a black image. You don't have to do the whole screen at a time, moving a black window around is fine.

You should never see any red, gree, blue or white spots in the black window.  These are hot dead pixels.

Testing for cold dead pixels is similar - use pure red, green, blue and white in turn ... this time look for black spots. This is much harder.

When you are using white, any pixels with a dead subpixel will look cyan, magenta or yellow, depending on which subpixel is dead.

Like I said though, its better not to know, unless it disturbs you in normal use.

[edit]

You can be pretty sure Samsung tested it because warranty repairs are expensive - they are funded directly out of profit, they really hurt the bottom line.

Its lower cost to catch these things before they are shipped

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## satimis

 *NeddySeagoon wrote:*   

> satimis,
> 
> First of all - don't test it for dead pixels.  Use the display for a week or two and see if you notice any in normal use. If not its fine.
> 
> The thing about dead pixels is that once you know where they are, you will see them and they will annoy you. Even if its just one or two, which is inside the tolerance for dead pixels.
> ...

 

Hi NeddySeagoon,

Lot of thanks for your detail info

B.R.

satimis

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## eccerr0r

I guess I was a fairly early adopter of LCD displays (meaning, my crappy displays costed an arm and a leg) and ended up with a 19" and a 17" both with a bad pixel.  Because TFTs actually turn pixels *black* (subtractive) a dead transistor is unable to blacken a particular subcolor of a pixel.  The two monitors I have that have a defect have a blue and a green pixel stuck on, respectively.  Keep in mind there are three transistors per pixel, and I'd be unhappy with white, cyan, magenta, or yellow bad pixels (because that would mean two or all three transistors at that pixel were defective).

I've actually gotten fairly used to the bad pixels, I don't really notice them anymore -- to the point that sometimes I have to actually hunt for the pixel again.  Luckily they're only one per display and is just a speck.  Of course when I have a black screen showing (say, with xsetroot -solid black ... and minimizing all your windows), they become fairly clear of their locations.

The unfortunate thing is my 17" -- the first one I got before I had to RMA it due to becoming unusable -- had no bad pixels (one weak one however, as it doesn't turn fully on/off but was nevertheless functional).  The swapped display has one completely bad green transistorl  :Sad: 

I've rarely seen "stuck on" pixels where they stay black.  Seems like a fairly uncommon failure mode as far as I understand about LCD technology?

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## NeddySeagoon

eccerr0r,

I was involved in the development of early LCD displays and 10 bad pixels on a 640x480 display was considered OK!

There were some complex rules abut how they were spread out over the display but perfect displays in those days were rare.

LCDs can be made both ways, the transistor serves to rotate the plane of polarised light passing through the cell.

If the display uses parallel polarisers, turning the transistor on makes the cell dark, if the display uses crossed polarisers, turning the transistor on makes the cell light.

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