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Today, I've cleaned my laptop screen and noticed white spot that can be seen behind a white background pixels. There are about 3 or 4 of these little spot on the right and left side of screen. It seems like they are formed when something had hit with screen. Any idea what can they be, if they are permanent?

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Well... with LCD, white isn't so much a color as it is the absence of any filtering in the liquid crystal, allowing just the white background to reflect light through. That said, you can see the white spots behind white background pixels. What happens when there is a color over that spot. Do you still see some kind of mark there? Are you sure it is not something on the screen (sneeze debris, etc) that didn't come off when you cleaned it?

If they stay white regardless of the color there, but only show when the screen is on, then they are most likely now dead pixels.

But it is really hard to mark up the background reflector that is behind the LCD panel.

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I've just tested it behind pure red, green, blue background and yes they are there. But behind a black background they cannot be seen, it is just black. – user103231 May 4 '12 at 2:31
The white reflector behind an LCD panel is typically hard plastic, albeit thin plastic... as opposed to a simple paper that could be torn or marked up due to a gentle impact. You have definitely determined it is neither dead pixels nor in front of the LCD layer. It could be something between the last layer of the LCD panel and the reflector. Not that I'd advocate you take apart the LCD panel (I've done this many times tho), but can you localize when the damage first appeared? – Bon Gart May 4 '12 at 2:36
Yes, I have marked these white spot with red in this sample where they appear on my screen. – user103231 May 4 '12 at 2:51
How about when? They almost appear to be hand damage... as if you (or someone) was holding the laptop when it was closed, and their fingers or edges of their palms were at those points on the lid. The edges of laptop LCDs are typically sealed with foil tape, and the panels aren't known to be loosely assembled, but excessive screen twist (opening the lid from one corner repeatedly) can separate the layers of an LCD, and theoretically allow debris between the reflector and last layer, provided the foil tape was also damaged, but that's a stretch. – Bon Gart May 4 '12 at 2:55
I guess it could be about an year ago when I used to take my laptop out with me and I remember when somebody tried to grab it from screen. But today it is the first time I noticed them. You have to look closely in that area to able to notice them. Is there a chance that it will become more damaged by just little pressure? – user103231 May 4 '12 at 3:12

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