I have a late 2009 iMac 21". I recently noticed what I thought was a water stain on the screen, but then I realised it wasn't on the surface of the glass, so I got interested.
There's what looks like a water stain, and a verticle line of sorts - but: it doesn't show when the screen is white, or when pure RGB colours are displayed. It only shows for anything with greyscale, or CMYK colours. So I notice it most when watching films, where the colours are variable.
In the following screenshots, you have white, RGB pure yellow/red/blue, and lastly 50% grey (where the blemish and line are obvious towards the left of the screen). It barely shows using CMYK yellow, but is more obvious in CMYK magenta or cyan (I didn't get photos of those, since the grey tells the story).
Something's definitely rotten, I must get it looked at.
_________________________ If it's brokenless, don't suffix it...
Try pulling off the glass and see if it is just dirt. If I'm seeing it right the curved watermark thing going on doesn't look electrical or like the LCD screen losing traces.
I happen to have a suction cup like in the vid, it's used for placing tiles when doing a floor. Any decent kind will do. Mine cost $7 US. You can probably use a couple of those clear little ones that stick to a window to hang stuff too.
Make sure you a have a good lint free cloth to wipe it with if your the anal type. Any dust underneath can be seen when the screen is off, but usually disappears when the screen is lit up. A can of compressed air helps. Blow it off then quickly put the glass back on.
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