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.

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.

OMG! Are you kidding me? I would have never thought the glass was a panel held on by magnets. LOL.
Kewl.

On another note.. Pad... late 2009?. I guess you didn't extend the warranty?

Dang...I'm tempted to pull my Imacs glass now....
It's gonna bug me now.... Must Pull Glass...Must Pull Glass...

Heading to Harbor Freight to see if they have a large suction cup.

Tempting, isn't it?

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.

You all can have fun with that last part.

It's still (just barely) under AppleCare warranty. I'm going to take it in next week and let them do the sucking and blowing for me .

I'm intrigued as to why it's only visible when the colours aren't "pure", though. I love a mystery.

Me too. I'd figure dirt would be more visible when it's shut off and would show up with any color. Let us know what happens.

Good for you that it's still under warranty.

A little late to this discussion, but I've had the same issue with my Cinema Display from, lo, those many years ago. Only mine has been slowly migrating across the screen from left to right.

Checking around, the theories that makes most sense are that it's some sort of contamination of the LCD material or perhaps de-lamination of some sort ...