I know this comes up occasionally, and is a well known design flaw/"feature" - this may be common knowledge, but just in case it helps someone.. This is the second iMac I've had that developed this problem in a not particularly dusty environment. It's a late 2012 model (refurb) i bought direct from Apple.. And I've been outside of warranty for years. Anyway, it's a darkish grey tinge around the top corners of the display with weird patterns in the dark patches. (it's not under the glass front panel, it's inside the display, between the backlight and the lcd part - which would be hellish to clean and requires ungluing and untaping things) Apparently screen contamination is replaceable within *6 years* of purchase in the UK due to how we deal with EU consumer law.. Crazy! So if anyone's got this problem, and they bought their iMac from Apple, just get an appointment at the nearest Apple store and they'll likely replace the LCD panel (£280 on the receipt) for free! This must include all the newer models too. I'm really happy not to have to either pay, or clean it out manually! It does mean I'm without the iMac for a week, but it's better than having a nasty looking display..