iMAC screen FOG

Just wondering if anyone out there is having an issue with screen FOG? I have a new iMAC with 27" screen bought primarily for photography editing.

I find if I turn it off overnight then turn it back on the screen will slowly develop a fog which then takes about 2 hours to clear. I can repeat the problem so I need to know if all iMAC screens do this if turned off?? - it is obviously condensation inside the screen and I can't edit my photos until it clears - very disruptive to photography editing. thanks - in anticipation of some advice! I did speak to my local supplier who said it isn't a "known" problem but I have seen a few comments about similar problems on the Web. Helen