I just had the same experience. Received my new (late 2012) 27" iMac a couple weeks ago. I had it set up side by side with my older (late 2009) 27" iMac. I noticed quite a difference in brightness and color. The new iMac is, as you say, at least a couple notches dimmer than the older unit. I am particularly concerned about how white is an slightly off grey. I spoke with Apple and they kindly agreed to replace the iMac although they had not heard of this issue. Just fired up the replacement and it's the same issue again. It is disappointing. Clearly not a faulty display but the specs must fall short of the older models.
Now I need to figure out how I am going to calibrate so that I can get something close to accurate and match my older unit. I have the older unit connected as an extended display with a MiniDP cable so I guess if I need brighter, accurate colors I can work in that space.
I went into the Apple store with my old machine and compared it to the new ones and found the same issue with the store models. I was told that because of the new isp display and how thin they are that they are entirely different technology than the older model LCD screens.I had to re-calibrate my imac 3 times because over a month the colors shifted from a very low contrast grey to blue tones. So far my last calibration that was done in late Feb seems to be fine.
I think that these screens need to fully burn in after many hours of use but compared to my 2009, i never had any of these issues. I haven't sold my 2009 yet as i'm slightly afraid that something still could happen to the new one so i've been holding on to it as an emergency backup. I also had major issues with my MBPR which had to be replaced due to screen problems too.
Its great having things super thin but how bout holding off a few more months and get the tech working correctly rather than having apple customers pay top dollar to be beta testers.
good luck with yours. let me know what happens with the replacement.
I received my replacement very promptly. Unfortunately, same problem. New unit remains dimmer than the 2009 and colors are out of sync. I have checked again the color profile and drivers, and set auto brighness levels. Just waiting to find a few hours to spare so I can call Apple support and perhaps talk with advanced support to try calibrating my display wti their guidance. I had given some thought to going out and buying a screen calibration unit like Spyder but have not done so yet. Ideally I will get these two units matched side by side.
I have a late 2009 21.5 iMac.
Last night my friend brought his mid 21.5 2010 iMac here so I could upgrade him to Yosemite ( I have very fast internet).
Anyway, side by side, the 2010 iMac is EXTREMELY bright compared to my late 2009 iMac.
Here is a photo that I took of them to give an idea. I posted a question about it elsewhere here but no responses yet.
You can see the color profiles for each. The 2010 is on the left, 2009 on the right. I wonder if there's a way to adjust the color profile to match the 2010?
If you look ...you can see mine has a grey tinge ....(it looks yellow on my monitor but on the camera and to my eyes, mine is definitely a grey tinge. The 2010 is bright white no matter where you look at it.