Adjusting iMac 24″ Screen Brightness
If you need to calibrate the iMac 24″’s screen, use ColorEyes. I use a Gregtag i1 Display v2 and there’s simply no way to bring down the brightness level of the screen luminance to 120cd/m2 when profiling with the supplied software. The lowest the screen would go using the iMac’s brightness control is about 250cd/m2 which is still to bright. ColorEyes will automatically adjust for the screen brightness when calibrating the display which makes viewing much more pleasant.
There is a demo version available.