Many end users with Apple iMacs are reporting that the monitor will not dim down enough to reach the 120 cd/m2 level with the standard Apple controls. Some of these same end users have found a third party program to allow them to reach the desired luminance level. I would suggest that you check out Shades, from Charcoal Design in the UK. This program is freeware, and gives you a control that begins dimming the brightness level, where the Apple control ends. We don't manufacture this software, and we are not endorsing its use, but it has been useful for many of our end users.