Many owners of iMac computers, particularly the aluminum fronted Intel models, have found that their monitor will not dim down sufficiently to reach the recommended LCD luminance value of 120 cd/m2. The design of the Apple iMac is such that there is no immediately apparent way to change this.
Some people will then explore in their System Preferences to see if they can find some "hidden" buttons to further lower the monitor's brightness. This will often cause image quality problems. We are not aware of any correct method of making the iMac monitor dimmer, once you have the brightness slider all the way to the left (minimum).
We are aware of some third party utility programs that claim to lower the brightness of the monitor even further than Apple's own controls. They do an effective job of reducing screen brightness, but at the expense of color temperature accuracy. X-Rite cannot recommend the use of any of these program, and cannot troubleshoot the color issues that they might introduce.
The design strategy of Apple monitors in general is to be minimal in the number of controls that are available to the end user. This doesn't mean that the monitor is any less capable of excellent color reproduction on screen. The lack of adjustment range in brightness is more often found on iMacs, rather than on Apple Cinema monitors.