Apple's LED Cinema Display (27-inch) includes a built-in ambient light sensor, which makes automatic brightness adjustments on the display based on current lighting conditions.
To calibrate these models using the Apple Display Calibrator Assistant, temporarily turn off ambient light sensing in the Displays System Preferences:
- Choose System Preferences from the Apple () menu.
- Choose Displays from the View menu.
- Disable the option "Automatically adjust brightness as ambient light changes".
You should also turn off ambient light sensing while you make ColorSync profile adjustments with ColorSync Utility or while you calibrate with any display calibration products not made by Apple. This should help ensure a consistent graphics workflow, regardless of lighting changes that may occur while you create the profile or calibrate the display. Re-enable ambient light sensing after you complete your adjustments.