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In a multiple-monitor system, the luminance of all monitors must match must match so that color perception is not affected.
The first time you calibrate the monitors, check the maximum luminance level of each monitor and then set the luminance for all monitors to 200 or 10% below the maximum luminance value of the monitor with the lowest maximum luminance if not achieving 200.
Note: Replace the monitor when it can no longer achieve a luminance level of 120.