Mac Book Pro with OS X Lion installed on the system by default dims the display when it is on the battery power source. This is an useful feature and reduces the draining of battery power when laptop is on battery power source. But if you want to disable this feature then you can use the option provided as part of Energy Saver.
Click the Apple icon and select System Preferences from the menu list.
In the System Preferences window, navigate to Hardware section and click the Energy Saver option.
Then un mark the check box with label as “Slightly dim the display when using this power source“
Please note that disabling this feature would quickly start draining your Mac Book Pro battery.