If you are running your laptop on the battery, and want to temporarily prevent the screen from dimming after a short period of inactivity, you can use the 'Disable Screensaver' hot corner (set in the Desktop & Screen Saver System Preferences panel) to also prevent the screen from dimming.
In my tests, the screen would dim after 60 seconds of inactivity when display sleep was set to five minutes.
You can also permanently turn off screen dimming by opening the Energy Saver System Preferences panel. Select Settings for Battery, click the Options tab, and uncheck 'Automatically reduce the brightness of the display before display sleep.'
Mac OS X Hints
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