I assume you have held down the function key and press the up arrow key?
When a Dell™ computer is running on battery power, you can conserve power by setting the brightness to the lowest comfortable setting by pressing <Fn> and the up- or down-arrow key on the keyboard.
NOTE: Brightness key combinations only affect the display on your portable computer, not monitors or projectors that you attach to your portable computer or docking device. If your computer is connected to an external monitor and you try to change the brightness level, the Brightness Meter may appear, but the brightness level on the external device does not change.
You can press the following keys to adjust display brightness:
Press <Fn> and the up-arrow key to increase brightness on the integrated display only (not on an external monitor).
Press <Fn> and the down-arrow key to decrease brightness on the integrated display only (not on an external monitor).
Taken from http://www.solano.edu/technology/data/D630/display.htm