Your phone has a sensor (also called an ambient light sensor) that automatically adjusts the brightness on the screen depending on how bright it is where you happen to be. (That's the "ambient light" part.) You'll likely notice your phone's screen doing dim or brightening throughout the day. The idea is to keep it low, but not too low, to save on battery life.
But what if it's just not bright enough for you? Go to Menu>Settings>Display>Brightness and uncheck the "Automatic brightness" box. You'll now have a slider with which you can adjust your screen.
Most phones (especially those on Froyo) also have widgets that let you adjust to low, medium and high brightness.
We don't recommend running around with your phone's screen turned up all the way, but you're free to do so if you want. Just don't come complaining to us about how quickly your battery drains.
- Tips and Guides