Hi there, As other users have pointed out, the light sensor does adjust screen brightness depending on ambient light, but there is a delay of a couple of seconds. The other thing it does is activation of the backlight on the keyboard. So in bright light the backlight is off, and it comes on in dim lighting. I have also installed Simple Brightness Applet (good app!) which is useful for changing the screen brightness in large steps. This control is normally hidden in the settings menu. It doesn't disable the ALS since the ALS only makes small changes (I think one step up and one step down).