I have a Lenovo U460 laptop running Ubuntu 10.10. During normal use, the brightness of the screen will suddenly start fluctuating rapidly up and down for up to 30 seconds at a time. This occurs once every few minutes, and it happens in a wide range of ambient light levels. During this time, the screen brightness indicator will appear in the upper right corner of the screen and the indicator bar will flicker back and forth as the brightness changes. While this is happening, the Fn- hotkeys that control brightness have no effect as if they are being interrupted/ignored. When this spontaneous brightness fluctuation is not happening, however, I can control the brightness with the hotkeys as expected.
I can also use gnome-power-preferences (Preferences > Power Management) to change brightness. Strangely, gnome-power-preferences seems to set the screen brightness but does not get the current value. When started, it shows a value for "Set display brightness to:" that does not match the actual screen brightness, and when the screen brightness changes (either due to this spontaneous flickering bug or when I press the hotkeys), the value shown is not updated. Setting the screen brightness using gnome-power-preferences does not stop the screen brightness from fluctuating later.
I want to stop the spontaneous fluctuation of screen brightness. I suspect that it might have something to do with the ambient light sensor, but I can't find where the ambient light sensor is located on the laptop; if I did, I would try shining a flashlight on it or blocking it to see if this changes the screen brightness. I also don't know if the ambient light sensor is enabled at a software level in Ubuntu Linux, and if so, where I could disable it completely.
$ uname -a
Linux alice 2.6.35-23-generic #40-Ubuntu SMP Wed Nov 17 22:14:33 UTC 2010 x86_64 GNU/Linux