So... I 've been tired searching and searching on internet and nothing specific appears to solve the following problem.
I use a Sony Vaio laptop. The model is the VGN-FZ31E. To not make someone searching what is the graphics card, I tell you it's the NVIDIA GeForce 8400M GT.
The problem is that I can't control in any way the screen's backlight. Not the brightness of the appeared objects on the display but the light. One more hint is when I plug out the DC adaptor and the laptop continues on battery mode, although the "Reduce backlight brightness" in power manager is checked, the backlight brightness does not reduce.
I booted with Ubuntu 10.04 from a USB flash drive and the backlight brightness could be increased and decreased. So this tells me that it can work. The problem is how. I use now Ubuntu 10.04 and this was updated from Ubuntu 9.10 at least, but I don't remember if it was updated to 9.10 from 9.04. When I say update from 9.10, I mean without format. Just update from the update manager.
Hope someone helps...!