Can you please help me ?
I tried everything I found and nothing worked. For example, I modified grub configuration file, installed xbacklight and tried setpci command.
Your best bet would be to reference this post, while it doesn't reference your exact machine many users have used this patch to fix their screen brightness problems. If you are starting to wonder about Toshiba Satellite backlit keys as well (just taking a jump here), its a known bug; and it has been flagged with a low priority by Canonical.
Type su and enter root password in terminal
And then type this :
echo "x" > /sys/class/backlight/acpi_video0/brightness
where x = 0 - 7
-If use nVidia just open dash home and type "NVIDIA X server setting",
-Then click "X Server Color Correction" under X Screen O,
-Reduce the Brightness level,
-Click "Confirm current changes" before the time runs out,
-Choose Quit to save the change.
It worked for me with the following spesifications:
Toshiba Satellite L745
Intel Core i3
NVidia GeForce with CUDA
RAM 6 GB