So, I started to find out workarounds:
There is a built-in software called gnome-control-center, so I tried to find out the solution from it.

We can see that there is a bar controlling monitor's brightness. Drag to the left for darkening, and vice versa.
I've downloaded the source code for 12.04 [site], but I still couldn't find the code snippet which controls the monitor's brightness.
Could anyone help me? Thanks.
xbacklight, for examplexbacklight -set 80but typically this means that you have some problem with your drivers. Which driver did you have installed ? – user2485710 May 06 '14 at 17:50Software & Updates -> Additional Driverswhich is basically the oldjockey-gtk. Other methods are likely to be unsupported and you are supposed to know what you are doing as you do it. Wait for someone experienced with Nvidia software. – user2485710 May 06 '14 at 18:05