This post is not intended for those who can adjust their screen brightness via GUI without much difficulties. I mention this command line option considering a lot of complaints from users controlling the brightness through GUI. Disregard this if you are comfortable with controlling brightness via GUI. If your hardware drivers(especially regarding screen) are free from errors, then you can adjust your screen brightness with the help of xbacklight utility.  xbacklight may not be installed by default. So you must download and install the utility. Don’t worry, it is of negligible size(may be less than 1 MB).

After installing run the following command
xbacklight -get
which displays your brightness %.
To set brightness to a particular % run
xbacklight -set X
where X is the required %.
You can get to know more options regarding xbacklight by running
man xbacklight