This is the most general problem that when you install fresh Ubuntu OS or any Ubuntu based OS like Linux Mint , Zorin OS etc , you will face problem of adjusting brightness of your PC / laptop. Even if you will try to reduce brightness from System Setting , brightness will not be reduce .
If you are facing the same problem , and finding a good solution for that then you are in right place.Below I have described every steps that need to be done to adjust brightness.
The software you have to install is Brightness Controller which is written in Python language.

For Ubuntu

If you are using Ubuntu Os then all you have to do is open Software Center and search for the Brightness Control . That’s it install the software named brightness control.When you run the software you will see the scroll which indicate brightness of your screen , use it to adjust brightness.

For Other Linux

If you are using Other linux based on Ubuntu Distro then first of all you have to install python.Run the followin command to install python.

$ sudo apt-get install python-wxgtk2.8

After that download brightness controller from git repository.Here is the link to Brightness controller and after that unzip it or You can execute following commands.

wget https://github.com/lordamit/Brightness/archive/master.zip
unzip master.zip

Now to run the software navigate to src directory and then give executable permission to brightness.py file and the run that file.Just follow below commands.

cd Brightness-master/src/
chmod 777 brightness.py
./brightness.py

When you run the software you will see the scroll which indicate brightness of your screen , use it to adjust brightness.
Update
I am using sony vaio with “NVIDIA GRAPHIC CARD” and I have a problem that Function keys to adjust brightness is not working on Linux Mint , I have recently solved this problem without using Brightness controller app.I think that brightness controller is not actually reducing but it’ll create effect like reducing brightness or create layer having tranceperancy.So I have decide to find genuine solution on Linux Mint Forum which solve my problem.Below is the trick to solve problem.
First of all look for xorg.conf file in /etc/X11 . If that file is not there then don’t worry just run the following command.

sudo nvidia-xconfig

This command is basically create xorg.conf file in /etc/X11 directory.Now look at that file and search for the text Section “Device”.In that section add below line.

Option "RegistryDwords" "EnableBrightnessControl=1"

After that restart OS or restart Xserver.Now I think your function keys for brightness will work.
I have found other solutions also but those are not working in my laptop.I am listing those below.
Run following command in terminal.

sudo nano /etc/default/grub

Now find the line GRUB_CMDLINE_LINUX_DEFAULT=”quite splash“
and try to edit that line as same as one of the following line.

GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash acpi_osi=Linux”
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
GRUB_CMDLINE_LINUX_DEFAULT="quite splash acpi_osi=Linux acpi_backlight=vendor"

At last don’t forget to run following command.And then reboot your PC/Laptop and test your function keys.

sudo update-grub

Let me know if you have any queries.

External Sorces

Linuxg
Zip archive for Brightness Controller