There is a file in Ubuntu which stores numerical integer value of brightness.
you will find 3 files in the directory /sys/class/backlight/<VGA>directory
replace directory with intel_backlight for intel cards.

You will find the max brightness value in max_brightness file and according to that value set the brightness in brightness file.

You can install xcalib and then type the command:

xcalib -co 50 -a

Use xcalib -h for help regarding it's options.

As Colin Ian King said, the levels of brightness are hardware related.

Some laptop screens have 8 levels of brightness and new ones have 16 levels.

On Windows OSes you can change the levels with 1% steps, but they will be rounded to the nearest hardware level. It depends on the screen type.

My solution was to install 'xbacklight'

Then I created keyboard shortcuts such as

• 
  ctrlshift+ with command xbacklight +5
  


• 
  ctrlshift- with command xbacklight -5
  

So when you use those shortcuts they call the related command
however I could not replace the system inbuilt brightness buttons to use these commands
You can use almost any keyboard shortcut button combination you like with xbacklight.

The brightness levels are generally under ACPI control with the levels defined in firmware. For example the ACPI control method _BCL "Query List of Brightness Control Levels Supported" informs the kernel how many brightness levels are supported. You can't realistically change this.

In my case (Lenovo T500) the problem is, that the brightness regulating key-press is applied twice - once by using the Xwindows, but also independently in the lower level, by kernel's graphic driver itself. So the brightness steps are twice as big as usual.

The working solution is to deny the low-level functionality, by adding this line to /etc/rc.local (just before the line with 'exit 0'):

echo -n 0 > /sys/module/video/parameters/brightness_switch_enabled

This way it will perfectly work in logged-in X session, unfortunately it will remove the key-press brightness regulation functionality in all other cases (console terminal, X login screen etc).

On Ubuntu 16.04, with Intel onchip graphics (i5) under LXQt, it is sufficient to assign xbacklight commands to the brightness function keys XF86MonBrightness(Down|Up) using the global key shortcut settings. These seem to override the defaults even if /sys/module/video/parameters/brightness_switch_enabled is set to Y, while the original functionality on virtual terminals is retained.

For me xbacklight did not work (EDIT: probably because I did not reboot my machine), therefore I created my own script which uses the built-in gsd-backlight-helper.

This script uses smaller steps, especially when the screen is quite dark already.

Step 0: check if it works (optional)

To see if it works you could try:

pkexec /usr/lib/gnome-settings-daemon/gsd-backlight-helper --get-brightness

This should output a number.

Step 1: save script

Save the following script in an .sh file, for instance in /home/me/scripts/brightness.sh.

#!/bin/bash



max=$(pkexec /usr/lib/gnome-settings-daemon/gsd-backlight-helper --get-max-brightness)

cur=$(pkexec /usr/lib/gnome-settings-daemon/gsd-backlight-helper --get-brightness)

if [ $(($max/$cur)) -ge 25 ]

then

    step=$((max/300+1))

else if [ $(($max/$cur)) -ge 5 ]

    then

        step=$((max/50+1))

    else

        step=$((max/20+1))

    fi

fi

case "$1"

in

+) new=$((cur+step));;

-) new=$((cur-step));;

esac

pkexec /usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness $new

Step 2: assign shortcuts

Open Keyboard Shortcuts from the Settings menu and assign shortcuts:

name:     Darker

command:  sh /home/me/scripts/brightness.sh -

shortcut: Super + F5



name:     Brighter

command:  sh /home/me/scripts/brightness.sh +

shortcut: Super + F6

Brightness cannot controlled with large precision. I'm afraid that you're stuck with those brightness levels. To be sure, try controlling the brightness using these terminal commands.

You could try setting it manually. First you have to get the PCI-ID of the VGA device:

lspci

Then try this (in my case the PCI-Device is 00:02.0)

sudo setpci -s 00:02.0 f4.b=FF

The 2 letters at the end of the Line set the new brightness ranging from 00-FF (0-255)

If you are using Nvidia Graphic card..you can easily use my new method of brightness changing,and edit values in Code.. theres two files and two options within each one.

find these two lines:

if ( $value > 0.0) { $value = $value - 0.30 };  

if ( $value2 > 1.1) { $value2 = $value2 - 0.08 }; 

change values of ($value > 0.0) & ( $value2 > 1.1)
and see whats happens!

meanwhile if you dont want to change values and wish to use my method originally you can
change brightness in a wide range and 5 steps.
wish you like it

link of method: https://askubuntu.com/a/179063/82136

For Xubuntu LTS

Firstly, install xbacklight

$ sudo apt install xbacklight

Secondly, check whether you have control over the backlight.

$ xbacklight -1

$ xbacklight +5

Should these commands result in a No outputs have backlight property error,
then follow these remediating steps before proceeding.

Once xbacklight -1 and xbacklight +1 work from the command line, proceed with assigning these commands to respectively the XF86MonBrightnessDown and XF86MonBrightnessUp keys. This is done by hitting those keys when asked by the Settings → Keyboard → Application Shortcuts application.

Finally, reboot for these changes to take effect.

Official documentation:

https://www.x.org/archive/X11R7.5/doc/man/man1/xbacklight.1.html

inc percent
Increases brightness by the specified amount.

dec percent
Decreases brightness by the specified amount.

Example:

xbacklight -inc 10%

xbacklight -dec 10%