Try this:

1. Open a terminal (Ctrl + Alt + T).
2. Then type sudo nano /etc/default/grub. It will ask for your password. Type it in.
3. Around the 11th line, there will be something like: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash". Change it to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
4. Save the file by Ctrl+O followed by Ctrl+X. Then run sudo update-grub in the terminal.
5. Reboot and see if backlight adjustment works. If not, undo the changes you did above, by invoking the text editor as in steps 1 and 2.

Hope it helps.

Works for Acer Aspire v3-571,Acer Aspire v3 571g,Hewlett Packard Bell EasyNote TS,Acer Aspire 4755G,Acer Aspire 5750-6866, Acer Aspire 5739, Lenovo T540p

OP reported in Revisions 2 & 3 of the question that the following worked for him.

I figured it out from different sites, it fixes backlight.

Run the following command in Terminal:

gksu gedit /etc/default/grub

then change

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

to

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
GRUB_CMDLINE_LINUX="acpi_osi=Linux"

then save and run:

sudo update-grub 

and then restart the system for changes to take effect.

Ubuntu 14.04 (13.10+) with intel graphics

How to check if graphics card is intel

First, check if your graphics card is intel. You can check it from System Settings->Details->Graphics or with following command:

ls /sys/class/backlight

You should see something like:

ideapad  intel_backlight