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Reactivating the Backlight
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Overview

Some people - particularly those using laptops - are encountering a problem where the screen brightness is too dim upon replacing Microsoft Windows with a Linux distribution as their main operating system. Although it affects all Linux distributions, the problem is actually due to the computer's BIOS settings. Certain hardware manufacturers have set it up so that if Windows is not detected running on their systems, the backlight is automatically disabled.
Reactivating the Screen Backlight

This problem can be easily fixed by ensuring that the GRUB bootloader re-activates the backlight. To do this, first open up your terminal, and enter the following command:

sudo sed "s/\(GRUB_CMDLINE_LINUX=\)\"\"/\1\"acpi_osi=Linux acpi_backlight=vendor\"/" /etc/default/grub -i

You will also have to enter your password to continue. Now enter the second and final command:

sudo update-grub

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Brightness hotkeys

Problems with brightness keys are most often vendor and bios dependent.

Acer

    sudo gedit /etc/default/grub

Add the following parameters to grub configuration:

    GRUB_CMDLINE_LINUX="acpi_osi=Linux acpi_backlight=vendor"

Update Grub:

    sudo update-grub