How to fix uneven screen brightness with Debian on the Acer Aspire 4739 laptop

This guide assumes a Debian-based Linux distribution like Linux Mint. Some steps may be different in distinct bases.

I had this issue with my laptop. It wasn't a hardware issue, since during the entire boot process the entire screen brightness was ok. To fix this issue, you need to add a kernel boot parameter. You can do that through:

Summary

Graphical interface
Command-line

Graphical interface

1. Open the System Administration program. Under the "Boot parameters" section, click on the "+" (plus) sign.

System Administration program in Linux Mint

2. Type acpi_backlight=intel_backlight and click Save. If that doesn't work after restarting the computer, replace it with acpi_backlight=native.

Adding a kernel boot parameter with the System Administration program in Linux Mint

3. Click Save and wait until it finishes updating GRUB (the boot manager).

4. Restart the computer.

Command-line

1. Edit the GRUB (boot manager) settings with a text editor like Nano:

sudo nano /etc/default/grub

2. There, add the parameter acpi_backlight=intel_backlight and save. If that doesn't work after restarting the computer, replace it with acpi_backlight=native.

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=intel_backlight"

3. Update GRUB:

sudo update-grub

4. Restart the computer.