Linux Mint: Can't change my laptop's screen brightness! edit

Edit /etc/default/grub and add acpi_osi, acpi_backlight options.

GRUB_CMDLINE_LINUX_DEFAULT="i915.modeset=1 acpi_osi= acpi_backlight=vendor"

Also checkout the kernel parameters under /sys/class/backlight/

References

http://forums.linuxmint.com/viewtopic.php?f=109&t=113900