I recently installed Linux Mint 17.1 and was reminded of a problem I believe I have noticed with previous versions of LM as well.
When I had the laptop connected to a monitor with HDMI cable, I noticed that if I unplugged the laptop, I lost the ability to control the brightness. I could dim the screen but I could not make it brighter.
What seems to have done the trick for me was from a forum thread at LM http://forums.linuxmint.com/viewtopic.php?f=49&t=142017
Changing the grub variable
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor acpi_osi=Linux"
Seems to have fixed this. There are other suggestions in the forum that is worth trying if the problem persists.
Just remember to: